SUMMARY The near-nerve sensory nerve conduction in the medial and lateral plantar nerves was studied in 25 cases of tarsal tunnel syndrome. Sensory nerve conduction was abnormal in 24 cases (96%) The most common abnormalities were slow nerve conduction velocities and dispersion phenomenon (prolonged duration of compound nerve action potentials). These two electrophysiological abnormalities are indicative of a focal segmental demyelination as the primary pathological process in tarsal tunnel syndrome.In 1979, we published a method of obtaining sensory nerve conductions in the medial and lateral plantar nerves using the surface electrode, which was able to confirm the diagnosis of tarsal tunnel syndrome in 90% of cases.' For the past four years we have used the near-nerve needle sensory nerve conduction technique in diagnosis of this disorder. We report here our experience with this technique. Materials and meThe technique of the near-nerve needle sensory nerve conduction in the digital and interdigital nerves of the foot was described in detail in a previous report.2 Sensory compound nerve action potentials (CNAPs) were recorded orthodromically using the near-nerve needle and signal averaging techniques. The active needle recording electrode was inserted close to the medial plantar fascicle near the posterior tibial nerve in the ankle behind the medial maileolus above the flexor retinaculum. This site was chosen with the aid of a surface stimulating electrode. The adequacy of the needle position was determined by stimulating the nerve through the active electrode. When the great toe (digit I) was contracting minimally with less than 3 mA for a stimulus of 0-05 ms duration, the needle was considered to be adequately positioned. The reference needle electrode was placed subcutaneously at the same level as the active electrode at a transverse distance 3-4 cm. The I and V digital nerves were stimulated with ring electrodes and various interdigital nerves with interdigital stimulating surface electrodes which were specially designed to stimulate two branches of the interdigital nerve simultaneously. Each interdigital nerve was stimulated separately by placing the interdigital stimulating electrodes between the toes. Stimulus duration used was 0-05-0-1 ms. Supramaximal stimulation was assured by increasing it 25-30% beyond the maximal stimulation intensity when the CNAP was observed on each stimulus. The supramaximal stimulation intensity was usually at least three times the sensory threshold level and above 60 mA for a stimulus of 0-05 ms. Thus, these stimulus parameters were used when the CNAP was not observed on each stimulus. In every case, at least 2 digital or interdigital nerves in the medial plantar nerve and one digital or interdigital nerve in the lateral plantar nerve were tested. 64 to 256 stimuli were averaged with a signal averager in each recording. When no recognisable and constant potential was noted after 256 stimuli had been averaged in three recordings, we concluded that there was no CNAP. Surface ...