Value of somatosensory evoked potentials in saphenous entrapment neuropathy

Neuralgia of the saphenous nerve (SN) is a rare clinical syndrome simulating a vascular disorder of the lower extremities. In four cases, the presenting complaint was persistent pain on the medial aspect of the knee. Examination revealed tenderness over the site of exit of the SN form the femoral canal. Femoral nerve motor conduction, quadriceps H-reflex and EMG of the leg muscles were normal. The sensory nerve action potential of the SN in the leg was not obtained in some patients, even in the unaffected leg. SEP were therefore preferred for diagnosis and performed at the infrapatellar and descending branches of the right and left SN and recordings from the Cz'-Fz electrode. Latency and amplitude differences were evaluated and compared with a control group of healthy subjects. An alteration in the SEP from one branch was observed on the painful side. Posterior tibial responses were normal. In one case, pain resolved immediately after neurolysis, confirming SN entrapment above the femoral canal, before its division. Pain resolved in two other cases and persisted in the last after medical treatment. SEP studies are valuable in the diagnosis of an isolated lesion of the SN.