When the piriformis muscle shortens or spasms due to trauma or overuse, it can compress or strangle the sciatic nerve beneath the muscle. Generally, conditions of this type are referred to as nerve entrapment or as entrapment neuropathies; the particular condition known as piriformis syndrome refers to sciatica symptoms not originating from spinal roots and/or spinal disc compression, but involving the overlying piriformis muscle.
In many of the normal population the sciatic nerve passes through or pierces the piriformis muscle, rather than underneath it.
It has been theorized that people who sit lengthy hours, regularly exercise by running, bicycling, and other forward-moving activities may be more susceptible to developing piriformis syndrome if they do not engage in lateral stretching and strengthening exercises. When not balanced by lateral movement of the legs, repeated forward movements can lead to disproportionately weak hip abductors and tight abductors. Thus, disproportionately weak hip abductors/gluteus medius muscles, combined with very tight abductor muscles, can cause the piriformis muscle to shorten and severely contract. This means the abductors on the outside cannot work properly and strain is put on the piriformis. However, it is also possible that such people are actually experiencing small herniation's in a spinal disc which then impinge on the sciatic nerve and cause the piriformis to spasm secondarily. Evidence for a specific relationship between the strength or weakness of certain hip muscles and sciatic nerve pain centered around the piriformis muscle remains scant. Also, this sports-related explanation is useless for understanding piriformis syndrome in those who are not unusually physically active (which is often the case).
The result of the piriformis muscle spasm can be impingement of not only the sciatic nerve but also the pudendal nerve. The pudendal nerve controls the muscles of the bowels and bladder. Symptoms of pudendal nerve entrapment include tingling and numbness in the groin and saddle areas, and can lead to urinary and fecal incontinence.
Piriformis syndrome may also be associated with direct trauma to the piriformis muscle, such as in a fall.