Abstract
Tibial mononeuropathy at the ankle, also known as tarsal tunnel syndrome (TTS), and the tibial branch neuropathy referred to as Baxter neuropathy (BN) are both entrapment neuropathies of branches of the posterior tibial nerve. Clinical diagnosis of TTS can be difficult and often misleading, as the foot symptoms are common to several peripheral polyneuropathies and even some lower lumbosacral radiculopathies. This chapter will help review the anatomical basics through diagnosis and management options available for TTS.
Keywords
Entrapment neuropathy, tarsal tunnel syndrome, tibial mononeuropathy at the ankle
| Synonyms | |
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| ICD-10 Codes | |
| G57.50 | Tarsal tunnel syndrome, unspecified lower limb |
| G57.51 | Tarsal tunnel syndrome, right lower limb |
| G57.52 | Tarsal tunnel syndrome, left lower limb |
Definition
Tarsal tunnel syndrome (TTS) can be described as a constellation of signs and symptoms caused by entrapment or compression of the tibial nerve or any of its branches within the tarsal tunnel, the region beneath the flexor retinaculum on the medial aspect of the ankle ( Fig. 94.1 ). The tibial nerve branches that may be involved within the tarsal tunnel include the medial plantar nerve, lateral plantar nerve, Baxter’s nerve (also known as the first branch of the lateral plantar nerve or inferior calcaneal nerve), and medial calcaneal nerve. Anatomically, the tarsal tunnel is a fibro-osseous structure that begins just posterior to the medial malleolus; the roof is the flexor retinaculum (also called the laciniate ligament), and the floor is formed by the tendons of the posterior tibialis, flexor digitorum longus, and flexor hallucis longus muscles. The tibial nerve usually divides into three branches at the level of the ankle: the medial plantar nerve, the lateral plantar nerve, and the medial calcaneal nerve. However, Baxter’s nerve usually branches from the lateral plantar nerve (but can branch off the tibial nerve) just distal to the origin of the medial calcaneal nerve at the level of the tarsal tunnel. Baxter’s nerve then traverses laterally across the anterior aspect of the heel and terminates with motor branches to the abductor digiti quinti (or minimi) pedis muscle. It is likely that TTS occurs infrequently compared with other well-known focal entrapment neuropathies, such as carpal tunnel syndrome, ulnar neuropathy at the elbow, and peroneal (fibular) neuropathy at the knee. In fact, in a retrospective review of isolated tibial neuropathies in the foot, the incidence of Baxter’s neuropathy (17%) was much greater than that of TTS (5%).
There are generally considered to be five basic categories that account for the etiology of TTS: trauma and post-traumatic changes, mass or space-occupying lesions causing compression, systemic diseases, biomechanical causes related to joint structure or deformity, and idiopathic causes. In addition, the underlying pathophysiologic mechanism of TTS remains elusive; a portion of the literature supports the process of demyelination, whereas other sources implicate axonal degeneration as the primary process. It is thought that the tibial nerve may be entrapped proximally within the tarsal tunnel, or one of its branches (e.g., the medial plantar nerve) may be entrapped distally in its own calcaneal chamber. Entrapment of the first branch of the lateral plantar nerve (i.e., Baxter’s nerve) has also been described as a cause of heel pain. Therefore, in a case of clinically suspected TTS, the tibial nerve and its major terminal branches (including the medial plantar nerve, lateral plantar nerve, and Baxter’s nerve) should be thoroughly evaluated.
In the current literature, there is no mention of an age or gender preference in patients with TTS. One possible explanation for this is the relatively low incidence and various causes of TTS.
Symptoms
The patient usually presents with pain or paresthesias along with numbness over the sole of the foot. Pain is typically described as burning or a dull ache, but it may also be expressed as throbbing, cramping, or even tightness, and pain may extend proximally to the medial calf. Symptoms are often exacerbated by prolonged standing or walking and may be worse at night but may not be well localized. However, if the distribution of sensory disturbance is limited to a particular region of the foot, these symptoms could correspond to a specific tibial nerve branch that is involved (e.g., medial sole of foot due to medial plantar nerve involvement). Obvious weakness of the intrinsic foot muscles is an uncommon patient complaint and may be manifested only if the resulting foot deformity is grossly noticeable or so severe that it causes an unstable gait pattern. Patients with TTS generally present with unilateral symptoms.
Physical Examination
Sensory examination of a patient with TTS should reveal decreased light touch or pinprick over the plantar aspect of the foot corresponding to the distribution of one or all of the tibial nerve branches involved ( Fig. 94.2 ). Motor examination of the intrinsic foot muscles is challenging because it is often difficult for patients to selectively activate these muscles. However, one may be able to appreciate muscle atrophy of the involved foot because its appearance may be asymmetric compared with the other foot. A patient with TTS often has a Tinel sign over the tibial nerve or one of its branches in the tarsal tunnel ( Fig. 94.3 ). However, this exam finding may be of no diagnostic value for tibial nerve entrapment at the ankle due to its low sensitivity and only moderate specificity. On occasion, percussion over the tibial nerve at the ankle will elicit pain extending proximally along the course of the tibial nerve; this sign is called the Valleix phenomenon. There may also be palpable tenderness over the tibial nerve in the tarsal tunnel. Two other provocative maneuvers that may reproduce symptoms in the foot or ankle are extension of the great toe and sustained passive eversion of the ankle. Muscle stretch reflexes in both lower extremities (including patellar, medial hamstring, and Achilles) should be normal and symmetric. Peripheral pulses (posterior tibial and dorsalis pedis) are usually palpable and unremarkable. If the biomechanical configuration of the foot is altered severely enough, gait deviations can be observed.
