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HETEROPHORIA
AND
VERGENCE
ABNORMALITIES
Heterophoria
Heterophoria may present clinically with
associated visual symptoms , particularly
at times of stress or poor health, when the
fusional amplitudes are insufficient to
maintain alignment .
Both esophoria and exophoria can be
classified by the distance at which the
angle is greater ( respectively :
convergence excess or weakness
,divergence weakness or excess, and
mixed ) ; vertical phoria are caused by
abnormal ocular motility .
Treatment involves the following :
Orthoptic treatment is of most value in
convergence weakness exophoria .
- Any significant refractive error should be
appropriately corrected .
- Symptom relief may otherwise be obtained
using temporary stick-on Fresnel prisms
and may be subsequently incorporated
into spectacles ( maximum usually 10-12
∆ split between the two eyes ).
- Surgery may occasionally be required for
larger deviations .
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VERGENCE ABNORMALITIES
Convergence insufficiency
Convergence insufficiency typically affects
individuals with excessive visual demand such as
students .
1- signs : reduced near point of convergence
independent of any heterophoria .
2- treatment : involves orthoptic exercises aimed
at normalizing the near point and fusional
amplitudes . With good compliance , symptoms
should be eliminated within a few weeks but if
persistent can be treated with base-in prisms.
3- accommodative insufficiency : is occasionally
also present . It may be idiopathic ( primary ) or
post-viral and typically affects school age
children .
The minimum reading correction is prescribed to
give clear vision but is often difficult to discard .
Near reflex insufficiency
1- paresis of the near reflex presents as an
exaggerated of convergence and
accommodation insufficiency . Mydriasis may be
seen on attempted near fixation . In the absence
of neurological signs treatment involves reading
glasses , base-in prisms and possibly Botulinum
toxin ( orthoptic exercises have no effect ) but it
is difficult to eradicate .
2- complete paralysis in which no convergence or
accommodation can be initiated may be of
functional origin or caused by midbrain disease
or follow head trauma ( recovery possible ) .
Near reflex spasm
Spasm of the near reflex is a functional condition
affecting patients of all ages ( mainly females ).
1- signs :
- Diplopia , blurred vision and headaches are
accompanied by esotropia , pseudomyopia and
miosis .
- The spasm may be triggered when testing ocular
movements .
- Observing miosis is the key to the diagnosis .
- Refraction with and without cycloplegia confirms
the pseudomyopia , which must not be corrected
optically .
2- treatment involves reassurance and
advising the patient to look away and
cease the activity that triggers the
response . If persistent , atropine and a
full reading correction are prescribed but it
is difficult later to abandon treatment
without recurrence . Patients usually seem
to live a fairly normal life despite the signs
and symptoms .
Divergence insufficiency
Divergence paresis or paralysis is a rare
condition associated with underlying
neurological disease , such as intracranial
space-occupying lesions, CVA and head
trauma .
Presentation may be at any age and may be
difficult to differentiate from sixth nerve
palsy , but is primarily a concomitant
esodeviation with reduced or abscent
divergence fusional amplitudes . It is
difficult to treat ; prisms are the best
option .
ESOTROPIA
Esotropia ( manifest convergent deviation ) may
be concomitant or incomitant .
In a concomitant esotropia the variability of the
angle of deviation is within 5 ∆ in different
horizontal gaze positions.
In an incomitant deviation the angle differs in
various positions of gaze as a result of abnormal
innervation or restriction .
All squints are different and not all fit neatly into a
classification .
For example a microtropia may occur with a
number of the other categories .
It is more important to understand the part played
by binocular function , refractive error and
accommodation in the pathophysiology of each
individual squint and to tailor treatment
accordingly .
EARLY ONSET ESOTROPIA
Up to 4 months of age infrequent episodes of
convergence are normal . After 4 months ocular
misalignment is abnormal .
Early onset ( congenital , essential , infantile )
esotropia is an idiopathic condition developing
within the first 6 months of life in an otherwise
normal ifant with no significant refractive error
and no limitation of ocular movements.
Diagnosis
- The angle is usually fairly large ( > 30 ∆ ) and
stable .
- Fixation in most infants is alternating in the
primary position .
- There is cross-fixation in side gaze , so that the
child uses the left eye in right gaze and the right
eye in left gaze . Such cross fixation may give a
false impression of bilateral abduction deficits ,
as in bilateral sixth nerve palsy .
- Abduction can, however, usually be
demonstrated either by the doll’s head
manoeuvre or by rotating the child .
Should these fail , uniocular patching for a few
hours will often unmask the ability of the other
eye to abduct .
- Nystagmus is usually horizontal .
- Latent nystagmus is only seen when one eye is
covered and the fast phase beats towards the
side of the fixing eye . This means that the
direction of the fast phase reverses according to
which eye is covered.
- Manifest latent nystagmus is the same except
that nystagmus is present with both eyes open ,
but the amplitude increases when one is
covered.
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-
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The refractive error is usually normal for
the age of the child ( about + 1 to +2D ).
- Asymmetry of optokinetic nystagmus .
Inferior oblique overaction may be present
initially or develop later .
- Dissociated vertical deviation ( DVD )
develops in 80 % by the age of 3 years .
INITIAL TREATMENT
Early ocular alignment gives the best chance of the
child developing some form of binocular
function.
Ideally , the eyes should be surgically aligned by
the age of 12 months , and at the very latest by
the age of 2 years , but only amblyopia or
significant refractive errors have been corrected.
The initial procedure can be either recession of
both medial recti or unilateral medial rectus
recession with lateral rectus resection .
Very large angles may require recessions of 6.5
mm or more .
Associated inferior oblique overaction should
also be addressed . An acceptable goal is
alignment of the eyes to within 10 ∆
associated with peripheral fusion and
central suppression .
This small-angle residual strabismus is often
stable , even though bifoveal fusion is not
achieved .
SUBSEQUENT TREATMENT
1-undercorrection may require further recession of
the medial recti , resection of one or both lateral
recti or surgery to the other eye .
2- inferior oblique overaction may develop
subsequently , most commonly at age 2 years.
The parents should therefore be warned that
further surgery may be necessary despite an
initially good result . Initially unilateral , it
frequently becomes bilateral within 6 months .
Inferior oblique weakening procedures include
disinsertion , recession and myectomy .
3- DVD may appear several years after the initial
surgery , particularly in children with nystagmus
. It is characterized by the following :
- up-drift with excyclotorsion of the eye when
under cover or spontaneously during periods of
visual inattention .
- When the cover is removed the affected eye will
move down without a corresponding down-drift
of the other eye. Thus DVD does not obey
Hering law . Although it is usually bilateral , it
may be asymmetrical .
Surgical treatment is indicated when the condition
is cosmetically unacceptable .
Superior rectus recession with or without posterior
fixation sutures or inferior oblique anterior
transposition are useful for DVD , although full
elimination is seldom possible .
4- amblyopia subsequently develops in
about 50% of cases as unilateral fixation
preference commonly develops
postoperatively .
5- an accommodative element should be
suspected if the eyes are initially straight
or almost straight after surgery and then
start to reconverge . It is therefore
important to perform repeated refractions
on all children and to correct any new
accommodative elements accordingly .
DIFFERENTIAL DIAGNOSIS
1- congenital bilateral sixth nerve palsy, which is
rare and can be excluded as described above .(
video )
2- secondary ( sensory ) esotropia due to organic
eye disease .
3- nystagmus blockage syndrome in which
convergence dampens a horizontal nystagmus .
Nystagmus can be elicited on abduction and the
infant adopts a face turn to fixate in the adducted
position .
4- Duane syndrome type 1 and 3 ( video )
5- Mobius syndrome .( video )
6- strabismus fixux .
ACCOMMODATIVE ESOTROPIA
Near vision involves both accommodation and
convergence . Accommodation is the process by
which the eye focuses on a near target , by
altering the curvature of the crystalline lens .
Simultaneously the eyes converge , in order to
fixate bi-foveally on the target . Both
accommdation and convergence are
quantitatively related to the proximity of the
target , and have a fairly constant relationship to
each other ( AC/A ratio ). Abnormalities of the
AC/A ratio are important cause of certain types
of esotropia .
Refractive accommodative esotropia
Here the AC/A ratio is normal and esotropia
is a physiological response to excessive
hypermetropia , usually between +2 and
+7 D.
The considerable degree of accommodation
required to focus clearly , even on a
distant target , is accompanied by a
proportionate amount of convergence ,
which is beyond the patient’s fusional
divergence amplitude.
It can not therefore be controlled and a
manifest convergent squint results .
The magnitude of the deviation varies little (
usually < 10 ∆ ) between distance and
near . The deviation typically presents at
the age of 18 months to 3 years ( range of
6 months to 7 years ) .
1- fully accommodative esotropia is
eliminated by optical correction of
hypermetropia and BSV is present at all
distances with glasses but the deviation is
present when glasses are not worn .
2- constant accommodative esotropia is
reduced , but not eliminated , by full
correction of hypermetropia .
Amblyopia and bilateral congenital superior
oblique weakness are frequent .
Most cases show suppression of the
squinting eye although ARC may occur ,
but of lower grade than in microtropia .
Non-refractive accommodative esotropia
This is associated with a high AC/A ratio in which a
unit increase of accommodation is accompanied
by a disproportionately large increase of
convergence . This occurs independently of
refractive error , although hypermetropia
frequently coexists. It can be subdivided into :
1- convergence excess :
- High AC/A ratio due to increased accommodative
convergence ( accommodation is normal ,
convergence is increased ) .
- Normal near point of accommodation .
- Straight eyes with BSV for distance .
- Esotropia for near, usually with suppression .
- Straight eyes through bifocals .
2- hypoaccommodative convergence excess:
- High AC/A ratio due to decreased
accommodation ( accommodation is weak
, necessitating increased effort , which
produces overconvergence ) .
- Remote near point of accommodation .
- Straight eyes with BSV for distance .
- Esotropia for near , usually with
suppression .
Medical treatment
Refractive error should be corrected , as previously
described . In children under the age of 6 years
, the full cycloplegic refraction revealed on
retinoscopy should be prescribed ( with a
deduction only for the working distance ) . In
the fully accommodative refractive esotrope this
will control the deviation for both near and
distance .
After the age of 8 years , refraction should be
performed without cycloplegia and the maximal
amount of ‘ plus ‘ that can be tolerated (
manifest hypermetropia ) prescribed .
For convergence excess esotropia bifocals
may be prescribed to relieve
accommodation ( and thereby
accommodative convergence ) , thus
allowing the child to maintain bi-foveal
fixation and ocular alignment at near .
The minimum add required to achieve this is
prescribed .
The most satisfactory form of bifocals is the
executive type in which the intersection
crosses the lower border of the pupil .
The strength of the lower segment should be
gradually reduced and eliminated by the early
teenage years .
Bifocals are best suited to hypoaccommodative
esotropia and where the AC/A ratio is not overly
excessive , when there is a reasonable chance of
discarding bifocal correction with time .
At higher levels surgery is the better long-term
option .
The ultimate prognosis for complete withdrawal of
spectacles is related to the magnitude of the
AC/A ratio and also the degree of hypermetropia
and associated astigmatism . Spectacles may be
needed only for close work .
Surgery
The aim of surgery is to restore or enhance BSV or
to improve the appearance of the squint .
Surgery should only be considered if spectacles do
not fully correct the deviation and every attempt
has been made to treat amblyopia .
- Bilateral medial rectus recessions are performed
in patients in whom the deviation for near is
greater than that for distance .
- If there is no significant difference between
distance and near measurements, and equal
vision in both eyes , some perform unilateral
medial rectus recession combined with lateral
rectus resection , whereas others prefer bilateral
medial rectus recessions.
- In patients with residual amblyopia surgery is
Usually performed on the amblyopic eye .
- In constant accommodative esotropia
surgery to improve appearance is best
delayed until requested by the child to
avoid early consecutive exotropia and
should only aim to correct the residual
squint present with glasses on .
TREATMENT OF AMBLYOPIA IS VERY
IMPORTANT BEFORE CONTEMPLATING
SURGERY .
MICROTROPIA
Microtropia ( monofixation syndrome ) ,may be
primary or follow surgery for a large deviation .
It may occur in apparent isolation , but it is
often associated with other conditions such as
anisometropic amblyopia .
Microtropia is more a description of binocular
status than a specific diagnosis .
For example a patient with fully accommodative
esotropia may control to a microtropia rather
than true bifoveal BSV with glasses .
It is characterized by the following :
1- very small angle manifest deviation measuring
8∆ or less , which may or may not be detectable
on cover testing .
2- central suppression scotoma of the deviating
eye .
3- ARC with reduced stereopsis and variable
peripheral fusional amplitudes .
4- anisometropia is often present , commonly with
hypermetropia or hypermetropic astigmatism .
5- symptoms are rare unless there is an associated
decompensating heterophoria .
6- treatment involves corrections of refractive
errors and occlusion for amblyopia as indicated .
Most patients remain stable and symptom free .
OTHER ESOTROPIAS
NEAR ESOTROPIA
1-signs
- No significant refractive error .
- Orthophoria or small esophoria with BSV
for distance .
- Esotropia for near but normal or low AC/A
ratio .
- Normal near point of accommodation .
2- treatment is usually bilateral medial
rectus recessions .
DISTANCE ESOTROPIA
This typically affects healthy young adults who are
often myopic .
1- signs
- Intermittent or constant esotropia for distance .
- Minimal or no deviation for near .
- Normal bilateral abduction .
- Fusional divergence amplitudes may be reduced
.
- Absence of neurological disease .
2- treatment is with prisms , until spontaneous
resolution, or surgery in persistent cases .
NB It is important to distinguish this from sixth
nerve paresis , which may be difficult on clinical
grounds and investigation should be considered.
Acute ( late onset ) esotropia
This presents for no apparent reason around 5-6
years of age .
1- signs
- Sudden onset of diplopia and esotropia.
- Normal ocular motility and no significant
refractive error .
- Underlying sixth nerve palsy must be excluded .
2- treatment is aimed at quickly re-establishing
BSV to prevent suppression with prisms,
botulinum toxin or surgery .
Secondary ( sensory ) esotropia
This is caused by a unilateral reduction in VA
which interferes with or abolishes fusion , such
as cataract , optic atrophy or hypoplasia ,
macular scarring or retinoblastoma .
FUNDUS EXAMINATION UNDER MYDRIASIS IS THEREFORE ESSENTIAL
IN ALL CHILDREN WITH STRABISMUS .
CONSECUTIVE ESOTROPIA
This follows surgical overcorrection of an
exodeviation . If it occurs following surgery for
an intermittent exotropia in a child it should not
be allowed to persist for more than 6 weeks
without further intervention .
Cyclic esotropia
This is a very rare condition characterized by
alternating manifest esotropia with
suppression and BSV , each lasting 24
hours. The condition may persist for
months or years and the patient may
eventually develop a constant esotropia
requiring surgery .
Earlier correction of the full manifest angle
can be successfully performed during the
intermittent phase .