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COMMON ENTRAPMENT NEUROPATHIES

Median Neuropathy at the Wrist (Carpal Tunnel Syndrome)

Carpal tunnel syndrome (CTS) is the most common of all entrapment neuropathies. 

Anatomy

Just proximal to the wrist, the Palmar Cutaneous Sensory branch leave the median nerve to run subcutaneously to supply sensation over the thenar eminence. The median nerve then enters the wrist through the carpal tunnel. Carpal bones make up the floor and sides of the carpal tunnel, with the thick transverse carpal ligament forming the roof. In addition to the median nerve, nine flexor tendons traverse the carpal tunnel as well. In the palm, the median nerve divides into motor and sensory divisions. The motor division travels distally into the palm supplying the First and Second lumbricals. In addition, the Recurrent Thenar Motor Branch is given off. This branch turns around (hence, recurrent) to supply muscular branches to most of the thenar eminence including the opponens pollicis, abductor pollicis brevis and superficial head of the flexor pollicis brevis. The sensory fibers of the median nerve that course though the carpal tunnel supply the medial thumb, index, middle and lateral half of the ring finger. 

Above: cross sectional anatomy through the wrist at the carpal tunnel (median nerve in red).

Etiology

Most cases are idiopathic. In most cases, edema, vascular sclerosis and fibrosis are seen, findings consistent with repeated stress to connective tissue. Demyelination follows compression and ischemia of the median nerve, and if severe enough, Wallerian degeneration and axonal loss ensue. Occupations or activities which involve repetitive hand use clearly increase the risk of CTS. Other predisposing etiologies include certain systemic disorders, most notably hypothyroidism, rheumatoid arthritis and amyloidosis.

Clinical Presentation

Patients with CTS may present with a variety of symptoms and signs. Women are more often affected than men. Although usually bilateral clinically and electrically, the dominant hand is usually more severely affected, especially in idiopathic cases. Patients complain of wrist and arm pain associated with paresthesias in the hand. The pain may be localized to the wrist, or may radiate to the forearm, arm or rarely the shoulder; the neck is not affected. Some patients may describe a diffuse, poorly localized ache involving the entire arm. Paresthesias are frequently present in a median distribution (medial thumb, index, middle and lateral ring finger). While many patients report that the entire hand falls asleep, if asked directly about little finger involvement, most will subsequently note that the little finger is spared.

Above: typical distribution of sensory symptoms in carpal tunnel syndrome.

Symptoms are often provoked when either a flexed or extended wrist posture is assumed. Most commonly, this occurs during ordinary activities, such as driving, holding a phone, book or newspaper. Nocturnal paresthesias are particularly common. During sleep, persistent wrist flexion or extension leads to increased carpal tunnel pressure, nerve ischemia and subsequent paresthesias. Patients will frequently awaken from sleep and shake or ring out their hands, or hold them under warm running water.

Sensory fibers are involved early in the majority of patients. Pain and paresthesias usually bring patients to medical attention. Motor fibers may become involved in more advanced cases. Weakness of thumb abduction and opposition may develop, followed by frank atrophy of the thenar eminence. Some patients describe difficulty buttoning shirts, opening a jar, or turning a doorknob. However, it is unusual to develop significant functional impairment from loss of median motor function in the hand.

The sensory examination may disclose hypesthesia in the median distribution. Sensation over the thenar area in spared, as this area is innervated by the Palmar Cutaneous Sensory Branch, arising proximal to the carpal tunnel. The Tinel's sign, tested by tapping over the median nerve at the wrist, and the Phalen's maneuver, holding the wrist passively flexed, may both provoke symptoms. 

The motor examination involves inspection of the hand looking for wasting of the thenar eminence (severe cases) and testing the strength of thumb abduction and opposition.

Ulnar Neuropathy at the Elbow

Ulnar neuropathy at the elbow (UNE) is second only to median nerve entrapment at the wrist (i.e., carpal tunnel syndrome) as the most common entrapment neuropathy affecting the upper extremity. Lesions of the lower brachial plexus or C8-T1 roots may result in similar symptoms to UNE.

Anatomy

The ulnar nerve is essentially derived from the C8 and T1 roots. All ulnar fibers travel through the lower trunk of the brachial plexus and then continue into the medial cord. The terminal extension of the medial cord becomes the ulnar nerve. The ulnar nerve then travels medially and distally toward the elbow. At the elbow, the nerve enters the ulnar groove formed between the medial epicondyle and the olecranon process. Slightly distal to the groove in the proximal forearm, the ulnar nerve travels under the tendinous arch of the two heads of the flexor carpi ulnaris muscle, known as the humeral-ulnar aponeurosis (HUA) or "cubital tunnel." Muscular branches to the flexor carpi ulnaris and the medial division (fourth and fifth digits) of the flexor digitorum profundus are then given off.

The nerve then descends through the medial forearm, giving off no further muscular branches until after the wrist. Slightly proximal to the wrist, the dorsal ulnar cutaneous sensory branch exits to supply sensation to the dorsal medial hand and the dorsal fifth and medial fourth digits. The nerve next enters the medial wrist to supply sensation to the volar fifth and medial fourth digits and muscular innervation to the hypothenar muscles, the palmar and dorsal interossei, the third and fourth lumbricals, and two muscles in the thenar eminence, the adductor pollicis and the deep head of the flexor pollicis brevis.

Etiology

UNE usually occurs as a result of chronic mechanical compression or stretch, either at the groove or at the cubital tunnel. Although rare cases of ulnar neuropathy at the groove are caused by ganglia, tumors, fibrous bands, or accessory muscles, most are caused by external compression and repeated trauma. Elbow fracture, often years before, and subsequent arthritic change of the elbow joint may result in so-called tardy ulnar palsy. In addition, chronic minor trauma and compression (including leaning on the elbow) can either exacerbate or cause ulnar neuropathy at the groove. Ulnar neuropathy at the groove is also common in patients who have been immobilized because of surgery or who sustain compression during anesthesia or coma.

Distal to the groove is the cubital tunnel, the other major site of compression of the ulnar nerve in the region of the elbow. Although some use the term cubital tunnel syndrome to refer to all lesions of the ulnar nerve around the elbow, it more properly denotes compression of the ulnar nerve under the HUA. Some individuals have congenitally tight cubital tunnels that predispose them to compression. Repeated and persistent flexion stretches the ulnar nerve and increases the pressure in the cubital tunnel, leading to subsequent ulnar neuropathy.

Clinical Presentation

UNE caused by compression at the groove or at the cubital tunnel may present in a similar manner. In contrast to carpal tunnel syndrome in which sensory symptoms predominate, motor symptoms are more common in ulnar neuropathy, especially in chronic cases. In some patients, insidious motor loss may occur without sensory symptoms, particularly in those with slowly worsening mechanical compression. As most of the intrinsic hand muscles are ulnar innervated, weakness of these muscles leads to loss of dexterity and to decreased grip and pinch strength. These are often the complaints that bring the patient to medical attention. There may be atrophy of both the hypothenar and thenar eminences (the ulnar-innervated adductor pollicis and deep head of the flexor pollicis brevis are in the thenar eminence). However, thumb abduction is spared (median and radial innervated).

In moderate or advanced cases, examination often shows the classic hand postures that occur with ulnar muscle weakness. The most recognized is the "Benediction posture" (see photo above). The ring and little fingers are clawed, with the metacarpophalangeal joints hyperextended and the proximal and distal interphalangeal joints flexed (from third and fourth lumbrical weakness), while the fingers and thumb are held slightly abducted (from interossei and adductor pollicis weakness). Patients with ulnar neuropathy may not be able to flex the distal fourth and fifth fingers completely when making a grip; in contrast, the median-innervated second and third distal digits flex normally.

In UNE, sensory disturbance, when present, involves the volar and dorsal fifth and medial fourth digits, and the medial hand. The sensory disturbance does not extend proximally much beyond the wrist crease. Sensory involvement extending into the medial forearm implies a higher lesion in the plexus or nerve roots (i.e., this is the territory of the medial antebrachial cutaneous sensory nerve, which arises directly from the medial cord of the brachial plexus). 

Pain, when present, may localize to the elbow or radiate down to the medial forearm and wrist. Paresthesias may be reproduced by placing the elbow in a flexed position or by applying pressure to the groove behind the medial epicondyle. The ulnar nerve may be palpably enlarged and tender. Especially in patients with ulnar neuropathy at the cubital tunnel, the nerve may be palpably taut with decreased mobility.

Radial Neuropathy at the Spiral Groove

Of the major upper extremity nerves, compression of the radial nerve is less common. However, the radial nerve is susceptible to external compression and can result in a classic syndrome of wrist and finger drop.

Anatomy

The radial nerve receives innervation from all three trunks of the brachial plexus and, correspondingly, a contribution from each of the C5-T1 nerve roots. After each trunk divides into an anterior and posterior division, the posterior divisions from all three trunks unite to form the posterior cord. The posterior cord gives off the axillary, thoracodorsal, and subscapular nerves before becoming the radial nerve. In the high arm, the radial nerve first supplies the three heads of the triceps brachia before wrapping around the posterior hummers in the spiral groove. Descending into the region of the elbow, muscular branches are then given off to the brachioradialis, and then all the extensors of the wrist and fingers. In addition, the superficial radial sensory nerve is given off to supply sensation over the lateral dorsum of the hand as well as part of the thumb and the dorsal proximal phalanges of the index, middle, and ring fingers.

Etiology

The most common radial neuropathy occurs at the spiral groove. Here, the nerve lies juxtaposed to the hummers and is quite susceptible to compression, especially following prolonged immobilization (see figure above). One of the times this characteristically occurs is when a person has draped an arm over a chair or bench during a deep sleep or while intoxicated ("Saturday night palsy"). The subsequent prolonged immobility results in compression and demyelination of the radial nerve.

Clinical Presentation

Clinically, marked wrist drop and finger drop develop in radial neuropathy at the spiral groove. Notably, elbow extension (triceps) is spared. Sensory disturbance is present in the distribution of the superficial radial sensory nerve, consisting of altered sensation over the lateral dorsum of the hand, part of the thumb, and the dorsal proximal phalanges of the index, middle, and ring fingers.

Personal Neuropathy at the Fibular Neck

Peroneal neuropathy often occurs from compression at the fibular neck, where the nerve is quite superficial and vulnerable to injury. Patients usually present with a footdrop and sensory disturbance over the lateral calf and dorsum of the foot. However, patients with sciatic neuropathy, lumbosacral plexopathy, or L5 radiculopathy may present with a similar pattern of numbness and weakness.

Anatomy

The peroneal nerve is derived predominantly from the L4-S1 nerve roots, which travel through the lumbosacral plexus and eventually the sciatic nerve. Within the sciatic nerve, the fibers that eventually form the common peroneal nerve run separately from those that distally become the tibial nerve. The sciatic nerve bifurcates above the popliteal fossa into the common peroneal and tibial nerves. The common peroneal nerve winds around the fibular neck and passes through the fibular tunnel between the peroneus longus muscle and the fibula. The common peroneal nerve then divides into superficial and deep branches. The deep peroneal nerve innervates the dorsiflexors of the ankle and toes. It also supplies sensation to the web space between the first and second toes. The superficial peroneal nerve innervates the ankle evertors and then supplies sensation to the mid- and lower lateral calf.

Etiology

Peroneal neuropathy can be seen as a result of a variety of conditions. Acute peroneal neuropathy often follows trauma, forcible stretch injury, or compression from prolonged immobilization. In the hospital, this occurs most often postoperatively in patients who have received anesthesia or heavy sedation. Slowly progressive lesions often suggest a mass lesion, such as a ganglion or nerve sheath tumor. Entrapment of the peroneal nerve at the fibular tunnel, although quite uncommon, may also present in a progressive manner.

Several other circumstances predispose one to peroneal neuropathy. Habitual leg crossing may repetitively injure the peroneal nerve at the fibular neck, where it is quite superficial. Similarly, repetitive stretch from squatting, for example, by gardeners has also been associated with peroneal neuropathy. In addition, patients who are thin or who have recently lost a substantial amount of weight may be prone to peroneal palsy, probably because of the lack of protective supporting adipose tissue at the fibular neck.

Clinical Presentation

Patients with peroneal neuropathy at the fibular neck present with a characteristic neurologic picture. Most often, both the deep and superficial peroneal nerves are affected. Involvement of the deep peroneal nerve leads to weakness of toe and ankle dorsiflexion, resulting in a foot and toe drop. Dysfunction of the superficial peroneal nerve results in weakness of foot eversion. Clinically, weakness of these muscles results in a stereotyped set of symptoms. Patients note a slapping quality of their foot as it hits the ground while walking. Weakness of eversion leads to a tendency to trip, especially on uneven sidewalks or curbs, and an increased risk of sprained ankles. When observed while walking, patients have a so-called "steppage gait" whereby they bring their knee up higher than usual so that the dropped foot clears the floor.

Sensory disturbance develops over the mid- and lower lateral calf and the dorsum of the foot. Local pain and a Tinel's sign may be present over the lateral fibular neck. In isolated peroneal neuropathy at the fibular neck, function of the sciatic, tibial, and sural nerves remains normal. Most important, ankle inversion is spared, mediated by the tibialis posterior (L5, sciatic-tibial nerve). Finally, all reflexes, including the ankle reflex, remain normal in an isolated peroneal neuropathy.

It is important to note that lesions of the sciatic nerve, lesions of the lumbosacral plexus, and L5 radiculopathy may also present with a footdrop and numbness over the lateral calf and dorsum of the foot. Indeed, these lesions, especially early on, occasionally mimic a peroneal palsy almost exactly, including abnormalities of sensation. It is in these cases that EMG studies are especially helpful.

Lateral Femoral Cutaneous Sensory Neuropathy (Meralgia Paresthetica)

Entrapment of the lateral femoral cutaneous sensory nerve is the most common entrapment neuropathy in the lower extremity. It is associated with a classic clinical syndrome, known as meralgia paresthetica. 

Anatomy

The lateral femoral cutaneous nerve is a pure sensory nerve that is derived from the L2-L3 roots and runs under the inguinal ligament near the superior iliac spine, where it may be injured or entrapped. The lateral femoral cutaneous nerve only supplies sensation to a large oval area of skin over the lateral and anterior thigh.

Etiology

This entrapment is more common in patients who are obese, wear tight underwear or pants, or who have diabetes. Although the vast majority of cases are due to an entrapment at the inguinal ligament, rare cases have resulted from tumors and other mass lesions compressing the upper lumbar plexus more proximally.

Clinical Presentation

The clinical syndrome, known as meralgia paresthetica, results in a painful, burning, numb patch of skin over the anterior and lateral thigh, sometimes worst in the standing position. Because there is no muscular innervation from this nerve, there is no associated muscle atrophy, weakness, or loss of reflexes.