1-P-D-12. Tarsal tunnel syndrome (TTS) diagnosed by the double-peaked compound muscle action potential (CMAP) of the tibial nerve

Electrodiagnosis of tarsal tunnel syndrome (TTS) is often difficult because sensory nerve action potential (SNAP) of the planter nerves may be very small especially in aged patients and motor nerve conduction study of the tibial nerve often lacks diagnostic utility. We report a patient with TTS affecting only the medial planter nerve branch, diagnosed by the double-peaked compound muscle action potential (CMAP) of the tibial nerve. A 66 year-old woman noticed paresthesia in the medial and anterior quadrant of the sole and the planter aspects of the 1st to 3rd toes. Tinel's sign was positive at the ankle. The tibial CMAP following ankle stimulation was double-peaked. Separate stimulation of the medial or lateral planter nerve was achieved by manipulating the stimulating electrode position at the ankle, which showed that the later peak was from the medial planter nerve. SNAP of the medial planter nerve was absent only on the affected side. An intraneural ganglion compressing the medial plantar nerve was found during operation. After surgery, her symptoms improved and the double-peaked CMAP disappeared. The double-peaked CMAP of the tibial nerve could be a finding to quickly suspect TTS affecting only the medial planter nerve.