Tarsal Tunnel Syndrome Caused by an Occult Schwannoma of the Posterior Tibial Nerve: Avoidance of Delay in Diagnosis.

Tarsal tunnel syndrome (TTS), resulting from compression of the posterior tibial nerve (PTN) within the tarsal tunnel, is a relatively uncommon entrapment neuropathy. Many cases of tarsal tunnel syndrome are idiopathic; however, some causes, including space-occupying lesions, may lead to occurrence of TTS symptoms. Schwannoma, the most common tumor of the sheath of peripheral nerves, is among these space-occupying lesions, and may cause TTS when it arises within the tarsal tunnel, and it may mimic TTS even when it is located outside the tarsal tunnel and cause a significant delay in diagnosis. The possibility of an occult space-occupying lesion compressing the PTN should be kept in mind in the differential diagnosis of TTS, and imaging studies that are usually not used in entrapment neuropathies may be of importance in such patients. This case report presents a 65-year-old woman with TTS symptoms and neurophysiologic findings secondary to an occult schwannoma of the PTN proximal to the tarsal tunnel. Avoidance of delay in diagnosis in secondary cases is emphasized.