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Anomalous flexor digitorum superficialis muscles in the hand are an uncommon phenomenon, and therefore present challenges in diagnosis and management. We report a case of a 16-year-old girl presenting with a painful, slowly enlarging palmar swelling. The swelling was investigated with ultrasound and magnetic resonance imaging, and was found to be an anomalous muscle belly of the flexor digitorum superficialis muscle. After careful consideration, multidisciplinary discussion, and thorough imaging, the patient was treated successfully without surgical exploration or excision, in comparison to previously reported cases. The patient was pain free and had no concerns at 8-month follow-up, demonstrating the value of conservative management in these cases.
A 16-year-old right-handed girl presented to the hand clinic with a 6-month history of swelling to the right palm. The mass had been slowly increasing in size, and although occasionally painful, there was no functional impairment. On examination, a soft swelling was noted over the tendon sheath to the right index and middle fingers distal to the thenar eminence. The swelling moved with flexion of the proximal interphalangeal (PIP) joint to the index finger. There was no neurovascular dysfunction. The swelling had been investigated before referral to us via ultrasonography (Fig. 1), revealing a solid lesion that was thought to possibly be a giant cell tumor, although on review by our musculoskeletal radiologists it was actually felt to be muscular in nature. An urgent magnetic resonance imaging (MRI) scan was arranged to further assess the mass.
MRI revealed a swelling that was isointense to neighboring muscle in all sequences, again suggestive of a muscular origin (Fig. 2). The patient was therefore diagnosed with an anomalous flexor digitorum superficialis (FDS) muscle in the palm. At follow-up, there was still residual swelling and aching, causing the patient to take time off school. The patient was keen to have the swelling either debulked or excised, but was advised that the muscle was providing sole flexor action to her index finger PIP joint, and it was therefore unwise to interfere with the muscle surgically. With careful reassurance, the patient was managed conservatively, accepting that the aching in her palm was likely due to an increased awareness of the muscle. At further 8-month follow-up, her pain had resolved, with no change in the mass. She was therefore discharged, on the proviso that we would like to see her again should there be any change in the size, shape, or consistency of the mass.
Anomalous palmar FDS muscle bellies are uncommon, and therefore present a diagnostic challenge. The majority of patients present in their teenage years or 20s with a slowly enlarging mass in the palm,1 which may also cause pain2 and functional compromise.3 The index finger of the right hand is most commonly affected, with a female predisposition.1 Carpal tunnel syndrome is also reported as a presenting feature, which may or may not be accompanied by a palmar mass.4 There have also been reports of patients presenting with triggering of the wrist and finger5 and rupture of the associated flexor digitorum profundus (FDP) tendon.6 On examination, a soft palmar mass that becomes more firm and moves proximally with finger flexion is usually found, although these findings are not ubiquitous.
The genetic development of these anomalies has been discussed recently by Bhat et al,1 and it is thought they may be atavistic in nature. The authors also propose an updated classification system for these anomalies, allowing categorization of the full range of anatomical variants and acknowledging their phylogenetic origin (Table 1). Normally, the FDS is described as having two heads: a humeroulnar head, arising from the medial epicondyle of the humerus, the ulnar collateral ligament of the elbow, and the coronoid process of the ulna, and a radial head, arising from the anterior body of the radius. In the distal forearm, these muscles form four tendons to the four fingers, which split to insert onto the radial and ulnar aspects of the proximal half of the middle phalanx of each finger.7
In this case, the anomalous muscle belly originated just proximal to the distal crease of the flexor retinaculum, replacing the normal FDS longus tendon, and became tendinous again at the level of the A1 pulley (Fig. 2B), where the anomalous FDS brevis tendon joined the FDP tendon, before inserting normally at the proximal half of the middle phalanx of the index finger. Using Bhat et al's classification system, our case demonstrates the Type IIa variant.
The correct choice of investigation in these palmar masses is essential to guide management and highlight sinister differentials, and varies between reports, with plain film radiography, ultrasound, computed tomography, and MRI all used to varying extents. The imaging of these pseudotumors should follow the principles for all upper extremity tumors, as previously outlined by Peterson et al.8 Initial investigation with plain film radiographs may be performed, and although these will not provide a diagnosis of a muscular anomaly, they can help rule out other differentials for a soft tissue swelling.
Sonography may also be considered as a first-line investigation when assessing these masses, and is rapidly accessible, cost-effective, and noninvasive.9 Ultrasound can be used to define the size, location, and extent of a mass, and can differentiate between solid and cystic lesions. On ultrasound, anomalous muscles are hypoechoic, with fibroadipose septa and a striated appearance.10 Ultrasound also allows dynamic investigation of a mass, and a change in shape with joint movement is particularly suggestive of an anomalous muscle.
MRI is often considered the gold standard when assessing tumors of the hand. In comparison to ultrasound, MRI offers a more accurate diagnosis of the tumor type, through analysis of both the position and tissue character of the tumor.11 Anomalous muscles are isointense to normal muscles within the hand. For this reason, they are often overlooked as they do not stand out as abnormal. It is therefore important to analyze the origins and insertions of suspected anomalous muscles, while observing their relation to the surrounding anatomy.
The majority of previously reported anomalous FDS muscles in the palm have been managed surgically. Indeed, in a recent review, of the 31 out of 37 cases that had their treatment detailed, all underwent at least some degree of surgical intervention.1 Surgical treatments have included simple exploration, partial excision, and decompression. However, as demonstrated in our case, surgical intervention may not be required to achieve satisfactory results. Our management included thorough imaging with close correspondence between hand surgeons and musculoskeletal radiologists, and regular follow-up with careful reassurance, sparing the patient an operation. The patient had a positive outcome and her symptoms had resolved themselves over time with no further concerns.
We present a case that illustrates the challenges in diagnosing and managing anatomical variants of the FDS muscle. The patient was treated successfully without the need for surgical intervention, in comparison to the majority of previously reported cases that have undergone surgical exploration or excision. We believe that conservative management is a valid option when treating these masses, requiring meticulous use of imaging and multidisciplinary discussion between musculoskeletal radiologists and hand surgeons.
Conflict of Interest None.