Upper Limb Nerve Variations May Alter Clinical Diagnosis: A Scoping Review

Abstract

Upper limb nerve variations may be related to the absence of a nerve, an interconnection between two nerves or a variant course. The purpose of this review is to screen the existing literature on upper limb nerve variations that may alter the neurologic diagnostic process. A scoping review was performed following PRISMA for Scoping Reviews guidelines. Initially, 1331 articles were identified by searching Pubmed and Web of Science until the 22nd of October 2022. After screening, reading, and additional searching 50 articles were included in this review. Variations were divided into two categories: 1) variations causing a different innervation pattern involving sensory, motor, or both types of fibers, and 2) variations causing or related to compression syndromes. Two-thirds of the included articles were cadaver studies. Nine articles were diagnostic studies on symptomatic or healthy individuals involving medical imaging and/or surgery. Nerve variations that may cause a different innervation pattern concern most frequently their interconnection. The connection between the median and musculocutaneous nerve in the upper limb and the connection between the median and ulnar nerve in the forearm (Martin-Gruber) or hand (Riche-Cannieu) may be present in half of the population. Injury to these connections may cause compound peripheral neuropathies a result of variant sensory and motor branching patterns. Muscular, vascular, or combined anomalies in the forearm were reported as causes of entrapment neuropathies. These nerve variations may mimic classical entrapment syndromes such as carpal tunnel syndrome or compression at ulnar canal (Guyon's canal). Knowledge of frequent nerve variations in the arm may be important during the diagnostic process and examination. Variant innervation patterns may explain non-classical clinical signs and/or symptoms during provocative tests. Classical nerve compression syndromes in the arm may warrant for differential diagnosis, especially in the case of persistent or recurrent symptoms.