Tarsal Tunnel Syndrome

Patients with pain and numbness in the foot often are referred to the electromyography (EMG) laboratory for evaluation of possible tarsal tunnel syndrome (TTS). TTS results from entrapment of the distal tibial nerve under the flexor retinaculum at the medial ankle. Superficially, it might seem that tibial nerve entrapment under the flexor retinaculum at the ankle would be analogous to median nerve entrapment under the flexor retinaculum at the wrist [i.e., carpal tunnel syndrome (CTS)]. However, in contrast to CTS, which is very common, TTS is exceptionally rare. Although electrophysiology can be useful in demonstrating focal slowing at the tarsal tunnel in those rare cases of true TTS, every electromyographer should be aware that significant technical difficulties are often encountered when studying the distal tibial nerve and the muscles it innervates, especially in older patients.

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 ( Figure 24–1 ). 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 and tendons of the flexor hallucis longus, flexor digitorum longus, and tibialis posterior pass through the tarsal tunnel. The distal tibial nerve then divides into three or four branches. One or two branches ( medial and lateral calcaneal sensory nerves ) are purely sensory and provide sensation to the heel of the sole ( Figure 24–2 ). 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. 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.

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 ( Figure 24–3 ). 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 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). TTS caused by hypertrophy of the flexor retinaculum from repetitive use (akin to CTS) is unusual. One or more of the three nerve branches (calcaneal, medial plantar, and lateral plantar) may be involved.

Etiology

The incidence of TTS is widely debated. Some podiatrists believe that TTS is rather common, whereas most neurologists 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). TTS caused by hypertrophy of the flexor retinaculum from repetitive use (akin to CTS) is unusual. One or more of the three nerve branches (calcaneal, medial plantar, and lateral plantar) may be involved.

Differential Diagnosis

The differential diagnosis of TTS includes local orthopedic problems of the foot (especially tendinitis 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 had either a normal electrophysiologic examination (and may have had a local orthopedic problem) or were found to have a mild distal polyneuropathy.

Electrophysiologic Evaluation

Nerve Conduction Studies

Evaluation of suspected TTS is greatly simplified if one side is symptomatic and the other side is normal. This situation allows for side-to-side comparison studies ( Box 24–1 ). The important nerve conduction studies to perform include bilateral tibial distal motor latencies to both the AHB and ADQP, for the medial and lateral plantar nerves, respectively, stimulating the tibial nerve proximal to the tarsal tunnel at the medial malleolus ( Figure 24–4 ). Compound muscle action potential (CMAP) amplitudes and distal latencies are compared from side to side. Theoretically, if there is demyelination across the tarsal tunnel, the distal latencies on the involved side should be markedly prolonged. In axonal loss lesions, the CMAP amplitudes will be reduced, and the latencies will be normal or only slightly prolonged.

Box 24–1

Recommended Nerve Conduction Study Protocol for Tarsal Tunnel Syndrome

Routine studies:

1

Distal tibial motor (medial and lateral plantar) studies, stimulating tibial nerve at medial malleolus and recording abductor hallucis brevis (medial plantar) and abductor digiti quinti pedis (lateral plantar). Comparison with contralateral side is required
2

Routine tibial motor study, recording abductor hallucis brevis, stimulating medial ankle and popliteal fossa
3

Routine peroneal motor study, recording extensor digitorum brevis and stimulating ankle, below fibular neck, and lateral popliteal fossa
4

Medial and lateral plantar mixed or sensory studies (plantar mixed and sensory responses usually require averaging several potentials). For mixed studies, stimulate medial sole and record medial ankle (medial plantar mixed); stimulate lateral sole and record medial ankle (lateral plantar mixed). For sensory studies, stimulate great toe and record medial ankle (medial plantar sensory); stimulate little toe and record medial ankle (lateral plantar sensory). Comparison with the contralateral side is required, using identical distances between the stimulating and recording sites
5

Sural sensory response, stimulating posterior calf, recording posterior ankle
6

Tibial and peroneal F responses
7

H reflexes, bilateral studies (may be abnormal in S1 radiculopathy or polyneuropathy but not in tarsal tunnel syndrome)

Surface sensory and mixed nerve studies are difficult to perform, even in normal healthy subjects, but they increase the sensitivity of making the electrodiagnosis of TTS. Orthodromic surface sensory studies can be performed stimulating the great and little toes (medial and lateral plantar sensory nerves, respectively) and recording over the tibial nerve at the medial ankle proximal to the tarsal tunnel. The potentials are usually extremely small in amplitude, making it necessary to average many potentials. Antidromic surface sensory studies also can be performed, but they have similar technical limitations. Surface recording of the mixed plantar nerves is slightly easier ( Figure 24–5 ). Both the medial and lateral plantar mixed nerves can be stimulated in the sole, recording over the tibial nerve at the medial ankle (proximal to the tarsal tunnel). Averaging is still required to measure these small potentials, and in older individuals they may be absent. Often, medial and lateral plantar sensory and mixed nerve potentials are unobtainable even in normal subjects . Consequently, an absent or low-amplitude potential should not be considered abnormal unless a clear side-to-side difference is found using identical distances between the stimulating and recording sites. No diagnostic significance should be attributed to bilaterally absent plantar mixed or sensory nerve responses, especially in middle-aged or older individuals. It is important to emphasize that the plantar mixed and sensory nerves are the most distal nerves in the lower extremities. As such, their conduction velocities normally are slower than those of more proximal nerves and are more susceptible to the effects of temperature and cooling.