Symptomatic compression neuropathy of the median nerve is a frequent entity.1 The most common cause of the median nerve compression is carpal tunnel syndrome (CTS) due to thickening of the anterior annular ligament.2 Nerve compression by anomalous masses located at the wrist and distal forearm is an infrequent condition.3

Accessory muscles and other masses, like lipomas, may compress underlying structures in the carpal tunnel region, causing pain and paresthesias, leading to the diagnosis of CTS, which can mislead to a wrong diagnosis of thickening of the anterior annular ligament.

We present three cases of patients with symptoms and clinical tests (Tinel's and Phalen's) compatible with compression of the median nerve in the carpal tunnel who showed, on clinical examination, a soft tissue mass in the distal region of the forearm. The ultrasound (US) evaluation revealed a mass, of different aethiologies, compressing the median nerve.

A 31-year-old woman, whose electromyogram revealed a sensory latency of the left median nerve in the wrist, compatible with mild CTS, presented a soft tissue mass that, on US, corresponded to an accessory muscle. A wrist MRI showed a “muscle mass, which may correspond to an anatomical variant at this level and the median nerve shows a hypersignal proximal to the carpal tunnel.” A surgical exploration of the carpal tunnel was performed, allowing the identification and release of the median nerve and the excision of an exuberant muscular mass that corresponded to an accessory flexor digitorum superficialis indicis. After two months of follow-up, the patient referred total resolution of the complaints of paresthesia and neuropathic pain (Fig. 1A–D).

Fig. 1.

Clinical case 1. A: Swelling of the forearm (circle); B: Magnetic resonance imaging showing a muscle mass, which may correspond to the anatomical variant (arrow); C: Ultrasound of normal forearm; D: Ultrasound of accessory flexor digitorum superficialis indicis muscle (arrow). Clinical case 2. E: Swelling of the forearm (circle); F: Magnetic resonance imaging showing mild hypersignal of the pronator quadratus (arrow); G: Ultrasound showing a hypoechoic mass (arrow); H: Ultrasound of normal forearm; I: Surgical exploration that revealed a lipoma. Clinical case 3. J: Swelling of the forearm (circle); K: Ultrasound of normal forearm; L: Ultrasound of a hypoechoic mass that compressed the median nerve, suggestive of lipoma or muscle mass (arrow).

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A 43-year-old woman, with an unremarkable electromyogram, was submitted to an US evaluation of the wrist, which revealed that the mass on the dorsal flexor surface corresponded to an hyperechoic mass compared to the surrounding hypoechoic fat, with fibrous capsule and no posterior acoustic enhancement. The MRI revealed “muscular groups of the volar surface of the distal forearm with mild hypersignal of the pronator quadratus, which may be due to overuse or translate innervation interfere”. The surgical exploration revealed a lipoma which was surgically extracted (Fig. 1E–I).

A 45-year-old woman, with a normal electromyogram, whose soft tissue mass on the forearm corresponded, on US evaluation, to a muscle mass that was compressing the median nerve. The patient is currently waiting for surgical releasing of the median nerve (Fig. 1J–L)

CTS is one of the most common peripheral neuropathies in the upper extremity. It is defined as an entrapment syndrome of the median nerve at the level of the distal forearm.2 Median nerve compression at the wrist can also result from accessory flexor tendons or hypertrophic muscles. Although rare, these anatomic variances need to be considered, especially in the presence of a mass in the distal region of the forearm, such as in the reported cases. Lipomas are the most frequent human tumors and are generally asymptomatic. However, as in the clinical case 2, they can compress some structures, including nerves.4 In literature, the reported cases of accessory muscles and other masses causing median nerve compression are mainly described in patients with residual symptoms after the surgical decompression of the carpal tunnel.4,5

US evaluation prior to the median nerve decompression surgery is of upmost importance since it allows an adequate differential diagnosis, helping identifying these cases and planning surgical approach.

Authors declare that the manuscript has not been submitted or published elsewhere with the exception of abstracts published with scientific meetings

These patients gave your informed consent in accordance with the 1964 Helsinki declaration.

The authors declare they have no conflicts of interest.