Piriformis syndrome diagnosed by cauda equina action potentials: report of two cases

Evaluation of recorded potentials from the lumbar spine after stimulation of the peroneal nerve in patients with piriformis syndrome is reported.

To report a new method of diagnosis of piriformis syndrome.

Clinically, the Freiberg and Pace signs were used to diagnose this entity. Among imaging examinations, computed tomography and magnetic resonance imaging have been used. An electrophysiological method using H reflex has also been reported. However, the best method for diagnosis of piriformis syndrome has yet to be clearly established.

After having an epidural electrode inserted into the lumbar spine, patients lay on their back. Cauda equina action potentials were recorded with the epidural electrode at L3/4 after stimulation of the peroneal nerve at the fibular head. As a control, the potentials were recorded with the hip and knee extended. Potentials were then also recorded with the hip flexed, and then with the hip flexed and internally rotated to stress the sciatic nerve by the piriformis muscle.

We observed changes in amplitudes and latencies of recorded potentials. Amplitude decreased to about 70% of the control value in one case and to 67% in the other. On the nonaffected side, however, this amplitude did not decrease below 90%. The latency was also delayed on the affected side, but not on the nonaffected side.

Recording of evoked potentials from the lumbar spine with stress to the affected leg was useful for diagnosing piriformis syndrome.