Age / Sex : 42 / M

Chief complaint: Palpable mass of wrist dorsum

1) What is your impression?

Two weeks later, you can see the final diagnosis with a brief discussion of this case.

(Quiz는 quiz일 뿐이오니 답안은 한개만 보내주시기 바라오며, 복수의 답안을 보내주시는 분은 정답이 포함되어 있더라도 부득이 semi-correct answer로 처리토록 하겠습니다.)

Courtesy: 박진균 (Park, Jin-Gyoon), 전남대학교병원(Chonnam National University Hospital)

Accessory muscle ( extensor digitorum brevis manus muscle)

Discussion

Findings:

1) US images show a hypoechoic mass with muscle echotexture.

2) Axial MR images of wrist reveal a mass between carpal bone and extensor digitorum tendons,  that is iso-intense relative to normal muscle on T1WI, T2WI, and contrast enhanced image.

Differential Diagnosis:

Diagnosis:  

Accessory muscle ( extensor digitorum brevis manus muscle)

Discussion:

Accessory muscles are anatomic variants representing additional distinct muscles that are encountered along with the normal complement of muscles. Historically, the majority of data regarding accessory musculature has been based on serendipitous findings at surgery. However, with the advent of modern cross-sectional imaging techniques such as ultrasonography, computed tomography , and magnetic resonance imaging, accessory muscles are regularly encountered and can be accurately identified noninvasively. Accessory muscles are commonly overlooked at imaging evaluation . Although they are typically asymptomatic and encountered as incidental findings, accessory muscles have been implicated as a potential source of clinical symptoms. Such symptoms are usually due to mass effect of the supernumerary muscle, with the patient presenting with either a palpable swelling or secondary compression of adjacent structures such as nerves, vessels, or tendons. Cross-sectional imaging can accurately demonstrate accessory muscles and help differentiate them from other soft-tissue masses.

An extensor digitorum brevis manus (EDBM) muscle is a well-described accessory muscle that occurs on the dorsum of the hand and wrist. An EDBM is rare: In a study of 128 cadavers by Rodriguez-Niedenfuhr et al, this accessory muscle was found in four limbs (1.6%). The muscle can be unilateral or bilateral. The origin of an EDBM can vary, although the most common origin is the dorsal wrist capsule deep to the extensor retinaculum. The muscle may also arise from the distal radius or the deep carpal fascia. It typically inserts onto the extensor hood of the index finger or middle finger but may also insert into the extensors of the fourth and fifth fingers by way of either a tendon or a slip. An EDBM can manifest clinically as a soft tissue swelling on the dorsum of the hand and wrist that becomes firm on extension of the fingers. It is often clinically diagnosed as a ganglion, synovitis, or a carpal boss. An EDBM is usually painless but may occasionally be associated with exercise induced pain or tenosynovitis of the extensor tendons. The EDBM tendon has been used in surgical restoration of function of ruptured tendons. At MR imaging, an EDBM typically appears as a masslike lesion that is iso-intense relative to normal muscle on T1WI, T2WI, and contrast enhanced image, usually located between the second and third metacarpals, with the bulk of the muscle at the level of the carpometacarpal joints. Because of its low T1 and T2 signal intensity, the muscle may mimic a giant cell tumor of the tendon sheath. US can be used to demonstrate the normal echotexture of the muscle as well as morphologic changes of the muscle upon flexion and extension, resulting in dynamic contraction-induced protrusion of the muscle between the extensor tendons. Treatment of a symptomatic EDBM may include extensor retinaculum release or excision of the muscle .

References:

1. Sookur PA, Naraghi AM, Bleakney RR,Talan R, Chan O, White LMl. Accessory muscles: anatomy, symptoms, and radiologic evaluation. Radiographics 2008;28:481-499

2. Rodriguez-Niedenfuhr M, Vazquez T, Golano P, Parkin I, Sanudo JR. Extensor digitorum brevis manus: anatomical, radiological and clinical relevance. Clin Anat 2002;15:286-292.

3. Cavdar S, Dogan T, Bayramicli M, Sehirli U, Yuksel M. An unusual variation of extensor digitorum brevis manus: a case report and literature review. J Hand Surg [Am] 1998;23:173-177.

4. Anderson MW, Benedetti P, Walter J, Steinberg DR. MR appearance of the extensor digitorum manus brevis muscle: a pseudotumor of the hand. AJR Am J Roentgenol 1995;164:1477–-1479.

5. Ouellette H, Thomas BJ, Torriani M. Using dynamic sonography to diagnose  extensor digitorum brevis manus. AJR Am J Roentgenol 2003;181:1224-1226.