Background Piriformis Syndrome is thought to account for about 5 to 10 percent of the cases of chronic low back and gluteal pain and could cause debilitating chronic suffering. However as at present there are no specific confirmatory tests, it remains mainly a diagnosis of exclusion. On the other hand musculoskeletal ultrasound is a rapidly developing imaging modality, particularly efficient in the evaluation of lesion in soft tissues.
Objectives To study the piriformis muscles by sonography in patients with unilateral “nonspecific” chronic low back and gluteal pain, referred or not the thigh and in subjects without such complaints. The findings from the study were used to determine possible sonographic diagnostic features for the Piriformis syndrome.
Methods We studied the piriformis muscles of 14 middle-aged patients (5 males, 9 females) with unilateral, regional chronic low back and gluteal pain of cause unidentified by clinical examination, conventional X-ray and routine laboratory tests. The contra lateral, non painful side of the same individuals as well as both piriformis muscles of another 12 adults (5 male, 7 female) of matching age, height and weight without such complaints served as control.
Results Patients were examined in prone position. The thickness, echogenicity and structure of the muscle were evaluated by an ultrasound scan parallel to the long axis of the muscle. Then the smoothness of muscle gliding over the iliac bone was observed by dynamic scanning of the muscle while the unilateral hip was passively and repeatedly externally and internally rotated. The anterior-posterior thickness of the piriformis muscle measured by ultrasound varied widely between individuals: from 5.8 to 11.5 mm in males and from 4.4 to 9.6 mm in females probably reflecting the different level of physical conditioning. However the difference between the piriformis muscles in the same individual was neglect able: mean 0.51, maximum 1.34 mm. On the other hand there was significant asymmetry in size between painful and non-painful piriformis muscle in the same patient (in 13/14 subjects) with mean difference 3.98, maximal 10.4 mm. Regarding echogenicity and structure, painful muscles were more often hypoechogenic (10/14), than non-painful ones (4/38) and with buldging upper and lower margins (9/14), than non-painful (6/38). Dynamic scanning of the non painful muscles showed smooth movement of the muscle over the iliac bone in all subjects. On the other hand dynamic examination of 9 of the 14 painful muscles showed non smooth movements with catching and jumping of the inferior surface of the muscle over the iliac bone (signs of piriformis muscle impingement) with reduced distance between lower margin of the muscle and the iliac bone. That was only sonographic sign which correlated with the presence of positive tests for piriformis stressing in the given patient (FAIR and Pace tests were used).
Conclusions Static and dynamic sonographic study of the piriformis muscle could be used to routinely evaluate patients with unidentified by other means of investigation cause for chronic low back and gluteal pain. Marked asymmetry in muscles thickness between both sides of the body, hypoechogenicity, as well as dynamic signs of non-smooth movement and impingement of the muscle against the iliac bone could be used as the most reliable signs for the presence of Piriformis syndrome.
Disclosure of Interest : None declared