Piriformis syndrome is a neuromuscular disorder wherein stretching of the piriformis muscle entraps and compresses the sciatic nerve. This rare, often controversial condition is characterized by persistent and radiating pain in the buttocks, the posterior part of the thigh and the lower leg.
The primary mode of presentation of piriformis syndrome is a persistent pain in the buttocks that may radiate to the hip, the posterior part of the thigh and the proximal part of the lower leg . This may be associated with prolonged low back pain and sensory deficits in the form of numbness and paresthesia over the affected regions  . Pain may also be present in the coccyx, groin or the distal parts of the leg . The patient may have difficulty in walking and may present with a limp .
The pain may be aggravated by activity or during sitting, squatting or standing. A certain number of patients may exhibit pain with bowel movements or during sexual intercourse . The pain might only be relieved by lying down or by bending the knee while walking. Some patients are unable to find pain relief in any position. However, walking does seem to provide them the maximum degree of relief.
This condition may be recognized by the short and externally rotated limb seen in a supine patient. This external rotation, also called splayfoot or a positive piriformis sign, is due to contraction of the piriformis muscle . On examination, point tenderness is often found at the sciatic notch where the sciatic nerve passes under the piriformis. Certain maneuvers that may stretch the piriformis muscle and compress the sciatic nerve further may be performed to elicit the pain. These include FABER (flexion, abduction, external rotation), FAIR (flexion, adduction, internal rotation), Freiburg and the Pace maneuvers  .
Some patients may present with leg swelling and sexual dysfunction. Motor deficits, in the form of muscle weakness and decreased/ absent reflexes, are usually not seen in piriformis syndrome. This is in contrast to the other causes of low back pain.
The diagnosis of piriformis syndrome is one of exclusion and is primarily made on clinical grounds. Imaging techniques, such as computed tomography (CT), magnetic resonance imaging (MRI), ultrasonography (USG) and electromyography (EMG), are used to rule out other conditions presenting in a similar manner . The differential diagnosis of piriformis syndrome includes spinal canal stenosis, lumbar muscle strain, herniated nucleus pulposus (HNP) and facet arthropathy .
A potentially useful modality for diagnosing piriformis syndrome is magnetic resonance neurography (MRN). This yet-investigational test can demonstrate the sciatic nerve irritation caused by the stretched piriformis muscle. Via its efficacy to show a split nerve/ muscle, this test may serve as a guide to effective therapy in the form of surgery or injections.
Image-guided injections may also be useful to make a diagnosis. These can be accomplished by MRI or 3D imaging techniques. Other modalities such as ultrasound, EMG, fluoroscopic guided or blind injections are usually not preferred owing to their lack of reliability and potential side-effects.