Case 6

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Case 6

HISTORY AND PHYSICAL EXAMINATION

Pain and numbness developed in the left big toe of a 28-year-old white man with rheumatoid arthritis. Symptoms were worse upon standing or walking. He had no leg weakness and experienced no similar symptoms in the hands or right leg. He denied any history of ankle trauma. He had been treated with oral methotrexate and chloroquine and was referred to the electromyography (EMG) laboratory.

On examination, the patient had slight swelling of the left ankle compared with the right. There was hypesthesia, most pronounced over the plantar surfaces of the big and second toes. There was possible weakness of left big toe flexion. Plantar flexion and dorsiflexion of left ankle were normal. He had a Tinel sign on percussion of the left tibial nerve over the flexor retinaculum. There was no sensory impairment or weakness in the right foot or hands. Deep tendon reflexes were normal and symmetrical.

An electrodiagnostic (EDX) examination was performed.

Please now review the Nerve Conduction Studies and Needle EMG tables.

QUESTIONS

1. The findings are consistent with:

A. Sensorimotor peripheral polyneuropathy.

B. Right tarsal tunnel syndrome.

C. Right S1/S2 radiculopathy.

D. Right proximal tibial mononeuropathy.

2. Difficulties in establishing the diagnosis of this entity, especially in the elderly, are due to all of the following except:

A. When bilateral, it can mimic early sensorimotor polyneuropathy.

B. Chronic denervation changes in intrinsic muscles of the foot are common.

C. Percutaneous plantar mixed nerve conduction studies are technically difficult.

D. When axonal and not associated with focal slowing, it can mimic S1/S2 radiculopathy.

E. It is a common entrapment neuropathy in the lower extremity.

3. Common underlying factors include all of the following except:

A. Rheumatoid arthritis.

B. Tenosynovitis.

C. Ankle trauma.

D. Recent anesthesia and surgery.

EDX FINDINGS AND INTERPRETATION OF DATA

Pertinent EDX studies include the following:

1. Low-amplitude left tibial compound muscle action potentials (CMAPs) with delayed distal latencies, recording abductor hallucis (AH) and abductor digiti quinti pedis (ADQP), with normal tibial proximal motor conduction velocities. This is evident when compared to normal values and to the asymptomatic right side. Note that the tibial study, recording AH, assesses the medial plantar terminal nerve, while the tibial study, recording ADQP, evaluates the lateral plantar nerve.

2. Delayed left medial and lateral plantar mixed nerve latencies across the ankle. These mixed responses assess both sensory and motor fibers in the corresponding nerves.

3. Chronic denervation restricted to the tibial-innervated muscles of the left foot.

Case 6: Nerve Conduction Studies

Case 6: Needle EMG

These findings are consistent with a right tibial mononeuropathy at, or distal to, the ankle, affecting both terminal branches (medial and lateral plantar nerves), and compatible with a left tarsal tunnel syndrome (TTS). The study is not consistent with an S1 radiculopathy due to the normal H reflex and lack of denervation in other S1-innervated muscles (medial gastrocnemius, flexor digitorum longus, extensor digitorum brevis, gluteus maximus, and lumbar paraspinal muscles). A sensorimotor polyneuropathy is unlikely because of the lack of denervation in the contralateral foot and normal H reflexes, right sural sensory nerve action potential (SNAP), and left peroneal and right tibial motor distal latencies and conduction velocities.

DISCUSSION

Applied Anatomy

After innervating the gastrocnemius, soleus, tibialis posterior, flexor digitorum profundus, and flexor hallucis longus in the calf, the tibial nerve passes through the tarsal tunnel at the medial aspect of the ankle. There, the tibial nerve is accompanied by the tibial artery and the tendons of the flexor digitorum longus, flexor hallucis longus, and tibialis posterior muscles. This tunnel’s roof is composed of a thin fascia, the laciniate ligament (flexor retinaculum), which connects the medial malleolus to the calcaneus.

The tibial nerve bifurcates into its two terminal branches within 1 cm of the malleolocalcaneal axis in 90+ of patients (Figure C6-1). Its terminal branches are: (1) the calcaneal branch, a purely sensory nerve, which has a variable takeoff in relation to the flexor retinaculum and explain its involvement or its lack of in nerve entrapments at that site (Figure C6-2); (2) the medial plantar nerve, which innervates the abductor hallucis, flexor digitorum brevis, and flexor hallucis brevis, in addition to the skin of the medial sole and, at least, the medial three toes; and (3) the lateral plantar nerve, which innervates the abductor digiti quinti pedis, flexor digiti quinti pedis, adductor hallucis, and the interossei, in addition to the skin of the lateral sole and two lateral toes. The innervation of the skin of the sole of the foot is provided primarily through the three terminal branches of the tibial nerves, with minimal contribution from the saphenous and sural nerves (Figure C6-3).

Figure C6-1 The distribution of location of the tibial nerve along the malleolocalcaneal axis and the flexor retinaculum.

(From Dellon AL, Mackinnon SE. Tibial nerve branching in the tarsal tunnel. Arch Neurol 1984;41:645–646, with permission from the American Medical Association.)

Figure C6-2 Patterns of origin of the medial calcaneal branch from the tibial nerve in 20 patients.

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