Neurogenic Disorders

Anatomy and Etiology

The tarsal tunnel is a fibroosseous tunnel within the posteromedial ankle and hindfoot in which the tibial nerve, posterior tibial artery, accompanying veins, posterior tibial tendon, flexor digitorum longus, and flexor hallucis longus tendons pass into the foot. The flexor retinaculum acts as the roof of this tunnel and extends from the medial malleolus to the medial side of the calcaneal tuberosity. The medial distal tibia, talus, and calcaneus make up the tunnel’s floor. Septa, which separate the posterior tibial, flexor digitorum longus, and flexor hallucis longus tendons, project from the fibrous roof to the calcaneus. Between the flexor digitorum longus and flexor hallucis longus tendons, the tibial nerve, posterior tibial artery, and accompanying veins pass to enter the foot.

Before reaching the foot, the tibial nerve divides into three terminal branches: the medial calcaneal nerve (MCN), lateral plantar nerve (LPN), and medial plantar nerve (MPN). Tibial nerve branch morphology varies, but typically the tibial nerve branches within the tunnel just proximal and deep to the upper edge of the abductor hallucis muscle. The MCN branches first, traveling posteriorly to the subcutaneous tissue. Coming off posteriorly, the first branch of the LPN passes under the abductor, over the medial fascia of the quadrates plantae, deep to the plantar fascia, and under the heel to the flexor digitorum brevis, where it sends a sensory branch to the central heel skin, and terminates in the abductor digiti quinti. The first branch of the LPN may branch from the main tibial nerve but still travels under the abductor with the LPN. Anterior to its first branch, the LPN passes deep to the abductor fascia and plantar fascia and over the quadrates plantae. Then it continues distally under the flexor digitorum brevis, terminating in the fourth web space and supplying a branch to the third web space. The LPN also supplies motor branches to the intrinsic muscles. Finally, the MPN innervates the abductor and continues under the abductor and the plantar fascia to form the common digital nerves, which terminate to the first, second, and third web spaces and motor branches to the interossei and lumbricals.

Historically, the term tarsal tunnel syndrome referred to tibial nerve entrapment beneath the flexor retinaculum. In 1987 Heimkes and colleagues first described distal tarsal tunnel syndrome, in which the distal tibial nerve branches are entrapped as they enter the foot. Together, proximal and distal tarsal tunnel syndromes encompass a spectrum of tibial nerve entrapment within the tarsal canal.

Sources of constriction beneath and adjacent to the tarsal tunnel include bone fragments, tenosynovitis, ganglia, soft-tissue encroachment in inflammatory arthritis, varicosities, neural tumors (neurilemmoma; Fig. 87.1 ), perineural fibrosis, tarsal coalition, and calcaneal osteotomies. Furthermore, a fixed valgus hindfoot can predispose to chronic traction neuropathy of the posterior tibial nerve or one of its branches.

Clinical Findings and Diagnosis

Clinical symptoms of tarsal tunnel syndrome vary, and this disorder should be kept in mind whenever unexplained dysesthesias are present in the plantar aspect of the foot, in the toes, or over the medial distal calf (Valleix phenomenon). Symptoms may be similar to those of plantar fasciitis, but unlike plantar fasciitis, nerve entrapment symptoms do not resolve quickly. Making the diagnosis difficult, typical neurogenic symptoms are not always present and symptoms may be present at night, during exercise, or at rest. Symptoms can be confined to the lateral plantar nerve, medial plantar nerve, or medial calcaneal nerve.

A thorough, detailed examination, aided with a good patient history, improves diagnostic accuracy. Careful examination for subtle sensory abnormalities or differences in temperature, sweating pattern, and skin abnormalities may lead to the correct diagnosis. Although a common complaint, sensory abnormalities often are difficult to detect. Dryness and scaliness of the skin may be present over only the lateral or medial plantar nerve distribution. Atrophy of the abductor hallucis, abductor digiti minimi, or both, often a difficult finding to detect, may be obvious compared with the asymptomatic foot. Point tenderness of the medial heel in the soft spot at the lower edge of the abductor hallucis may indicate distal entrapment of the tibial branches, and a Tinel sign over the flexor retinaculum may indicate proximal entrapment.

Abouelela and Zohiery described a “triple compression test” for diagnosing tarsal tunnel syndrome: the ankle is plantarflexed and the foot is inverted (increasing the tarsal tunnel compartment pressures), then digital compression is applied over the tibial nerve. Fifty patients with symptoms suggestive of tarsal tunnel syndrome and 40 asymptomatic patients were tested, and the tarsal tunnel diagnosis was confirmed with electrophysiologic testing. Test sensitivity was 86% and specificity was 100%.

Kinoshita et al. described another provocative maneuver to elicit symptoms suggesting tarsal tunnel syndrome. The dorsiflexion-eversion test is done by maximally everting and dorsiflexing the ankle passively, while all the metatarsophalangeal joints also are maximally dorsiflexed. This position is held for 5 to 10 seconds. In a comparison of 100 feet in asymptomatic volunteers with 44 feet in symptomatic patients, the authors found that the patients’ symptoms were exacerbated in 36 of the 44 feet. None of the feet in the control group exhibited symptoms with this maneuver. After release of the tarsal tunnel, the dorsiflexion-eversion test elicited tarsal tunnel symptoms in only three feet, all of which had calcaneal fractures.

Imaging modalities can assist the evaluation of tarsal tunnel syndrome. Plain radiographs demonstrate osseous abnormalities (e.g., fractures or talocalcaneal coalitions) that may contribute to tarsal tunnel symptoms. The use of ultrasound in the evaluation of tarsal tunnel has been reported; however, we prefer MRI because of the improved detail of the tarsal contents. MRI has been shown to identify the cause of the tarsal tunnel syndrome in up to 88% of patients. More important, MRI aids with surgical planning by providing detailed characteristics and location of space-occupying lesions.

Electrodiagnostic testing is indicated for any patient suspected of having compression of the tibial nerve beneath the flexor retinaculum. In an evidence-based review of the usefulness of electrodiagnostic testing in suspected tarsal tunnel syndrome, Patel et al. recommended the use of nerve conduction studies but found insufficient data to recommend the use of electromyography. Electrical studies also are useful in identifying an unsuspected peripheral neuropathy suggestive of a systemic, rather than localized, nerve injury. Despite evidence demonstrating their usefulness, electrodiagnostic studies can be normal in patients with tarsal tunnel syndrome, and negative electrodiagnostic testing does not provide a contraindication for surgery.

Treatment

Initially, 6 to 12 weeks of ankle immobilization in a night splint, antiinflammatory agents, and a wide, cushioned, comfortable shoe are recommended. If distal tarsal tunnel syndrome is suspected, an orthosis with a relief channel within the medial arch may be effective; a standard orthosis with a longitudinal arch may worsen symptoms. Caution is recommended in advising surgical treatment of tarsal tunnel syndrome in patients who are older (60 to 80 years old), have posttraumatic scarring within the tarsal canal, have no objective cause for symptoms (idiopathic), and those with protracted psychiatric illness. Symptoms caused by space-occupying lesions should be treated surgically. When conservative treatment fails, surgical treatment is indicated.

If surgical treatment is indicated, the tibial nerve and its branches must be meticulously exposed and unroofed. The release must include incision of 1 to 2 cm of the deep fascia above the proximal edge of the flexor retinaculum and following the medial and lateral plantar nerves beneath the abductor hallucis because one or both of these branches may pass through fascial slings as they enter the plantar surface of the foot. The dissection is refined by the use of magnification, a tourniquet, small dissecting scissors, and nontoothed forceps. Space-occupying lesions should be excised and alignment disorders corrected.

Patients with definite lesions generally respond better to surgical decompression than those with idiopathic or traumatic etiologies. With no identifiable cause, however, the relief of symptoms is less predictable, with approximately 25% of patients achieving little or no relief. Rarely, symptoms may even worsen after surgical release. Symptom duration of less than 1 year also has been cited as a predictor of better outcomes. There are few outcome studies of tarsal tunnel release, and the evidence generally is level IV or V.

Even closer scrutiny of the clinical factors that can influence a surgical decision is required for revision tarsal tunnel release because diagnosing an “inadequate release” often is difficult, even with knowledge of the original procedure. Failures can occur from incorrect diagnosis, inadequate release, poor technique, or a combination of any of these. To minimize the probability of inadequate release, a complete release of the tibial nerve and its branches is recommended, except in the case of a space-occupying lesion, for which a smaller, lesion-specific release is acceptable. Nerve scarring is likely in patients who had relief after their initial surgery but then experienced a slow return of symptoms. Neurolysis with saphenous vein or collagen wrapping to prevent nerve adherence has been recommended in this situation. Worsening symptoms immediately after surgery may indicate an iatrogenic nerve injury, from which a painful neuroma may develop. Despite careful attention to patient history and physical examination findings, combined with meticulous surgical technique, the outcomes of revision tarsal tunnel release are unpredictable, and appropriate counseling and caution are recommended before treatment.

Pathologic Findings

The following list summarizes reported pathologic findings:

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  Perineural fibrosis ( Fig. 87.7A )

  

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  Increased number of intrafascicular arterioles with thickened and hyalinized walls caused by multiple layers of basement membranes ( Fig. 87.7B ). Demyelinization and degeneration of nerve fibers with a decrease in the number of axis cylinders ( Fig. 87.7C )

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  Endoneural edema

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  Absence of inflammatory changes

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  Frequent presence of bursal tissue accompanying the specimen ( Fig. 87.7D )

Signs and Symptoms

The primary symptom of interdigital neuroma is pain, which is most often located in the region of the third metatarsal head, and frequently described as burning, aching, or cramping. Trauma may initiate the symptoms, and the duration of pain varies from a few weeks to many years. It is increased with walking and relieved by rest, removing the shoe, or massaging the forefoot. In exploring other possible pain sources, careful examination should include a Lachman test of the metatarsophalangeal joint, dorsal palpation, and palpation under the metatarsal head versus the web space to discern pain etiology. If the tenderness is mostly in the web space and not under the plantar aspect of the proximal phalangeal base or the dorsolateral aspect of the second metatarsophalangeal joint, a neuroma is likely. The palpable and audible click associated with reproducing the patient’s pain is helpful for diagnosis (Mulder’s click). This click frequently is present without pain on the opposite asymptomatic foot. Hence, the click alone is not diagnostic. Simulating weight bearing in narrow shoes can reproduce symptoms. Pressure is placed under the web space with the thumb dorsal followed by gentle medial-to-lateral squeezing of the forefoot by the examiner’s other hand. Repeating this maneuver followed by relaxation will reproduce painful clicking of the neuroma ( Fig. 87.8 ). Subjective numbness of the toes of the involved interspace is common, but objective signs of decreased sensibility are less common. Women are more commonly affected than men (8:1), and the condition usually is unilateral. The use of sensory action potentials to confirm the diagnosis objectively has yielded variable results. Diagnostic injections have been shown to extravasate into the adjacent web space, making them less sensitive for accurate diagnosis. Although MRI and sonography have been reported for diagnosing interdigital neuroma, they have been shown to be of limited value, so the diagnosis of interdigital neuroma is still a clinical one.

Treatment

Numerous reports have confirmed the efficacy of neuroma excision in the third web space. The results of nonoperative treatment, consisting of metatarsal bars or pads, local injection of a steroid preparation into the affected web space, and wide toe box shoes, have been unpredictable but successful often enough to warrant at least a trial period.

In a level 1 prospective, randomized study, Thomson et al. found that at 3-month follow-up, corticosteroid injections produced significantly improved visual analog scale (VAS) scores compared to the control group. However, Lizano-Díez et al. found no difference between corticosteroid with anesthetic injections and anesthetic injection alone at 3 and 6 months. Roughly half of the patients still requested surgical excision. Furthermore, caution is advised with the repetitive use of steroids, which can cause atrophy of the plantar fat pad or disruption of the adjacent joint capsule with resultant toe deviation.

Through nerve sclerosis, alcohol injections have been reported to improve symptoms in short-term follow-up, but Gurdezi et al. found no improvement at long-term follow-up (average, 61 months) in a study of 45 patients: only 29% reported improvement and 36% proceeded with surgical excision. Furthermore, 12 of the 45 patients had minor complications. Epinosa et al. found similar results: only 22% of their 32 patients reported improvement after alcohol-sclerosing injections. A recent systematic review found insufficient high-quality evidence supporting the use of alcohol injections for interdigital neuromas. Because of these poor or inconclusive results, we do not use sclerosing agents for neuroma treatment.

Other than corticosteroid and alcohol injections, less common nonsurgical treatments also have been described through small series. Reported treatments include extracorporeal shockwave therapy (ECSWT), radiofrequency ablation, cryoneurolysis, hyaluronic acid injections, and capsaicin injections. The sparse literature has not validated the safety and efficacy of these alternative modalities.

Surgical excision remains the most predictable management for successful treatment of an interdigital neuroma; however, an accurate diagnosis and patient counseling are critical to ensure optimal outcomes. In most of the series reported, 80% to 95% of the patients are completely asymptomatic and pleased with the result of surgery. However, Mann and Reynolds reported that 65% of patients still noted some local plantar tenderness after surgery, and 20% subjectively rated their result as less than a 50% improvement in symptoms. Another study reported limitation in footwear in 70% of patients, and Womack et al. found that the long-term outcomes of neuroma excision were not as successful as previously reported. More recently, Kasparek and Schneider reported that 76% of 98 feet had an excellent or good result an average of 15 years after neuroma excision. Detailed preoperative examination and careful patient selection for surgery are recommended. In addition, the patient must be informed before surgery that some symptoms may remain after surgery. The surgeon should look carefully for other causes of metatarsalgia that might coexist in a patient with an interdigital neuroma and should explain that any symptoms other than those caused by the interdigital neuroma would not be improved by its excision.

Opinions vary as to whether excision is the operative treatment of choice for interdigital neuroma. Gauthier performed epineural neurolysis using a longitudinal dorsal approach. The deep transverse intermetatarsal ligament was released to facilitate neurolysis. Overall, 28% of the patients either were unimproved or had enough continued symptoms to warrant further treatment. We have had no experience with this technique. Furthermore, the division of the deep transverse intermetatarsal ligament during this procedure is controversial. Whether its division results in a “dropped” metatarsal or splaying of the forefoot remains uncertain. Through the dorsal approach, the nerve is better exposed in its proximal portion if the ligament is divided. Through the plantar approach, the ligament does not impair exposure of the neuroma in its proximal portion and usually is left intact.

The choice of a dorsal or plantar incision for removing a neuroma probably is determined by the surgeon’s training. Either approach can be satisfactory, but patients with a plantar incision complain of soreness around the wound for many weeks, which gradually resolves. Each approach has advantages and disadvantages. For a recurrent interdigital neuroma, a plantar approach is recommended because the exposure is excellent. With a dorsal approach, identifying the stump of the neuroma amid the scar tissue can be difficult. The plantar approach allows the surgeon to identify the nerve in normal tissue and dissect it out to the involved tissue. Dorsal and plantar longitudinal approaches were compared in one study that assessed sensory loss, number of sick leave weeks, and further treatments. The overall patient satisfaction was not significantly different between the two approaches. Infection, missed nerves, and nerve stump neuroma occurred in the dorsal group, and three minor (2 × 3 mm) intraincisional keratoses occurred in the plantar group. More recently, Nery et al. reported 89% good results, 7% fair results, and 4% poor results at 7-year follow-up of 168 patients who had neuroma excision through a plantar incision.

Regardless of the approach, the nerve is transected as far proximally as the incision allows, but care must be taken not to injure the common digital branches to the second or fourth web spaces. Several plantarly directed nerve branches run from the common digital nerve into the forefoot pad. They are found in the highest concentration in the distal aspect of the common digital nerve, just proximal to its bifurcation into the proper branches. If the nerve is not transected well proximal to the deep transverse intermetatarsal ligament, these plantarly directed nerve fibers can tether or prevent retraction of the common digital nerve, leading to its adherence near the metatarsal head and causing recurring symptoms. Careful localization of the most tender area, and presumably the neuroma stump, before surgery helps the surgeon to locate the nerve during the dissection. Mann recommended the dorsal approach for a recurrent neuroma and for the original excision.

The results of surgery for “recurrent” interdigital neuroma or resection of a neuroma stump are not as predictably satisfactory as the results of primary resection. Forty-three percent of patients in one study were dissatisfied with their clinical outcome or had major reservations about the efficacy of the procedure. Revision surgery for neuroma excision should be approached cautiously, and the patient should be counseled regarding the possibility of an unsatisfactory result after revision surgery for interdigital neuroma.

We currently do not use endoscopy for decompression (release of the transverse metatarsal ligament) or excision of an interdigital neuroma because of the lack of evidence regarding its efficacy and safety. However, Kubota et al. and Lui have reported encouraging case reports and techniques.

Etiology

A cavus foot may result from many different processes, all of which create muscle imbalances that result in deformity. The imbalance may occur from a progressive or static process, leading to variable severities and treatments. Underlying neuromuscular disease, such as Charcot-Marie-Tooth (CMT) or ataxia, leads to more progressive deformities. Other processes (postpoliomyelitis, cerebral palsy, club foot, etc.) are more static, but worsening contractures can still lead to varying cavus deformities. Spinal cord disorders (spinal cord tumors, spinal cord dysraphism, syrinx, etc.) lead to muscle imbalance causing cavus deformity. Even traumatic injuries, resulting in varus malunions or postcompartment syndrome imbalances, can lead to cavus deformity.

Charcot-Marie-Tooth disease is the most common neuromuscular disease causing pes cavus. This disease is actually a heterogeneous group of hereditary sensory and motor neuropathies (HSMN) caused by neural protein mutations affecting peripheral nerve conduction through axon or myelin irregularities. Our understanding and classification of this disease continues to advance. All types are inheritable, but nearly half of the cases result from a new mutation. Characterized by axon demyelination, CMT-1 is the most common type, occurring in over 50% of CMT patients. CMT-1 can be further subdivided into CMT-1A, CMT-1B, and CMT-1C. CMT-2 is the second most common type and may have near-normal nerve conduction velocities. There is striking variability of penetration and clinical expression of this autosomal-dominant disease, even in immediate family members and extended family. CMT-X is found in 10% to 20% and demonstrates an X-linked inheritance pattern. CMT-4 exhibits an autosomal recessive inheritance pattern and is very rare.

Physical Findings

Regardless of the specific type, all types result in muscle imbalance affecting the lateral and anterior muscle compartments. The clinical expression varies from individual to individual, but the classic muscle imbalances involve weakness of the tibialis anterior while sparing the peroneus longus, resulting in a plantar flexed first metatarsal ( Fig. 87.14 ). To accommodate this forefoot pronation, the hindfoot assumes a varus posture ( Fig. 87.15 ). Additionally, peroneus brevis weakness countered by the near-normal tibialis posterior strength leads to worsening inversion of the midtarsal joints and hindfoot varus deformity. Developing contractures of the plantar fascia and the varus-oriented Achilles tendon further contribute to the deformity. Last, imbalance of the intrinsic-extrinsic muscles enhances the deformity. The intrinsic muscles in the plantar aspect of the foot generally flex the metatarsophalangeal joints and extend the interphalangeal joints. Any weakness of these muscles relative to their antagonists causes plantar flexion of the metatarsals, hyperextension of the metatarsophalangeal joints, and hyperflexion at the interphalangeal joints (toe clawing).

Patients with cavus deformity as a residual of poliomyelitis usually have different physical findings from patients with CMT disease. Fortunately, with the success of vaccination in the 1950s, postpoliomyelitis cavus is less commonly encountered. Depending on the level of injury and inconsistent recovery, poliomyelitis can cause differing patterns of muscle imbalance and subsequent deformity ( Fig. 87.16C ). Unlike CMT disease, a weak gastrocnemius-soleus muscle opposite a strong anterior tibial muscle may cause a calcaneal deformity of the hindfoot, with or without hindfoot varus or valgus, depending on which muscle groups have preserved strength. Because of intact sensation and the nonprogressive nature of the deformities, patients with postpoliomyelitis cavus feet typically have a better prognosis than patients with CMT disease.

Traumatic cavus deformity can be caused by deep posterior compartment syndrome or by malunion of midfoot and hindfoot fractures. The deformity may not appear for several months after the local ischemia and muscle fibrosis of the deep posterior compartment, or it may appear as a mild, barely perceptible abnormality that progresses relentlessly to a rigid cavovarus, claw toe deformity. Soft-tissue injuries from crush mechanisms or severe burns can lead to contractures and muscle imbalances that result in cavus deformity. Through altered bony anatomy and joint mechanics, hindfoot malunions (i.e., calcaneal and talar neck malunions) also can create posttraumatic cavus deformities.

In some patients with symptomatic cavus deformities, no definite cause is discovered ( Fig. 87.17 ). In patients with idiopathic deformities, the underlying pathologic mechanism of the cavus deformity is believed to be an imbalance of the extrinsic-intrinsic muscles. As previously discussed, the intrinsic minus foot demonstrates metatarsal plantar flexion and toe clawing in which the hindfoot assumes a varus posture to accommodate the rigid plantar flexed first ray (see Fig. 87.14 ). Furthermore, patients may display mild, subtle cavovarus as a normal variation within the bell curve for arch height and foot posture. Despite this “normal” variation, patient symptoms still require recognition and appropriate treatment.

Radiographic Findings

Standard radiographic workup involves a standing series of the foot and ankle. Many angles and measurements have been described to help classify foot posture. These specific angles and values are likely most useful in research data collection rather than daily application, but the overall concept in recognizing these abnormal relationships is critical for understanding deformities and formulating treatment plans.

The standing lateral radiograph allows estimation of the contribution of the hindfoot (talus and calcaneus), midfoot (navicular and cuboid-cuneiform), and forefoot (Lisfranc) to the cavus deformity. The standing lateral view allows assessment of calcaneal pitch (see Fig. 87.16A ), talo–first metatarsal (Meary’s) angle, lateral talocalcaneal angle, and medial-cuneiform height. The extension deformity of the phalanges on the metatarsal heads during weight bearing helps determine the severity of the fixed forefoot deformity ( Figs. 87.18 and 87.19 ). The standing hindfoot alignment view displays the relationship of the calcaneus to the tibia, demonstrating varying degrees of hindfoot varus.

Similarly, the standing anteroposterior view allows estimation of the contribution from the hindfoot, midfoot, and forefoot. This view shows talar head overcoverage, metatarsus adductus, talocalcaneal angle, and the talo–first metatarsal angle. In flexible deformities, obtaining a standing anteroposterior foot radiograph with the hindfoot corrected (via Coleman block; see Fig. 87.22 ) helps corroborate any metatarsus adductus component suspected clinically ( Fig. 87.20 ). The talocalcaneal angle (Kite angle) is determined on this view. The closer the talocalcaneal angle approaches zero, the more parallel the talus is in relation to the calcaneus, indicating hindfoot varus.

Other radiographic findings that may be helpful include (1) degenerative changes in the tibiotalar, subtalar, or midtarsal joints; (2) rotation of the talus in the ankle mortise exhibited by a posterior fibula; and (3) dystrophic ossification in soft tissue suggesting tendon or ligament injury ( Fig. 87.21 ).

Although of questionable benefit for initial conservative treatment, CT scans are invaluable for surgical planning. The advancements in weight-bearing CT and three-dimensional reconstructions help surgeons conceptualize the deformity and localize the apex of the deformity for surgical correction.