Chapter 33 Entrapment Neuropathies

• Entrapment neuropathy occurs when a peripheral nerve is subjected to repetitive movement in a constricted space. Conservative treatment involves reducing the movement; surgical treatment enlarges the space.

• Carpal tunnel syndrome is by far the most common entrapment neuropathy and the commonest cause of neurological symptoms in the hand. The symptoms are not always confined to the hand and can spread proximally.

• In cubital tunnel surgery, when comparing simple ulnar nerve decompression versus anterior subcutaneous transposition techniques, one can expect similar outcomes but fewer complications with simple ulnar decompression.

• Diagnosis of entrapment neuropathy can usually be made from the clinical history, and can then be confirmed by physical signs, electromyography, nerve conduction studies, and magnetic resonance imaging (MRI).

• Surgery for entrapment neuropathy is indicated if conservative measures fail or if the clinical picture is one of severe compression with significant pain and muscular weakness and wasting.

Some peripheral nerves, irrespective of whether they are motor, sensory, or of mixed type, pass through narrow, constricted areas in the arms or legs. Under certain circumstances, these nerves are susceptible to compression at these sites,¹^–³ and this compression can eventually clinically manifest as entrapment neuropathy.⁴^–⁶ Entrapment of nerves usually occurs as they pass beside a joint, such as the elbow, wrist, or hip and only very occasionally elsewhere in the limbs. This, along with the fact that entrapment neuropathy seldom occurs in the head or trunk, suggests that repetitive motion is a major factor that precipitates entrapment in an anatomically constricted segment.

Two types of physical constrictions predispose to entrapment neuropathy. The first type (Fig. 33.1A) is a fibro-osseous tunnel. The space available for the nerve within the tunnel becomes constricted either because the contents of the tunnel become larger or hypertrophic, as when a patient with tenosynovitis has carpal tunnel syndrome, or because the walls of the tunnel encroach upon the tunnel’s lumen, as when fractured fragments of a carpal bone displace into the carpal canal. Compression of a nerve in a tunnel is an example of static compression. The second type (Fig. 33.1B) involves dynamic compression of the nerve as it passes through a fibrotendinous arcade. The nerve is flanked by two bellies of a muscle that under static conditions do not compress the nerve. When they contract, however, they cause a shutter-like closure of the arcade, compressing the nerve. For example, this can occur at the arcade of Frohse in the supinator muscle, the two heads of the flexor carpi ulnaris at the entrance to the cubital tunnel, or the two heads of the flexor digitorum sublimis forming the “sublimis bridge.”

FIGURE 33.1 A, Example of a fibro-osseous tunnel—the carpal tunnel at the wrist. The median nerve and the tendons of the long flexor muscles are the main contents of the tunnel. B, Example of fibrotendinous arcade—the cubital canal at the elbow. The ulnar nerve enters the fibroaponeurotic arcade formed by the two heads of the flexor carpi ulnaris (Osborne’s ligament).

Pathology of Nerve Compression ⁷

The pathophysiological changes following nerve compression ⁸^–²¹ are dependent on the degree, rate, and duration of compression. Loss of function of the nerve as a result of compression is manifested clinically by motor paralysis, paresthesia, or numbness. In physiological terms, mild and brief compression produces a transient and reversible conduction block within the nerve. Sustained compression over a long period causes structural changes. Not all components of the nerve are equally susceptible to a given degree of compression. Nerve fibers that have a greater amount of epineurium compared to the nerve fascicles are less susceptible to compression than those with larger fascicles and scanty epineurium (Fig. 33.2). Also, within a given nerve, not all fibers undergo degenerative changes to the same extent. The superficially located fibers tend to bear the brunt of the compression, while the central fibers are relatively spared. Large, heavily myelinated fibers subserving light touch and motor function are more sensitive to compressive changes than unmyelinated fibers subserving pain sensation.

FIGURE 33.2 Nerves having a greater amount of epineurium (A) are less susceptible to compression than those having scanty epineurium (B).

Impediment to microvascular flow appears to be a major factor in the pathophysiology of nerve impingement.⁷ Capillary blanching and venular obstruction herald progressive compression. This leads to nerve ischemia, which in turn leads to endothelial impairment and progressive edema; the edema compounds the ischemia and swelling of the nerve (Fig. 33.3). Critical swelling of a nerve within the constraints of its surroundings may lead to further nerve compression, a phenomenon that can be called a mini-compartmental syndrome.

FIGURE 33.3 A, Magnetic resonance imaging (MRI) of right elbow showing ulnar neuropathy at the cubital canal (enlarged ulnar nerve with high short tau inversion recovery (STIR) signal). 1, Ulnar nerve; 2, medial epicondyle of humerus; 3, triceps; 4, brachial artery.

B, MRI of the wrist showing median nerve changes (increased STIR signal) characteristic for carpal tunnel syndrome. 1, Median nerve; 2, tendons of flexor digitorum superficialis and profundus; 3, carpal bones; 4, flexor retinaculum.

Nerve compression blocks axonal transport. The antegrade transport from the nerve cell to the axon toward the synapse can be divided into fast and slow components; the fast component carries the membrane-associated materials and the slow components carry the cytoskeletal proteins. Nerve compression impedes both the fast and slow components of the antegrade flow, resulting in a swelling of the nerve proximal to the compression as a result of the damming up of the moving axoplasm within the fibers. Thus, the distribution of cytoskeletal elements, axolemma constituents, and the transmitter substances required for synaptic conduction are all impaired by a block of antegrade flow. Retrograde axonal flow from the synaptic level to the cell body of the nerve is similarly blocked by compression of the nerve. This results in a loss of transfer of neuronotropic factors to the nerve cell body. The impairment of retrograde axonal transport results in certain changes in the nerve cell body comparable to those that occur after peripheral nerve section (wallerian degeneration). Thus, changes noted in the cell body are an eccentric nucleus, dispersion of Nissl substance (chromatolysis), and a decrease in nuclear and whole cell volumes. The overall result of the impediment to axoplasmic flow is impaired membrane permeability and conduction block.

With acute and severe compression one observes a characteristic sequential invagination or telescoping of the myelin sheath (Fig. 33.4). The polarity of invagination is reversed at the edges of the compression. With chronic compression, segmental demyelination occurs within the compressed segments, accounting for the slowing of conduction velocity of the nerve. In the early phases, the nerve fibers distal to the compression show normal morphology. With sustained compression, axolysis occurs within the compressed segment, leading to distal wallerian degeneration.

FIGURE 33.4 Telescoping of myelin sheath with acute and severe nerve compression.

Double Crush Syndrome

If a nerve is compressed proximally, its distal part is more susceptible to compression than a normal nerve would be, because the antegrade axonal flow is blocked by the first compression. In a similar manner, if there is distal compression, the nerve cell body undergoes degeneration more quickly if a second compression is present proximally, because of impediment of retrograde flow. This latter syndrome is called a reverse double crush syndrome.²²

Nerve Compression Syndrome in Diabetics

Patients with diabetic neuropathy are more susceptible to compression, presumably because of accumulation of sorbitol, a metabolite of glucose, and the formation of endoneurial edema.

The Entrapment Syndromes

The entrapment sites, the nerves involved at each site, and the corresponding syndromes are listed in Table 33.1. This chapter will cover the most common entrapment syndromes: carpal tunnel syndrome, cubital tunnel syndrome, meralgia paresthetica, suprascapular nerve entrapment, and tarsal tunnel syndrome. Some patients can develop multiple entrapment neuropathies.²³

TABLE 33.1 Entrapment Syndromes

Carpal Tunnel Syndrome ²⁴

The carpal tunnel syndrome ²⁵^–²⁷ is the most common entrapment neuropathy encountered in clinical practice. It results from compression of the distal median nerve within the carpal tunnel, located in the proximal part of the palm of the hand.²⁸ The carpal tunnel is bounded dorsally by the carpal bones and ventrally by the transverse carpal ligament. The carpal bones form a shallow trough that is converted into a tunnel by the carpal ligament. The contents of the tunnel are the median nerve and tendons of the long flexor muscles (see Fig. 33.1A). Any lesion affecting the synovial sheath tends to compromise the cross-sectional diameter of the carpal canal and may induce compressive neuropathy.²⁹ Recent studies that include magnetic resonance imaging (MRI) and computed tomography (CT) scans show that patients with carpal tunnel syndrome tend to have small carpal canals. The small size of the carpal canal, measured by the decrease in its cross-sectional diameter, is a congenital or developmental phenomenon.³⁰ Its small size in women may account for their higher incidence of carpal tunnel syndrome.

Clinical Features ³¹

Women are more commonly affected than men, by a ratio of 7:3. Most patients are middle-aged at the onset of symptoms. The predominant symptom is an aching, burning, tingling, numb sensation in the hand, ordinarily in the lateral half of the hand and the outer three or four digits.³² Frequently there may be an aching pain in the proximal forearm or even in the arm up to the shoulder and it can lead to confusion with cervical radiculopathy. Patients typically wake up at night with increased pain, and they may shake their hand to obtain relief. The symptoms are often bilateral. With severe or advanced compression patients complain of weakness of grip and a tendency to drop things.

In the early stages of the syndrome, at which time most patients are seen in contemporary practice, there are few objective findings. Two mechanical tests can be performed. Tinel’s sign may be elicited by lightly tapping over the median nerve at the wrist crease, which results in a tingling in the distribution of the median nerve if positive. Phalen’s test consists of asking the patient to flex the wrist to 90 degrees for about 60 seconds, which will precipitate paresthesia in the distribution of the median nerve if positive.³³^–³⁵ Neither test is conclusive and both results are often absent. Perception of light touch, pinprick, and two-point discrimination in the tips of the fingers in the median nerve distribution may be impaired. In advanced cases there may be atrophy of the thenar muscles, especially in the abductor pollicis brevis. A recently proposed scratch collapse test for evaluation of carpal and cubital canal syndrome may be a significant addition for clinicians but it needs to be evaluated by independent reviewers.³⁶

The clinical history, especially of nocturnal pain, is usually the most reliable diagnostic clue. There are several local and systemic risk factors that precipitate the symptoms of carpal tunnel syndrome (Table 33.2).³⁷^–⁴⁴

TABLE 33.2 Risk Factors in the Pathogenesis of Carpal Tunnel Syndrome

Local Factors	Systemic Factors
Increased volume of the contents of the carpal canal Hypertrophic tenosynovitis Masses: neurofibroma, hemangioma, lipoma, ganglion cyst, gouty tophus, xanthoma Anomalous muscles and tendons ⁴⁰ Persistent median artery with or without thrombosis, aneurysm, arteriovenous malformation Acute palmar space infections Hemorrhage ⁴³ Reduction in the capacity of the carpal canal Congenitally small carpal canal ³⁰ Idiopathic or familial thickening of the transverse carpal ligament ³⁹ Malunion or callus following Colles’ fracture or fracture of the carpal bones Unreduced dislocations of the wrist or intercarpal joints Improper immobilization of the wrist (“cotton loader position”) Compression by cast Exostoses Other local factors Burns at the wrist	Increased susceptibility of nerves to pressure Alcoholic or diabetic polyneuropathy Hereditary neuropathy with liability to pressure palsies Amyloidosis Proximal lesions of the median nerve (“double crush” syndrome) Other polyneuropathies Factors unique to women Pregnancy and lactation ^41,⁴³ Menstrual cycles Contraceptive pills Menopause Toxic shock syndrome Eclampsia Other hormonal factors Myxedema Acromegaly Other systemic factors Obesity Raynaud’s disease Athetoid-dystonic cerebral palsy Long-term hemodialysis ^41,⁴⁴ Inflammatory and autoimmune disorders Rheumatoid arthritis ³⁷ Dermatomyositis Scleroderma Polymyalgia rheumatica Metabolic disorders Mucopolysaccharidoses Mucolipidoses Amyloidosis Chondrocalcinosis Gout

Local Factors

Systemic Factors

Increased volume of the contents of the carpal canal

Reduction in the capacity of the carpal canal

Other local factors

Burns at the wrist

Increased susceptibility of nerves to pressure

Alcoholic or diabetic polyneuropathy

Hereditary neuropathy with liability to pressure palsies

Amyloidosis

Proximal lesions of the median nerve (“double crush” syndrome)

Other polyneuropathies

Factors unique to women

Pregnancy and lactation ^41,⁴³

Other hormonal factors

Myxedema

Acromegaly

Other systemic factors

Obesity

Raynaud’s disease

Athetoid-dystonic cerebral palsy

Long-term hemodialysis ^41,⁴⁴

Inflammatory and autoimmune disorders

Rheumatoid arthritis ³⁷

Dermatomyositis

Scleroderma

Polymyalgia rheumatica

Metabolic disorders

Mucopolysaccharidoses

Diagnosis

The most important diagnostic tests are electromyography and study of nerve conduction velocity.⁴⁵ The earliest and most significant finding is the prolongation of sensory latency due to demyelination. The sensory evoked response will show diminution of amplitude and may even be absent. Motor latency abnormalities occur late in the course of the disease. Needle electromyography may show loss of motor unit potentials and the presence of denervation potentials in the median-innervated muscles in the thenar eminence due to axonal loss. Clinically it corresponds to the impairment of two-point discrimination.

Treatment

Current management strategies based on the evidence-based medicine approach were summarized in the American Academy of Orthopaedic Surgeons practice guidelines published in 2009 ⁴⁶ and are summarized in Table 33.3.⁴⁷^–⁵¹

TABLE 33.3 Management of Entrapment Syndromes: Evidence-Based Medicine Results

Study/Review	Conclusions
Carpal Tunnel Syndrome
Ono S, et al. Optimal management of carpal tunnel syndrome.⁴⁷ (A review of RCTs and systematic reviews.) Jarvik JG, et al. Surgery versus non-surgical therapy for carpal tunnel syndrome: a randomized parallel-group trial.⁵¹	A trend toward recommending early surgery for cases with and without median nerve denervation. Symptoms in both groups improved, but surgical treatment led to better outcome than that with nonsurgical treatment. However, the clinical relevance of this difference was moderate.
Cubital Canal Syndrome
Bartels RHMA, et al. Prospective randomized controlled study comparing simple decompression versus anterior subcutaneous transposition for idiopathic neuropathy of the ulnar nerve at the elbow.⁴⁸ Chung K. Treatment of ulnar nerve compression at the elbow.⁴⁹ (A review of RCTs and systematic reviews.)	The outcomes were equivalent, but simple decompression was associated with fewer complications. Use of this approach is advised even in the presence of (sub)luxation. Different transposition techniques (subcutaneous and submuscular) yielded results similar to those with simple decompression. All mentioned randomized controlled trials had relatively small samples.
Tarsal Tunnel Syndrome
Patel AT, et al. Usefulness of electrodiagnostic techniques in the evaluation of suspected tarsal tunnel syndrome: an evidence-based review.⁵⁰	Nerve conduction studies may be useful for confirming the diagnosis of tibial neuropathy at the ankle (recommendation level C).

In early cases with minimal symptoms or in individuals in whom the syndrome is expected to be transient, conservative treatment should be instituted. This consists of a wrist splint in neutral position at night (initial relief in approximately 50%), a course of vitamin B₆, and anti-inflammatory drugs. Injection of local anesthesia and steroids around the median nerve may be beneficial,^52,⁵³ but accidental injection directly into the nerve may result in annoying paresthesias in the distribution of the median nerve.

Surgical therapy is indicated when conservative measures fail.⁵¹ The surgical procedure can be performed by either the open method⁵⁴ or an endoscopic technique.⁵⁵ The steps in the surgical sectioning of the transverse carpal ligament are shown in Figure 33.5. Usually local or regional anesthesia (Bier block) is used. General anesthesia may be used if the patient is extremely nervous.