Clinical History
A 38-year-old man presented with a prominence in the dorsum of the right hand that was more evident during the active extension of the fingers and painful after doing heavy manual work.
Imaging Findings
A 38-year-old man presented with a pain in the dorsum of the right wrist, which he had developed while practising heavy manual work in the past month. At physical examination, a prominence in the
dorsum of the right hand, more exactly in the distal part of the wrist, between the extensor tendons of the index and middle fingers, was detected. The prominence was more evident when the fingers
were fully extended. Suspecting a ganglion, a sonography of the dorsum of the right hand and wrist was performed, which evidenced a soft tissue mass with muscle-like echographic texture (Fig. 1) that
showed morphological changes during the active extension of the fingers (Fig. 2). MRI demonstrated the presence of an ovoid mass similar to a normal muscle (Figs. 3 and 4) in the dorsum of the hand.
According to these findings in sonography and MRI scan, the final diagnosis made was that of a right extensor digitorum brevis manus muscle.
Discussion
The extensor digitorum brevis manus (EDBM) is a relatively rare supernumerary muscle of the dorsum of the hand. There is a slight predominance of occurrence in males. It is estimated to be two to
three of the cases and there are no significant differences of it being bilateral in 54 locations between both hands. It is a single belly muscle originating in the distal epiphysis of the radius,
the dorsal radiocarpal ligament, or the wrist-joint capsule. It has an insertion most commonly into the hood of the second extensor, followed by the hood of the third extensor and sometimes the
second and third fingers. The tendon of the extensor of the index is absent in the cases in which the EDBM muscle inserts into the second finger. The EDBM was innervated by the posterior interosseous
nerve and its blood supply was provided by the posterior interosseous artery. EDBM usually is asymptomatic, although repetitive movements of the wrist and hand may cause pain in the dorsum of the
wrist. The pain is likely to be a consequence of muscle hypertrophy and of impingement of the muscle against the extensor retinaculum. In the physical examination, the EDBM may be misdiagnosed as a
ganglion or soft-tissue tumors. A characteristic finding of the EDBM is that the prominence in the dorsum of the hand increases in size with the active extension of the fingers. Sonography reveals
the presence of a soft-tissue mass with muscle-like echo texture, which suffers morphological changes during the active extension of the fingers. MRI scans are helpful in distinguishing, in a
non-invasive way, EDBM from tumors, thereby avoiding surgery. The treatment depends on the severity of the symptoms. When the symptoms are severe, conservative treatment with anti-inflammatory drugs
and immobilization may be unsuccessful. Although division of the retinaculum may provide symptomatic relief, the excision of the muscle is usually recommended. However, resection of EDBM should be
avoided in those cases in which it compensates for the absence of the extensor indicis propius.
Differential Diagnosis List
Accessory extensor digitorum brevis manus muscle.
Final Diagnosis
Accessory extensor digitorum brevis manus muscle.
