Anconeus Epitrochlearis Muscle Causing Ulnar Neuropathy at the Elbow: Clinical and Neurophysiological Differential Diagnosis

Ulnar neuropathy at the elbow is the second most frequent entrapment neuropathy and is considered idiopathic in most patients. However, several anatomic variations, including the anconeus epitrochlearis muscle, have been reported to cause ulnar nerve compression. The anconeus epitrochlearis muscle is a common anatomic variation, with a prevalence of up to 34%, but the clinical diagnosis of ulnar neuropathy of the elbow as a result of this variation is rare, with an unknown prevalence. It is a congenital accessory muscle between the medial humeral epicondyle and the olecranon that covers the posterior aspect of the cubital tunnel and is usually an operative finding, not a preoperative diagnosis. Ulnar neuropathy as a result of the anconeus epitrochlearis muscle usually has different characteristics than idiopathic disease, including younger age at onset, more rapid progression with a short duration of symptoms, distinct neurophysiology with velocity drop or conduction block of the ulnar nerve, and edema of the anconeus epitrochlearis muscle on magnetic resonance imaging. Neurophysiologic findings in anconeus epitrochlearis-associated ulnar neuropathy indicate subacute onset of symptoms rather than the chronic demyelinating process that is seen in idiopathic ulnar neuropathy. Medial elbow pain may be more exacerbated in these patients rather than the more common sensorial symptoms. This is probably the result of static compression of the nerve and increased cubital tunnel pressure, even when the elbow is in extension. This article describes a case of ulnar nerve entrapment of the elbow in a 28-year-old woman as a result of compression by the anconeus epitrochlearis muscle and includes magnetic resonance imaging findings, surgical correlations, and clinical and neurophysiologic findings.