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Excerpt
Motor nerve conduction studies have been extensively studied to the abductor pollicis brevis and abductor digiti minimi. A few studies have explored the first lumbrical, whereas others have investigated the second lumbrical-interosseous distal motor latency difference in the diagnosis of carpal tunnel syndrome (CTS). Our objective is to establish control values for latency and amplitude of the first lumbrical, second lumbrical, and the interossei. Absolute values as well as comparison ratios are presented. Orthodromic stimulation was performed at the wrist 12 cm proximal from the recording electrode. We examined 43 consecutive hands without a history of CTS or peripheral neuropathy. A sensory nerve conduction carpal tunnel screen was performed on each hand before recording of the lumbricals or interossei. The lumbrical and interossei latencies were found to be <4.3 ms. The lumbrical amplitudes were between 1 and 4 mV, whereas the interossei amplitudes ranged from 2 to 10 mV. Volume conduction was a frequent problem in the lumbrical CMAP when stimulating the ulnar nerve. When recording over lumbricals with ulnar nerve stimulation, the issue is which ulnar innervated muscle is recorded. Amplitudes are the best indictors of which intrinsic muscle is being recorded. Establishing normal values for median and ulnar innervated hand intrinsics could allow subsequent studies to evaluate trauma and entrapment of nerve branches in the hand and add an additional technique to use for individuals suspected of CTS.
