Abstract
The median-to-ulnar communicating branch (MUC) is an asymptomatic variant of the upper limb innervation that can lead to interpretation errors in routine nerve conduction studies. The diagnosis of carpal tunnel syndrome (CTS) or ulnar nerve lesions can be complicated by the presence of MUC. In this study, we describe electrophysiological features of MUC in CTS patients presenting to our clinic. We enrolled MUB cases from consecutive CTS patients referred to our laboratory between the years 2014 and 2019. MUC was present in 53 limbs (36 patients) from the studied population. MUC was bilateral in 53% of patients. MUC type II was the most common subtype (74%), followed by types III and I; more coexisting MUC types were found in the majority of tested limbs. A positive correlation was demonstrated between the severity of CTS and the presence of positive onset, faster CV, or a double component of the compound muscle action potentials. We emphasize the importance of suspecting the presence of MUC in CTS in the presence of a positive onset or a double component in routine motor conduction studies.
Highlights
The median-to-ulnar communicating branch (MUC) [1], known as “MartinGruber anastomosis/communication,” is a common anatomical innervation variant of the upper limb in which a crossover of axons passes from the median (MN) to the ulnar nerve (UN) in the forearm; this condition occurs asymptomatically in about 5–40% of the population and is bilateral in 10–40% of cases [2,3,4,5,6,7,8,9,10]
We describe the electrophysiological features of median-to-ulnar nerve communication in a cohort of carpal tunnel syndrome (CTS) patients to better characterize MUC’s electrophysiological features and its impact in clinical practice
We evaluated several neurophysiological parameters as reliable measures of the communication entity: the presence of a positive onset, a double potential, conduction velocity (CV), compound motor action potential (CMAP) amplitude, “gain” in CMAP amplitude obtained by a distal UN stimulation compared to a proximal stimulation, and “drop” in CMAP amplitude obtained by a proximal MN stimulation compared to a distal stimulation
Summary
The median-to-ulnar communicating branch (MUC) [1], known as “MartinGruber anastomosis/communication,” is a common anatomical innervation variant of the upper limb in which a crossover of axons passes from the median (MN) to the ulnar nerve (UN) in the forearm; this condition occurs asymptomatically in about 5–40% of the population and is bilateral in 10–40% of cases [2,3,4,5,6,7,8,9,10]. In the presence of CTS, the compound motor action potential (CMAP) at the elbow could present an initial positive deflection, leading to an apparently fast nerve conduction velocity (CV) of the MN in the forearm [5,8,11,21]. These alterations are not always easy to recognize despite marked discrepancies between clinical and electrodiagnostic findings. Greater CMAP amplitude over FDI recording when stimulating the ulnar nerve at the wrist compared to the elbow. MUC—median-to-ulnar communicating branch; NCS—nerve conduction studies; FDI—first dorsal interosseus muscle; ADM—abductor digiti minimi muscle; APB—abductor brevis pollicis muscle
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