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Brain 2010: 133; 1537–1551
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BRAIN
A JOURNAL OF NEUROLOGY
The symptom of functional weakness: a controlled
study of 107 patients
Jon Stone,1 Charles Warlow1 and Michael Sharpe2
1 Department Clinical Neurosciences, School of Molecular and Clinical Medicine, University of Edinburgh, Western General Hospital, Crewe Rd,
Edinburgh EH4 2XU, UK
2 Psychological Medicine Research, School of Molecular and Clinical Medicine, University of Edinburgh, Royal Edinburgh Hospital, Morningside Park,
Edinburgh EH10 5HF, UK
Correspondence to: Jon Stone,
Department Clinical Neurosciences,
Western General Hospital, Edinburgh,
EH4 2XU, UK
E-mail: [email protected]
Functional weakness describes weakness which is both internally inconsistent and incongruent with any recognizable neurological disease. It may be diagnosed as a manifestation of conversion disorder or dissociative motor disorder. Other names
include psychogenic or ‘non-organic’ paralysis. We aimed to describe the incidence, demographic and clinical characteristics of
cases with functional weakness of less than 2 years duration, and to compare these with controls with weakness attributable to
neurological disease. Both cases and controls were recruited from consultant neurologists in South East Scotland. Participating
patients underwent detailed assessments which included: physical examination, structured psychiatric interview (Structured
Clinical Interview for the Diagnostic and Statistical Manual of Mental Disorders), measures of symptoms, disability and distress
[Short Form (36) Health Survey, Hospital and Anxiety Depression Scale], and assessment of their illness beliefs using an
augmented version of the Illness Perception Questionnaire. In total, 107 cases (79% female, mean age 39 years, median
duration of illness 9 months) were recruited. This number suggests a minimum annual incidence of 3.9/100 000. Forty-six
controls (83% female, median age 39 years, duration 11 months) were also recruited. Compared to controls, cases had similar
levels of disability but more physical symptoms, especially pain. They had a higher frequency of psychiatric disorders, especially
current major depression (32 versus 7%, P50.0001), generalized anxiety disorder (21 versus 2%, P50.005), panic disorder (36
versus 13%, P50.001) and somatization disorder (27 versus 0%, P50001). There was no difference in median self-rated
anxiety and depression scores. Paradoxically, they were less likely than controls to agree that stress was a possible cause of
their illness (24 versus 56%, P50.001). Cases were twice as likely as controls to report that they were not working because of
their symptoms (65 versus 33%, P50.0005). Functional weakness is a commonly encountered clinical problem. Patients with
this symptom are as disabled as patients with weakness of similar duration due to neurological disease. There is a paradox
between the frequency of depression and anxiety diagnoses and the patient’s willingness to accept these as potentially relevant
to their symptoms. We discuss the theoretical and practical implications of these findings for the concept of conversion disorder.
Keywords: conversion disorder; functional weakness; case–control study; psychogenic
Abbreviations: DSM = Diagnostic and Statistical Manual of Mental Disorders; HADS = Hospital and Anxiety Depression Scale;
IPQ = Illness Perception Questionnaire; SCID = Structured Clinical Interview for DSM-IV—Axis 1 disorders; SF-36 = Short Form (36)
Health Survey
Received December 9, 2009. Revised February 18, 2010. Accepted March 9, 2010. Advance Access publication April 15, 2010
ß The Author (2010). Published by Oxford University Press on behalf of the Guarantors of Brain. All rights reserved.
For Permissions, please email: [email protected]
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Introduction
Weakness that is clinically inconsistent or incongruous with any
recognized neurological disease is a relatively common clinical
problem for neurologists. It is typically diagnosed when the weakness has an internally inconsistent quality (e.g. with a global distribution, give way quality, or with a positive Hoover’s sign) (Stone
and Sharpe, 2001). A variety of terms have been used to describe
this phenomenon, including psychogenic or non-organic weakness, motor conversion disorder and dissociative motor disorder.
However, the diagnosis of conversion disorder encompasses diverse symptoms including non-epileptic attacks, movement disorders and sensory and visual disturbance. In this article we use
the straightforward term ‘functional weakness’ to describe the
specific symptom and to avoid making assumptions about its
aetiology.
The limited evidence available indicates that functional weakness is relatively common. The estimated population incidence is
5/100 000 per annum (Binzer et al. 1997), the estimated frequency in new neurology outpatients is 1.5% (Binzer et al.,
1997; Stone et al., 2009) and in neurology inpatients 2% (Parry
et al., 2006).
Despite its frequency there have been only a small number of
case series of functional weakness (Baker and Silver, 1987;
Crimlisk et al., 1998; Heruti et al., 2002; Stone et al., 2003)
and only one case-control study (of 30 patients) published
(Binzer et al., 1997, 1998; Binzer and Eisemann, 1998).
Furthermore, the generalizability of these studies to neurological
services has been limited by their being based on samples of specific groups (e.g. neurological inpatients, patients referred to
psychiatry and patients without somatization disorder) (Binzer
et al., 1997, 1998; Binzer and Eisemann, 1998). It is also difficult
to draw conclusions from studies of patients described as having
conversion disorder (Barnert, 1971; Merskey and Trimble, 1979;
Watson and Buranen 1979; Wilson-Barnett and Trimble, 1985;
Spitzer et al., 1999; Roelofs et al., 2002) as these have included
the diverse symptoms included under this diagnosis (e.g. weakness, blackouts and movement disorder) (Stone et al., 2004c). In
summary, we need to know more about the frequency, clinical
features and aetiology of functional weakness.
We aimed to determine the incidence, demographic and clinical
characteristics of patients with recent onset functional weakness
who had been recruited from inpatient and outpatient National
Health Service (NHS) neurological services in South East
Scotland. The variables assessed included: age of onset; socioeconomic deprivation; symptoms and clinical signs; disability
(including use of aids and appliances); physical and mental
health status [Short Form (36) Health Survey; SF-36], Diagnostic
and Statistical Manual of Mental Disorders (DSM)-IV axis-1 psychiatric disorder; self-rated anxiety and depression; and illness beliefs and work status (including litigation and financial benefits).
We also aimed to test whether there were differences in these
characteristics between cases and a control group of patients
with recent onset weakness attributed to neurological disease (to
control for the effect that experiencing weakness might have on
these variables, as well as factors leading to hospital assessment).
J. Stone et al.
Based on the available literature, we hypothesized that compared
to controls, patients with functional weakness would have many
other physical symptoms (Binzer et al., 1997; Crimlisk et al.,
1998), more self-rated anxiety and depression, a higher frequency
of interview-rated psychiatric disorder and be more commonly
involved in litigation (Binzer et al., 1997). We did not expect to
find a substantial difference in disability, illness beliefs, socioeconomic deprivation category and work status (Crimlisk et al., 1998;
Stone et al., 2003, 2004a).
Materials and methods
Ethical approval was obtained from the Lothian Research Ethics
Committee and all patients provided written consent to participate.
Recruitment and selection
Cases with functional weakness
Patients with functional weakness were recruited over a
28-month-period (2000–2003) by referral from all of the nine consultant neurologists based in Edinburgh at the time. These neurologists
provided the exclusive NHS neurology service to South East Scotland,
a population of around one million where there is very little commercial practice. The study was promoted to clinicians by means of personal reminders and newsletters.
The inclusion criteria for cases were: (i) a complaint of weakness
that suggested a neurological disease; (ii) symptoms judged by a consultant neurologist to be definitely unexplained by organic disease; (iii)
symptom onset within the previous 2 years (to provide a more homogenous sample); (iv) weakness judged not to be solely a result of pain
or fatigue; and (v) weakness not in part due to a known neurological
disease. We did not apply or record the DSM-IV criteria for conversion
disorder because of the practical difficulty in applying the criteria; we
had no reliable way to exclude feigning and viewed the criterion
requiring the association of psychological factors as too vague to
apply. Rather we wished to focus on the symptom of functional weakness without selecting cases on the basis of any aetiological assumptions. In order to recruit patients representative of those diagnosed by
consultant neurologists, no constraints were placed on the extent of
the investigation that the patient had to undergo prior to diagnosis for
them to be eligible for the study.
Controls with weakness attributable to neurological
disease
These were selected by examining consecutive outpatient and inpatient letters written by three of the nine general adult consultant neurologists working in the same service. The same inclusion criteria used
for the patients with functional weakness were applied, but for the
controls the weakness had to be judged to be entirely explained by
organic disease. We did not seek to match the age and sex of the
control group.
Exclusions
The exclusion criteria for both groups were: (i) age less than 16 years;
and (ii) unable to communicate with the researcher or complete questionnaires because of language difficulties, severe learning disability or
dementia.
The symptom of functional weakness
Assessment
All subjects were contacted by telephone. Following informed written
consent, face-to-face interviews and neurological examinations of
cases and controls were carried out by J.S. in the subjects’ homes
(72%), in the hospital ward (17%) or in the outpatient clinic (11%).
For patients seen at home, all questionnaires were given to the patient
and returned by post after the assessment [apart from the Illness
Perception Questionnaire (IPQ) which was completed before the interview]. Patients interviewed in hospital completed the questionnaires in
hospital. The assessment included interview and patient rated
measures.
Interviewer rated measures
Semi-structured interview
This collected information on sex, age, marital status, ethnicity, presenting symptoms (using a checklist), current disability (including use
of aids and appliances), work status, receipt of financial benefits, the
presence of litigation and the patient’s beliefs about their illness.
Postcode data were used to determine socioeconomic ‘deprivation category’, a measure of socioeconomic deprivation (McClone, 2004).
We also recorded which investigations had been performed by the
referring clinician.
Psychiatric diagnostic interview
This was the Structured Clinical Interview for DSM-IV-Axis 1 disorders
(SCID), a semi-structured diagnostic interview of proven reliability for
major mental disorders (excluding personality disorders) (First et al.,
1996). Prior to the start of the study J.S. was trained in its use over 12
months by a senior psychiatrist (M.S.). All study interviews and ratings
were also discussed with M.S. and re-rated if appropriate. Since there
is no diagnostic interview for factitious disorder, we relied on the presence of marked inconsistency in the history, or evidence of lying to
raise suspicion of this diagnosis. We supplemented the interview with
a question on previous self-harm. We did not attempt to assess malingering. Nor did we diagnose dissociative disorders that are not covered by the standard SCID. Finally we did not record diagnoses of
undifferentiated somatoform disorder, which we regarded as lacking
specificity (Mayou et al., 2005).
Neurological examination
A neurological examination was performed. This paid particular attention to the presence or absence of the following ‘positive’ signs of
functional disorder: collapsing weakness (of affected arm or leg);
Hoover’s sign (weakness of hip extension returning to normal with
contralateral hip flexion against resistance); midline splitting of sensory
modalities (sharp demarcation of light touch, temperature at the midline); altered vibration sensation across the forehead (difference in
vibration sensation across the forehead or sternum); and ‘la belle
indifference’ (unconcern about disability) (Stone et al., 2002c).
Physical disability
This was assessed using the interviewer-rated Barthel scale, which has
established validity and reliability across a wide range of physically
disabling illnesses (Wade and Collin, 1988).
Patient rated measures
Physical disability and mental health status
The SF-36 questionnaire scales of health status are the most widely
used self-report measures of physical and mental health status in medical research, and have undergone a large amount of testing for reliability, validity and responsiveness (McHorney et al., 1993).
Brain 2010: 133; 1537–1551
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Anxiety and depression
The Hospital and Anxiety Depression Scale (HADS) provides continuous measurements of anxiety and depression (Zigmond and Snaith,
1983). The scale has proven reliability and validity (Bjelland et al.,
2002). The score can be analysed both as a continuous and categorical
measure of anxiety and depression (Bjelland et al., 2002).
Illness beliefs
These were measured using the IPQ, which was developed to provide
theoretically derived information about five major components of illness beliefs (Weinman et al., 1996). These are: (i) identity—the symptoms the patient associates with their illness; (ii) cause—the patient’s
ideas about the aetiology of their illness; (iii) time line—the patient’s
perceived duration of the illness; (iv) consequences—the patient’s perception of the seriousness and consequences of their illness; and (v)
cure-control—how the patient feels they are able to control or recover
from their illness. The IPQ presents 39 statements which patients rate
on a five-point scale from ‘strongly agree’ to ‘strongly disagree’. It has
good internal reliability and acceptable levels of stability in chronic
disease (Weinman et al., 1996). Additional items were added to the
‘cause’ section of the IPQ to make it more relevant to patients with
neurological weakness as follows: undiscovered physical cause,
damage to the nervous system, inflammation in the brain, reversible
changes within the nervous system, problems to do with the bones in
the spine, something you experienced as a child.
Analysis
We estimated the incidence of functional weakness using the Scottish
2001 Census Online Website (http://www.scrol.gov.uk) to give population data from the referral catchment area of the study. We planned
comparisons between cases and controls for age, sex, marital status,
duration of symptoms, socioeconomic deprivation category, disability
(Barthel and SF-36), physical and mental health status (SF-36), frequency of interview-rated psychiatric disorder (SCID), self-rated anxiety and depression (HADS), illness beliefs (IPQ) and work status data.
Parametric and non-parametric tests were used as appropriate
(Statsdirect; http://www.statsdirect.com). Simple post hoc exploratory
correlation analysis of HADS data, IPQ (cause) and SF-36 disability
were performed to explore potential mechanisms and confounders.
Because of multiple statistical comparisons we suggest that P-values
greater than 0.01 are regarded with particular caution.
Results
Recruitment
Cases with functional weakness
Totally 192 patients were referred by consultant neurologists
during the 28-month study recruitment period. Out of them
67 were excluded for the following reasons: weakness of more
than 2 years duration (n = 48); referred erroneously as they never
had the symptom of weakness (n = 16); comorbid organic disease
(n = 12); and deaf mute (n = 1). A further eight eligible patients
were not included for the following reasons: uncontactable (n = 5);
refused to take part (n = 2); and did not complete the interview
(n = 1). The final sample was therefore 107 cases.
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Table 1 Demographics, and duration and location of weakness at time of interview
Demographics
Age (mean, years)
Sex (n, % female)
Marital status (n, % married or co-habiting)
Socio-economic deprivation categoryc (mean, 95% CI)
Weakness
Duration of symptoms (months) (median, interquartile range)
Hemiparesis
Monoparesise
Triparesis
Paraparesis
Tetraparesis
Leg involvement
Right arm and/or legf
Left arm and/or legf
Cases (n = 107)
Controls (n = 46)
Significancea
39.1
85 (79%)
62 (68%)
3.7 (3.4–3.9)
39.3
38 (83%)
26 (78%)
3.5 (3.3–3.8)
–
NSb
NS
NS
NSb
9 (4–16)
67 (63%)
17 (16%)
3 (3%)
11 (10%)
9 (8%)
101(94%)
40 (46%)
47 (54%)
11
8
11
1
15
11
43
11
9
(6 –21)
(17%)
(24%)
(2%)
(33%)
(24%)
(94%)
(55%)
(45%)
NSb
d
d
d
d
d
d
d
d
NS = not significant at P50.05 level. Cases had functional weakness, controls had weakness due to neurological disease.
a Fisher’s exact test except where stated.
b Mann–Whitney.
c 1 = least deprivation, 7 = most deprivation.
d Control data shown for comparison only.
e Arms = 6, legs = 11.
f Percentage represents proportion on left or right [of hemi, mono, triparesis (side with both arm and leg affected) patients].
Controls with weakness attributable to neurological
disease
Demographic and social characteristics
Forty-six eligible patients were approached to participate in
the study. All were contactable, took part and completed the
assessment. None were excluded because of symptom duration
or communication difficulties. Their diagnoses were: multiple sclerosis (n = 27); Guillain–Barré syndrome (n = 4); first episode of
demyelination (n = 3); sensory ganglionopathy (in all cases leading
to disabling ataxia) (n = 3); transverse myelitis (n = 3); and
other (n = 6) (cervical cord lesion, foot drop, malignant cord syndrome, motor neuron disease, myasthenia gravis and ulnar
neuropathy).
Table 1 shows the age, sex, marital status and socioeconomic
deprivation category for cases and controls.
The mean age and the sex distribution of cases and controls
were similar. The median duration of symptoms (this was not
normally distributed) was also similar (9 versus 11 months).
Some patients no longer had weakness by the time of the interview (functional weakness n = 8, neurological weakness n = 7) because it had improved since the time of referral. Apart from one
neurological control, all patients were white/Caucasian.
The mean age of onset of weakness was 38 years for both
groups. The age distribution is shown in Supplementary Fig. 1.
The onset of functional weakness peaked in the 30s and early
40s with few cases having onset after the age of 55 years.
There was no substantial difference in mean socioeconomic deprivation score between cases and controls (3.7 versus 3.5)
(1 = least deprivation, 7 = most deprivation) (Table 1). These
scores indicate average socioeconomic deprivation for South East
Scotland.
Completeness of data
There were complete data for all participating patients from the
semi-structured interview and SCID. The proportions completing
the self-rated questionnaires were as follows: SF-36: cases n = 101
(94%), controls n = 43 (93%); HADS: cases n = 96 (90%), controls
n = 40 (87%); IPQ: cases n = 102 (95%), controls n = 43 (93%).
Missing data were not imputed.
Estimate of incidence
The incidence of functional weakness was calculated for adults
over the age of 16 to be 3.9 /100 000/year [based on 116 eligible
cases from a referral population of 1 261 191 in 28 months
(including the deaf mute patient)]. This figure is bound to be an
underestimate (or minimum incidence) as it does not include patients who were not referred to the study, those referred to other
specialties and those aged less than 16 years.
Symptoms
Distribution of weakness and handedness
The distribution of weakness in cases and controls is shown in
Table 1. Hemiparesis or monoparesis were the most common presentation in cases (79%, n = 84). There was no statistically significant preponderance of left sided symptoms. Four out of the five
left-handed patients with unilateral functional weakness had
left-sided symptoms. There were unusual cases of transient complete paralysis, but mild relative weakness of one side compared to
the patient’s other side was the most common finding.
The symptom of functional weakness
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Table 2 Current symptoms ascertained by interview
Symptom
Fatigue
Sleep problems
Pain—(apart from affected limb)
Sensory symptoms
Memory/concentration problems
Gastrointestinal symptoms
Headache
Back pain
Visual disturbance
Pain in affected limb
Muscle pain
Bladder problems
Slurred speech
Joint pain
Dizziness
Neck pain
Blackoutsa
Word finding difficulties
Hearing problems
Tremora
Skin problems
Dystonic spasma
Mean number of symptoms from this list
Cases (n = 107)
Controls (n = 46)
n
%
n
%
88
80
68
68
64
52
43
39
38
35
30
30
30
29
29
20
15
11
8
7
4
1
9
82
75
64
64
60
49
40
36
36
33
28
28
28
27
27
19
14
10
7
7
4
1
30
19
16
27
19
9
4
8
12
9
6
18
10
4
10
1
1
2
1
2
3
0
5
65
41
35
59
41
20
9
17
26
20
13
39
22
9
22
2
2
4
2
4
7
0
Significance (Fisher’s exact test)
P = 0.03
P = 0.0001
P = 0.001
NS
P = 0.05
P = 0.001
P50.0001
P = 0.02
NS
NS
NS
NS
NS
P = 0.01
NS
P = 0.005
P = 0.05
NS
NS
NS
NS
NS
P50.0001b
Cases had functional weakness, controls had weakness due to neurological disease. NS = not significant.
a In every patient with functional weakness, blackouts and tremor and dystonia were also diagnosed as functional (i.e. not explained by disease).
b Unpaired t-test.
Other symptoms
The semistructured interview assessed the frequency of symptoms
other than weakness and these are presented in Table 2. Sleep
symptoms, pain symptoms, gastrointestinal symptoms and other
functional neurological symptoms [such as non-epileptic attacks
(n = 15), functional/psychogenic tremor (n = 7) and functional/psychogenic dystonia (n = 1)] were all significantly more common in
the cases. Blackouts and tremor (all ‘functional’ in the cases)
looked the most promising in terms of differentially predicting
cases. Fatigue and concentration difficulties were common in
both groups and only moderately more common in the cases.
Similarly an analysis of symptom ratings from the SF-36 questionnaire revealed more pain in the cases but similar low ‘energy’ in
both groups (Table 3).
Physical examination findings and
investigation
Table 4 shows the frequency of some putative ‘positive’ physical signs of functional weakness on examination in cases and
controls.
Routinely tested signs
La belle indifference: many of the cases could be said to have
had la belle indifference in that they initially appeared
unusually cheerful despite their symptoms. However, in
nearly all of these cases, during the course of the interview,
the patient admitted that they had initially been trying to
appear cheerful so that they weren’t thought by the examiner
to be depressed or ‘mentally ill’. In three cases however
la belle indifference was maintained. In these three cases
there was a strong suspicion of factitious disorder (symptoms
fabricated consciously in order to obtain care). La belle indifference was also seen in one control with multiple sclerosis (in
the absence of cognitive impairment).
Hoover’s sign: Hoover’s sign was found in 60 cases (56%).
There were reasons why it could not be found in a further
41% of cases: no weakness (7%); weakness too mild (13%);
bilateral weakness (12%); arm weakness only (5%); weak leg
but no hip extension weakness (2%); and distal leg weakness
only (2%). In the other 3% of patients with functional weakness it was absent. In the controls, only seven patients had
unilateral hip extension weakness of a degree that Hoover’s
might be found and one had a positive Hoover’s sign.
Dragging monoplegic gait: a striking ‘dragging gait’ in which
one leg is dragged with the knee extended and hip rotated
externally or internally (Fig. 1) was not seen often in the cases
(nine patients) and not at all in controls.
Collapsing weakness: this was common in cases, although it is
a sign particularly prone to error in patients with pain or who
have difficulty following instructions.
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Table 3 Disability, health status (SF-36), and anxiety and depression (HADS) data
Barthel Score (disability)
Barthel Score (median out of 20, IQR)
Barthel Score [n with score of 18 (%)]
SF-36 (health status) (median, IQR)
Physical functioning
Role physical
Pain
Energy
Social functioning
General health perception
Mental health
Role emotional
HADS (anxiety and depression) (median, IQR)
Anxiety score
Depression score
Cases (n = 107)
Controls (n = 46)
Significancea
20 (17–20)
35 (33%)
20 (19–20)
10 (22%)
NS
NSb
30
0
33
30
44
40
60
33
40 (18–70)
0 (0–50)
56 (33–94)
40 (20–55)
56 (28–78)
50 (35–75)
68 (52–80)
67 (0–100)
NS
NS
P50.0001
NS
P50.05
P50.05
NS
NS
(14–55)
(0–0)
(22–44)
(15–50)
(22–67)
(30–56)
(44–84)
(0–100)
8 (4–13)
7 (3–11)
6 (3–10)
5 (2–7)
NS
NS
Cases had functional weakness,controls had weakness due to neurological disease. A lower score in the Barthel (range 0–20) or SF-36 (range 0–100) equals worse disability
or distress or more pain; a higher score in the HADS indicates more anxiety or depression. IQR = interquartile range; NS = non-significant.
a Mann–Whitney test unless specified.
b Fisher’s exact test.
Table 4 Frequency of ‘positive physical signs’ of functional
weakness
Cases (n = 107)
Controls (n = 46)
Examination findings
n
%
n
%
La belle indifference
Collapsing weakness
Hoover’s sign
Dragging monoplegic gait
Hand strike
Midline split
Increased vibrationa
Decreased vibrationa
Decreased temperature
sensation a
3
74
60
9
1
20
7
41
25
3
70
56
9
1
19
7
39
24
1
1
1
0
0
1
3
5
4
2
2
2
0
0
2
7
11
9
Cases had functional weakness, controls had weakness due to neurological disease. No statistical comparisons made.
a On affected side.
Diminished ticklishness: cases often mentioned spontaneously
(and were found to have) diminished ‘ticklishness’ of the affected foot. This is not proposed as a discriminatory sign but is
of theoretical interest.
Arm protection: some cases tended to ‘protect their arm’ in a
flexed position in the absence of pain, either laying it across
their lap or holding it in a flexed position when walking for
reasons that they could not explain.
Investigations carried out in the cases
In 14% of cases, the diagnosis had been made by the referring
clinician without any brain or spine imaging. MRI brain scan,
n = 58 (55%), CT brain scan, n = 38 (36%), lumbar puncture,
n = 33 (31%) and neurophysiology, n = 15 (14%) were the most
common investigations. The reason given by the referring neurologists for not investigating was their clinical confidence in the
diagnosis.
Physical disability
Other so-called ‘signs’: of a functional disorder such as large
dark glasses or furry slippers would not be worth mentioning
had they not already been suggested (Hawkes, 1997). They
were observed only rarely (twice each in the cases and once in
the controls).
Other observations on the clinical examination in cases
with functional weakness
Some further qualitative observations were made.
‘Inverse pyramidal leg weakness’: while many cases had a
global rather than pyramidal pattern of weakness, they commonly had more difficulty with extension than flexion of the
leg muscles. This was especially the case for ankle plantarflexion, which was often subjectively weaker than dorsiflexion.
There was no statistically significant difference in severity of physical disability between cases and controls either on the
observer-rated Barthel score or on the self-rated SF-36 physical
function scale (Table 3).
However, the Barthel score proved to be a relatively insensitive
measure because of the ceiling effect (Supplementary Fig. 2). The
SF-36 had less of a ceiling effect and more clinically relevant
information (see Table 3, Supplementary Table 1 for data from
individual questions). For example, among the patients with functional weakness, the physical functioning scale items rated as
‘Yes, limited a lot’ included moderate activities (53%), lifting or
carrying shopping (50%) and bathing or dressing yourself (21%).
Despite cases and control groups having similar proportions with
leg weakness and similar severity of disability, cases reported using
The symptom of functional weakness
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Figure 1 Typical ‘dragging gait’ was seen in only 9 of 107 patients with functional weakness.
more aids and appliances than controls. These were: wheelchair
(10 versus 2%), bath chair (5 versus 3%); Zimmer frame (3 versus
0%); and a stair lift (2 versus 0%). Elbow crutches (9 versus 0%)
were noticeably more common in cases compared to controls,
who tended to use a stick (17 versus 11%) (Supplementary
Table 1).
Self-rated anxiety and depression and
interview-rated psychiatric disorder
Self-rated anxiety and depression
The HADS data (Table 4) indicated that self-reported anxiety and
depressive symptom scores were relatively high in both groups,
with no statistically significant difference between them. Analysis
of HADS data according to cut off score, SF-36 ‘mental health’
category and individual items are presented in Supplementary
Tables 2 and 3.
Interview rated psychiatric disorder
In contrast to the HADS data, the psychiatric diagnostic interview
found a striking excess of many psychiatric disorders in cases
compared to controls (Table 5). In addition to those cases who
met criteria for current major depression (n = 34, 32%) a number
had minor depression or cyclothymic disorder (n = 7, 7%) and a
significant number had ‘suspected’ major depression (n = 16,
15%). These latter patients had all the somatic symptoms of depression and at interview appeared depressed but strongly denied
low mood or anhedonia. Panic disorder (n = 38, 36%) and somatization disorder (n = 29, 27%) were also particularly common in
cases. Four cases were suspected of having factitious disorder because of inconsistencies in their histories, although in none could
this diagnosis be definitively confirmed. All were apparently seeking medical care and not monetary reward.
Comorbidity of psychiatric disorders
An additional analysis of psychiatric disorders in the cases shows
how the DSM-IV axis 1 disorders overlapped with each other
(Supplementary Fig. 3). Sixty-six of the cases were responsible
for most of the major psychiatric disorders. Another 35 only had
a history of one axis-1 disorder or more minor axis-1 disorder.
One in twenty of cases had no current or previous axis-1 psychiatric disorder.
Illness beliefs
Illness perception questionnaire—main domains
The main domains of the IPQ are shown in Table 6. In most of the
domains cases and controls were similar. Both groups mostly
agreed that their symptoms came and went in cycles, had major
consequences on their life and that their illness made them feel
depressed, angry or afraid. Endorsement of statements about the
positive effects of potential treatment and the degree to which the
illness could be influenced by the person themselves was low in
both groups (but similar for cases and controls). There were however differences in two domains: first, cases were less likely to
believe their illness was permanent than controls (timeline,
P50.0001). This was despite tending to agree overall that their
symptoms were likely to be long lasting. Second, cases were much
more likely to believe that their illness was a mystery (illness
coherence, P50.0001).
Illness perception questionnaire—cause
Table 7 shows frequency of endorsement of possible causes of
symptoms listed in the questionnaire. Significance testing took
into account the range of five responses from ‘strongly disagree’
to ‘strongly agree’. The striking findings were firstly that cases
were less likely than controls to agree that stress was a cause
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J. Stone et al.
Table 5 Psychiatric disorders and frequency of previous self harm as assessed using SCID interview
Any Axis-1, lifetime
Current major depression
Suspected major depression
Minor depression/cyclothymic
disorder
Any current affectivea
Past major depression
Panic
Agoraphobia without panic
Generalized anxiety disorder
Somatisation disorder
Hypochondriasis
Social phobia
Obsessive compulsive disorder
Post traumatic stress disorder
Previous self harm
Factitious disorder
Bipolar disorder
Alcohol dependence/abuse
Cases (n = 107)
Controls (n = 46)
n
%
n
%
102
34
16
7
95
32
15
7
32
3
0
0
65
42
38
7
22
29
10
20
7
8
10
4
2
2
61
39
36
7
21
27
9
19
7
7
9
4
2
2
5
20
6
6
1
0
1
6
1
0
2
0
0
3
Significance
(Fisher’s exact test)
Odds ratio
(95% confidence interval)
71
7
0
0
P50.0001
P50.001
P50.005
NS
8.9 (2.7–34)
6.7 (1.9–36)
n/a
n/a
11
43
13
13
2
0
2
13
2
0
4
0
0
7
P50.0001
NS
P50.01
NS
P50.005
P50.0001
NS
NS
NS
NS
NS
NS
NS
NS
9.7 (3.4–34)
0.8 (0.4–1.8)
3.7 (1.4–12)
0.5 (0.1–1.8)
11.6 (1.7 –491)
n/a
4.6 (0.6–206)
1.5 (0.5–5.0)
3.2 (0.4–145)
n/a
2.3 (0.5–11)
n/a
n/a
0.3 (0–2.5)
Cases had functional weakness, controls had weakness due to neurological disease. There were no cases of substance abuse or schizophrenia in either group. n/a = not
applicable.
a Includes minor depression, mixed anxiety and depression, cyclothymic disorder.
Table 6 Illness beliefs of cases and controls—main domains of the IPQ [median scores (range 0–100)]
IPQ domain
Interpretation
Cases
(n = 102)
Controls
(n = 43)
95% CI difference
between medians
Significance
(Mann–Whitney)
Timeline
Higher score = agrees with ‘my illness is
likely to be permanent rather than
temporary’
Higher score = agrees with ‘my illness
comes and goes in cycles’
Higher score = agrees with ‘my illness has
major consequences on my life’
Higher score = agrees with ‘I have the
power to influence my illness’
Higher score = agrees with ‘my treatment
will cure my illness’
Lower score = agrees with ‘my illness is a
mystery to me’
Higher score = agrees with ‘my illness
makes me feel afraid/depressed’
60
77
(–23 to –10)
P50.0001
68
63
(–5 to 10)
NS
73
75
(–10 to 3)
NS
57
61
(–10 to 0)
NS
60
62
(–4 to 4)
NS
40
60
(–28 to –16)
P50.0001
67
69
(–10 to 3)
NS
Timecycle
Consequences
Personal control
Cure
Illness coherence
Emotional representation
Cases had functional weakness, controls had weakness due to neurological disease. NS = not significant; CI = confidence interval.
(24 versus 56%) (P50.0001). Secondly, cases had a tendency to
endorse all causes to a lesser extent than controls. Even items such
as chance or bad luck were endorsed less by cases.
Cases and controls were also asked to name the three ‘most
important causes of their illness’. The responses were grouped into
four categories (Supplementary Table 4): physical, psychological,
bad luck and ‘don’t know’. Only 12% of cases (compared to 19%
of controls) chose a primary cause which could be seen as psychological (even though this encompassed concepts of external
pressure such as ‘stress’ or ‘overwork’). Even when all three
chosen ‘causes’ were taken in to consideration, only 25% of
causes endorsed by cases fell into the ‘psychological’ category.
Work status, financial benefits and
litigation
Table 8 shows work status, receipt of benefits and frequency of
pending litigation. The cases were twice as likely as controls to
have given up work as a result of ill health [n = 68 (65%) versus
The symptom of functional weakness
Brain 2010: 133; 1537–1551
| 1545
Table 7 Illness beliefs regarding cause
Stress or worry
Damage to the nervous system
Inflammation in the brain
Altered immunity
Chance or bad luck
Something I experienced as a child
My emotional state
Family problems or worries
Undiscovered physical cause
Reversible changes in the nervous system
Accident or injury
Problems to do with the bones in the spine
Overwork
Poor medical care in my past
My mental attitude
My own behaviour
My personality
Didn’t agree with any of the above
Cases (n = 102)
Controls (n = 43)
% agree
% agree
n
%
n
%
24
34
10
11
30
4
17
10
46
25
23
18
17
11
8
6
3
11
24
33
10
11
29
4
17
10
45
25
23
18
17
11
8
6
3
11
24
32
20
20
28
4
10
10
18
16
7
7
7
4
3
3
3
1
56
74
47
47
65
9
23
23
42
37
16
16
16
9
7
7
7
2
Significance
Odds ratio
P50.0001
P50.0001
P50.0005
P50.0005
P50.005
P50.05
P50.05
P50.05
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
4.1 (2–10)
5.8 (2.5–14)
8 (3–22)
7.2 (2.8–19)
4.5 (2–10)
2.5 (0.4–14)
1.5 (0.6–4)
2.8 (0.9–8.2)
0.9 (0.4–1.9)
1.8 (0.8–4.2)
0.7 (0.2–1.8)
0.9 (0.3–2.5)
1 (0.3–2.7)
0.9
0.9
1.2
2.48
5.0 (0.6–41)
Cases had functional weakness, controls had weakness due to neurological disease. Significance was determined by a Chi Square test of trend across all five responses
(strongly disagree, disagree, neither, agree, strongly agree). For analysis of ‘first choice’ cause see Supplementary Table 4. Higher odds ratios correspond with beliefs more
common in controls. NS = not significant.
Table 8 Employment, receipt of benefits and litigation
Cases (n = 107)
Work status prior to symptom onset
Working (including parenting)
Not working due to ill health
-off sick
-retired on ill health
Other reasons for not workingb
Work Status at time of interview
Working (inc parenting)
Not working due to ill health
-off sick
-retired on ill health
Other reasons for not workingb
Pending litigation
Receiving state benefits
Disability living allowance
Incapacity benefit
Income support
Controls (n = 46)
Significance (Fisher’s exact test)
n
%
n
%
82
20 a
8
12
5
78
19
8
11
3
41
1
0
1
4
89
2
0
2
3
NS
P50.005
–
–
–
35
68a
51
17
4
9
55
35
12
8
33
65
49
16
3
9
52
33
11
8
28
15
8
7
3
3
19
16
1
2
61
33
17
15
7
7
41
35
2
4
P50.005
P50.0005
–
–
–
NS
NS
–
–
–
Cases had functional weakness, controls had weakness due to neurological disease. NS = not significant.
a Looking for work, housewife (not parenting) or retired.
b Includes two patients with learning disability who were unable to continue with normal day centre/college activities.
n = 15 (33%)]. This is a meaningful comparison in the context of
the same duration of symptoms, and similar severity of physical
disability between cases and controls. Prior to the onset of symptoms, 20 cases (19%) were already off sick or retired because of ill
health compared with only one (2%) of the controls. Of the cases,
only one had a clear additional organic disease reason for this
(systemic lupus erythematosus).
Despite the disparity in proportion working, a similar proportion
of cases and controls were in receipt of disability benefits at the
time of the interview [cases n = 55 (52%) versus controls n = 19
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| Brain 2010: 133; 1537–1551
(41%)]. The interviews suggested that this may have been firstly
because some of the patients with functional weakness had been
unsuccessful or had not applied for benefit, and secondly because
some of the controls were still working part time but also legitimately claiming benefits. There was no excess of ongoing litigation
in the cases [n = 9 (9%) compared with controls n = 3 (7%)].
Association between distress, disability
and beliefs
J. Stone et al.
1992; Binzer et al., 1997; Teasell and Shapiro, 1997; Ziv et al.,
1998) in contrast to patients with dissociative seizures who characteristically develop attacks in their mid-20s (Stone et al., 2004c).
Patients can also present at older ages; for example one series of
patients presenting to an acute stroke unit had a mean age of 50
(Nazir et al., 2005). In our study, referral bias would almost certainly have prevented older patients reaching our neurology
services.
Socio-economic deprivation
Exploratory analysis within the cases revealed an association between being more likely to believe stress was a cause of their
illness and having more symptoms of anxiety and depression on
the HADS (r = 0.24; P = 0.02) and also with having less physical
disability on the SF-36 (r = 0.38; P = 0.001).
We did not find cases to be more socioeconomically deprived than
the controls. The idea that patients with conversion symptoms
have less ‘sophisticated’ symptoms and therefore may tend to
come from lower socioeconomic groups, was previously reported
by one controlled study (Folks et al., 1984) but not by two others
(Roy, 1979; Ewald et al., 1994).
Discussion
Distribution of weakness
This article reports the minimum incidence, demographic characteristics, age of onset, socioeconomic deprivation category, symptoms, clinical features, disability, self-rated anxiety and depression,
frequency of interview-rated psychiatric disorder, illness beliefs and
work status of 107 patients with functional weakness and compares these to those of patients with weakness attributed to
neurological disease.
Frequency of functional weakness
The minimum annual incidence of 3.9/100 000/year in adults is
comparable both to the only previous estimate for motor conversion disorder of 5/100 000 (Binzer et al., 1997), and to incidence
figures for brain tumour and multiple sclerosis from population
based studies (MacDonald et al., 2000). If, however, we had
looked for patients with functional weakness among children, in
primary care and in other specialties such as orthopaedics we
would have found a much higher incidence.
Gender
We found a preponderance of females in the cases (79%) but also
in the controls (83%). Since most studies of conversion disorder
also reported this finding it is perhaps surprising that the literature
on functional weakness has not previously reported any gender
predominance. Our analysis of seven studies of a total of 272
patients with functional paralysis yielded a mean proportion of
53% females (Ljungberg, 1957; Baker and Silver, 1987; Apple
Jr, 1989; Binzer et al., 1997; Cantello et al., 2001; Heruti et al.,
2002; Nazir et al., 2005) One possible explanation is that some of
the previous studies had been carried out in military institutions
where males predominate.
Age of onset
The average age of onset in cases in this study was 39 years.
Functional weakness has been reported to occur in a wide range
of ages from children aged five (Grattan-Smith et al., 1988) to the
mid-70s (Zhang et al., 1987). The average reported age of onset
in previous studies clusters in the mid-30s (Ehrbar and Waespe,
In our study the most common presentation of functional weakness was hemiparesis (63%); there were fewer cases of monoparesis (16%) than reported in other case series. Pooling data from
the 253 patients included in all the previous studies that reported
distribution of weakness gives the following figures: 33% hemiparesis, 26% monoparesis, 12% tetraparesis, 28% paraparesis,
51% brachial diplegia and 1% triplegia. (Fallik and Sigal, 1971;
Knutsson and Martensson, 1985; Apple Jr, 1989; Lempert et al.,
1990; Ehrbar and Waespe, 1992; Binzer et al., 1997; Heruti et al.,
2002). In keeping with our previously reported systematic review
of laterality of functional weakness (Stone et al., 2002a), we
found only a slight and non-significant excess of left-sided symptoms in this study.
Physical signs
The phenomenon of la belle indifference has been much discussed but was a rare finding in our study. We did come across
patients who were said to have la belle indifference by nursing or
medical staff. In these cases it was usual for the patient to report
that they had been ‘putting on a brave face’, apparently to avoid
the perceived stigma of being told the problem was ‘in their
mind’. Further assessment suggested that they were distressed
by their symptoms (rather than indifferent to them). This finding
suggests that this is a sign with little discriminating value (Stone
et al., 2006). Hoover’s sign did however appear to be discriminating, although—like any physical sign—it has a number of potential
pitfalls (Stone and Sharpe, 2001). We propose that further empirical work on the value of these and the other physical signs
described in this article is required (Ziv et al., 1998).
Symptoms
We found a high frequency of physical symptoms, especially pain
symptoms and sleep disturbance, in the cases. Other functional
neurological symptoms, such as non-epileptic attacks and functional tremor were present almost exclusively in cases suggesting
they may be useful discriminators for diagnosis. The nature and
frequency of other symptoms in patients with functional weakness
has only been inconsistently reported. In previous studies of
The symptom of functional weakness
patients with motor symptoms, pain (55–97%) (Knutsson and
Martensson, 1985; Teasell and Shapiro, 1997; Birket-Smith and
Mortensen, 2002; Stone et al., 2003) and sensory disturbance
are commonly mentioned (30-48%) (Knutsson and Martensson,
1985; Crimlisk et al., 1998; Stone et al., 2003). Pseudoseizures
(23%) (Crimlisk et al., 1998), bladder symptoms (25%) (Teasell
and Shapiro, 1997), dysphonia (5%) (Crimlisk et al., 1998) and
dysarthria (11%) (Teasell and Shapiro, 1997) are mentioned in
some series, confirming that other functional neurological symptoms commonly co-occur with weakness.
Disability and health status
Despite similar median duration of symptoms (and by chance,
similar ages and sex distribution), we did not find a significant
difference in either interview-rated or self-rated disability between
cases and controls. There was, however, considerable heterogeneity in the severity of impairment and disability in the cases. We
also cannot exclude the possibility that there may have been
worse mobility in cases or controls despite similar scores. There
is only one previous report measuring disability in patients with
functional weakness. In this study of 30 inpatients, all patients had
moderate or severe disability with 27% in the severe disability
group compared to 17% of disease controls (Binzer et al., 1997).
It is also rare to find reports of disability in the literature on
conversion disorder. In our 12-year follow-up study of patients
with motor or sensory symptoms, 38% were limited in moderate
activities and their SF-36 scores at follow-up were comparable to
outpatients with multiple sclerosis (Stone et al., 2003). One third
had taken medical retirement. In a Dutch study of 45 patients with
conversion disorder receiving in patient treatment, 82% used
wheelchairs or crutches, 15% used more than two devices and
24% had had their houses adapted (Moene et al., 2002). The
existence of small local series of wheelchair using patients with
functional weakness highlights how commonly this scenario can
occur (Davison et al., 1999; Allanson et al., 2002).
Interview-rated psychiatric disorder and
self-rated anxiety and depression
Patients with functional weakness had a significantly higher frequency of interview-rated Axis-1 psychiatric disorder (including
current major depression, panic disorder and somatization disorder) than controls. However a third had only minor Axis-1 psychiatric disorder or no disorder. This is consistent with other
studies that have found a frequency of depression of 27% in patients with functional weakness (Binzer et al., 1997) and 5–50% in
patients diagnosed with conversion disorder (Purtell and Robins,
1951; Stephens and Kamp, 1962; Lewis and Berman, 1965;
McKegney, 1967; Barnert, 1971; Guze et al., 1971; Stefansson
et al., 1976; Bishop and Torch, 1979; Folks et al., 1984;
Wilson-Barnett and Trimble, 1985; Marsden, 1986; Lecompte
and Clara, 1987; Lempert et al., 1990; Tomasson et al., 1991;
Ebel and Lohmann, 1995; Kapfhammer et al., 1998; Al-Habeeb
et al., 1999; Chand et al., 2001; Allanson et al., 2002;
Birket-Smith and Mortensen, 2002; Roelofs et al., 2002; Kuloglu
et al., 2003).
Brain 2010: 133; 1537–1551
| 1547
One important difference we found between cases and controls
was in the frequency of panic disorder (36 versus 13%). This has
been reported in only one previous study of conversion disorder
(Roelofs et al., 2002). Our impression is that the panic disorder
tended to present with predominantly somatic symptoms but not
with anxiety, so called non-fearful panic (Chen et al., 2009) . This
may have led to the diagnosis being missed.
It was striking that whilst a great excess of depression and anxiety disorders were found at interview in the cases, the median
self-rated anxiety and depression scores were similar in cases and
controls. Why might this have occurred? First, interviewer
non-blinding may possibly have led to an overestimate of psychiatric disorder in cases, although this is made less likely by all being
re-rated. Second, perhaps as the illness belief data suggest, patients with functional weakness may have tended to play down
the role of psychological factors, leading to artefactually low scoring on the self-rated HADS.
Illness beliefs
The concept that ‘hysterical’ paralysis may depend in large part on
an ‘idea’ was probably first suggested by Reynolds as long ago as
1869 (Reynolds, 1869). Despite this, patients’ ideas and beliefs
have been little studied. Strong beliefs about the presence of disease has previously been observed in small groups of patients with
functional weakness (Binzer et al., 1998) and in patients with
non-epileptic attacks (Stone et al., 2004a). In our study, only
the ‘cause’, ‘illness coherence’ (‘my illness is a mystery’) and ‘timeline’ (‘my illness is permanent’) domains of the IPQ differed between cases and controls. The fact that patients with functional
weakness were more often mystified by their symptoms than the
controls is hardly surprising, given the nature of the diagnosis.
Many doctors themselves report being mystified by functional
symptoms (Kanaan et al., 2009). We did not specifically study
what the doctors had said to the patients in this study, but this
would undoubtedly have shaped their beliefs. For example, a patient who believes they have a physical disorder and is told they
have a psychological problem may be especially mystified. The
similarity of most of the domains suggests a group of patients
experiencing symptoms as if they did actually have a neurological
disease diagnosis. This is consistent with other studies comparing
disease groups with ‘non-disease’ counterparts which have tended
to find relatively little difference in health beliefs between patients
with and without disease if they have the same symptoms
(Trigwell et al., 1995; Stone et al., 2004a).
Regardless of their origins, illness beliefs appear to be important
in this patient group. In our follow-up study of 718 neurological
outpatients with symptoms unexplained by disease, illness beliefs
(of permanence and that the condition was not psychological) as
well as the receipt of disability benefits were the main independent
predictor of poor 12-month outcome (Sharpe et al., 2010).
The cases’ illness beliefs do however give rise to some interesting and potentially paradoxical findings. Despite having a much
higher frequency of interview-rated psychiatric disorder, cases
were much less likely than controls to agree that stress was a
cause of their symptoms. We propose several potential explanations for this finding.
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| Brain 2010: 133; 1537–1551
(I) The presence of a diagnosis allows the patient to consider
aetiological factors including ‘stress’. In other words,
anyone in diagnostic ‘limbo’, whether they have a disease
or not, may be reluctant to consider stress, or any other
possible aetiological factor until they have a diagnosis. This
is in keeping with the finding that cases tended to endorse
all causes (even ‘damage’ and ‘bad luck’) to a lesser extent
than controls.
(II) Patients with functional weakness tend to deny emotional
symptoms. The conversion hypothesis, that distress is converted to a greater or lesser extent into physical symptoms,
has held sway for nearly 100 years. The relative denial of
stress as a causal factor observed in the cases in this study
might be seen as consistent with this hypothesis.
Additionally, the post hoc finding that patients with functional weakness with more physical disability were less likely
to agree that stress is a factor could be seen to support it as
well. However many cases endorsed symptoms of anxiety
and depression on self-report. Furthermore, the correlation
between more disability and less agreement with stress as a
factor may alternatively be explained by stress being a less
socially acceptable explanation for a totally paralysed limb
than a mildly weak limb.
(III) Societal stigma. Patients with functional weakness may
avoid suggestions that the problem is stress-related because
they have found that other people in general and doctors, in
particular, tend not to believe their symptoms are ‘real’ if
they do. Society tends to stigmatize physical symptoms
which are thought to be ‘psychosomatic’ in nature (Peters
et al., 1998; Stone et al., 2002b, 2004b). For patients with
functional weakness, admitting a role for stress risks a diagnosis of mental illness. For the controls with disease, admitting a role for stress does not detract from the validity of an
organic disease diagnosis.
It may not be possible to produce scientific data that can either
prove or refute the conversion hypothesis. We do not present data
on conflicts, dilemmas or ‘stress’ prior to symptom onset that
might be useful in further testing this hypothesis more clearly.
Also, our data on illness beliefs are cross sectional so inferring
causality in relation to emotional disorder and the onset of weakness is not possible. However we suggest that a simpler and alternative hypothesis to the conversion hypothesis is that patients
with functional weakness do not convert their emotional distress
into physical symptoms and are not in denial about it—they just
do not want to tell the doctor about their emotional state. Since
doctors find explaining limb paralysis on the basis of emotional
disorder a difficult theoretical leap, and society tends to accuse
people seen as having a ‘psychosomatic’ weak limb of ‘faking
it’, we should perhaps not be surprised if patients are reluctant
to endorse emotional symptom and causes.
Work status, benefits and litigation
Cases were less likely to be working than controls. This also appeared to be the case before symptom onset. This finding has
many possible explanations. They may have been more likely to
J. Stone et al.
be unable to work because they had more physical and emotional
symptoms. Also work itself may have been perceived to be a
causal factor in the production of the symptoms which may
have led them to avoiding returning. It was interesting to note
that despite the large difference between the proportions of patients in work (65 versus 33%) there was much less disparity between the groups in terms of who was in receipt of state benefits
(52 versus 41%). This may reflect the difficulty obtaining benefits
for a poorly defined illness compared to a well-known condition
like multiple sclerosis. The lack of any excess litigation in the cases
argues against the stereotype of the patient with functional weakness typically developing symptoms for ‘secondary gain’.
Limitations
Our study is the largest reported case-control study of patients
with the symptom of functional weakness. The patients were representative of outpatient and inpatient neurological practice and
were not selected on psychiatric or other criteria. The relatively
short and homogenous duration of symptoms, similarity of age,
sex distribution, social deprivation and severity of disability in the
cases and controls, all reduce the risk that our findings are a result
of confounding.
However, all case–control studies are subject to bias (Sackett,
1979). What might be the biases on our study? First did we obtain
true and representative cases? We defined cases by a clinical diagnosis of functional weakness made by a consultant neurologist.
Might some cases have been incorrectly diagnosed? We do not
think this is likely to have been an important limitation as data
from our own systematic review (Stone et al., 2005), local 12-year
follow-up data (Stone et al., 2003) and Scottish National
follow-up study (Stone et al., 2009) all suggest that misdiagnosis
is uncommon (less than 4% of cases) . Similar concerns may also
be raised regarding contamination of our sample by patients who,
unknown to us, may have been malingering or had factitious disorder (in addition to the four patients we suspected of having
factitious disorder). Were our cases representative? They were
not truly consecutive referrals to the neurology service even
though they were consecutive referrals to the study. Also, cases
of functional weakness may present to services other than neurology such as stroke units (Nazir et al., 2005), accident and emergency (Scott and Silbergleit, 2003) and orthopaedics (White et al.,
1988). This ‘neurology’ selection bias may have led to us missing
some patients, for example, those with a regional pain syndrome
and comorbid functional weakness (Birklein et al., 2000), those
with only mild weakness seen in primary care, and patients over
65. Furthermore, neurologists vary in their willingness to make a
diagnosis of a functional disorder (Nieman, 1990; Perkin, 1990)
and may have been especially reluctant to make the diagnosis in
patients without obvious psychiatric comorbidity, men and older
people. This bias could have inflated the frequency of
interview-rated psychiatric disorders in the cases. Cases with
very transient symptoms, for example after non-epileptic attacks
may also have been disproportionately missed.
Second, was our choice of the controls appropriate? They were
selected to control for the presence of the symptom of weakness
and for the experience of being a patient. They were selected
The symptom of functional weakness
from the same service but from only three of the nine consultants
who referred cases. This could have introduced some bias,
although their socioeconomic deprivation category was the
same. The majority had inflammatory diseases of the nervous
system, usually multiple sclerosis. Because of local service configuration, stroke patients were not included. Additionally, disease
may have confounded some comparisons. For example, there is
some evidence that multiple sclerosis, the commonest diagnosis in
our controls, is associated with a high lifetime incidence of major
depression (Sadovnick et al., 1996; Patten et al., 2003). We also
did not control for possible differences in experience of the neurology service, for example inpatients versus outpatients.
Furthermore, the controls had diseases such as multiple sclerosis,
which may be associated with unusual disease specific illness
beliefs.
Third, did we suffer from measurement bias? This may have
arisen from non-blinding of the interviewer (J.S.) carrying out
the structured psychiatric interviews, although these were all discussed and re-rated where necessary. There may have also been a
tendency for cases to communicate their desire not to be seen as
‘mentally ill’ in their completion of self-rated questionnaires.
Fourth, were the differences we found between cases and controls reliable? We advised that results between P50.01 and
P50.05 should be treated with caution because of the multiple
statistical comparisons made. Alternatively, the relatively small
sample size may also have led to type 2 false negative errors.
Conclusion
Functional weakness is a common presentation to neurologists,
usually accompanied by many other symptoms, substantial physical disability, a high frequency of psychiatric disorders and illness
related unemployment. Patients with functional weakness generally view their illness in a similar way to patients whose weakness
is due to neurological disease, but despite having a more emotional disorder are much less likely to agree that this is a cause of
their symptoms. These data are of clinical and potential theoretical
relevance to understanding patients with the symptom of functional weakness.
Acknowledgements
Thanks to Ann Houliston, Ian Deary, Alan Carson, Rustam
Al-Shahi Salman, Rosemary Anderson, Adam Zeman, Bob Will,
Richard Knight, Colin Mumford, Richard Davenport, Peter
Sandercock, Christian Lueck, Roger Cull, Greg Moran, Robin
Grant, Pamela Martis, David Perry, Steff Lewis, Brian Pentland,
Matthew Hotopf, Markus Reuber and the patients who took part.
Funding
Chief Scientist Office, Scottish Executive, Edinburgh EH1 3DG,
Scotland UK.
Brain 2010: 133; 1537–1551
| 1549
Supplementary material
Supplementary material is available at Brain online.
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