Nerve Entrapment Syndromes Clinical Presentation

Symptoms of nerve compression vary based on the particular nerve involved. In general, however, the temporal sequence of neurologic manifestations is as follows:

• Irritation or inflammation (sensory nerves): pain, paresthesia

• Ablative symptoms: numbness (sensory nerves), weakness and atrophy (motor nerves)

In a major mixed nerve (both sensory and motor) such as the sciatic or median nerve, signs of sympathetically mediated features may be prominent in chronic cases. These changes manifest as the following:

• Dry, thin, hairless skin

• Ridged, thickened, cracked nails

• Recurrent skin ulceration

Most entrapment syndromes involve mixed sensory and motor nerves and thus conform to the aforementioned pattern. Exceptions include the deep branch of the ulnar nerve at Guyon canal and PIN (both predominantly motor) and the lateral femoral cutaneous nerve (LFCN; pure sensory) near the anterior superior iliac spine (ASIS).

Median nerve at the wrist (carpal tunnel syndrome)

Carpal tunnel syndrome (CTS) is the most commonly encountered nerve entrapment condition. Median nerve compression at the wrist is seen at the transverse carpal ligament (TCL), which attaches to and arches between the pisiform and hamate on the ulnar side and the scaphoid and trapezium on the radial side. The palmar fascia is fused to the TCL proximally, then fans out to the soft tissue of the palmar skin as the palmar aponeurosis. Combined layers of the TCL and the proximal palmar fascia form the flexor retinaculum.

The palmaris longus tendon inserts into the palmar aponeurosis and lies directly over the median nerve just proximal to the TCL but is absent in approximately 15% of individuals. In these patients, the nerve can be found beneath a fascial membrane between the flexor carpi radialis and flexor digitorum superficialis tendons.

The palmar cutaneous branch originates from the radial side of the median nerve proximal to or just deep to the flexor retinaculum, then transverses superficially to the flexor retinaculum to innervate the thenar eminence (thumb) and the palm, roughly up to the vertical line overlying the fourth metacarpal.

The recurrent motor branch to the thenar muscles leaves the median nerve radially just beyond the distal edge of the flexor retinaculum, but variant nerves may pierce through the flexor retinaculum or may arise from the ulnar aspect of the median nerve, and an accessory motor branch may even emerge proximal to the flexor retinaculum. ^[9]

Occasionally, the ulnar nerve (10%) and artery (4%) lie radial to the hook of the hamate superficial to the flexor retinaculum, placing them at risk for direct or indirect injury during carpal tunnel surgery (eg, from retractor pressure).

Patients note a dull, aching pain at the wrist that may extend up the forearm to the elbow. Often, this pain is associated with distressing paresthesias in the thumb and index finger, particularly upon awakening. Typically, patients rub their wrists or shake their hands to try to "get the blood back into their wrists." ^[10] The pain is typically worse at night and disturbs the patient's sleep. As symptoms worsen, sensation may be decreased at the volar pads of the thumb and index, middle, and ring fingers. These symptoms are the result of compression of the median nerve as it passes through the wrist and the carpal tunnel. ^{[11, 12]}

Sensation in the palmar surface of the lateral 3.5 fingers is often affected; however, the palm is supplied by the palmar cutaneous branch, which does not travel through the carpal tunnel. Therefore, if palmar sensation is lost, the nerve injury is most likely located more proximally.

In more chronic or severe cases, weakness in the hand or atrophy may be evident. The median nerve at the wrist supplies the following functions to LOAF muscles in the hand:

• L: Lumbricals 1 and 2 are affected.

• O: Opponens pollicis is affected.

• A: Abductor pollicis brevis (APB) loss results in weakness and atrophy, causing thinning of the lateral contour of the thenar bulk.

• F: Flexor pollicis brevis (FPB) typically is dually innervated by both median and ulnar nerves; therefore, compression of only the median nerve does not usually cause appreciable symptoms because of the ulnar nerve contribution.

Forced wrist flexion causes increasing paresthesia and pain (Phalen test), as does extreme wrist extension (reverse Phalen test), due to compression of the nerve in the carpal tunnel. Symptoms can be elicited by applying steady pressure with the thumbs over the flexor retinaculum (compression test). Gentle tapping of the median nerve over the flexor retinaculum (wrist) produces paresthesias (percussion test).

Acute carpal tunnel syndrome is a surgical emergency that requires decompression. Progressively worsening pain and sensory disturbances in the median nerve distribution distinguish acute carpal tunnel syndrome from the less severe median nerve neurapraxia. Neuropathy can occur primarily at the time of injury, as the result of unreduced fracture fragments or callus, or following prolonged immobilization in palmar flexion. Prompt diagnosis and treatment of acute injury to the median nerve after wrist trauma are key to a successful outcome. ^[13]

Ulnar nerve at the elbow (cubital tunnel)

The ulnar nerve travels on the medial side of the brachial artery in the upper arm, pierces the medial intermuscular septum at mid arm, and continues toward the elbow on the medial head of the triceps. At the elbow, it passes through the cubital tunnel—a groove between the medial humeral epicondyle and the olecranon. The nerve travels beneath the aponeurotic arcade between the 2 heads of the flexor carpi ulnaris and down the forearm between the deep and superficial finger flexors.

The following 5 potential areas of ulnar nerve entrapment are found within its course into and out of the elbow:

• The arcade of Struthers (present in 70% of the population; differs from the ligament of Struthers, which can compress the median nerve) stretches from the medial head of the triceps to insert into the medial intermuscular septum. It is located approximately 6-8 cm above the medial epicondyle. It can be a factor in ulnar nerve compression after ulnar nerve transposition.

• The medial intermuscular septum presents a sharp edge that can indent the nerve (particularly after anterior transposition) in which the nerve may be kinked.

• The cubital tunnel is floored by the medial collateral ligament of the elbow and is roofed by the arcuate ligament (cubital tunnel retinaculum) that stretches between the medial humeral epicondyle and the medial aspect of the olecranon.

• The arching band of aponeurosis between the 2 heads of the flexor carpi ulnaris (so-called Osborne band) may compress the nerve, especially during repetitive contraction of the muscle.

• The aponeurotic covering between the flexors digitorum profundus and superficialis is occasionally a site of compression.

Ulnar neuropathy at the elbow may result from a posttraumatic or nontraumatic etiology. Trauma may be caused by a single event or, more typically, may be due to mild repetitive injuries. The resulting pathophysiologic basis for the traumatic neuropathy is likely due to scarring and adhesion at the cubital tunnel, compression at the heads of the flexor carpi ulnaris aponeurosis, or both. ^[14]

Patients with an ulnar neuropathy from a nontraumatic etiology often perform activities that require repetitive elbow flexion or prolonged resting of the elbow on a hard surface. Elbow flexion creates narrowing of the cubital tunnel as a result of traction on the arcuate ligament and bulging of the medial collateral ligament. Elbow flexion may also contribute to the injury by increasing intraneural pressure. With scarring and adhesion on the epineurium, elongation accentuates the tethering effect on the axons. These effects may be accentuated at night when the patient sleeps with the elbow in flexion.

Spontaneous subluxation or dislocation of the ulnar nerve out of the cubital tunnel occurs in up to 15% of the population, occasionally aggravating symptoms of entrapment through a rubbing action against or over the bony surfaces (ie, medial epicondyle). ^[15]

Signs and symptoms of the clinical presentation include the following:

• Pain typically presents as a deep ache around the elbow region.
• Pain is exacerbated when the medial elbow is impacted.
• Intermittent paresthesias and numbness may occur in the ring finger and in the little finger.
• Hand weakness may be noted, especially with gripping objects.
• Sensation over the palmar portion of the fifth digit and the ulnar half of the fourth digit specifically is decreased to the following stimuli: pinprick, light touch, and 2-point discrimination
• Sensory loss can also be detected along the dorso-ulnar aspect of the hand (due to involvement of the dorsal cutaneous branch of the ulnar nerve, which arises proximal to the wrist).
• Late symptoms include dense numbness, profound weakness, and atrophy of intrinsic hand muscles.
• Weakness may be detected in finger abductors and adductors (interossei) and in the thumb (adductor pollicis), whereas thumb abduction is normal.
• An ulnar claw hand may be present with extension of little and ring fingers.
• Extension at the metacarpophalangeal joints and flexion at the intraphalangeal joints is caused by loss of lumbricals 3 and 4.
• Provocative tests: A gentle tapping of the nerve at and around the cubital tunnel elicits distressing electrical shock, tingling, or both, down into the ulnar fingers (percussion test). Sustained elbow flexion combined with gentle digital pressure on the cubital tunnel causes paresthesias and pain.

Ulnar nerve compression at the wrist (Guyon canal)

At the wrist, the ulnar nerve runs above the flexor retinaculum lateral to the flexor carpi ulnaris tendon and medial to the ulnar artery. At the proximal carpal bones, it dips between the pisiform and the hook of the hamate at the entrance to the Guyon canal, roofed over by an extension of the TCL between these 2 bones. ^{[16, 17]}

Three zones of the ulnar nerve within the distal ulnar tunnel have been defined as follows:

• Zone 1 – Ulnar nerve proximal to the bifurcation

• Zone 2 – Deep branch

• Zone 3 – Superficial branch or branches

These anatomic zones correlate with clinical symptoms. Patients with zone 1 compression can present with motor, sensory, or mixed lesions; those with zone 2, motor lesions, and with zone 3, sensory.

After entering the Guyon canal, the deep (motor) branch first supplies the abductor digiti minimi (ADM), then crosses under one head of the flexor digiti minimi (FDM), supplies this muscle, and crosses over to supply the opponens digiti minimi (ODM) before rounding the hook of the hamate to enter the mid palmar space and supply other hand muscles. Depending on the exact site of compression within the Guyon canal, the ADM or both the ADM and the FDM may be spared. The ODM is always affected, together with the interossei, lumbricals 3 and 4, and the adductor pollicis.

Compression of the deep branch is most common and usually occurs at the level of the fibrous arch of the hypothenar muscles. The distal canal is also a common site for ganglions arising from the wrist.

The superficial branch supplies sensation to the hypothenar skin ulnar to the vertical line at the base of the ring finger and ends as the 2 ulnar digital nerves for the little finger and the ulnar half of the ring finger. Its only motor fibers extend to the palmaris brevis, which wrinkles the hypothenar skin to cup the palm.

Patients typically have repeated trauma or compression at the wrist. Examples of this are as follows:

• Paraplegics who use hand crutches that have a horizontal bar across the palm, or those using wheelchairs

• Motorcyclists or bicyclists who firmly grasp the handlebar

• Operators of pneumatic drills

• Carpenters who perform repetitive activities such as hammering

The classic presentation is a young man with painless atrophy of hypothenar muscles and interossei with sparing of the thenar group. Sensory loss and pain involving the ulnar 1.5 digits may be present.

This site can be differentiated from ulnar compression at the elbow, in which sensation over the dorsum of the ulnar half of the ring finger and the little finger (from the dorsal cutaneous branch, which leaves the ulnar nerve prior to entering Guyon canal, approximately 6-8 cm proximal to the wrist) is affected.

A positive Phalen test and percussion tenderness over the course of the ulnar nerve at the wrist may be present.

Radial nerve in the proximal forearm – posterior interosseous nerve syndrome

At mid arm, the radial nerve descends behind the humerus, deep to the long head of the triceps, then spirals around the humerus between the medial and lateral heads of the triceps in the spiral groove. Approximately 5-10 cm above the lateral humeral epicondyle, the nerve pierces the lateral intermuscular septum to gain the anterior compartment of the arm. Here, it immediately enters the deep, muscular groove bordered medially by the biceps and the brachialis and laterally by the brachioradialis, the extensor carpi radialis longus (ECRL), and the extensor carpi radialis brevis (ECRB). The nerve then courses immediately in front of the radiocapitellar joint capsule, where it divides into the (motor) deep branch of the radial nerve and the sensory superficial radial nerve (SRN).

Branches to the brachioradialis and ECRL come off before the bifurcation, and the nerve to the ECRB comes off the deep branch of the radial nerve. A leash of arterial branches (of Henry) that arises from the recurrent radial artery cross over the deep branch of the radial nerve just before the arcade of Frohse. This nerve continues as the PIN in the radial tunnel. The PIN traverses a musculo-tendinous arcade—the arcade of Frohse, which is formed by the upper free border of the superficial head of the supinator. Within the tunnel, the PIN rests on the deep head of the supinator.

After emerging from the radial tunnel beneath the supinator, the PIN lies posteriorly to the interosseous membrane of the forearm and innervates the extensor digiti minimi, the extensor carpi ulnaris, and the extensor digitorum communis medially, as well as the extensor indicis proprius, the extensor pollicis longus and brevis, and the abductor pollicis longus laterally.

PIN compression is most commonly associated with tendinous hypertrophy of the arcade of Frohse and with fibrous thickening of the radiocapitellar joint capsule. Vascular compression by the leash of Henry has been reported. Lesions such as lipoma, synovial cyst, rheumatoid synovitis, and vascular aneurysm have been found in some cases. Hobbies or occupations associated with repetitive and forceful supination predispose the individual to PIN neuropathy. Chronic trauma to the flexion surface of the forearm likewise causes problems. For example, the constricting rings of the Canadian crutches, which exert direct pressure over the supinator surface, typically cause PIN neuropathy in patients with paraplegia.

Brachial plexitis tends to have more diffuse involvement affecting certain sites more commonly (nb, electromyography may be especially helpful). However, brachial plexitis may affect only 1 nerve territory; in these cases, distinction of plexitis from entrapment may be difficult.

The PIN is predominantly a motor nerve. It has pain fibers supplying the wrist but has no cutaneous innervation of the skin. Paralysis of the extensor muscles is heralded by a feeling of fatigue during finger extension and elbow supination. The extension in the metacarpophalangeal joints is weakened, but it is not weakened in the interphalangeal joints because the lumbricals are intact.

The index and fifth fingers receive both their own extensor tendon and the tendon branch from the common extensor and are less affected than the extension of the third and fourth digits. Thus, in the early stage of entrapment, the hand exhibits a characteristic pattern upon finger extension, in which the middle 2 fingers fail to extend, while the index and little fingers can be extended ("sign of horns").

Progression of paralysis eventually causes weakness in all finger extensors and in thumb abduction. Radial wrist extensors are intact because of the proximal innervation of the extensor carpi radialis muscles. No sensory symptoms are present. Dull, aching pain is sometimes present over the front of the elbow, and palpation over the radiohumeral joint often aggravates the pain, probably via irritation of the nervi nervorum of the PIN.

Suprascapular nerve entrapment

The suprascapular nerve arises from the lateral aspect of the upper trunk of the brachial plexus, runs across the posterior triangle of the neck together with the suprascapular artery and the omohyoid muscle, dips under the trapezius, and then passes through the suprascapular notch at the superior border of the scapula. As the nerve enters the supraspinous fossa, it supplies the supraspinatus muscle, then curls tightly around the base of the spine of the scapula, enters the infraspinous fossa, and supplies the infraspinatus. ^{[18, 19, 20]}

A stout, strong suprascapular ligament closes over the free upper margins of the suprascapular notch. Suprascapular nerve entrapment is caused by this ligament, often in conjunction with a tight, bony notch. The only sensory fibers in the suprascapular nerve supply the posterior aspect of the shoulder joint. These articular fibers are the source of the ill-localized, dull shoulder pain associated with the syndrome. The syndrome often afflicts athletes, particularly those involved in basketball, volleyball, weightlifting, and gymnastics.

Signs and symptoms include the following:

• Pain with insidious onset

• Deep, dull aching pain in the posterior portion of the shoulder and in the upper periscapular region

• Non-circumscribed pain

• Lack of neck or radicular symptoms

• Shoulder weakness

• Weakness confined to the supraspinatus, which initiates shoulder abduction, and/or to the infraspinatus, which externally rotates the arm. This pattern of weakness must be distinguished from rotator cuff disease and C5 radiculopathy.

• Atrophy manifesting as hollowing of the infraspinous fossa and prominence of the scapular spine. Supraspinatus atrophy may not be obvious because of the overlying trapezius. Deep pressure over the midpoint of the superior scapular border may produce discomfort.

Lateral femoral cutaneous nerve (meralgia paresthetica)

The lateral femoral cutaneous nerve (LFCN) arises from the ventral rami of the L2 and L3 nerve roots. This purely sensory nerve is formed just deep to the lateral border of the psoas muscle, then descends into the pelvis over the iliacus muscle deep to the iliacus fascia. Just medial to the ASIS, the nerve exits the pelvis by passing through deep and superficial bands of the inguinal ligament as they attach to the ASIS. The nerve is almost horizontal while still within the pelvis before it traverses the inguinal ligament, but it then takes a vertical course out to the surface of the thigh.

This almost 90º kink of the nerve is often exaggerated by a thickened ridge in the iliacus fascia, where it attaches to the posterior aspect of the inguinal ligament. Beyond the groin, the nerve quickly enters the fascial covering of the sartorius, which originates from the ASIS. The most constant relationship of the LFCN is with the medial border of the sartorius about 2-5 cm distal to the ASIS. After this, the nerve usually crosses over the muscle and divides into anterior and posterior branches, supplying sensation to the anterolateral surface of the thigh down to the top of the patella.

A protruding, pendulous abdomen, as seen in obesity and pregnancy, compresses the inguinal ligament downward and onto the nerve, causing it to be kinked. This angulation of the nerve is further exaggerated with extension of the thigh and is relaxed with flexion. Extension also tenses the deep fascia and may add to compression from the front. The nerve may also be compressed directly by tight belts or pants, or by pressure when a patient is prone in spine surgery.

The main symptoms include uncomfortable numbness, tingling, and painful hypersensitivity in the distribution of the LFCN, usually in the anterolateral thigh down to the upper patellar region. Symptoms are often accentuated by walking down slopes and stairs; by engaging in prolonged standing in the erect posture; and, sometimes, by lying flat in bed. The patient learns to relieve symptoms by placing a pillow behind the thighs and by assuming a slightly hunched posture while standing.

Decreased appreciation of pinprick is elicited, together with a hyperpathic reaction to touch and even an after-discharge phenomenon of persistent, spontaneous tingling after the touch. Deep digital pressure medial to the ASIS may set off shooting paresthesia down the lateral thigh. The diagnosis is confirmed with a nerve block using 0.5% bupivacaine injected a finger's breadth medial to the ASIS. Resulting anesthesia over the sensory territory of the LFCN should be concomitant with complete cessation of pain and tingling. Differential diagnosis includes lumbar disc herniation at L2/L3 levels, which may require magnetic resonance imaging (MRI).

Common peroneal nerve entrapment

The common peroneal nerve is 1 of the 2 terminal divisions of the sciatic nerve. It is smaller and lateral to the tibial nerve. It descends obliquely along the lateral side of the popliteal fossa medial to the tendon of the biceps femoris. Posterior to the head of the fibula, it lies superficial to the lateral head of the gastrocnemius. It winds around the lateral aspect of the neck of the fibula deep to the peroneus longus (fibular tunnel), where it divides into superficial peroneal, deep peroneal, and articular branches. Entrapment occurs when the nerve is in close relationship to the neck of the fibula.

In the thigh, it supplies the short head of the biceps femoris and contributes to the sural nerve. In the leg, it supplies the muscles of the lateral and anterior compartments of the leg and sensation on the dorsum of the foot.

Signs and symptoms include the following:

• Pain radiating from the knee region to the dorsal aspect of the foot

• Sensory loss on the dorsum of the foot

• Foot drop (loss of dorsiflexion of the foot) and loss of extension of the toes, as well as eversion of the ankle (this is differentiated from an L5 radiculopathy, in which posterior tibialis function [inversion in plantar flexion] is affected)

• Tinel sign at the fibular neck: The differential diagnosis is broad, and lumbar radiculopathy (L4 or L5) must be considered.

Tarsal tunnel syndrome

Compression of the tibial nerve behind the medial malleolus, or tarsal tunnel syndrome (TTS), is an uncommon entrapment neuropathy. ^[21]

The roof of the tunnel is formed by the flexor retinaculum stretched between the medial malleolus and the calcaneus. The tarsal bones are the floor. Numerous fibrous septae between the roof and the floor subdivide the tunnel into separate compartments at various points. The contents of the tarsal tunnel at its proximal end are, from front to back, as follows:

• Tibialis posterior tendon

• Flexor digitorum longus tendon

• Posterior tibial artery and vein

• Tibial nerve

• Flexor hallucis longus tendon.

• Tibial nerve with 3 terminal branches. It bifurcates into medial and lateral plantar nerves within 1 cm of the malleolar-calcaneal axis in 90% of cases; in the other 10%, the medial and plantar nerves are found 2-3 cm proximal to the malleolus.

• Calcaneal branch, which usually comes off the lateral plantar fascicles (but around 30% leave the main nerve trunk just proximal to the tunnel). Distally, the medial and lateral plantar nerves travel in separate fascial compartments. The medial branch supplies the intrinsic flexors of the great toe, the first lumbrical, and sensation over the medial plantar surface of the foot inclusive of at least the first 3 toes. The lateral branch supplies all interossei and the lateral 3 lumbricals, as well as sensation over the lateral plantar surface of the foot. The calcaneal branch, which traverses its own tunnel, provides sensation to the heel.

Early symptoms include burning, tingling, and dysesthetic pain over the plantar surface of the foot. Characteristically, pain is set off by pressing or rubbing over the plantar skin, sometimes with after-discharge phenomenon. Percussion tenderness (Tinel sign) is often evident over the course of the main nerve or its branches, and pain may be aggravated by forced eversion and dorsiflexion of the ankle.

In advanced cases, the intrinsic flexors of the great toe are weak and atrophied, producing hollowing of the instep. The lateral toes may also show clawing due to paralysis of the intrinsic toe flexors. The calcaneal branch may be spared because of its proximal takeoff.

Thoracic outlet syndrome

Thoracic outlet syndrome is a rare condition (1-3 per 100,000) caused by neurovascular compression at the thoracic outlet. This disorder can be classified as neurogenic, arterial, or venous based on the compressed structure(s). Patients develop an objectively verifiable form of thoracic outlet syndrome secondary to congenital abnormalities such as cervical ribs or fibrous bands originating from a cervical rib. Neck trauma or repeated work stress can cause scalene muscle scarring or dislodging of a congenital cervical rib that can compress the brachial plexus. Nonsurgical treatment includes anti-inflammatory medication, weight loss, physical therapy/strengthening exercises, and botulinum toxin injections. The most common surgical treatments include brachial plexus decompression, neurolysis, and scalenotomy with or without first rib resection. ^[22]

The first thoracic ventral ramus joins the eighth cervical ventral ramus to form the lower trunk of the brachial plexus. This runs near the subclavian artery on top of the pleural apex to enter the axilla between the clavicle anteriorly and the first rib posteriorly. ^[23]  Structures crossing on top of the first rib, from anterior to posterior, include subclavian vein, scalenus anterior, subclavian artery, lower trunk of brachial plexus, and scalenus medius. The lower trunk and/or the subclavian artery could be compressed by different structures: fibrous bands deep to the scalenus anterior muscle, thickened suprapleural membrane (Sibson fascia), a cervical rib (bony or fibrous), or an elongated transverse process of C7.

In the neural syndrome, patients typically present with pain and paresthesias along the ulnar aspect of the forearm, hand, and medial 2 fingers. Symptoms are often exacerbated by overhead activities. Patients with neurogenic symptoms often have no objective neurologic deficits clinically or electrophysiologically. On rare occasions, true neurologic loss may be seen with clinically apparent weakness and atrophy, as well as electrophysiologic denervation of finger and hand muscles supplied by the lower trunk (C8 and T1). The Gilliatt-Sumner hand characteristic of the neurogenic thoracic outlet syndrome consists of atrophy affecting thenar and hypothenar eminences.

In the venous syndrome, patients present with arm swelling, cyanosis, and pain; in the arterial syndrome, episodic muscle cramping, coldness, and blanching of the hand, especially with arm elevation, may occur. ^[24]

Provocative maneuvers include the Adson test (obliteration of the pulse with chin elevation and head turning to the ipsilateral side). Although this was Adson’s original description, pulse obliteration could occur more frequently with head rotation to the contralateral side. In 1966, Roos popularized the 90º abduction in external rotation stress test. In the upper limb tension test of Elvey, the arms are abducted at 90º and the wrists are dorsiflexed, while the head is tilted to the contralateral side. Pulse obliteration with provocative maneuvers could occur in 9-53% of normal people. Tenderness with percussion may be noted over the lower elements of the brachial plexus in the supraclavicular fossa. ^{[25, 26]}

Other tests include electrodiagnostic studies; vascular laboratory studies, including Doppler ultrasonography combined with provocative maneuvers; magnetic resonance imaging (MRI)/magnetic resonance angiography (MRA); and, rarely, angiography.

(Chest radiographs could show a cervical rib, as depicted in the image below, or prolonged transverse process of C7.)