Deep gluteal syndrome describes pain going down the leg from the lower back due to nerve irritation in the buttocks (rather than the spine or pelvis). In medical terms, this sciatica is non-discogenic extrapelvic entrapment of the sciatic nerve in the deep gluteal space. It is an extension of non-discogenic sciatic nerve entrapment beyond the traditional model of piriformis syndrome. Where sciatic nerve irritation in the buttocks was once thought of as only piriformis muscle, it is now recognized that there are many other causes. Symptoms are pain or dysthesias (abnormal sensation) in the buttocks, hip, and posterior thigh with or without radiating leg pain. Patients often report pain when sitting. The two most common causes are piriformis syndrome and fibrovascular bands (scar tissue), but many other causes exist. Diagnosis is usually done through physical examination, magnetic resonance imaging, magnetic resonance neurography, and diagnostic nerve blocks. Surgical treatment is an endoscopic sciatic nerve decompression where tissue around the sciatic nerve is removed to relieve pressure.
Gluteal space definition The gluteal space is defined by anatomic landmarks. A simple way to think of the boundaries is the buttocks, or the tissue anterior to the whole of the gluteus maximus muscle. The specific boundaries (top, bottom, left, right, front, back) are defined as Posterior (back): the gluteus maximus
Anterior (front): posterior acetabular column + hip joint capsule + proximal femur
Lateral (side further from center): gluteal tuberosity + lip of linea aspera
Medial (side closer to center): sacrotuberous ligament
Superior (top): inferior margin of the greater sciatic notch
Inferior (bottom): proximal origin of hamstrings at the ischial tuberosity
Anatomy The hip has five external rotators: The piriformis, superior gemellus, obturator internus, inferior gemellus, and quadratus femoris. (External rotation is an anatomical term of motion to describe rotation away from the center of the body.) The inferior gluteal nerve/artery, sciatic nerve, pudendal nerve, posterior femoral cutaneous nerve, obturator internus nerve, superior gemellus nerve, quadratus femoris nerve, and inferior gemellus nerves exit the greater sciatic foramen underneath the piriformis. While any of the nerves which exist in this deep gluteal space can be entrapped by deep gluteal space problems, the existing definition of the syndrome tends to focus on sciatic nerve pathology specifically.
Signs and symptoms As deep gluteal syndrome is defined as entrapment of the sciatic nerve, patients will have pain along the distribution of the nerve, also known as sciatica. These general sciatica symptoms include unilateral, though sometimes bilateral, radiating pain or dysthesias in the affected legs. There may also be abnormal reflexes or weakness of the affected leg. However, significant localized neurological symptoms like foot drop are not typical. Patients also frequently report persistent or intermittent pain or dysthesias in posterior hip, buttocks, or thigh.
Unlike discogenic sciatica (caused by the spine), patients with deep gluteal syndrome report exacerbation of symptoms with pressure in the buttocks, such as tenderness or pain on deep palpation, or pain on prolonged sitting. Often patients will be unable to sit more than 20–30 minutes. When patients sit for long periods of time, they may exhibit the antalgic position, where weight is shifted to avoid pressure on the affected side in the hopes of reducing pain. Pain is often increased by activities or positions involving hip flexion on the affected side. Patients with deep gluteal syndrome may have a history of trauma, such as a fall on the buttocks.
Causes Fibrous bands
Fibrous bands are the most common cause of deep gluteal syndrome, and are frequently observed intraoperatively during endoscopic sciatic nerve explorations. Fibrous tissue, also known as scar tissue, is dense, inelastic tissue that can form after the body heals from an injury. Fibrous bands are fibrous tissue with a long, thin shape like a rope or a band. It's often not clear how internal scarring would materialize. However, many deep gluteal syndrome patients do have a history of falls or trauma in the gluteal space.
Fibrous bands can be compressive / bridge-type bands, adhesive / horse-strap bands, or have an undefined distribution. The bridge-type bands act as a barrier preventing the sciatic nerve from moving past a given point with compression, like a seat belt. The adhesive bands strongly anchor the nerve and completely limit movement beyond a certain point with traction, like a leash. Fibrous bands with an undefined distribution anchor the sciatic nerve in multiple directions, like glue.
Piriformis syndrome
Piriformis syndrome is the second most common cause of deep gluteal syndrome, and the most commonly involved among the various musculotendinous structures in the pelvis. There are several mechanisms proposed where the piriformis entraps the sciatic nerve: hypertrophy (muscle size squeezes tissue around it), dynamic nerve entrapment at the muscle (muscle pinches the nerve with certain movements), anomalous course of the nerve, anomalous attachments of the muscle, iatrogenic injury, and trauma. Anatomic variations of sciatic nerve branching were speculated to play a role, however this is unproven, as surgery groups for deep gluteal syndrome tend to have the same prevalence of sciatic nerve branching anomalies found in non-patient cadavers.
Other causes There are many other causes of deep gluteal syndrome although less frequent. They include hamstring conditions, gluteal muscles, gemelli-obturator internus syndrome, vascular abnormalities (e.g. dilated veins), quadratus femoris/ischiofemoral impingement, space-occupying lesions (neuroma, ganglion cyst), fibrosis after classic open surgery of piriformis, trauma, and orthopedic causes. Sometimes rare, pathological anatomic variations may be the cause, such as a muscle fiber piercing a nerve.
Pathophysiology
The sciatic nerve is highly mobile in the deep gluteal space with hip and even knee movements. For example, hip flexion with knee extension (also called a straight leg raise) causes the sciatic nerve in the deep gluteal space to move 28mm towards the center of the body. Hip movements may also create dynamic impingement between muscles. For example, hip flexion, adduction, and internal rotation stretch the piriformis muscle and reduce the space between the piriformis and superior gemellus as well as the piriformis and sacrotuberous ligament.
Normally the sciatic nerve can stretch, glide, and accommodate moderate compression associated with normal hip joint and knee movements, but it may be impeded by various pathologies. Diminished or absent sciatic nerve mobility is believed to be the precipitating cause of sciatic neuropathy in deep gluteal syndrome. This impaired mobility induces strain and compression on the sciatic nerve during various hip movements. Even a 6% stretch on a nerve can result in impaired conduction.
Diagnosis
The general workup involves excluding lumbar, pelvic, and hip pathologies, physical examination, magnetic resonance neurography (MRN) imaging, and diagnostic injections. The use of MRN and diagnostic injections are relatively new diagnostic tools that allow making precise diagnoses where standard diagnostic modalities has failed. Nerve testing such as EMG and NCS can be done but there is little evidence it's helpful. Differential diagnoses (different conditions with similar symptoms) include pudendal nerve entrapment, ischiofemoral impingement, greater trochanter ischial impingement, and ischial tunnel syndrome.
Exclude hip/spine/pelvis pathologies early Diagnosing deep gluteal syndrome is often is a clinical challenge because the symptoms can have considerable overlap with symptoms of pelvic, hip, and spine pathology. In particular lumbar pathology should be excluded early as sciatica that originates in the spine is thought to be more common than sciatica that originates in the deep gluteal space. When assessing the possibility of lumbar pathology, magnetic resonance imaging (MRI) should not be the sole basis for a diagnosis as it has very high sensitivity which may cause non-discogenic sciatic nerve entrapment to be overlooked due to false positive diagnosis. On MRI, disc lesions are present in many asymptomatic people.
Intrapelvic problems should be excluded by covering a gynecologic or urologic history. High resolution imaging can also rule out some forms of intrapelvic problems such as endometriosis or vascular abnormalities.
Physical examination The clinical signs should determine the diagnostic approach. For example, sitting pain is associated with sciatic nerve entrapment under the piriformis, but pain lateral to the ischium when walking is associated with ischiofemoral impingement. The core of the physical examination is palpation and stretch/activation tests of the external hip rotators.
As patients often have tenderness in the buttocks, palpation can provide information on which structures may be involved. For example, tenderness over the gluteal muscle at the greater sciatic notch may be piriformis syndrome; lateral to the ischial tuberosity may be ischiofemoral impingement; medial to the ischial tuberosity may be pudendal nerve entrapment. Stretch/activation tests of the external hip rotators try to create dynamic impingement with hip/knee movements. The most used ones are the FADIR test (flexion, adduction, and internal rotation), seated piriformis challenge test, and the active piriformis test. Additional tests include Lasegue test (known as the straight leg raise test), Pace's sign, Freiberg's sign, and the Beatty test.
Imaging Magnetic resonance imaging (MRI) and magnetic resonance neurography (MRN) are the diagnostic procedures of choice for deep gluteal syndrome. MRN provides additional information that MRI alone can't by visualizing structural properties of the sciatic nerve. As an example of the diagnostic improvement of MRN, when MRI is used to assess piriformis muscle asymmetry, it has 46% sensitivity and 66% specificity for piriformis syndrome. When MRN is used and includes unilateral sciatic nerve hyperintensity at the sciatic notch, the sensitivity increases to 64% and the specificity increases to 93%. MRN's advantage is in identifying anatomic nerve abnormalities by visualizing neural structures such as nerve diameter, nerve fascial edema, fascicular appearance, perifascicular and endoneural signal intensity. Diffusion tensor imaging / Magnetic resonance tractography is expected to be another powerful clinical tool for diagnosis of deep gluteal syndrome because it can reveal additional physiological information about the nerves, but is still in the research phase.
Diagnostic injections