Low amplitude dorsal ulnar cutaneous nerve sensory nerve action potential from cross over innervation. Can nerve ultrasound help?

The ulnar dorsal aspect of the hand is predominantly innervated by the dorsal ulnar cutaneous nerve with variable input from the superficial radial cutaneous nerve. This cross innervation can cause difficulty in interpreting low amplitude sensory nerve action potential for the dorsal ulnar cutaneous nerve particularly when facing suspected ulnar neuropathyat the elbow. In three subjects with low dorsal ulnar cutaneous sensory nerve action potential amplitude due to cross over with the superficial radial nerve, we compared amplitude with nerve circumference and fascicular count as measured by ultrasound. Dorsal ulnar cutaneous nerve circumference was significantly smaller where there was low sensory nerve action potential amplitude and showed fewer fascicles. Nerve ultrasonography may be a useful additional test modality to determine if low dorsal ulnar cutaneous nerve amplitude is physiological.

Electrophysiologic Findings of Ulnar Neuropathy at the Elbow According to the Level of the Lesion

OBJECTIVE: To determine whether electrophysiologic findings of ulnar neuropathy at the elbow (UNE) are associated with anatomic location or a pathophysiologic mechanism, electrophysiologic findings of ulnar neuropathy above the elbow (UNAE) and below the elbow (UNBE) were compared. METHOD: Electrophysiologic findings of 56 patients with UNE were analyzed: segmental ulnar motor conduction study with abductor digiti quinti (ADQ) and first dorsal interosseous (FDI) recordings, ulnar and dorsal ulnar cutaneous nerve (DUCN) sensory action potentials, and needle electromyographic findings. Based on anatomic location, lesions were divided into UNAE and UNBE. Based on pathophysiologic findings, they were classified into three groups (focal demyelination, axonal degeneration, and mixed lesion). RESULTS: Twenty-eight patients were diagnosed with UNAE, and 28 with UNBE. Of the patients with UNAE, 4 had focal demyelination, 2 showed axonal degeneration, and 22 were of mixed lesions. Of patients with UNBE, 5 had focal demyelination, 6 showed axonal degeneration, and 17 were of mixed lesions. No significant differences in pathophysiologic mechanisms, or in electrophysiologic findings, were observed between UNAE and UNBE. The proportion of positive findings of focal demyelination was higher in FDI recording than in ADQ recording; however, this finding was not statistically significant (p>0.05). Thirty of 31 patients with abnormal DUCN had axonal degeneration with or without focal demyelination, whereas 9 of 25 patients with normal DUCN had focal demyelination only (p<0.05). CONCLUSION: Electrophysiologic findings did not relate to the anatomic location of UNE, but could relate to the pathophysiologic severity or fascicular involvement of the lesion.

Dorsal Ulnar Cutaneous Nerve Conduction Study in Entrapment Ulnar Neuropathy at the Elbow

OBJECTIVE: To evaluate the electrophysiologic findings of the dorsal ulnar cutaneous nerve (DUCN) in ulnar neuropathy at the elbow and to correlate the sensory nerve action potential (SNAP) amplitude of DUCN to those of ulnar compound muscle action potential (CMAP) and SNAP. METHOD: The subjects were 39 patients diagnosed as ulnar neuropathy at the elbow. Electrophysiologic classification consisted of 3 groups: conduction block (CB); conduction block and axonal degeneration (CBAD); axonal degeneration (AD). Findings of DUCN conduction study were evaluated according to the electrophysiologic classification. The amplitude of DUCN was correlated to those of CMAP and SNAP. RESULTS: The DUCN conduction study was normal in all cases of CB (4 cases), but was abnormal in 21 cases of CBAD (23 cases) and in 10 cases of AD (12 cases). Even in the cases with AD, 4 cases showed normal DUCN potentials. Correlations of DUCN to ulnar SNAP and CMAP were r=0.48, (p<0.01) and r=0.33, (p<0.05), respectively. CONCLUSION: Findings of DUCN may be related to the fascicular involvement of the ulnar nerve at the elbow. Although it is possible for the DUCN to be spared in ulnar neuropathy at the elbow, electrophysiologic measurement of the DUCN response remains useful, but only in those cases where it is abnormal, which suggests the lesion to be at a more proximal site.