tarsal tunnel syndrome

**Anatomy:** The tarsal tunnel is a fibro-osseous canal on the medial ankle. The floor is formed by the talus, calcaneus, and medial malleolus. The roof is the flexor retinaculum (laciniate ligament). Contents, from anterior to posterior, follow the mnemonic "Tom, Dick, And Very Nervous Harry": - Tibialis posterior tendon - Flexor Digitorum longus tendon - posterior tibial Artery - posterior tibial Vein - tibial Nerve - Flexor Hallucis longus tendon

The tibial nerve divides within or just distal to the tunnel into the medial plantar, lateral plantar, and medial calcaneal nerves.

**Etiology:** - Space-occupying lesions: ganglion cyst, lipoma, tenosynovitis, varicose veins, accessory muscle (accessory abductor hallucis), osteophytes, tumors. - Trauma: ankle fracture or sprain, with scarring or post-traumatic edema. - Biomechanical: excessive foot pronation or pes planus, with hindfoot valgus stretching the nerve. - Systemic: rheumatoid arthritis, diabetes, hypothyroidism, and any cause of edema or inflammation. - Many cases are idiopathic.

**Pathophysiology:** Increased pressure in the tunnel causes ischemia and demyelination of the nerve, producing sensory symptoms first and, in later stages, motor involvement.

**Clinical features:** - Burning, tingling, or shooting pain and numbness over the plantar foot, toes, and sometimes the heel. - Symptoms are worse with standing, walking, or at night, and may improve with rest. - Positive Tinel sign over the tibial nerve behind the medial malleolus. - Dorsiflexion-eversion test reproduces symptoms. - Late findings include weakness or atrophy of the intrinsic foot muscles (abductor hallucis) and clawing of the toes. - Sparing of the dorsal foot helps distinguish it from peroneal or L5 pathology. - The medial calcaneal branch often exits proximal to the tunnel, so heel sensation may be spared.

**Diagnosis:** - Primarily clinical. - Nerve conduction studies/EMG may show prolonged distal motor and sensory latencies of the medial and lateral plantar nerves. They are often normal, so a negative study does not rule out the diagnosis. - MRI or ultrasound identifies space-occupying lesions.

**Differential diagnosis:** - Plantar fasciitis (heel pain worse with first steps in the morning) - Morton neuroma (pain between the 3rd and 4th toes) - Peripheral neuropathy (stocking-glove distribution) - S1 or L5 radiculopathy - Peripheral artery disease

**Management:** - Conservative: rest, NSAIDs, orthotics or arch supports to correct pronation, activity modification, and corticosteroid injection. - Surgical: tarsal tunnel release (decompression of the flexor retinaculum) with removal of any mass lesion, for refractory cases or a clear structural cause. Outcomes are best when a discrete mass is the cause.

**High-yield pearl:** Think "carpal tunnel of the foot." The key board association is the tibial nerve (L4-S3) passing behind the medial malleolus, with burning plantar foot pain and a positive Tinel sign. Don't confuse it with the anterior tarsal tunnel syndrome, which involves the deep peroneal nerve under the inferior extensor retinaculum and causes pain on the dorsum of the foot and first web space.