Medial antebrachial cutaneous nerve conduction study, a new tool to demonstrate mild lower brachial plexus lesions. A report of 16 cases.

OBJECTIVE: This was to demonstrate the ability to electrodiagnosed mild lower brachial plexus lesion only through abnormal medial antebrachial cutaneous nerve (MABCN) conduction study.

METHODS: We report 16 cases of unilateral, atypical pains and paresthesias of the upper limbs without motor deficit or atrophy. Patients were referred as carpal tunnel syndrome in 12 cases. All patients had needle examination of the impaired upper limb from C5 to T1. Motor and sensory conductions of median and ulnar nerves were bilaterally studied. MABCN was antidromically (16 cases) and orthodromically (9 cases) studied at the elbow in the both sides. MABCN abnormality was defined by an interside amplitude ratio of the sensory nerve action potential equal or greater than 2 (mean + 3 SD).

RESULTS: No patient had a definitive and accurate diagnosis, before MABCN abnormality determination. MABCN testing was abnormal in all the 16 cases with a mean interside amplitude ratio of 7.2 (mean + 25 SD), when all other motor and sensory nerve conductions were normal. All except four patients showed normal needle examination from C5 to T1. In 5 cases, an obvious cause (traumatic and neoplastic) explained the mild lower brachial plexus lesion. In 2 cases, a mild neurogenic thoracic outlet syndrome (NTOS) was confirmed by surgical findings. In the 9 other cases, the mild lower brachial plexus lesion defined by MABCN findings, was without cause and was considered as a mild NTOS.

CONCLUSIONS AND SIGNIFICANCE: These 16 cases, support a new electrodiagnostic pattern to define a mild lower brachial plexus lesion: comparatively low or low MABCN SNAP amplitude, normal median and ulnar SNAP/Compound motor action potential amplitudes and normal or slightly reduced interference pattern in some C8-T1 innervated muscles. This pattern can be found in patients with 'carpal tunnel syndrome like' symptoms who have normal electro-diagnostic examination, or in patients with clinical features suggesting a lower brachial plexus lesion.