359

Electrophysiologic Evidence of Piriformis Syndrome M. Fishman, MD, Patricia A. Zybert, PhD

Loren

ABSTRACT. Fishman LM, Zybert PA. Electrophysiologic evidence of piriformis syndrome Arch Phys Med Rehabil 1992;73:359-64. l Piriformis syndrome (PS) is defined by a loose cluster of symptoms arising from entrapment of one or both divisions of the sciatic nerve as they pass the sciatic notch. This paper presents a method of using the H-reflex as an aid in the diagnosis of PS. Forcible pressure from the piriformis muscle on the sciatic nerve can be induced by internal rotation of an affected limb in an adducted, flexed position. This pressure is reflected in a delay of the H-reflex. The length of delay seen in 39 legs of 34 patients who met the criteria for PS is compared with that seen in 13 unaffected contralateral limbs and 14 limbs from able-bodied subjects. Mean delay of H-reflex was 2.66msec for affected legs and .36msec for the combined control groups (1 = 7.45, p < .OOl). There were no differences in the H-reflexes themselves between groups. Physical therapy aimed at reducing mechanical impingement was successful in I 1 of 12 patients on followup at three to nine months. 1~ 1992 by the American

Congress of Rehabilitation

Medicine

and the American Academy of Physical

Medicine

and

Rehabilitation

Freiberg and Vinke’s’ classic paper defines piriformis syndrome (PS) as the triad of positive Lasegue sign. tenderness at the sciatic notch. and improvement with conservative treatment. Along with most others writing about PS since the late 19th century,7-x Freiberg and Vinke described it as accompanied by no other neurologic signs or positive results from the laboratory.“‘0 Apart from one case report using scintigraphy,-’ two using evoked potentials.“.” and our own early attempts”,‘” using the H-reflex, no relevant clinical test has been reported. This paper describes the use of the H-reflex in the diagnosis of PS. The piriformis muscle forms along the ventrolateral surface ofthe sacrum. where its origins interdigitate with sacral nerves S9, S3, S4. the anterosuperior sacroiliac joint capsule. and the sacrotuberous ligament. It inserts on the superior aspect of the greater trochanter, along with the gemelli, obturator internis, and quadratus femoris, which, in ascending order. are all more powerful external rotators of the hip. In flexion. the piriformis muscle abducts the hip. The posterior tibia1 and common peroneal nerves, usually grossly united at the sciatic notch. are histologically distinct as they pass close to the bone. Anatomic investigators (table I ) report lOo/ to 21.5% variation in the nerves crossing the piriformis muscle. Relative frequencies of the most common situations are estimated in figure I. A case of PS results from a degree of entrapment of one or both divisions of the sciatic nerve as they emerge from the Fromthe Flurhlng. leg.

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