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Skeletal Radiol
DOI 10.1007/s00256-015-2124-6
REVIEW ARTICLE
Deep gluteal syndrome: anatomy, imaging, and management
of sciatic nerve entrapments in the subgluteal space
Moisés Fernández Hernando & Luis Cerezal & Luis Pérez-Carro &
Faustino Abascal & Ana Canga
Received: 28 November 2014 / Revised: 26 January 2015 / Accepted: 15 February 2015
# ISS 2015
Abstract Deep gluteal syndrome (DGS) is an underdiagnosed entity characterized by pain and/or dysesthesias in
the buttock area, hip or posterior thigh and/or radicular pain
due to a non-discogenic sciatic nerve entrapment in the
subgluteal space. Multiple pathologies have been incorporated
in this all-included Bpiriformis syndrome,^ a term that has
nothing to do with the presence of fibrous bands, obturator
internus/gemellus syndrome, quadratus femoris/ischiofemoral
pathology, hamstring conditions, gluteal disorders and orthopedic causes. The concept of fibrous bands playing a role in
causing symptoms related to sciatic nerve mobility and entrapment represents a radical change in the current diagnosis of
and therapeutic approach to DGS. The development of
periarticular hip endoscopy has led to an understanding of
the pathophysiological mechanisms underlying piriformis
syndrome, which has supported its further classification. A
broad spectrum of known pathologies may be located nonspecifically in the subgluteal space and can therefore also trigger
DGS. These can be classified as traumatic, iatrogenic, inflammatory/infectious, vascular, gynecologic and tumors/pseudo-
Electronic supplementary material The online version of this article
(doi:10.1007/s00256-015-2124-6) contains supplementary material,
which is available to authorized users.
M. F. Hernando (*) : L. Cerezal : L. Pérez-Carro : F. Abascal :
A. Canga
Department of Radiology, Diagnóstico Médico Cantabria (DMC),
Calle Castilla 6 Bajo, 39002 Santander, Cantabria, Spain
e-mail: [email protected]
M. F. Hernando : L. Cerezal : L. Pérez-Carro : F. Abascal : A. Canga
Orthopedic Surgery Department Clínica Mompía (L.P.C.),
Valdecilla University Hospital, Santander, Cantabria, Spain
M. F. Hernando : L. Cerezal : L. Pérez-Carro : F. Abascal : A. Canga
Department of Radiology, Valdecilla University Hospital,
Santander, Cantabria, Spain
tumors. Because of the ever-increasing use of advanced magnetic resonance neurography (MRN) techniques and the excellent outcomes of the new endoscopic treatment, radiologists must be aware of the anatomy and pathologic conditions
of this space. MR imaging is the diagnostic procedure of
choice for assessing DGS and may substantially influence
the management of these patients. The infiltration test not only
has a high diagnostic but also a therapeutic value. This article
describes the subgluteal space anatomy, reviews known and
new etiologies of DGS, and assesses the role of the radiologist
in the diagnosis, treatment and postoperative evaluation of
sciatic nerve entrapments, with emphasis on MR imaging
and endoscopic correlation.
Keywords Deepgluteal syndrome . MRneurography . Sciatic
nerve . Sciatic neuritis . Piriformis syndrome . Hip . Posterior
hip pain . External rotators . Injection test . MRI
Introduction
Posterior hip pain and sciatica in the adult are among the more
common diagnostic and therapeutic challenges for orthopedists and radiologists [1]. Over the past decade, much has
changed in the approach to diagnosing and treating hip pathology based on the development of imaging techniques and
further progress in hip arthroscopy [2–4]. To date, reports
including the term Bdeep gluteal syndrome^ (DGS) in orthopedic and radiologic journals have been limited. However,
many articles discuss or focus on piriformis syndrome. Currently, there are many known causes of sciatic nerve entrapment that have nothing to do with piriformis syndrome, which
is actually a subtype of DGS. In clinical practice, in addition,
causes of sciatic nerve entrapment are often overlooked because of the high sensitivity of lumbar spine magnetic resonance (MR) imaging. It is known that extraspinal sacral
Skeletal Radiol
plexus and sciatic nerve entrapments may result from a wide
variety of extrapelvic (within the subgluteal space) or
intrapelvic pathologies [5]. Due to this variation in anatomical
entrapment, the term Bdeep gluteal syndrome^ may be a more
accurate description of this non-discogenic sciatica [2, 3].
In this article, the anatomy, clinical findings, pathophysiology, etiology, MR neurography (MRN) appearances and management of DGS are discussed.
Anatomy
The subgluteal space is the cellular and fatty tissue located
between the middle and deep gluteal aponeurosis layers [2,
3] (Figs. 1, 2, 3, 4, 5 and 6). They are not clearly visible on
MRI since they are closely linked to the fascia. Its posterior
limit is the gluteus maximus muscle. At its inferior margin it
continues into and with the posterior thigh. Laterally it is demarcated by the linea aspera and the lateral fusion of the middle and deep gluteal aponeurosis layers extending up to the
tensor fasciae latae muscle via the iliotibial tract. The anterior
limit is formed by the posterior border of the femoral neck and
the greater and lesser trochanters. Within the space, superior to
inferior, the piriformis, superior gemellus, obturator internus,
inferior gemellus and quadratus femoris are included [2, 3]
(Figs. 2 and 6). The medial margin is comprised of the greater
and minor sciatic foramina (Fig. 5). The greater sciatic foramen is bounded by the outer edge of the sacrum, greater sciatic notch (superior) and sacrospinous ligament (inferior). The
Fig. 2 Normal anatomy of the subgluteal space. Consecutive axial T1weighted MR images illustrating the main anatomic relationship in the
subgluteal space. a Level of the piriformis muscle (P). Sciatic nerve
(arrow), GMgluteus maximus muscle. b Level of the obturator internus
muscle (OI). Sciatic nerve (arrow), posterior cutaneous nerve of the thigh
(arrowhead), inferior gluteal artery (asterisk), GM gluteus maximus
muscle, OI obturator internus muscle, CFA ascending circumflex
femoral artery, IS ischial spine, SSL sacrospinous ligament. c Level of
the quadratus femoris muscle (QFM). Sciatic nerve (arrow), GM gluteus
maximus muscle, IT ischial tuberosity, H hamstring tendons, ITB iliotibial
band, STL sacrotuberous ligament
Fig. 1 Normal anatomy of the subgluteal space. Diagram illustrates the
most important neurovascular structures within the subgluteal space. GM
gluteus maximus muscle, QFM quadratus femoris muscle, SN sciatic
nerve, SM semimembranosus tendon, B-ST conjoint tendon of the
biceps femoris-semitendinous, ITB iliotibial band, 1 ascending
circumflex femoral artery, 2 nerve to the short head of the biceps
femoris, 3vasa vasorum of the sciatic nerve, 4posterior cutaneous nerve
of the thigh, 5inferior gluteal artery and nerve, 6nerve to the long head of
the biceps femoris
limits of the lesser sciatic foramen are the lesser sciatic notch
(external), sacrospinous lower border (superior) and the upper
edge of the sacrotuberous ligament (inferior) [2, 3, 6]. The
greater sciatic foramen contains the piriformis muscle as a
satellite structure, which is inserted into the ventrolateral aspect of the sacrum (S2–S4) and into the medial aspect of the
greater trochanter. The superior gluteal artery and nerve run
within the suprapiriform space; the inferior gluteal artery/
nerve, sciatic, posterior femoral cutaneous, obturator
internus/gemellus superior and quadratus femoris/gemellus
inferior nerves run within the infrapiriform space [2, 3]
Skeletal Radiol
Fig. 3 Normal anatomy of the subgluteal space. Consecutive from
medial (a) to lateral (b) sagittal T1-weighted MR images show the
piriformis muscle (P), obturator internus-gemelli complex (OGC),
quadratus femoris muscle (QFM), gluteus minimus muscles (Gmin),
gluteus medium muscle (Gme), gluteus maximus muscle (GM) and its
relation with the sciatic nerve (arrows) in the middle area of the
subgluteal space and in the ischial tunnel
(Fig. 5). It is important to differentiate normal neurovascular
bundles and isolated nerves that normally run along the
subgluteal space and not to confuse them with fibrovascular
bands (Figs. 1 and 5). The lesser sciatic foramen contains the
obturator internus muscle [2, 3].
During a straight leg raise with knee extension, the sciatic
nerve experiences a proximal excursion of 28.0 mm medial,
toward the hip joint (video 1) [7]. Hip flexion, adduction and
internal rotation stretch the piriformis muscle and cause
narrowing of the space between the inferior border of the
piriformis, superior gemellus and sacrotuberous ligament.
Any abnormality that disrupts this normal excursion may trigger DGS [2].
Clinical assessment of patients with DGS is difficult since the
symptoms are imprecise and may be confused with other
lumbar and intra- or extraarticular hip diseases. It is usually
characterized by a set of symptoms and semiological data
occurring in isolation or in combination [2, 3, 8]. The most
common symptoms include hip or buttock pain and tenderness in the gluteal and retro-trochanteric region and sciaticalike pain, often unilateral but sometimes bilateral, exacerbated
with rotation of the hip in flexion and knee extension. Intolerance of sitting more than 20 to 30 min, limping, disturbed or
loss of sensation in the affected extremity, lumbago and pain
at night getting better during the day are other symptoms reported by patients [2, 3, 8]. An antalgic position is frequently
found (Fig. 7). Many patients have undergone lumbar spinal
surgery without improvement and require high doses of narcotics to control their pain [2, 3, 8]. As the causes are multiple,
many other symptoms characteristic of each disorder can be
overlain and in addition to DGS.
Physical examination tests have been used for the clinical
diagnosis of sciatic nerve entrapment including the Lasègue
test, Pace’s sign, Freiberg’s sign, Beatty test, FAIR test and
Fig. 4 Normal anatomy of the subgluteal space. Consecutive coronal T1weighted MR images illustrating the main anatomic relationship in the
subgluteal space from anterior (a) to posterior (b). Sciatic nerve (arrow),
SP sacral plexus, SGA/N superior gluteal artery and nerve, GM gluteus
maximus muscle, QFM quadratus femoris muscle, H hamstring tendons,
SIJ sacroiliac joint, OI obturator internus muscle, P piriformis muscle
Clinical examination and symptoms
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Fig. 5 Normal anatomy of the subgluteal space. Sagittal-oblique T1weighted MR image shows the greater and minor sciatic foramina and
its contents, including the piriformis muscle (P), obturator internusgemelli complex (OGC) and quadratus femoris muscle (QFM). The
superior gluteal artery and nerve (SGA/N) run within the suprapiriform
space; the inferior gluteal artery/nerve (IGA/N), tibial and peroneal
components of the sciatic nerve (arrows), posterior femoral cutaneous,
obturator internus/gemellus superior and quadratus femoris/gemellus
inferior nerves (asterisks from superior to inferior) run within the
infrapiriform space. GM gluteus maximus muscle, SSL sacrospinous
ligament, STL sacrotuberous ligament
seated piriformis stretch test [3, 8]. The active piriformis and
seated piriformis stretch tests reveal higher sensitivity and specificity for the diagnosis of sciatic nerve entrapments than the
other tests, especially when both are used in combination [8].
Fig. 6 Normal anatomy of the subgluteal space. The diagram illustrates
the main bone, ligament, muscle and tendon structures located in
subgluteal space in a back view. SP sacral plexus, SN sciatic nerve, STL
sacrotuberous ligament, P piriformis muscle, SG superior gemellus
muscle, OI obturator internus muscle, IG inferior gemellus muscle
Fig. 7 Typical aspect of antalgic position in patients with DGS, bearing
weight on the healthy ischium
Etiology
Multiple orthopedic and non-orthopedic conditions may manifest as a DGS [3, 5, 9]. Specific entrapments within the
subgluteal space include fibrous bands, piriformis syndrome,
obturator internus/gemellus syndrome, quadratus femoris and
ischiofemoral pathology, hamstring conditions, gluteal disorders or orthopedic causes.
(1) Fibrous and fibrovascular bands
The concept of fibrous bands, which may or not contain blood vessels, playing a role in causing symptoms
related to sciatic nerve entrapment represents a radical
change in the current diagnosis and therapeutic approach
for the all-inclusively used term Bpiriformis syndrome.^
Typically constricting fibrous bands are present in many
cases of sciatic nerve entrapment during endoscopy [2, 3].
Under normal conditions, the sciatic nerve is able to
stretch and glide in order to accommodate moderate strain
or compression associated with joint movement [10]. Diminished or absent sciatic mobility during hip and knee
movements due to these bands is the precipitating cause of
sciatic neuropathy (ischemic neuropathy) [8] (Fig. 8).
From the point of view of its macroscopic structure,
there are three primary types of bands: fibrovascular
bands, with vessels macroscopically identifiable by
MRN imaging and endoscopy (video 2), pure fibrous
bands, without identifiable macroscopic vessels, and pure
vascular bands, exclusively formed by a vessel without
surrounding fibrous tissue [2, 3].
Based on their location, they can be classified as proximal, which affect the sciatic nerve in the vicinity of the
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Fig. 8 Endoscopic view showing
the pathogenic mechanism of
DGS. (a) Edematous and
flattened sciatic nerve due to
fibrovascular entrapment (arrow)
in a patient with ischemic neuritis.
(b) Normal vascularization
recovery (arrowheads) after nerve
neurolysis
greater sciatic notch, distal, which affect it in the ischial
tunnel region between the quadratus femoris and proximal
insertion of the hamstrings, and middle bands, located at
Fig. 9 Pathogenic classification
of fibrous/fibrovascular bands. (a,
b) Compressive or bridge-type
bands limiting the movement of
the sciatic nerve from anterior to
posterior (type 1A) or from
posterior to anterior (type 1B). (c,
d) Adhesive or horse-strap bands
(type 2), which bind strongly to
the sciatic nerve structure,
anchoring it in a single direction.
They can be attached to the sciatic
nerve laterally (type 2A) or
medially (type 2B). (e) Bands
anchored to the sciatic nerve with
undefined distribution (type 3)
the level of the piriformis and obturator internus-gemelli
complex. In each of these three locations, these bands can
be located medial or lateral to the sciatic nerve [2, 3].
Skeletal Radiol
Depending on the pathogenic mechanism, bands can
be classified as (Fig. 9):
a. Compressive or bridge-type bands (type 1), which
limit the movement compressing the nerve from anterior to posterior (type 1A) (Fig. 10) or from posterior
to anterior (type 1B) (Fig. 11) (video 3). The former is
located in front of the sciatic nerve. These fibrous
bands usually extend from the posterior border of
the greater trochanter and surrounding soft tissues
(distal insertions are variable) to the gluteus maximus
onto the sciatic nerve and extend up to the greater
sciatic notch.
b. Adhesive bands or horse-strap bands (type 2), which
bind strongly to the sciatic nerve structure, anchoring
Fig. 10 Purely fibrotic proximal type-1A band in a 39-year-old female
with DGS. (a) Double sagittal-oblique PD-weighted MR image shows a
compressive or bridge-type band (arrowhead) limiting the movement of
the sciatic nerve (arrow) from anterior to posterior. (b) Coronal oblique
MDCT reconstruction of the same band (arrowhead) after the infiltration
test. The iodinated contrast around the sciatic nerve (arrow) allows better
delineation of this band. (c) Endoscopic image shows the sciatic nerve
decompression through resection of this band (arrowhead)
it in a single direction and not allowing it to perform
its normal excursion during hip movements (Fig. 12).
These bands can be attached to the sciatic nerve laterally from the major trochanter (type 2A) or medially
from the sacrospinous ligament (type 2B). Lateral
bands are the most common. Among those classified
as medial bands, a proximal location is more frequent.
c. Bands anchored to the sciatic nerve with undefined
distribution (type 3). These kinds of bands with an
erratic distribution are characterized by anchoring
the nerve in multiple directions (Fig. 13). As type-2
bands, they are secondary to a broad spectrum of
musculoskeletal and extraskeletal pathology.
Radiologists should be aware of this band's classification as it provides useful information enabling the
surgeon to choose the appropriate instruments, patient
positioning, endoscopic portals and intraoperative
management. Any band found should be classified
within these three groups. Because of its higher frequency, special attention must be given to branches of
the inferior gluteal artery (IGA) nourishing fibrous
bands in the proximity of the piriformis muscle [2, 3].
(2) Piriformis syndrome
Piriformis syndrome can be classified as a subgroup
of DGS but not all DGSs are piriformis syndrome [3].
Reported incidence rates for piriformis syndrome
among patients with low back pain vary widely from 5
to 36 % [11, 12]. Actual prevalence is difficult to determine accurately because it is often underdiagnosed or
confused with other conditions. Until now, piriformis
syndrome has probably been underdiagnosed rather than
overdiagnosed [13].
There are two types of piriformis syndrome, primary
and secondary. Primary piriformis syndrome has an anatomic cause (anatomical variations or anomalous attachments). Secondary piriformis syndrome occurs as a
result of a precipitating cause. Fewer than 15 % of cases
have a purely primary cause [14].
The potential sources of pathology related to the
piriformis muscle include:
a. Hypertrophy of the piriformis muscle.
Asymmetrically enlarged piriformis muscle with
anterior displacement of the sciatic nerve may be a
cause of DGS (Fig. 14). Muscle size asymmetry of
2 mm was present in 81 % of patients with no history
or clinical suspicion of piriformis syndrome and none
of the patients with asymmetry of 4 mm or more had
symptoms suggestive of piriformis syndrome according to Russell [15]. Muscle asymmetry singly had only
a specificity of 66 % and sensitivity of 46 % during
identification of patients with muscle-based piriformis
syndrome. Asymmetry associated with sciatic nerve
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Fig. 11 Proximal fibrovascular
type-1B band in a 44-year-old
female with DGS. (a) Axialoblique T1-weighted MR image
and (b) axial PD fat-suppressed
MR image show a fibrovascular
band with a macroscopically
identifiable artery (arrowheads)
located behind the sciatic nerve
(arrows). (c) Axial oblique
MDCT reconstruction of the same
fibrovascular band after the
infiltration test performance. The
iodinated contrast around the
nerve allows better delineation of
this band. (d) Endoscopic image
shows the sciatic nerve (arrow)
compression by a fibrovascular
band (arrowhead). Note the
bleeding vessel during resection
of the band (video 2). Resection
of the band resulted in complete
resolution of symptoms
hyperintensity at the sciatic notch revealed a specificity
of 93 % and sensitivity of 64 % in patients with
piriformis syndrome distinct from that which had no
similar symptoms [13].
First-line treatment consists of conservative measures, including rest, antiinflammatories, muscle relaxants and physical therapy for a 6-week period. The
infiltration test is valid for diagnosis and treatment. In
refractory cases, botox infiltration is the next step.
Botulinic toxin blocks presynaptic conduction, thereby
creating a temporary paresis, and induces a denervative
process and atrophy of the piriformis muscle. In a few
cases piriformis endoscopic resection is ultimately necessary [3, 4, 16–19].
b. Dynamic sciatic nerve entrapment by the piriformis
muscle.
Dynamic entrapment of the sciatic nerve by the
piriformis is not uncommon [2]. Often the only finding at imaging that can be shown is nerve signal
hyperintensity in edema-sensitive sequences
(Fig. 15a). The definitive diagnosis is endoscopic,
demonstrating the entrapment during dynamic maneuvers (Fig. 15b). Endoscopic piriformis release, initially reported by Dezawa et al. [18], and subsequent
verification of correct nerve mobility are required.
Because some cases are transient, an infiltration test
is the first step in the treatment sequence, which leads
to diagnosis and facilitates a high percentage of
healing without resorting to surgery, which is reserved for recurrent cases [3, 4, 16, 19].
c. Anomalous course of the sciatic nerve (anatomical
variations).
Descriptions of variations concerning the relationship between the piriformis muscle and sciatic nerve
have been limited [20–23]. The sciatic nerve may divide into its common fibular and tibial nerve components within the pelvis [22]. Prevalence was 16.9 % in
a meta-analysis of cadaveric studies and 16.2 % in
published surgical case series [21].
Six categories of anatomic variations of the relationship between the piriformis muscle and sciatic
nerve were originally reported in 1938 by Beaton
and Anson [20] (Fig. 16). Smoll presented the overall
reported incidence of these six variations in over 6,000
dissected limbs. Relationships A, B, C, D, E and F
occurred in 83.1, 13.7, 1.3, 0.5, 0.08 and 0.08 % of
limbs, respectively [21]. Therefore, with the exception
of relationship A (normal course), the B-type
piriformis-sciatic variation is the most commonly
found (Fig. 17) (video 4). An additional and unreported B-type variation consisting of a smaller accessory
piriformis with its own separate tendon is often seen
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(Fig. 16). These variants are easily identifiable on MR
imaging.
The anomaly itself may not always be the etiology
of DGS symptoms as some asymptomatic patients
present these variations and some symptomatic patients
do not. A subsequent event such as any etiology reported in this article or prolonged sitting, direct trauma to
the gluteal region, prolonged stretching, overuse,
pelvic/spinal instability or orthopedic conditions may
then precipitate sciatic nerve neuropathy [2, 3].
The infiltration test is helpful to achieve an improvement or resolution of symptoms in many cases. Otherwise, endoscopic tenotomy of the piriformis muscle is
indicated but must be performed both proximally and
distally to the variant, because if only the distal section
is tenotomized retraction of the muscle may drag the
nerve medially, resulting in clinical worsening [2–4].
d. Anomalous attachments.
Anomalous attachments include variations in
proximal (intra- or extra-pelvic) or distal insertions
[24, 25] (Fig. 18). Described variants of the origin
of the piriformis muscle are an intraforaminal insertion, the presence of a larger than normal insertion, a
wider insertion than normal, and the existence of an
intrapelvic or extrapelvic accessory muscle belly. The
latter is inserted into the posterior and superior aspect
of the iliac blade next to the sacroiliac joint and fuses
with the distal myotendinous junction, and it sometimes joins the gluteus maximus (Fig. 18a) [24, 25].
Underlying pathophysiologic mechanisms include
the disturbance of normal alignment of the axis contraction of the muscle or direct impingement of the
roots of the sacral plexus. Most of the distal anomalous insertions include full/partial insertion onto the
obturator internus or conjoint piriformis/obturator distal attachment onto the greater tuberosity of the femur
[24, 25]. The treatment in these cases initially consists
of conservative measures such as a physical therapy
program. Intramuscular botulinum toxin injection
may be a useful treatment in some cases. However,
endoscopic resection of the muscle is frequently required [3, 4].
e. Sciatic nerve entrapment secondary to fibrosis after
classic open surgery of piriformis syndrome. This
kind of surgery has a relatively high rate of
Fig. 13 Type-3 scarred bands in a 51-year-old female with DGS. (a)
Axial T1-weighted MR image shows fibrous irregular bands
(arrowheads) with undefined distribution in the perisciatic fat,
anchoring the sciatic nerve (arrow) in multiple directions. (b) The
endoscopic image shows the sciatic nerve decompression, more
complex when this type of bands is resected
Fig. 12 Adhesive and lateral type-2A fibrous band in a 31-year-old
female with DGS. Axial PD-weighted MR image (a) and endoscopic
view (b) show a fibrous band (arrowheads) attached to the sciatic nerve
(arrow) laterally from the major trochanter
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Fig. 14 DGS secondary to
hypertrophy of the piriformis
muscle in a 34-year-old female
with DGS. Axial (a) and coronal
(b) PD-weighted fat-saturated
MR images show a left
hypertrophic piriformis muscle
(P) and associated sciatic neuritis
(arrow in b)
Fig. 15 Dynamic entrapment of the sciatic nerve by the piriformis
muscle in a 45-year-old male with DGS. (a) Double sagittal-oblique fatsuppressed PD-weighted MR image shows sciatic neuritis (arrow) with
no other associated abnormalities. The definitive diagnosis in this patient
Fig. 16 Anatomic variations of
the relationship between the
piriformis muscle and sciatic
nerve. (a-f) Diagrams illustrate
the six variants, originally
described by Beaton and Anson.
(a) An undivided nerve comes out
below the piriformis muscle
(normal course). (b) A divided
sciatic nerve passing through and
below the piriformis muscle. (c) A
divided nerve passing above and
below an undivided muscle. (d)
An undivided sciatic nerve
passing through the piriformis
muscle. (e) A divided nerve
passing through and above the
muscle heads. (f) An undivided
sciatic nerve passing above an
undivided muscle. (g) Diagram
showing an unreported additional
B-type variation consisting of a
smaller accessory piriformis (AP)
with its own separate tendon. SN
sciatic nerve, P piriformis muscle,
SG superior gemellus muscle
was endoscopic, demonstrating the sciatic nerve entrapment. (b)
Endoscopic piriformis release (arrowheads) resulted in sciatic nerve
decompression (arrow) and complete relief of symptoms
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Fig. 17 Right type B of Beaton and Anson piriformis-sciatic complex
variation in a 34-year-old female with DGS. (a) Coronal PD-weighted
MR image shows a high sciatic nerve division (arrows) with its two
branches passing through and below the piriformis muscle bellies
(asterisks). (b) Sagittal oblique MDCT reconstruction after performing
a double infiltration test. (c) The endoscopic image confirmed this sciatic
nerve variation
postoperative fibrosis with subsequent entrapment of
the sciatic nerve by the retracted stump of the muscle
belly (Fig. 19) (video 5). The endoscopic neurolysis
of the sciatic nerve is the treatment of choice. The
infiltration test shows very poor results in these cases
(video 5) [3, 4].
f. Trauma- or overuse-related conditions: avulsions,
tendinosis, strains, calcifying tendinosis or spasm,
as may occur in any other tendon. Treatment usually consists of physical therapy. MR imaging is
required to detect complications, inflammatory
changes or the formation of fibrous bands in which
case infiltration or endoscopic debridement of the
sciatic nerve may be required. Piriformis spasm is
a diagnosis of exclusion that should only be distinguished from dynamic entrapment not associated
with neuritis on MR imaging. In both cases, the
treatment of choice is an infiltration test or intramuscular injection of botox if there is no improvement [3, 17].
Fig. 18 Anomalous attachments of the piriformis muscle. (a) Sagittaloblique MDCT reconstruction in a 35-year-old female after performing
botulinic toxin infiltration (asterisk) shows an anomalous extrapelvic
accessory muscle belly of the piriformis muscle (arrow). (b) Axial
oblique MDCT reconstruction in a 42-year-old female reveals an
intraforaminal insertion of the piriformis muscle (arrow) entrapping a
sacral root (arrowhead). (c) Sagittal oblique MDCT reconstruction in a
44-year-old male after botulinic toxin infiltration (asterisk) shows a larger
than normal proximal insertion of the piriformis (arrows). The toxin
solution contains a small amount of iodinated contrast to assess the
location of injection more accurately (a, c)
(3) Gemelli-obturator internus syndrome
Obturator internus/gemelli complex pathology is rare.
Because of its proximity and similarity in both structure
and function, most treatments for piriformis syndrome
also affect the internal obturator [26]. Dynamic compression of the sciatic nerve caused by a stretched or altered
dynamic of the obturator internus muscle should be included as a possible diagnosis for DGS [27] (Fig. 20). As
the sciatic nerve passes under the belly of the piriformis
and over the superior gemelli/obturator internus, a
scissor-like effect between the two muscles can be the
source of entrapment [28]. Insertional pathology and variations concerning the sciatic nerve and obturator
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Fig. 19 Recurrence of DGS after classic open tenotomy of piriformis
muscle in a 37-year-old female. (a) Axial fat-suppressed PD-weighted
MR image shows a sciatic neuritis (arrow) due to the entrapment by
postsurgical fibrosis around the retracted stump of the resected and
atrophic piriformis (arrowhead). (b) Endoscopic view of sciatic nerve
(SN) entrapment by type-3 fibrous bands (arrows) in the same patient
(video 5). Resection of these bands resulted in complete resolution of
symptoms
internus/gemelli are also possible, the most frequent being the obturator internus penetrating the nerve (video 6).
A hypertrophied obturator internus is rarely found. Endoscopic neurolysis of the sciatic nerve and resection of
the obturator internus eradicate symptoms in these patients. Management of these pathologies is identical to
the different types of piriformis syndrome [3, 4, 28].
(4) Quadratus femoris and ischiofemoral pathology
Published cases of quadratus femoris muscle tears involve the distal myotendinous junction. Isolated
quadratus femoris muscle strains or tears are uncommon
[29]. With severe edema, irritation of the adjacent sciatic
nerve may cause DGS [30]. Chronic inflammatory
changes and adhesions causing scar tissue between the
muscle and the sciatic nerve result in entrapment during
hip motion. In these cases, endoscopic neurolysis of the
sciatic nerve is required [3, 4]. Isolated dynamic entrapment of the sciatic nerve by the quadratus femoris muscle
has not been reported.
Narrowing of the ischiofemoral space leads to muscular, tendon and neural changes in the ischiofemoral impingement [30, 31]. Characteristic findings are a decreased ischiofemoral space compared to healthy
controls (the ischiofemoral space measures 23 ± 8 mm
and femoral space 12 ± 4 mm) and altered signals from
the quadratus femoris muscle, which results in edema,
muscular rupture or atrophy [30, 31]. The syndrome may
occur acutely because of inflammation/edema (Fig. 21)
or chronically because of fibrous tissue formation that
traps the nerve (Fig. 22). Endoscopic decompression of
the ischiofemoral space appears useful in improving
function and diminishing hip pain in patients with IFI
although conservative treatment is always the first step
in the treatment algorithm [3].
(5) Hamstring conditions
The sciatic nerve can be affected by a wide spectrum
of hamstring origin enthesopathies appearing either isolated or in combination: partial/complete hamstring
strain (acute, recurrent or chronic), tendon detachment
(Fig. 23), avulsion fractures (acute or chronic/
nonunited), apophysitis, nonunited apophysis, proximal
tendinopathy, calcifying tendinosis and contusions [32].
In the acute phase, edema predominates as a characteristic imaging finding resulting in sciatic nerve irritation.
Chronic inflammatory changes and adhesions causing
scar tissue between tendons or muscles and sciatic nerve
Fig. 20 Dynamic entrapment of
the sciatic nerve by the obturator
internus (OI) in a 48-year-old
male with clinical diagnosis of
DGS. (a) Double sagittal-oblique
fat-suppressed PD-weighted MR
image shows sciatic neuritis
(arrow). (b) Endoscopic view
confirms the dynamic entrapment
of the sciatic nerve (arrow) by the
obturator internus muscle
(arrowhead). The OI muscle was
found to be very tight and slightly
hyperemic
Skeletal Radiol
Fig. 21 DGS secondary to acute ischiofemoral impingement in a 57year-old female. Axial fat-suppressed PD-weighted MR image shows a
narrowing ischiofemoral space causing impingement of the quadratus
femoris muscle with secondary muscle edema (arrowhead). Sciatic
neuritis is also shown (arrow)
result in entrapment during hip motion (ischial tunnel
syndrome) [32]. Congenital fibrotic bands have also
been reported. Imaging findings are specific to each injury. MR imaging is essential to select the appropriate
treatment.
(6) Gluteal disorders
The most common gluteus disorder causing sciatic
nerve entrapment is gluteal contracture [33]. It occurs
in school-age children, with lifetime sequelae, and follows multiple intramuscular injections in the buttocks.
Gluteal contracture manifests characteristic features on
MR imaging, including an intramuscular fibrotic cord
extending to the thickened distal tendon with atrophy
of the gluteus maximus muscle and posteromedial displacement of the iliotibial tract. In advanced cases, medial retraction of the muscle and its tendon results in a
Fig. 22 Left DGS secondary to chronic ischiofemoral impingement in a
53-year-old female. Axial PD-weighted MR image shows bilateral
narrowing of the ischiofemoral spaces. On the left side, quadratus
femoris muscle atrophy and a residual fibrous type-2 band (arrowhead)
anchored to the sciatic nerve (arrow) are seen
Fig. 23 Right DGS secondary to acute hamstring detachment in a 32-yearold female. Coronal fat-suppressed PD-weighted MR image shows acute
inflammatory changes in the perisciatic fat and secondary sciatic neuritis
(arrow), causing sciatic-like pain in this patient with acute proximal
hamstring detachment and hematoma (arrowheads) on the left side
depressed groove at the muscle-tendon junction and external rotation of the proximal femur [33]. Entrapment
syndrome often occurs years later in patients suffering
from gluteal contracture. A constitutional fibrogenic diathesis has also been reported as an important factor [33].
Regardless of whether this is traumatic or not, any other
gluteal pathology can be a cause of perisciatic fibrosis
and DGS. Endoscopic neurolysis does not always allow
for the resection of all fibrous bands, and in severe cases
(Fig. 13) (video 4), open surgery may sometimes be required [3].
(7) Orthopedic causes
Changes in osseous alignment of the pelvis, femur
and spine may affect normal sciatic nerve excursion
and trigger DGS [34]. The excursion of the sciatic nerve
with straight-leg hip flexion performed by Coppieters
et al. [7] did not indicate the osseous parameters of the
hip, such as femoral version, which raises the question of
how the sciatic nerve excursion may be affected by them.
The femoral neck is normally anteverted to the
bicondylar femoral plane at an angle of 35° to 50° at
birth; this angle gradually decreases by 1.5 degrees per
year and reaches an approximate value of 16 ° at 16 years
of age and 10° in the adult [35]. This, together with other
orthopedic disorders, accounts for the few cases of DGS
in which there are no imaging or other endoscopic findings evident, that is, excluding neuritis. Pelvic instability
and alterations of the sagittal balance of the spine are
other poorly studied orthopedic causes that may trigger
or predispose subjects to the syndrome.
Skeletal Radiol
Fig. 26 Botox infiltration. Sagittal-oblique MDCT reconstruction after
performing botulinic toxin infiltration shows the final distribution of the
solution (arrows) centered on the thickness of the muscle belly to avoid
extramuscular extension
Fig. 24 Double injection of anesthetic and corticosteroid technique
(infiltration test). Axial MDCT image shows the sectional plane chosen
for CT-guided infiltration of the perisciatic space (arrow) through the
piriformis muscle. The solution contains a small amount of iodinated
contrast to assess the location of injection more accurately
Injection test
The injection test is a useful tool to treat this syndrome, enabling diagnosis and excluding articular pathology or disorders
within other spaces [3]. Guided injections with the patient lying prone, utilizing ultrasound, CT (Fig. 24) or open MR imaging, have been used [3, 16, 17]. A variant of the doubleinjection technique of an anesthetic and corticosteroids reported by Pace and Nagle [36], with CT guidance and high-volume
injection through the piriformis muscle into the perineural sciatic fat or within the nerve sheath using a 22-gauge spinal
needle is recommended. The solution is injected at a single
point under the ischial spine, which distributes easily along
the perisciatic fat with hip movements (Fig. 25) (video 7). Care
Fig. 25 Infiltration test. (a) Sagittal-oblique and (b) coronal-oblique
MDCT reconstructions after performing the infiltration test show the
final distribution of the solution (arrows), which is arranged around the
should be taken not to infiltrate a sciatic nerve split, the pudendal bundle or inferior gluteal vessels, as this may produce
abundant bleeding. Most patients experience a significant immediate post-injection decrease in the symptoms [16, 19]. In
case of partial improvement or lack of improvement, it can be
repeated twice. Intramuscular infiltration with botox (using the
same procedure to guide infiltration) can be a temporary solution prior to surgery, especially in patients with piriformis muscle abnormalities (dynamic entrapment, insertional variants,
anatomical abnormalities or spasm) [17] (Fig. 26).
MR neurography imaging protocol. Sciatic neuritis High resolution and high field magnetic resonance neurography (MR
neurography, MRN) has shown to have excellent anatomic
capability [34, 37–40].
The normal sciatic nerve is a well-defined oval structure
with 40 to 60 fascicles, uniform in size and shape, isointense
to adjacent muscle tissue on T1-weighted images.
Perifascicular and perineural high signal intensity from fat
sciatic nerve (asterisk) throughout its length along the subgluteal space.
The solution contains a small amount of iodinated contrast to assess the
location of injection more accurately
Skeletal Radiol
Fig. 27 Planning of the oblique planes included in the DGS protocol
(necessary to sequences with and without fat suppression). (a)
Following the long axis of the piriformis muscle on an axial image and
(b) perpendicular to the axis of the piriformis muscle on a pure coronal
plane, respectively. These sequences are recommended to identify fibrous
bands, anatomical variants and sciatic neuritis
makes nerves conspicuous on T1-weighted images [41]. On
T2-weighted or fast spin-echo inversion recovery images, the
normal sciatic nerve is isointense or mildly hyperintense to
muscle and hypointense to regional vessels, with clearly defined fascicles separated by interposed lower signal connective tissue. Normal endoneural fluid is thought to be primarily
responsible for the normal signal intensity pattern of nerves at
T2 weighting [42].
Neural enlargement, loss of the normal fascicular appearance, increased perifascicular and endoneural signal intensity
on T2-weighted or STIR images and blurring of the
perifascicular fat are morphologic changes that suggest neural
injury. However, increased signal intensity in the nerve does
not always indicate underlying disease. The magic angle effect
is a well-recognized artifact in sciatic nerve imaging. Unlike in
tendons, however, where the magic angle artifact can be overcome with longer echo times (>40 ms), spurious high nerve
signal intensity related to the magic angle can persist at higher
echo times (66 ms) as well as on STIR images. These anglespecific signal changes must be kept in mind, particularly
when increased nerve signal intensity is the sole abnormality.
A high signal intensity that is focal or similar to that of adjacent vessels is more likely to be significant (Figs. 11b, 14b, 15,
19, 20, 21, 23) [42].
Double sagittal-oblique PD-weighted MR images, with
and without fat suppression, with thin slices and no or a minimal gap between the slices, oriented with the long and perpendicular axis of the piriformis muscle (Fig. 27), are specially recommended to recognize fibrous bands, anatomic variants and sciatic neuritis (Figs. 3, 5, 10a, 15a, 20).
With the increasing use of 3-T MR scanners, new phasedarray surface coils, and parallel imaging aids in the acquisition
of high-resolution and high-contrast images in short imaging
times, it is now possible to discriminate which particular small
elements of the lumbosacral-sacral plexus and sciatic nerve
are involved in DGS [43].
The presence of perineural fibrovascular bands is best
depicted on longitudinal high-contrast isotropic 3D images
reconstructed along the course of the nerve using multiplanar
reconstruction (MPR) or curved-planar reconstruction. T2weighted SPAIR images have provided more homogeneous
fat suppression than frequency-selective T2 fat-suppressed
images in off-center areas [43, 44].
Finally, diffusion tensor imaging and especially diffusion
tensor tractography of the sciatic nerve will bring more physiologic information in patients with sciatic nerve entrapments
in the near future. Sciatic nerve normative quantitative diffusion data such us the apparent diffusion coefficient (ADC) and
fractional anisotropy (FA) should be collected as a reference
[45].
Conclusions
DGS is an underrecognized condition. Its etiology is multifactorial. Two common and underdiagnosed causes are fibrovascular bands and entrapment related to the external rotator
muscles.
Piriformis syndrome can be classified as a subgroup of
DGS, meaning that not all DGSs are piriformis syndrome.
MR imaging is the diagnostic procedure of choice for
assessing DGS and may substantially influence management
of these patients.
Perineural injections of the sciatic nerve with corticosteroid
and local anesthetic have both a diagnostic and therapeutic
function.
Endoscopic decompression of the sciatic nerve appears
useful in improving function and diminishing hip pain in sciatic nerve entrapments within the subgluteal space.
Radiologists must be aware of the imaging anatomy and
pathologic conditions of the subgluteal space as well as factors
that predispose to or cause nerve injury.
Acknowledgments The authors are grateful to Dr. Suzanne Anderson
for her contribution to the revision of the translation of this article.
Conflict of interest The authors declare they do not have any conflict
of interest.
Skeletal Radiol
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