Sciatic neuropathy

Sciatic neuropathy can be the result of nerve compression or traction injury of the sciatic nerve which might occur at several levels along its course. The peroneal division of the nerve is more commonly affected than the tibial division due to the more superficial location and two different fixation sites ^1,2. Entrapment of the sciatic nerve within the deep gluteal space is referred to as deep gluteal syndrome or subgluteal space entrapment of which piriformis syndrome is one condition ³.

Epidemiology

Sciatic neuropathy is a very common neuropathy of the lower extremity even if not quite as frequent as common peroneal neuropathy ³.

Associations

Sciatic neuropathy is associated with hip arthroplasty with an incidence of up to 1-3% ^2,3,5.

Diagnosis

The diagnosis can be made on clinical grounds and might be confirmed with electrodiagnostic tests like nerve conduction studies and electromyography ¹. Differential diagnoses as lumbosacral radiculopathy should be ruled out. Imaging studies such as MRI and ultrasound might show denervation changes in the affected muscles and help to identify the location of an entrapment site and provide clues about the etiology ².

Clinical presentation

The clinical symptom of sciatic neuropathy is sciatica and includes sharp pain and paresthesia along the posterior thigh and lower leg including the lateral and plantar aspects of the foot. Numbness and weakness often develop more gradually than pain ³. Motor symptoms include foot drop and weakness in knee flexion and hip extension due to affection of the hamstring muscles ^1-4. Achilles and hamstring reflexes might be diminished ².

Clinical symptoms will be sometimes less obvious and resemble peroneal nerve neuropathy due to incomplete injury to the nerve ².

Nerve conduction studies will reveal reduced amplitudes of the superficial peroneal and sural sensory stimulations as well as low tibial and peroneal motor responses but without evidence of conduction block at the knee ^2,4.

Pathology

The sciatic nerve is a mixed motor and sensory nerve formed by the L4-S3 nerve roots supplying motor innervation to the posterior thigh muscles and the vast majority of motor and sensory innervation to the lower leg via the tibial and common peroneal nerves ^1,2,4. It can be subject to direct traumatic injury including severance, chronic compression or traction injury along its whole course from the deep gluteal region to the popliteal fossa ^1,2,4. The injury affects more often the peroneal division due to a more superficial course and fixation at two locations at the sciatic foramen and the fibular head, whereas the tibial division is only fixed at the sciatic foramen ¹.

Etiology

Potential causes of sciatic neuropathy include the following ^1-8:

• traumatic injury (most common)

  • hip fractures, pelvic fractures, sacral fractures

  • posterior hip dislocation

  • gunshot or stab wounds

  • lacerations

• iatrogenic injury

  • hip surgery (e.g. total hip replacement)

  • positioning

  • traction injury

• chronic compression due to mass effect

  • fibrovascular bands

  • piriformis syndrome

  • external hip rotators (gemelli, obturator internus, quadratus femoris)

  • tumors

  • heterotopic ossification

  • hematoma

  • sciatic hernia

• vasculitis

Location

Locations of sciatic nerve entrapment include are more common in the pelvic and gluteal region than in the thigh and include the following ^1-4:

• greater sciatic foramen

• piriformis muscle – see piriformis syndrome

• thigh

Radiographic features

Ultrasound

Ultrasound can reveal nerve enlargement, hypoechogenicity and caliper changes as well as partial or complete loss of the fascicular pattern 6. It can be used to guide a nerve block ³.

CT

CT can be of aid in the search for an etiology ⁷.

MRI

On MRI of the pelvis the sciatic nerve can be easily visualized due to its large size in the greater sciatic foramen underneath the piriformis muscle, then lateral to the hamstring origin and superficial to the ischial spine and external hip rotator muscles underneath the gluteus maximus. In the posterior thigh, it can be assessed between the adductor magnus and hamstring muscles posterior to the femur ¹.

Displacement or course deviation, enlargement and increased signal intensity of the nerve are MR features of neuropathy ^1,4. An obliteration of fat planes around the nerve, loss of its fascicular pattern or adjacent mass lesions are other possible findings ¹.

Denervation changes might be seen in the hamstring muscles in the posterior thigh including the short head and long head biceps femoris muscle, semimembranosus and semitendinosus as well as the hamstring component of the adductor magnus muscle and in all of the lower leg muscles ¹.

Radiology report

The radiology report should include a description of the following:

• abnormal appearance of the sciatic nerve and location

• denervation changes especially

• neuroma formation

• space-occupying lesions compressing the nerve

Treatment and prognosis

Management depends on the etiology and includes conservative measures including physiotherapy, stretching local anesthetics and nonsteroidal anti-inflammatory drugs ⁴. Surgery might be required for the removal of space-occupying lesions and involves decompression and neurolysis.

Differential diagnosis

The differential diagnosis of sacral neuropathy includes ^1-4:

• lumbar spinal canal stenosis

• L5 or S1 radiculopathy

• lumbosacral plexopathy

• tibial neuropathy

• common peroneal neuropathy

Practical points

Denervation changes of the short head of the biceps femoris muscle can be helpful in situations where the peroneal division of the sciatic nerve is injured because this muscle is innervated by the peroneal division of the sciatic nerve proper and can be thus used for differentiation of a common peroneal nerve injury ².