Tarsal Tunnel Syndrome

Anatomy

As the tibial nerve descends distal to the medial malleolus, it runs beneath the flexor retinaculum on the medial side of the ankle, through the tarsal tunnel ( Fig. 27.1 ). The tarsal tunnel is a fibro-osseous tunnel below the medial malleolus with a bony floor and a roof formed by the flexor retinaculum. In addition to the tibial nerve, the tibial artery, the tibial veins and tendons of the flexor hallucis longus (FHL), flexor digitorum longus, and tibialis posterior pass through the tarsal tunnel. The distal tibial nerve typically divides into three branches. One branch, the medial calcaneal sensory nerve, is purely sensory and provides sensation to the heel of the sole ( Fig. 27.2 ). The other two branches, the medial and lateral plantar nerves , contain both motor and sensory fibers that supply the medial and lateral sole of the foot, respectively. Typically, the medial plantar nerve supplies the first three and a half toes (including the great toe), whereas the lateral plantar nerve supplies the little toe and the lateral fourth toe. The first branch of the lateral plantar nerve is the inferior calcaneal nerve (a.k.a., Baxter’s nerve). Both plantar nerves innervate the intrinsic muscles of the foot. The muscles that are most accessible to study using needle EMG are the abductor hallucis brevis (AHB), flexor hallucis brevis (FHB), and flexor digitorum brevis (FDB) for the medial plantar nerve and the abductor digiti quinti pedis (ADQP) for the lateral plantar nerve via the inferior calcaneal nerve.

Clinical

The most frequent symptom in patients with TTS is perimalleolar pain. Ankle and sole pain often is described as burning and often is worse with weight bearing or at night. Paresthesias and sensory loss involving the sole of the foot may occur due to compression of the plantar or calcaneal nerves ( Fig. 27.3 ). There are few other reliable clinical signs. Intrinsic foot muscle atrophy may occur but is not specific to TTS. For example, atrophy of intrinsic foot muscles may occur in L5–S1 radiculopathy, proximal tibial neuropathy, or polyneuropathy. It is extremely difficult to assess strength of the intrinsic foot musculature, because most of the important toe and ankle functions are subserved by the long extensors and flexors in the lower leg, which are innervated by the proximal peroneal and tibial nerves. Finally, many consider a Tinel’s sign over the tarsal tunnel to be suggestive of TTS. Unfortunately, like Tinel’s signs elsewhere, this is a nonspecific sign and may occur in some normal subjects. Significantly, the ankle tendon reflex, which is mediated by the tibial nerve proximal to the tarsal tunnel, is normal in TTS, as is sensation over the lateral foot (sural nerve) and the dorsum of the foot (superficial peroneal nerve).

Etiology

The incidence of TTS is widely debated. Some podiatrists believe that TTS is rather common, whereas most neurologists, physiatrists, and electrophysiologists believe that it is quite rare. Lesions of the medial and lateral plantar nerves most often occur as a result of trauma (including sprain and fracture) or occasionally from degenerative bone or connective tissue disorders. Rare cases of TTS are caused by varicosities or other unusual mass lesions (e.g., lipomas, ganglions, cysts, exostoses, varices). TTS caused by hypertrophy of the flexor retinaculum from repetitive use (akin to CTS) is unusual. One or more of the three nerve branches (medial calcaneal, medial plantar, and lateral plantar) may be involved.

Differential Diagnosis

The differential diagnosis of TTS includes local orthopedic problems of the foot (especially tendonitis and fasciitis), proximal tibial neuropathy, and, especially early on, mild polyneuropathy. S1 radiculopathy or lumbosacral plexopathy may cause sensory loss over the sole, but neither is typically associated with local foot pain. It is not unusual for patients who first present with polyneuropathy to be misdiagnosed with TTS. Most patients studied in our laboratory referred for possible TTS have either a normal electrophysiologic examination (and may have had a local orthopedic problem) or are found to have a mild distal polyneuropathy.

Electrophysiologic Evaluation