A rare cause of bilateral carpal tunnel syndrome

INTRODUCTION

Since the first description of anomalous flexor digitorum superficialis (FDS) muscle during cadaveric dissection, understanding of anatomical variations in the carpal tunnel has evolved significantly. Magnetic resonance imaging (MRI) has revolutionized the evaluation of carpal tunnel anatomy, offering superior soft tissue contrast and multiplanar capabilities that allow detailed assessment of neuromuscular variations. The role of advanced imaging has become particularly crucial in cases with atypical presentations or suspected anatomical variations.

CASE REPORT

A 31-year-old male weightlifter experienced bilateral wrist pain and numbness in both hands over the course of 2 months, particularly during his regular gym sessions. Nerve conduction study was negative for carpal tunnel syndrome (CTS). MRI was obtained, which revealed the presence of bilateral accessory FDS muscle bellies extending into the carpal tunnel. This was associated with compression of the median nerve, as evidenced by increased T2 signal intensity and flattening of the nerve on the MRI.

This uncommon finding of bilateral accessory muscles as the underlying cause for the patient’s CTS highlights the importance of considering anatomical variations when evaluating patients with bilateral symptoms.

MRI was performed on a 3 Tesla scanner using a dedicated wrist coil. The imaging protocol incorporated T1-weighted spin-echo, T2-weighted fast spin-echo, proton density (PD) sequences with fat suppression, and short tau inversion recovery sequences.

The examination was conducted with the patient in prone position and arm extended overhead. The imaging field of view was optimized to provide optimal visualization of carpal tunnel structures.

Bilateral T1-weighted sequences revealed accessory muscle bellies originating from the FDS muscle in the distal forearm. These accessory muscles demonstrated signal intensity isointense to normal muscle tissue on all pulse sequences, a characteristic finding confirming their muscular nature. The muscle bellies coursed through the carpal tunnel parallel to the main FDS tendon bilaterally [Figures 1 and 2].

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Figure 1:

Magnetic resonance images (a) Axial fat-suppressed proton density (PD) image, (b) Coronal PD image, and (c) Axial PD image of the left wrist at the level of the carpal tunnel demonstrate an accessory flexor digitorum superficialis muscle (arrows in a-c) located within the carpal tunnel.

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Figure 2:

Magnetic resonance images (a) Axial fat-suppressed image, (b) Axial proton density image, and (c) Coronal fat-suppressed image of the right wrist at the level of the carpal tunnel show an accessory flexor digitorum superficialis muscle (arrows in a-c) located within the carpal tunnel.

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T2-weighted and PD imaging with fat suppression demonstrated normal signal intensity within the accessory muscles without evidence of denervation changes or edema [Figures 1 and 2]. The median nerves appeared flattened and displaced on the ulnar side bilaterally, exhibiting a “pancake-like” configuration considered diagnostic for compression neuropathy.^[1] The nerves demonstrated mild signal hyperintensity bilaterally, with the flexor retinaculum showing palmar bowing consistent with increased carpal tunnel pressure.

Left wrist: The anomalous muscle measured 3.5 mm in width in the carpal tunnel. The median nerve demonstrated a cross-sectional area of 15 mm², exceeding the normal threshold of 9 mm².^[2] Right wrist: The anomalous muscle measured 5.5 mm in width. The median nerve demonstrated a cross-sectional area of 15 mm² [Figure 3].

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Figure 3:

Magnetic resonance images (a and b) Axial fat-suppressed images of the left wrist showing the accessory flexor digitorum superficialis muscle within the carpal tunnel, measuring 3.5 mm (a) and 5.5 mm (b) in width. The median nerve exhibits a fascicular cross-sectional area of approximately 15 mm² (orange oval in a and b) in both images.

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The patient was managed conservatively and responded well to medications without any surgical intervention. The patient was asymptomatic after 1 year of follow-up.

DISCUSSION

The presence of bilateral accessory FDS muscles causing simultaneous CTS represents an exceptionally rare anatomical finding. Fewer than 0.25% of CTS cases are attributed to bilateral anatomical variations,^[3] with bilateral accessory FDS muscles being among the most uncommon presentations.^[4] This case represents one of the few documented instances in literature where bilateral accessory FDS muscles led to concurrent bilateral CTS, making it particularly noteworthy from both clinical and anatomical perspectives.

The MRI findings in this case demonstrate the value of high-resolution imaging in characterizing such anatomical variants. The multiplanar capabilities of MRI proved essential for delineating the precise course and relationships of the accessory muscle, information critical for surgical planning.^[5] The ability to measure cross-sectional areas provided objective parameters for assessing compression severity.^[6]

Recent literature suggests that specific MRI signs, including nerve flattening ratio, cross-sectional area, and T2 signal intensity, correlate strongly with clinical severity and electrodiagnostic findings.^[7] The presence of an accessory FDS muscle represents a significant anatomical variation that can contribute to CTS development through direct mechanical compression, as demonstrated in this case.^[8]

Research conducted by Börekci et al. examined the surgical implications of accessory muscles in carpal tunnel procedures.^[9] The investigators found that the presence of these muscles overlying the transverse carpal ligament presents unique surgical challenges, especially when attempting minimally invasive endoscopic techniques. In certain instances, surgeons needed to switch to traditional open-surgical methods due to these anatomical variants. Their work reinforces the critical role of pre-operative imaging in surgical preparation and demonstrates that inadequate management of these accessory muscles during surgery may lead to ongoing post-operative symptomatology.

CONCLUSION

This case highlights a rare cause of bilateral CTS due to accessory FDS muscles, underscoring the need to consider uncommon anatomical variations. MRI plays a crucial role by providing detailed anatomy and identifying nerve changes, making it essential for diagnosis and surgical planning in complex CTS cases.