This prospective study following American Association of Electrodiagnostic Medicine recommended criteria in the diagnosis of Carpal Tunnel Syndrome(CTS) evaluated the sensitivity of multi-electrodiagnostic parameters and usefulness of residual latency in CTS.

In 45 symptomatic hands of 26 patients with clinical diagnosis of CTS, 8 electrodiagnostic parameters-median motor distal latency, median motor residual latency, median sensory onset latency, median sensory peak latency, median to ulnar sensory onset latency difference(digit 4), median to ulnar sensory peak latency difference(digit 4), median to radial sensory onset latency difference(digit 1), median to radial sensory peak latency difference(digit 1)-were compared to the normative data obtained from the age-matched control group.

In 31 CTS hands without polyneuropathy, median to ulnar sensory latency difference(digit 4), median to radial sensory latency difference(digit 1), median sensory peak latency have same sensitivity(71.0%).

In 8 CTS hands with delayed proximal median motor nerve conduction velocity which were indiscernible from polyneuropathy in routine nerve conduction study, residual latency was more sensitive than median to ulnar sensory latency difference and median to radial sensory latency difference.

Sensitivity difference between sensory onset latency and sensory peak latency was negligible in the electrophysiologic diagnosis of CTS.

We concluded that residual latency measurement was a very useful and convenient method in the diagnosis of CTS, especially in the patients with delayed proximal median motor conduction velocity.

Objective: To determinate the reference values of residual latencies of motor nerves and to evaluate the early diagnostic value of residual latency.

Method: The subjects were 129 diabetes mellitus patients and 60 controls with no known neurological disorders. The patients were divided into two groups based on the conventional nerve conduction study: Group 1, 75 patients without neuropathy; Group 2, 54 patients with neuropathy. The group 2 patients were subdivided into 4 sub- groups on the basis of conduction velocity and residual latency of the median nerve. Residual latencies were measured in all subjects and glycosylated hemoglobin percentages (HbA1c) were measured in the diabetes patients. In group 2, each nerve conduction parameter was correlated with the duration of diabetes and HbA1c. The duration of diabetes, HbA1c, and amplitude of median nerve response were compared between the subgroups of group 2 patients.

Results: Motor residual latencies obtained from the controls were 1.93⁑0.28 msec, 1.53⁑0.24 msec, 2.46⁑0.43 msec, 2.21⁑0.53 msec in median, ulnar, deep peroneal and posterior tibial nerves, respectively. In group 1, motor residual latencies of median & deep peroneal nerves were significantly delayed compared with those of the controls. In group 2, motor residual latencies of median, ulnar, deep peroneal and posterior tibial nerves were significantly delayed more than those of the controls and group 1.

In group 2, increased HbA1c correlated to the decreased conduction velocities of median, deep peroneal, posterior tibial nerves but not to the residual latencies.

In the subgroup of group 2 (2-D), the nerve involved more distally showing lower compound muscle action potential and higher HbA1c.

Conclusion: Residual latency measurement can be a useful diagnostic method for the early detection of diabetic neuropathy.

Residual latency is the difference between the expected and measured terminal latencies in nerve conduction study. The main contributors to the residual latency are the nerve tapering in the hand and fingers and the neuromuscular delay. We measured median motor and sensory residual latencies in the controls and in patients with diabetes mellitus(DM) to establish the normal values, to evaluate the diagnostic value of the residual latency in diabetic polyneuropathy. we studied 50 healthy controls and 100 diabetic patients with or without polyneuropathy.

The normal residual latency values were 1.42⁑0.41 msec(mean⁑SD) in motor part and 0.44⁑0.20 msec in sensory part of median nerve. The standard deviation of residual latency in median motor nerve was decreased by 12% as compared with that of distal latency in the patient with diabetic polyneuropathy. Duration of DM and age were not related to the residual latency of median nerve.

The results suggest that the residual latency of median motor nerve provides a narrower normal range in the diagnosis of diabetic polyneuropathy irrespective of duration of DM or age.