Nerve conduction studies: orthodromic vs antidromic latencies.

Variables known to affect the sensory nerve action potential (SNAP) include the method of measuring the SNAP, electrode placement and local impedance, volume conduction, temperature, age, nerve fiber composition, and duration dependent phase cancellation. There is debate, however, concerning the effect of orthodromic vs antidromic stimulation on the latency of the SNAP. This question was studied with the same methodology of those who reported a difference between antidromic and orthodromic latencies, but with control of the interelectrode distance. Twenty-five normal hands were studied. The median and ulnar digital nerves were analyzed both orthodromically and antidromically. There was careful control of interelectrode distance, which was the same for both the recording and stimulating electrodes. Studies were done with both a 3-cm and 4-cm interelectrode distance. Parameters recorded were onset latency, peak latency, terminal latency, and amplitude (baseline to negative peak, and peak to peak). Extensive statistical analysis was performed for all parameters. No differences were found between antidromic and orthodromic studies when the interelectrode distance was the same.

Meningitis-related hearing loss evaluated with evoked potentials.

Sixty-seven children with meningitis, ranging in age from 1 month to 14 years, were studied using brainstem auditory evoked potentials. Abnormal results were present in 60% of the subjects. Serial studies were obtained in 30% of the abnormal patients, 75% of whom demonstrated continued abnormalities on subsequent examination. Abnormal studies were found only in those patients who were 6 years of age or younger. The most prominent finding was an increased interwave interval resulting from increased wave V latencies. Additional abnormalities included an increased slope of the latency-intensity function suggestive of sensory-neural hearing loss. These studies clearly indicate the significance of early detection by the use of evoked potentials in the early diagnosis of hearing disorders in children with meningitis.