A Patient with Tarsal Tunnel Syndrome Associated with the Flexor Digitorum Accessorius Longus Muscle

The flexor digitorum accessorius longus muscle (ALM) can be overlooked as the eliciting factor in patients with tarsal tunnel syndrome (TTS), an entrapment neuropathy of the posterior tibial nerve that elicits sole numbness and pain. Most elicitations are idiopathic, however, mass lesions within the tarsal tunnel can be also implicated. We report an 80-year-old woman whose flexor digitorum ALM led to the onset of bilateral TTS. She had suffered numbness in both soles for 3 years. Magnetic resonance imaging (MRI) of the bilateral tarsal tunnel showed that the posterior tibial nerve was compressed by the arteriovenous complex and in contact with the flexor digitorum ALM. We diagnosed bilateral TTS based on her symptoms and imaging findings, and performed bilateral decompression surgery of the posterior tibial nerve under local anesthesia. The artery on both sides was dislocated for nerve decompression. Because the posterior tibial nerve on the right side was strongly compressed in ankle plantar flexion we excised a portion of the tendon compressing the nerve. Postoperatively her symptoms gradually improved and she reported surgical satisfaction 6 months after the operation. In patients with flexor digitorum ALM-related TTS, the effect of dynamic factors on MRI findings and on surgical treatment decisions must be considered. Intraoperatively, not only the flexor digitorum ALM, but also other potential etiologic factors eliciting TTS must be kept in mind.