J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.48.10.999 on 1 October 1985. Downloaded from Journal ofNeurology, Neurosurgery, and Psychiatry 1985;48: 999-1003 The near-nerve sensory nerve conduction in tarsal tunnel syndrome SHIN J OH, HYUN S KIM, BASHIRUDDIN K AHMAD From the Department ofNeurology, University ofAlabama at Birmingham, Birmingham, Alabama, USA 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 elec- trophysiological 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 sen- simultaneously. Each interdigital nerve was stimulated sory nerve conductions in the medial and lateral separately by placing the interdigital stimulating electrodes plantar nerves using the surface electrode, which between the toes. Stimulus duration used was 0-05-0-1 ms. was able to confirm the diagnosis of tarsal tunnel Supramaximal stimulation was assured by increasing it Protected by copyright. syndrome in 90% of cases.' For the past 25-30% beyond the maximal stimulation intensity when four years the CNAP was observed on each stimulus. The supramax- we have used the near-nerve needle sensory nerve imal stimulation intensity was usually at least three times conduction technique in diagnosis of this disorder. the sensory threshold level and above 60 mA for a stimulus We report here our experience with this technique. of 0-05 ms. Thus, these stimulus parameters were used when the CNAP was not observed on each stimulus. In Materials and me every case, at least 2 digital or interdigital nerves in the medial plantar nerve and one digital or interdigital nerve in The technique of the near-nerve needle sensory nerve the lateral plantar nerve were tested. conduction in the digital and interdigital nerves of the foot 64 to 256 stimuli were averaged with a signal averager in was described in detail in a previous report.2 Sensory com- each recording. When no recognisable and constant poten- pound nerve action potentials (CNAPs) were recorded tial was noted after 256 stimuli had been averaged in three orthodromically using the near-nerve needle and signal recordings, we concluded that there was no CNAP. Surface averaging techniques. The active needle recording elec- skin temperature under the arched portion of the medial trode was inserted close to the medial plantar fascicle near plantar surface of the foot was controlled at 32°C using a the posterior tibial nerve in the ankle behind the medial skin temperature control unit. Latency was measured from maileolus above the flexor retinaculum. This site was cho- the stimulus onset to the first positive peak for the max- sen with the aid of a surface stimulating electrode. The imum nerve conduction velocity (NCV), to the highest adequacy of the needle position was determined by negative peak for the negative peak NCV, and to the last http://jnnp.bmj.com/ stimulating the nerve through the active electrode. When negative peak of the potential for the minimum NCV. the great toe (digit I) was contracting minimally with less Conduction velocity was calculated by dividing the dis- than 3 mA for a stimulus of 0-05 ms duration, the needle tance by the latency. Distance was measured by caliper. was considered to be adequately positioned. The reference The amplitude was measured from peak to peak. The dura- needle electrode was placed subcutaneously at the same tion was measured from the onset of the first positive peak level as the active electrode at a transverse distance 3- to the baseline return of the last component of the poten- 4 cm. The I and V digital nerves were stimulated with ring tials. electrodes and various interdigital nerves with interdigital When the NCVs and the duration of CNAP in patients stimulating surface electrodes which were specially deviated by more than 2 standard deviations from the on October 6, 2021 by guest. designed to stimulate two branches of the interdigital nerve normal mean values, they were considered abnormal. Address for reprint requests: Shin J Oh, MD, Department of Abnormally prolonged duration of CNAP is termed "dis- Neurology, University of Alabama at Birmingham, University persion phenomenon". When the amplitude of CNAP was Station, Birmingham, Alabama 35294, USA. lower than the lowest normal range, it was considered abnormal. Received 7 December 1984 and in revised form 1 March 1985. The motor terminal latency of the medial and lateral Accepted 8 March 1985. plantar nerves was determined following the method pre- 999 J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.48.10.999 on 1 October 1985. Downloaded from 1000 Oh, Kim, Ahmad Table 1 Sensory nerve conduction abnormalities in 25 had bilateral tarsal tunnel syndrome. The ages of patients cases oftarsal tunnel syndrome ranged from 14 to 69 years. Clinically, medial plantar neuropathy was diagnosed in 10 patients, lateral plantar Plantar nerve Medial Lateral neuropathy in two, and calcarine neuropathy in one. None Normal 1* 1 of the patients had any motor weakness or wasting of the Abnormal 24 13 intrinsic foot muscles. Absent potential 2 6 In all cases, subjective burning and tingling paraesthesia Slow NCV 16 4 Maximum 13 3 over the medial or lateral plantar nerve territory was pres- Negative peak 14 3 ent. For the diagnosis of tarsal tunnel syndrome, objective Minimum 10 1 sensory impairment over the medial or lateral plantar Dispersion 14 5 nerve territory and a positive Tiners sign on these nerves Low amplitude 10 5 at the ankle were present in 20 cases; subjective symptoms *Number of cases and a Tiners sign in three cases; and subjective symptoms and objective sensory impairment in two cases. Lumbar radiculopathy and peripheral neuropathy were viously described.' 6 The distance between the recording ruled out by neurological and electrophysiological evalua- electrodes and the stimulating electrodes was kept con- tions when clinically indicated. stant: 10 cm in the medial and 12 cm in the lateral plantar nerve. Results Twenty-five cases of tarsal tunnel syndrome were diag- nosed in 21 patients, 11 male and 10 female. Four patients Among the 25 cases of tarsal tunnel syndrome, , 34.8 34.9 ii' A H I Protected by copyright. iI~~~~~~~~~~~~~~~~~r III-IV |f I .r^ 40.9 40-0 , 34 *6 34 2 A II eV / '-"' 39.8 IV-V I ., II 34.92u349V {I I X 2ms http://jnnp.bmj.com/ 1I-111i V .I1 I i 40.8 |1 -36.0 42.9 2 ms on October 6, 2021 by guest. Fig 1 Normal maximum and negative-peak nerve conduction velocity (NCV) in a 29-year-old normal control. Roman numerals indicate digital and interdigital nerves. Numbers under the compound nerve action potentials (CNAP) represent maximum NCVs and numbers above the CNAPs represent negative-peak NCVs respectively. Two ,uV calibration for the I and 1-1I interdigital nerves and I p4V calibration for all other nerves. J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.48.10.999 on 1 October 1985. Downloaded from The near-nerve sensory nerve conduction in tarsal tunnel syndrome 1001 26.1 23 3 - I -. , lII I-Iv ,r -2 .I 30.7 27.8 24.6 24.7 I-II _-> X Iv-v 30.2 31. 31.1 23 8 _ 25.0 _'Ilk b -X 1-I1 1 ,UV 3.26 28.9 Iim Protected by copyright. 2 ms 3266 Fig 2 Slow maximum and negative-peak NCVs in medial plantar nerve and normal maximum and negative-peak NCVs in lateral plantar nerve in a case oftarsal tunnel syndrome. abnormal sensory nerve conduction was observed in Mean values of the various nerve conduction 24 (96%): abnormal sensory nerve conduction in parameters are presented in table 2. Maximum and the medial plantar nerve in 24 cases and in the lat- negative-peak NCVs and duration of CNAP in the eral plantar nerve in 13 cases (table 1). Thus, diag- various interdigital nerves in tarsal tunnel syndrome nosis was confirmed electrophysiologicaily in 96% are significantly different from normal values. How- of the cases of tarsal tunnel syndrome. ever, the amplitudes of CNAP are only significantly In 18 cases, various combinations of sensory con- lower than normal values in the I digital and I-II duction abnormalities (for example, slow NCV and interdigital nerves. Slowing in minimum NCVs is dispersion phenomenon) were noted. In seven cases, barely significant in the I, II-III, and V nerves. (p < a single sensory nerve conduction abnormality was 0-05) noted: low amplitude in three, slow negative-peak Terminal latency of the plantar nerves was pro- NCV in two, dispersion phenomenon in two, and longed in only four of 24 tested cases (17%): one in absent CNAPs in two. the medial plantar nerve (<5.38 ms) and three in http://jnnp.bmj.com/ Slow nerve conduction velocities (NCVs) and dis- the lateral plantar nerve (<6-26 ms) Mean terminal persion phenomenon were the most prominent latencies were 4-00 + 0-77 (SD) ms (vs. normal, abnormalities (figures 1-3). In 12 cases, slowing was 4-10 0.64) in the medial plantar nerve and 4-81 _ observed both in maximum and negative-peak 1-06 ms (vs. normal, 4 70 0.78) in the lateral NCVs. In two cases, negative-peak NCV alone was plantar nerve. The amplitude of the compound slow, emphasising the value of the negative-peak muscle action potential was abnormally low in one NCV.