Nerve entrapment syndromes of the leg and foot

INTRODUCTION — Increased pressure on a nerve as it traverses a closed space causes an entrapment neuropathy. The mechanism of nerve damage is not completely understood but includes pressure, friction and ischemia. There are three major manifestations of nerve entrapment: Pain Paresthesia Weakness distal to the site of entrapment

The entrapment syndromes that involve the leg and foot are discussed here. Entrapment syndromes of the elbow, forearm, shoulder, and neck are discussed separately. (See "Overview of the nerve entrapment syndromes").

RISK FACTORS — There are several risk factors for the development of lower extremity entrapment neuropathies: Peroneal palsy (foot drop) may result from occupations requiring crouching, squatting, or kneeling (eg, agriculturalists, miners, shoe salesmen). Increased pressure at the popliteal fossa may occur in those who tilt back in chairs. The tarsal tunnel syndrome may result from the use of shoes with an improper arch support. Kneeling with the toes flexed inside tight shoes may cause interdigital nerve injury (eg, in electricians or carpet layers).

Nerves that previously have been affected by another process such as diabetes or alcoholism seem to be more susceptible to entrapment syndromes.

TARSAL TUNNEL SYNDROME — Tarsal tunnel syndrome refers to an entrapment neuropathy of the posterior tibial nerve as it passes through the tarsal tunnel beneath the flexor retinaculum on the medial side of the ankle (show figure 1) [1]. Beneath this retinaculum (or laciniate ligament) lies a tunnel containing the tendons of the flexor digitorum longus and flexor hallucis longus muscles, the vascular bundle, the posterior tibial nerve, and the medial and lateral plantar nerves [2].

Etiology — The most common cause of tarsal tunnel syndrome is a fracture or dislocation involving the talus, calcaneus, or medial malleolus [2]. In these cases, scar tissue, bone or cartilage fragments, or bony spurs may be found compressing the nerve. Etiologies other than injury include rheumatoid arthritis, other causes of inflammation, and tumors [3-5]. As an example, any disorder that results in tenosynovitis of one or more of the tendons within the tarsal tunnel can cause a compression neuropathy.

Pronation related to loss of the plantar arch is another risk factor. I have seen many elderly patients with flatfeet develop the syndrome when going barefoot around the house; slippers with arches or wedges are helpful in this situation [6]. In addition, increased tibial nerve tension in an unstable foot (thereby increasing the risk of the syndrome) may occur during eversion, dorsiflexion, combined dorsiflexion-eversion, and cyclical load with increasing internal rotation [7].

Increases in pressure within the tarsal tunnel occur when the foot is placed in maximal eversion or inversion, as was illustrated in a study in 10 cadavers [8]. This may provide an explanation for the clinical observation of aggravation of symptoms in these positions and improvement of symptoms with neutral immobilization in some patients with tarsal tunnel syndrome. (See "Joint protection program for the lower limb".)

Clinical manifestations — Patients with tarsal tunnel syndrome typically present with aching, burning, numbness, and tingling involving the plantar surface of the foot, the distal foot, the toes, and occasionally the heel. The pain may radiate up to the calf or higher [9]. The discomfort is often most aggravating at night, may be worse after standing, and sometimes leads to the desire to remove the shoes, even while driving. Bilateral involvement is more common when the syndrome is due to systemic inflammation. Any nocturnal foot symptoms and foot pain that radiates out to the toes should suggest an entrapment neuropathy [10,11].

Physical examination seldom reveals swelling or atrophy. Sensory nerve loss is variable and often absent. The Tinel sign and the dorsiflexion-eversion test are useful in assessing for posterior tibial nerve compression.

Tinel test — The Tinel's test (sign), in which the nerve is tapped with a finger or reflex hammer at the flexor retinaculum posterior and inferior to the medial malleolus often reproduces the symptoms. Tapping must be performed over the entire course of the posterior tibial nerve or one of its branches. Firm rolling pressure across the nerve occasionally may be required to reproduce the symptoms. A tourniquet applied just above the ankle can reproduce the symptoms by creating venous engorgement of the tarsal tunnel.

The dorsiflexion-eversion test — the dorsiflexion-eversion test, is a physical examination maneuver in which the tibial nerve is compressed as it runs beneath the flexor retinaculum behind the medial malleolus. In this test, the ankle is passively maximally everted and dorsiflexed while all of the metatarsophalangeal joints are maximally dorsiflexed; the position is maintained for five to 10 seconds. The diagnostic and prognostic value of this test was illustrated by a study in which it was performed on fifty normal volunteers (100 feet) and on 37 patients (44 feet) whose signs and symptoms were correlated with surgical findings [12]. The major findings were as follows: Signs and symptoms of tarsal tunnel syndrome were intensified or induced by the maneuver in fifteen of the twenty feet of the patients who reported numbness, in 15 of the 17 feet of those who reported pain alone, and in six of the seven feet of those who had combined numbness and pain. Local tenderness was intensified in 42 of 43 feet, and it was induced in one foot in which it had been previously absent. A Tinel sign (see "Tinel test" above) became more pronounced in forty-one feet, and the sign was induced in three feet in which it had been absent previously. Intraoperative observation confirmed that the tibial nerve was stretched and compressed beneath the laciniate ligament when the ankle was dorsiflexed, the heel was everted, and the toes were dorsiflexed. Among those who underwent decompressive surgery, preoperative signs and symptoms disappeared a mean of approximately 3 months after the operation, and they could not be induced by repeating the test except in three patients, all of whom had tarsal tunnel syndrome subsequent to a fracture of the calcaneus. In the normal volunteers, no symptoms or signs could be induced by the test.

Thus the physical examination maneuver was positive in 36/44 (82 percent) of symptomatic feet and in none of the 100 controls. When combined with a Tinel test over the posterior tibial nerve, performed at the same examination, the dorsiflexion-eversion test appears to be both sensitive and specific for the tarsal tunnel syndrome.

Children rarely may develop the tarsal tunnel syndrome. Their symptoms differ from those of adults: Night pain is less common Involvement is unilateral rather than bilateral The symptoms most typically include burning pain in the sole of the foot while walking, or recurrent spontaneous sudden sharp pain in the foot.

Paresthesias in the sole of the foot of affected children result when the nerve is percussed behind the medial malleolus. Children often walk with the affected foot in supination, allowing only the lateral border of the sole to contact the ground [13]. I have never seen a child with a tarsal tunnel syndrome and some neurologic and neurosurgical textbooks doubt that it exists.

Diagnosis — The diagnosis of tarsal tunnel syndrome is usually suspected from the history and physical examination, and may be by benefit resulting from conservative treatment. Then if symptoms persist imaging with MRI and electrodiagnostic tests are warranted. Radiographs are useful to detect bony abnormality. Electrodiagnostic testing, particularly nerve conduction studies (NCS), may be useful in assessing the severity of nerve compression.

Magnetic resonance imaging — High resolution MRI can demonstrate the contents of the tarsal tunnel and other soft tissue compartments of the ankle [14-17]. As an example, in one study of 40 feet in 33 patients with tarsal tunnel syndrome, the surgical findings in 19 patients confirmed MRI findings of tenosynovitis, dilated veins or varicosities, fracture or soft tissue injuries, or fibrous scars. Normal findings were also noted in six feet [14]. However, conservative measures should precede imaging studies and electrodiagnostic testing in most cases, unless there has been a recent injury or the symptoms continue to be disabling after all conservative measures have failed.

Electrodiagnostic studies — The tibial nerve conduction velocity normally is 49.9 ± 5.1 milliseconds, and latency from the malleolus to the abductor hallucis muscle is 4.4 ± 0.9 milliseconds. Prolonged latency in excess of 6.1 milliseconds for the medial plantar nerve, and 6.7 milliseconds for the lateral plantar nerve, is suggestive of disease [2,18,19]. However, normal values do not exclude the syndrome, particularly early in its course [19]. In addition, outcome does not necessarily correlate with abnormalities seen on nerve conduction studies.

In one study of electrodiagnostic studies on 111 feet with tarsal tunnel syndrome, distal motor latency (DML) was abnormal in 74 percent (82/111 feet); sensory conduction velocity (SCV) was abnormal from the big toe in 82 percent; from the 5th toe in 73 percent; only 7 percent had normal electrodiagnostic findings in this series [20].

The cost of these studies is prohibitive for the uninsured. Electromyography (EMG) does not contribute useful information beyond what nerve conduction studies provide; abnormalities in the EMG are rare in these patients in my experience.

Laboratory testing — Tarsal tunnel syndrome most often occurs in the absence of inflammatory rheumatic disease. However, if other features are suggestive, an erythrocyte sedimentation rate and tests for rheumatic disease, including rheumatoid factor and antinuclear antibody, should be performed.

Differential diagnosis — A peripheral neuropathy may present similarly to the tarsal tunnel syndrome, although reflexes are more likely to be diminished in the former. Complex regional pain syndrome (reflex sympathetic dystrophy), sciatica, and compartment syndromes should also be considered in the differential, but generally have other characteristic features. (See "Etiology, clinical manifestations, and diagnosis of complex regional pain syndrome in adults", see "Approach to the diagnosis and evaluation of low back pain in adults", and see "Overview of running injuries of the lower extremity").

Management — Nonoperative treatment of tarsal tunnel syndrome includes use of proper shoes (at least a one inch heel and cushioned sole if the flooring is concrete) and slippers, arch supports when indicated, ensuring proper gait and stride, orthoses if significant pronation is present, and nonsteroidal antiinflammatory drugs (NSAIDs). Patients should avoid the use of flat slippers or "deck" shoes.

Local injection — A corticosteroid–local anesthetic mixture injected in a fan-like pattern once or twice into the region inferior and posterior to the medial malleolus provides relief in most persistent cases; relief of pain with this procedure is also useful in confirming the diagnosis [21]. The mixture should include 20 mg methylprednisolone mixed with 1 percent procaine or lidocaine hydrochloride. Care should be taken not to inject the nerve.

Some practitioners report only a 30 percent response to corticosteroid injection and recommend decompression surgery [9].

Decompression surgery — The results of decompression surgery of the tarsal tunnel have been mixed. Examples include the following: A retrospective study of 30 patients (32 feet) with tarsal tunnel syndrome, reported that only 44 percent benefited from surgery [22]. Only 5 patients felt complete relief; those who had a mass lesion such as a ganglion cyst, an accessory navicular bone, or a medial talocalcaneal coalition seemed more likely to have a positive result, leading the authors to suggest that surgery be restricted to patients who have an associated lesion near or within the tarsal tunnel. In addition, 18 feet had electrodiagnostic studies suggestive of a tarsal tunnel syndrome in this report; however, the result had no relation to surgical outcome. Findings of varicosities, tight retinaculum, and synovitis, were similarly not well correlated with outcome. A second study of tarsal tunnel surgery found similar results [23]. Operative treatment in 45 patients (50 feet) was most successful in those with a tumor or talocalcaneal coalition. In contrast, patients with idiopathic and traumatic cases had the worst outcome. Results were best in those who had surgery within ten months after the onset of symptoms. A third study of tarsal tunnel surgery in 34 patients (37 feet) also found that outcomes were influenced by multiple characteristics, including fibrosis around the nerve, the preoperative severity of the condition, a history of sprained ankle, the duration of illness, and the demands of work [24]. More favorable results were observed with a short history of illness, the presence of a ganglion, no history of sprains, and light work demands. In addition, measurement of the terminal latency of the medial plantar nerve was valuable in assessing recovery in this [24] and other studies [25]. To ensure a complete release, division of the deep portion of the abductor hallucis fascia appears to be important [26]. In a fourth study, the results of surgery in 66 feet, all with positive electrodiagnostic studies, had improvement scores as follows [27]: preoperative MFS (Maryland Foot Scores) scores obtained preoperatively, were 61/100 (average), and postoperative MFS scores were 80/100 (average). Postoperative American Orthopaedic Foot and Ankle Society (AOFAS) scores were 80/100 (average). Patients with symptoms less than one year had postoperative MFS/AOFAS scores significantly higher (better) than those with symptoms greater than one year. The most common surgical findings included arterial vascular leashes indenting the nerve and scarring about the nerve. Varicosities and space occupying lesions were present also. The outcome of surgery was not affected by the presence or absence of trauma.

Endoscopic surgery has been proposed to lessen the patient trauma and hasten recovery [28]. Results of this procedure are preliminary at this point. Use of a radial forearm freeflap to cover the nerve was successful in two patients who had failed to benefit from standard decompressive surgery [29] Some surgeons use a posterior tibial nerve block preceding surgery; pain relief is said to be a predictor of a good surgical outcome.

Surgery for tarsal tunnel of longstanding duration may still be helpful. In one study of 18 patients with disease duration of 5 or more years, decompression resulted in total pain relief in 11 of 18 [24].

Summary — Patients with symptoms of tarsal tunnel syndrome and no history of trauma should be given a trial of conservative therapy, including NSAIDs, shoe modification, and in some cases orthotics. If the patient does not respond, corticosteroid injection may provide some relief and can be useful diagnostically. Those that continue to be symptomatic should have further diagnostic testing, including nerve conduction studies and/or MRI. Patients with mass lesions seen on MRI are more likely to benefit from tarsal tunnel surgery.

Local or systemic underlying disorders causing tarsal tunnel or similar symptomatic syndromes must be considered if symptoms persist despite therapy. These include vascular disease with venous stasis, diabetes mellitus with neuropathy, rheumatoid arthritis, myxedema, pregnancy, and amyloidosis [18,30]. However, one study suggests that surgery may provide significant pain relief for those with tarsal tunnel and diabetic neuropathy [30]. Fibromyalgia and tophaceous gout may also be present in patients with persistent symptoms; the former is a predictor of a poor surgical outcome in my experience.

INTERDIGITAL PLANTAR (MORTON'S) NEUROMA — Entrapment neuropathy, with or without an associated plantar neuroma, often develops between the third and fourth toes on the plantar surface of the foot (show figure 2). Anastomoses of the medial and lateral plantar nerves occur in this area. A neuropathy in this location is commonly called a Morton's neuroma. Similar involvement of other interdigital plantar nerves may also occur.

Etiology — Possible causes of neuropathy in this area include excessive mobility of the fourth metatarsal, nerve impingement between flattened metatarsal heads, or compression of the nerve as it is angulated over the transverse tarsal ligament. Chronic compression leads to neuroma formation. Similar signs and symptoms may be induced by an intermetatarsal bursitis rather than a neuroma since the neurovascular bundle lies close to the bursa [31]. (See "Bursitis: An overview of clinical manifestations, diagnosis, and management").

Clinical manifestations — Symptoms of a Morton's neuroma include hyperesthesia of the toes, numbness and tingling, and aching and burning in the distal forefoot. Pain radiates forward from the metatarsal heads to the third and fourth toes. It is aggravated by walking on hard surfaces and wearing tight or high-heeled shoes. The pain frequently persists for some time after cessation of weight bearing. Symptoms are unilateral in 85 percent of cases.

Physical examination reveals tenderness in the plantar aspect of the distal foot over the third and fourth metatarsals; compressing the forefoot reproduces the symptoms. The tenderness is occasionally aggravated by direct pressure to the plantar aspect of the third and fourth metatarsophalangeal joints; pressure may be applied by squeezing the metatarsal heads together with one hand, and simultaneously compressing the involved web space with the thumb and index finger of the opposite hand [32]. There should be a concomitant sensation of burning distally.

Diagnosis — The diagnosis of a Morton's neuroma is often clinical. Radiography may reveal lateral toe deviation, a faintly radiopaque shadow, and rarely, notching of the adjacent bone. Other procedures used include ultrasonography, computed tomography, MRI, and nerve conduction testing.

Of these modalities, MRI is the most effective to diagnose a Morton's neuroma. In one report, for example, MRI findings in 17 feet with suggestive symptoms of neuroma were corroborated by surgical findings [33]. In another study, a change in clinical diagnosis as well as in treatment plans occurred after MRI in 28 and 57 percent of feet initially thought to harbor a neuroma, respectively [34].

Resected nerves have no pathognomonic changes, although in one study of surgical specimens from patients with symptoms of intermetatarsal neuroma and plantar nerves obtained at autopsy the largest diameter nerves were surgical specimens while the smallest were from the autopsy group [35]. Nerve swelling was calculated to have a sensitivity of 78 percent and specificity of 80 percent. This provides some support for the usefulness of MRI assessment of nerve size as a confirmatory test.

Treatment — Conservative treatment should precede expensive diagnostic procedures; this involves decreasing stresses at the metatarsal heads with the use of a metatarsal support, metatarsal bar, or a comma-shaped metatarsal shoe insert. External appliances should be placed on both shoes so that the patient walks evenly, even when symptoms are unilateral. A broad-toed shoe that allows spreading of the metatarsal heads or an extra-depth shoe is helpful.

Injection of a local anesthetic–corticosteroid injection into the site of compression can be beneficial [32,36]; methylprednisolone 20 mg (0.5 mL) mixed with 0.5 mL 1 percent lidocaine should suffice. The injection should precede consideration for surgery since an intermetatarsal bursitis is common, and the injection and use of proper shoes may provide a cure. When the neuroma is evident, injection may only provide transient benefit.

Surgical removal of the neuroma and nerve may be required in those who are resistant to nonoperative therapy [37]; patients still benefit from wearing adapted shoes after surgery. In one post-operative study of 31 patients, for example, only 30 percent had no restrictions in the choice of their shoes at long term follow-up [38]. Others report surgical success rates of up to 80 to 90 percent [32,38,39].

OTHER ENTRAPMENT NEUROPATHIES OF THE LOWER EXTREMITY — Entrapment neuropathies also may occur in other areas of the lower extremity.

Anterior tarsal tunnel syndrome — Anterior tarsal tunnel syndrome results from entrapment of the deep peroneal nerve beneath the inferior extensor retinaculum at the anterior aspect of the tarsal tunnel [40]. Symptoms include paresthesias on the dorsum of the foot and often in the great toe. The discomfort is increased at night.

Relief following local infiltration with corticosteroids, along with an avoidance of boots, high-heeled shoes, or tight lacing, is helpful and confirms the diagnosis [41]. Surgical decompression is often necessary when the entrapment follows a crush injury [42].

Traumatic prepatellar neuralgia — Traumatic prepatellar neuralgia follows trauma to the anterior patella; it may be preceded by transient prepatellar swelling. Patients typically present a few weeks after the injury with exquisite tenderness over the medial outer border of the patella at the site of emergence of the neurovascular bundle; even slight stroking is exquisitely painful [6,43].

The diagnosis of this disorder is based upon clinical features. Tapping over the nerve (Tinel's sign) is positive when the paresthesia is reproduced or accentuated.

Treatment of traumatic prepatellar neuralgia with a local anesthetic or corticosteroid injection into the point of maximum tenderness is usually helpful [44]. Local application of capsaicin cream may also provide pain relief. Surgical excision of the subcutaneous tissue in the tender area can provide complete relief if necessary [43].

Compression of the common peroneal nerve — Foot-drop with an inability to dorsiflex the foot often results from peroneal nerve stretching forces during contact sports, or from compression due to crossing the leg, hanging the leg over a constricting rigid object, or from direct trauma. A tight boot or cast may also cause sensory or motor peroneal nerve loss, and a partial or complete foot-drop. Nerve conduction measurement can localize the site of entrapment, and distinguish peroneal nerve injury from other etiologies of foot-drop including mononeuritis, sciatica, and heavy metal poisoning.

Management of common peroneal nerve compression consists of primary treatment of the injury, use of a posterior foot-drop splint until active movement has recovered, physical therapy progressing from passive range of movement to passive assistive, active, and active resistant exercise under supervision with a physical therapist. Walking is particularly important therapy.

Approximately one-third of patients recover without surgery [45]. However, surgical consultation should not be delayed if no progress or worsening occurs within two months following the injury. In one study, for example, surgical decompression or interfascicular graft repair resulted in benefit in 97 percent of patients who had surgery at 2 months after the injury, compared to only 38 percent of those who had surgery 4 to 8 months after onset [25]. Less successful results are associated with increasing lengths of the interfascicular graft [45]. A constricting band often is found at the level of the fibular head and at the proximal origin of the peroneus longus muscle.

Joplin's neuroma — Perineural fibrosis of the plantar digital nerve may follow a bunionectomy or trauma to the first metatarsophalangeal joint. This results in pain and paresthesia at the plantar aspect of the first metatarsophalangeal joint of the great toe. A Hoffman-Tinel test may be positive beneath the first metatarsophalangeal joint. Relief occurs with foot rest or removal of the shoe. Surgical excision of the nerve may be necessary.


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