Piriformis syndrome is a neuromuscular disorder that occurs when the sciatic nerve is compressed or otherwise irritated by the piriformis muscle causing pain, tingling and numbness in the buttocks and along the path of the sciatic nerve descending down the lower thigh and into the leg. Diagnosis is often difficult due to few validated and standardized diagnostic tests, but two have been well-described and clinically validated: one is electrophysiological, called the FAIR-test, which measures delay in sciatic nerve conductions when the piriformis muscle is stretched against it. The other is magnetic resonance neurography, a type of MRI that highlights inflammation and the nerves themselves. Some say that the most important criterion is the exclusion of sciatica resulting from compression/irritation of spinal nerve roots, as by a herniated disc. However, compression may be present, but not causal, in the setting of sciatica due to piriformis syndrome.
The syndrome may be due to anatomical variations in the muscle-nerve relationship, or from overuse or strain.
Uncontrolled studies have suggested theories about the disorder, however a large scale formal prospective outcome trial found that the weight of the evidence-based medicine is that piriformis syndrome should be considered as a possible diagnosis when sciatica occurs without a clear spinal cause. The need for controlled studies is supported by studies of spinal disc disease that show a high frequency of abnormal discs in asymptomatic patients.