Peripheral Nerve Disease

Vinayak M. Sathe, MD

Neither Dr. Sathe nor any immediate family member has received anything of value from or has stock or stock options held in a commercial company or institution related directly or indirectly to the subject of this chapter.

ABSTRACT

This chapter discusses nerve entrapment at various sites in the foot and ankle and appropriate clinical examination, diagnosis and relevant investigations. Pathophysiology and treatment are also reviewed.

Introduction

The peripheral nerve disorders due to entrapments in the foot and ankle are less common than in the upper limb and can be difficult to diagnose. This chapter discusses the most common peripheral nerve disorders in the foot and ankle. These disorders can be more difficult to treat, and the response to intervention may be limited. It is important to diagnose and initiate early treatment to help prevent long-term effects. A comprehensive history, physical examination, and proper use of diagnostic tests can lead to early diagnosis.

Interdigital Plantar Neuralgia

Interdigital plantar neuralgia was first described as early as 1845 for pain in relation to the third interspace. This entity, commonly referred to as Morton neuroma, was described by Thomas Morton in 1876. He reported a peculiar and painful affection of the fourth metatarsophalangeal (MTP) joint which was treated by resection of that joint with the additional removal of soft tissue surrounding the joint including digital branches of the third and fourth web space nerves. He described removing the swollen portion of the tissue thinking this was a neuroma. We now know the term neuroma is a misnomer, as the nerve enlargement is not a tumor. The histological findings include interstitial sclerohyalinosis, degeneration of nerve fibers without Wallerian degeneration, intraneural and perineural fibrosis, and stromal changes with increased elastic fibers.¹^,² This condition has also been referred to as compression neuropathy.

Anatomy and Pathophysiology

There is no clear explanation of the etiology of this condition. Multiple theories have been put forward. The plantar aspect of the foot is supplied by the medial plantar nerve (MPN) which branches into the first, second, and third digital nerves and the lateral plantar nerve (LPN) which provides the common digital nerve to the fourth interspace and a proper digital branch to the lateral side of the fifth toe. The common digital nerve to each web space passes beneath the deep transverse metatarsal ligament (DTML). This ligament has been implicated as the site and cause of the nerve compression. The pathologic findings of intraneural fibrosis and degeneration occur distal to the DTML supporting this theory.² It has also been shown that during heel off and midstance phases of gait, the area of pathology is always distal to the DTML.³ Nerve biopsies performed on patients with interdigital neuralgia (IDN) compared with asymptomatic nerves at autopsy failed to show any significant difference between specimens.⁴

It has been previously postulated that the nerve in the third web space composed of communicating branches from the MPN and LPN is thicker and consequently more prone to injury,⁵ but this has not been supported clinically. Another study observed that the second and third intermetatarsal spaces are narrower than the first and fourth spaces. It has been theorized that this could be the reason for the more common entrapment of the nerves in these interspaces.⁶ A communicating branch between the third and fourth common digital nerves is present in up to 28% of feet, and an injury to one of these communicating branches could lead to pain and might be responsible for recurrent pain after neuroma resection.⁷

Another proposed contributing factor is the difference in mobility between the medial three rays compared with the lateral two rays. The first, second, and third metatarsals are more firmly fixed to the corresponding cuneiforms, compared with the fourth and fifth metatarsals which have more movement with their cuboid articulation. This difference may expose the common digital
nerve to trauma. Although, this theory can be argued for the third web space, it is negated by the presence of neuromas in the second interspace.⁸

Other causes of interdigital plantar neuralgia include direct injuries to the interdigital nerve such as stepping on sharp objects, crushing, or traction injuries. It has also been proposed that repetitive activities such as prolonged standing or walking on hard surfaces with noncushioned shoes might lead to IDN. In runners, dancers, or athletes, high forefoot forces during cutting, twisting, spinning, or jumping activities might lead to overuse injury of the interdigital nerve. Similarly, modern footwear can promote excessive dorsiflexion of the MTP joint, causing forced plantar flexion of the metatarsals and subsequent trauma to the nerves.⁹ Fat pad atrophy can also make the nerve more vulnerable. Rarely, the transverse metatarsal ligament may become thickened or have an aberrant band which when released will resolve the symptoms of IDN.⁹ Other extrinsic causes of nerve injury include presence of ganglion, lipoma, or MTP joint instability. In approximately 10% to 15% of patients, MTP joint capsule attenuation allows medial deviation of the third toe and consequent lateral shifting of the third metatarsal, reducing the third intermetatarsal space and causing interdigital plantar neuralgia.⁹ Injury to the plantar plate with subluxation or dislocation of the MTP joint may put additional strain on the nerve. Patients with proximal nerve compression may experience double-crush syndrome whereby the distal nerve becomes more sensitive to any pressure. Arthritis and synovitis of the MTP joints due to various causes and fracture sequelae such as malunion may also result in interdigital plantar neuralgia.

History and Physical Examination

The most common age for presentation of interdigital plantar neuralgia is reportedly 55 years (age range 29-81 years). Interdigital plantar neuralgia is 4 to 15 times more likely to be diagnosed in women than men.⁸^,¹⁰ Typically, the presenting symptoms are in the second or third interspace. The occurrence of symptoms in the first or fourth interspace is rare and atypical. Unilateral involvement is more common, but there is a 15% incidence of bilateral neuromas. A 3% incidence of two neuromas in the same foot has been reported.¹¹ Patients report burning, stabbing, tingling, electric-type shooting pain radiating into the corresponding affected toes. Removing tight-fitting shoes often relieves the symptoms. Walking barefoot on soft surfaces also helps to ease the pain. Some patients may describe fullness under the toes. The normal gait pattern of heel strike then rolling onto the ball of the foot may be lost as patients try to curl the toes during stance to reduce the pain.

Physical examination begins with observation of standing foot alignment to detect deviations in the toes, clawing, and swelling or fullness in the interspace compared with the contralateral side. The skin both on dorsal and plantar aspect is examined closely for the presence of corns, calluses, or erythema. It is also important to inspect the patient’s shoes because tight-fitting footwear is common. Careful palpation of the foot should be performed noting any areas of tenderness and/or fullness. Each MTP joint is evaluated for synovitis, range of motion, laxity, and instability. An MTP joint drawer test assesses stability. It is important to rule out pathology in the MTP joints as responsible for the painful symptoms. The intermetatarsal spaces are then examined individually with compression to identify the origin of pain.

Various tests and examination techniques have been described for interdigital plantar neuralgia. The most commonly reported clinical findings include plantar tenderness in 95%, radiation of pain into the toes in 46%, a palpable mass in 12%, and numbness and widening of the interspace in 3%.⁹ Another recent study showed that web space tenderness was positive in 95%, foot squeeze test positive in 88%, plantar percussion positive in 61%, and toe tip sensation deficit present in 67%.¹² A digital nerve stretch test has also been described with 100% sensitivity and 95% positive predictive value. To perform this test, both ankles are held in full dorsiflexion and the lesser toes on either side of the suspected web space are passively fully extended on both feet. The test is positive if the patient reports discomfort in the web space of the affected foot.¹³ The Mulder test is done by compression with mediolateral pressure to the corresponding metatarsal heads while palpating the plantar web space. A palpable “click” or “clunk” which reproduces patient symptoms is considered supportive of interdigital plantar neuralgia.¹⁴ Palpation of the affected toes rarely shows loss of sensation.

A gross motor examination is done including motor function and reflexes to rule out lumbar radiculopathy. Sensation is then tested for the sural, saphenous, and superficial peroneal nerves (SPNs).

Diagnostic Studies

AP, lateral, and oblique radiographs are obtained to evaluate for dislocation, subluxation, arthritis, foreign body, or other abnormalities. A recent study failed to show any relationship between metatarsal length and angular measurements in symptomatic IDN.¹⁵

Additional modalities that have been used for diagnosis of interdigital plantar neuralgia include ultrasonography and MRI. Both are controversial with routine use in diagnosing interdigital plantar neuralgia. The size of
the lesion is very important in detecting IDN using these modalities. In one study, both ultrasonography and MRI were found to be inaccurate. Specifically, ultrasonography was shown to have inaccuracies for lesions less than 5 mm.¹⁶ This study also concluded that relying on ultrasonography or MRI would have led to inaccurate diagnosis in 18 of 19 cases. A detailed clinical examination was found to be the most sensitive and specific diagnostic modality. Another report found ultrasonography to be very good at detecting intermetatarsal interdigital plantar neuralgia with 92% accuracy.¹⁷

No reliable electrodiagnostic studies are available to document the evidence of an interdigital plantar neuralgia. In one study using near-nerve needle sensory nerve conduction, an abnormal dip phenomenon was the most characteristic electrophysiologic diagnostic marker for interdigital plantar neuralgia.¹⁸ Overall, electrodiagnostic studies are mainly used for detection of more proximal nerve compression or if there is suspicion for radiculopathy.

Selective injections into the painful intermetatarsal space can be used as a diagnostic tool. Although complete relief may be obtained, it is advisable not to interpret this as a confirmation of interdigital plantar neuralgia without further support from the physical examination.

Nonsurgical Treatment

Early treatment involves fitting the patient with a wide, soft, laced shoe, preferably with a low heel. This type of shoe wear allows the toes to spread, thereby relieving local pressure and also eliminating chronic hyperextension of the MTP joints. A soft metatarsal support pad just proximal to the metatarsal heads may provide relief from pressure in the area and offload the forefoot reducing painful symptoms.

The use of corticosteroid injections may be helpful but usually does not provide long-lasting pain relief. Significant relief can be obtained after local injection in 60% to 80% of patients, with relief lasting up to 2 years in 30%.¹⁹ In a blinded randomized trial, patients were injected 40 mg methylprednisolone with 1% lidocaine under ultrasonography control by a radiologist. Compared with the control group, global assessment of foot health was better in the corticosteroid group at 3 months. The authors concluded that corticosteroid injections for interdigital plantar neuralgia can often provide symptomatic benefit for at least 3 months.²⁰ However, corticosteroid injections can be associated with serious side effects, especially if the injection is given at the wrong site. Injections should be used with some degree of caution. Atrophy of the subcutaneous fat pad and skin discoloration have been reported. Disruption of the joint capsule with resultant damage to the collateral ligaments and subsequent deviation of the toe medially or laterally can be a serious problem.

In a prospective randomized double-blinded placebo-controlled trial, two different types of injections were used to treat IDN. Corticosteroid with local anaesthetic or local anaesthetic alone was injected. Forty one patients were in two groups and followed for improvement of pain and function at 3 and 6 months. There was no statistical difference between the two groups in regard to request for neuroma excision, and the authors concluded that injection with corticosteroid plus local anaesthetic was not superior to local anaesthetic alone.²¹

Two studies reported good results with neuroma alcohol sclerosing therapy using ultrasonography guidance and multiple injections,²²^,²³ but a Level II prospective case series reporting on results with 5-year follow-up stated that alcohol injection did not provide permanent resolution of symptoms for most patients and can be associated with considerable morbidity.²⁴ A rat sciatic nerve injection study showed no demonstrable effect on cellular histology, apoptosis or cell survival after dehydrated alcohol injection raising questions about efficacy of using alcohol injection in humans.²⁵ Ultrasonography-guided hyaluronic acid injection has been used. In a retrospective study of 83 patients who underwent three weekly injections, the results showed that the American Orthopaedic Foot and Ankle Society (AOFAS) forefoot scale increased from 32.2 to 86.5 at 12 months with no complications.²⁶

Other nonsurgical options include NSAIDs, oral vitamin B₆ (200 mg daily for 3 months and then 100 mg daily), off-label use of tricyclic antidepressants, serotonin uptake inhibitors, and antiseizure medications. Overall, between 60% to 70% of patients in whom interdigital plantar neuralgia are diagnosed eventually undergo surgical intervention after failure of nonsurgical treatment.¹² A retrospective review attempted to identify factors predicting the need for further intervention following a single corticosteroid injection. Fifty-one percent required further treatment within 2 years. Neuroma size with a mean of more than 10 mm and younger age predicted a greater chance of surgical intervention. This study suggests surgical treatment for larger neuromas and younger patients rather than repeat injections.²⁷

Surgical Treatment

If nonsurgical treatment fails, then surgical intervention is indicated. Successful results after surgery have been reported to be between 51% and 93%.¹^,⁸^,²⁸^,²⁹^,³⁰^,³¹ Surgical excision of the nerve is the most frequent technique used to relieve pain from interdigital plantar neuralgia. Other options include neurectomy combined with burying the nerve stump into nearby nerve or muscle, nerve transposition, transverse intermetatarsal ligament release with
or without neurolysis, and endoscopic decompression of the transverse metatarsal ligament. An isolated intermetatarsal ligament (IML) release as primary surgical management has been described. A retrospective study of patients with isolated IML release with previous failed conservative treatment were followed after surgery. A study of 11 patients with isolated IML release with previous failed conservative treatment were followed after surgery. Visual analog scales (VAS) score pre- and postoperatively were compared and showed improvement down from 6.4 to 1.5 at final follow-up. All patients reported significant pain improvement and overall satisfaction. The study concluded that isolated IML release of chronic Morton neuroma showed promising short-term results.³²

Endoscopic decompression has been reported to provide excellent pain relief with low rate of complications.³³ An alternate technique without endoscopy was performed in 14 patients (17 nerve decompressions) using instrumentation designed for carpal tunnel release. The authors of this study reported complete pain relief in 11 of 14 patients, 26 months after surgery.³⁴

Typically, a dorsal incision is used for primary surgery, but plantar incision has been described as well. A recent Level I prospective randomized controlled study trial of plantar and dorsal incisions for surgical treatment of primary Morton neuroma demonstrated clinically good outcomes with both approaches, 87% plantar and 83% dorsal, with no difference in regard to pain, restrictions of daily activities, and scar tenderness. Scar complications were more commonly reported in the plantar group while resection of artery rather than nerve, wound infection and dehiscence, and postneurectomy pain were more commonly reported in the dorsal group³⁵ (Figure 1).

FIGURE 1 A, Intraoperative photograph of dorsal approach for neuroma excision. B, Intraoperative photograph of floor of the foot seen after neuroma excision.

In revision cases a plantar approach improves visualization of the residual plantar nerve (Figures 2 and 3). It is important to place the incision between the metatarsal heads to prevent a tender scar on the weight-bearing surface of the foot.

Regardless of the approach, care must be taken to identify and resect all plantar nerve branches which might tether the interdigital nerve, preventing its proximal retraction off the weight-bearing area of the forefoot. The nerve should be transected proximal to the level of the metatarsal heads. An uncut retained branch that originates proximally may be a conduit for persistent neuritic symptoms. Adjacent web space nerve resection should be avoided whenever possible because it may lead to dense sensory loss in the central toe.³⁶ A retrospective analysis of 674 consecutive primary neuroma excision surgeries showed that 38.9% pathology specimens included a digital artery.³⁷

Multiple outcome studies after interdigital nerve excision have been done. In one study of 56 patients with 76 interdigital plantar neuralgias, 71% became asymptomatic, 9% had substantial improvement, 6% had marginal improvement, and 14% failed. Still, 65% of the satisfied patients had residual plantar pain and 32% reported normal sensation in the web space.⁸ Another report on 66 patients with 5.8 years follow-up had a 85% satisfaction rate. Approximately 70% of these patients needed some modification in their shoe wear to remain pain free.²⁴ Another study of 120 patients with an average follow-up of 5.6 years used the Giannini neuroma score to evaluate results. Fifty-one percent had good to excellent results, 10% fair results, and 40% had poor results. The average VAS was 2.5.

FIGURE 2 A, Intraoperative photograph demonstrates vessel loops being placed beneath three nerves that were found in the area immediately deep to the plantar fascia within the adipose tissue between the flexor digitorum longus tendons. B, Intraoperative photograph of retraction that enables excellent visualization of these structures. C, Photograph of the resected plantar nerve from a patient with suspected recurrent and adjacent neuroma. (A and B reproduced with permission from Title CI, Schon LC: Morton’s Neuroma. Orthopaedic Knowledge Online Journal. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2005; 3[3]. C reproduced with permission from Title CI, Schon LC: Morton neuroma: Primary and secondary neurectomy. J Am Acad Orthop Surg 2008;16:550-557.)

FIGURE 3 Intraoperative photograph of plantar approach to neuroma excision.

A second web space interdigital plantar neuralgia was a prognostic indicator for poor outcome. The authors concluded that long-term outcomes are not as good as previously reported, possibly secondary to residual toe numbness.³¹

A retrospective study compared simple neurectomy (66 patients) with neurectomy with intramuscular implantation (33 patients) with a minimum follow-up of 6 months. This study concluded that both simple neurectomy and neurectomy with intramuscular implantation demonstrated significant improvement in terms of functional outcomes as measured with the FFI, SF-36, and VAS in patients with interdigital neuroma. The authors concluded that although requiring a longer operative time, neurectomy with intramuscular implantation technique might offer superior pain relief with comparable complications to the simple neurectomy technique.³⁸

Recurrent neuromas may occur, and the presenting symptoms are often identical to those of the initial presentation. The symptoms can result from inadequate proximal nerve resection or incomplete resection of tethering plantar nerve branches.³⁰^,³⁹^,⁴⁰ The bulb neuroma, which forms at the end of the cut nerve, takes approximately 12 months to become large enough to cause pain. Accordingly, patients may present with recurrence of symptoms several months to even years after the index surgery. Usually, patients with persistent or recurrent neuromas have a well-localized area of plantar tenderness. Palpation produces a Tinel sign with electric-like pain. Adjacent metatarsal head tenderness may be caused by the regenerating nerve innervations of the skin over the metatarsal heads. An option for revision surgery is the implantation of the nerve stump into the intrinsic muscles of the foot, which has been reported to provide pain relief in 80% of patients.⁴¹

Revision surgery should be undertaken with caution because the results are not as predictable as primary surgery. When evaluating these patients with recurrent symptoms of interdigital plantar neuralgia, the clinician should be suspicious of various causes including inadequate initial resection, formation of a true stump neuroma, misdiagnosis of correct web space, adjacent web space neuroma, and a proximal tarsal tunnel syndrome (TTS) or nerve entrapment resulting from spine pathology.

Tarsal Tunnel Syndrome

Tarsal tunnel syndrome (TTS) is an entrapment neuropathy of the tibial nerve or one of its branches as it passes through the tarsal tunnel. Originally described in 1960, the condition was dubbed TTS in 1962.⁴² This syndrome can result from space-occupying lesions or constriction
of the posterior tibial nerve. It has been compared with carpal tunnel syndrome in the hand because of name similarity, but in reality these two entities have little in common.

Anatomy

At the level of the ankle the flexor retinaculum or laciniate ligament is composed of the deep and superficial aponeuroses of the leg and creates a fibro-osseous tunnel posterior to the medial malleolus. Contents passing through this tunnel include the posterior tibial tendon, flexor digitorum longus tendon, flexor hallucis longus tendon, the posterior tibial artery, nerve, and vein. The floor of the tunnel is formed by the superior aspect of the calcaneus, the medial wall of the talus, and the distal-medial aspect of the tibia. The proximal and inferior borders of the tunnel are delineated by the inferior and superior margins of the flexor retinaculum. Within this tunnel, the posterior tibial nerve lies between the tendons of the flexor digitorum longus and flexor hallucis longus.

The tibial nerve ends by bifurcating into the medial and LPNs. This usually occurs within the tarsal tunnel (93% to 96%), with the remaining 4% to 7% occurring more proximally. Proximal division is considered a risk factor for TTS because of the increased volume of two nerves entering the canal causing the narrowing of the canal.⁴³ The medial calcaneal nerve usually branches off the tibial nerve. This nerve pierces the flexor retinaculum to provide sensory innervations to the medial and posterior heel (Figure 4). Variations include the nerve running superficial to the retinaculum⁴⁴ or arising from the LPN.⁴³

History and Physical Examination

Patients with TTS typically report burning pain and sometimes paresthesias along the medial and plantar aspects of the foot. Alternatively, radiating, diffuse, or poorly defined pain can be the presenting symptom. Typically, the pain increases with activity and improves with rest. It may also occur at night because of abnormal posture or pressure during sleep. Up to one-third of patients also report pain radiating proximally into the midcalf.⁹ Valleix points are seen as tenderness along the course of the nerve.

Precise and careful questioning is required to evaluate other potential sources of nerve pain. Differential diagnoses include rheumatologic conditions leading to chronic tenosynovitis, lumbar spine issues leading to radicular pain, and double-crush syndrome. With double-crush syndrome, proximal compression renders the nerve more susceptible for distal entrapment. Diabetes, vitamin deficiency, and alcoholism can also contribute to double-crush syndrome.

FIGURE 4 Illustration showing the contents of the tarsal tunnel. The flexor retinaculum passes posteriorly over the tarsal tunnel. The medial calcaneal nerve may originate from the tibial nerve proximal to the tarsal tunnel and may have multiple branches. The first branch of the lateral plantar nerve (LPN) may originate within the tarsal tunnel. The medial plantar nerve (MPN) frequently provides one or more calcaneal branches.

Physical examination may also provide insight into TTS etiology. The patient should be examined in standing position to assess hindfoot alignment. Heel valgus puts the nerve under tension, whereas varus may result in nerve compression. While the patient is seated, the medial aspect of the leg, ankle, and foot is examined for any masses, inflammation or swelling. Evaluation for a Tinel sign should be performed in the heel both in neutral and heel valgus position to assess sensitivity of the tibial nerve. Percussion along the course of the nerve may produce paresthesias. Inversion and eversion of the hindfoot can also influence tarsal canal pressure and affect symptoms.⁴⁵ Another provocative maneuver is a dorsiflexion eversion test, which can induce symptoms quickly.⁴⁶ Sensory testing to touch and use of a Semmes Weinstein monofilament may help to isolate terminal branches responsible for creating symptoms. Demonstration of plantar numbness and intrinsic motor weakness can be difficult, but loss of small toe abduction compared with the opposite, normal foot is more easily measurable and indicates loss of innervation of the abductor digiti quinti muscle.

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