A 50-year-old endurance runner complains of burning and tingling in the lateral aspect of her right leg. The symptoms seem to come on with long runs or cycling. If she hits or rubs her lower leg or applies pressure to it, she feels a “zing” going down the leg. There’s no numbness, but there’s tingling over the lateral calf and the dorsum of the foot. She denies back pain or symptoms proximal to the knee. Her family physician found no objective evidence of weakness or sensory loss; but tapping the fibular head produces a positive Tinel’s test with the pain tingling down the leg. She’s referred to a sports medicine physician familiar with nerve injuries.
Introduction
Entrapment and compression neuropathies are common in the general public as well as in athletes. Last month, we looked at nerve root and plexus injuries. This month, we move outwards to the limbs. Peripheral nerves can be damaged due to compression with resultant pain and loss of function. Injuries can be mild if only the outer myelin is damaged, or more severe if the actual axons are impacted. Virtually any
peripheral nerve can be compressed. Here we’ll discuss some of the more common entrapments.
Carpal tunnel syndrome
Median neuropathy at the wrist is one of the most frequently encountered entrapments. Golf, archery, cycling and wheelchair basketball are all associated with CTS.3 It’s often bilateral, but usually affects the dominant hand to a greater degree. Patients often complain of wrist pain and forearm pain, and occasionally radiation to the arm or shoulder. Nocturnal paresthesias are often pathognomonic of CTS and sensory symptoms frequently appear before motor fibres become involved.
Carpal tunnel can be seen as a condition of excess flexion, or even extension of the wrist. While sleeping, patients curl their wrists into maximal flexion, so they wake at night and must “flick” out their hands to ease the discomfort. The carpal tunnel is a tight space enclosed by the carpal bones and the transverse carpal ligament. The thumb and finger flexor tendons as well as the median nerve all travel through the carpal tunnel. The median nerve innervates the “LOAF” muscles of the thenar eminence; the first and second lumbricals, opponens pollicus, abductor pollicus brevis and part of flexor pollicus brevis. This nerve supplies sensation to the thumb, index, middle and half of the fourth finger. It’s important to note that sensation over the thenar eminence — the palm just proximal to the thumb — should be spared in carpal tunnel syndrome, as the palmar cutaneous sensory branch comes off before the carpal tunnel. If this sensory branch is involved, the median nerve may be compressed more proximally between the heads of the pronator teres.
Physical exam focuses on motor and sensory tests with weakness of the thenar eminence and sensory splitting of the fourth finger being highly suggestive of CTS. A positive Tinel’s sign with increased paresthesias when the examiner taps over the carpal tunnel is also suggestive of CTS, but it has a high false positive rate. Phalen’s sign is considered positive if tingling is reproduced with wrist flexion after 30 seconds to 2 minutes and is more sensitive than Tinel’s.
The main differential diagnosis is a C6 or C7 radiculopathy, which would be more likely with neck pain and radiation of pain down the arm with neck movements. Nerve conduction studies are a very sensitive tool to diagnose CTS and EMG can help to rule out a radiculopathy.
The most recent Cochrane review demonstrates that there are only three effective treatments for CTS: wrist splints, usually worn at night for six weeks to prevent wrist flexion; steroid injection into the carpal tunnel; and surgical release of the transverse palmar ligament.
Ulnar neuropathy
The ulnar nerve is most often compressed at the elbow as it enters the cubital tunnel, but it may also rarely become entrapped at the wrist through Guyon’s canal2. The ulnar first supplies the flexor carpi ulnarnis and flexor digitorum profundus of digits 4 and 5. It then enters Guyon’s canal, a tunnel between the pisiform and hook of hamate to supply the hypothenar muscles, interossei and 4th and 5th lumbricals.
Ulnar neuropathy at the elbow is usually caused by external compression and repeated trauma. Repetitive flexion and extension as seen in baseball pitchers and weight lifters can predispose to injury.3 Patients often present with motor complaints without sensory symptoms and have decreased grip and pinch strength. There may be a positive Tinel’s test, with tapping over the ulnar nerve at the elbow. There may be wasting over the first dorsal interossei and with severe cases, clawing of the 4th and 5th fingers.
Compression at the wrist in Guyon’s canal is classically associated with cycling, but there is a high likelihood of a ganglion cyst. The ulnar nerve branches into motor and sensory branches, so the clinical picture is quite variable.
Sensory testing is key to localizing ulnar nerve injury to the elbow or the wrist. Both injuries will cause decreased sensation in the 5th and medial half of the 4th digit. But if sensation is impaired over the dorsal medial hand in the dorsal ulnar cutaneous distribution, the nerve must be injured at the elbow as this branch comes off before the wrist. Similarly, with injury at the elbow, there will be weakness with wrist flexion in ulnar deviation, and flexion of the distal phalanges of digits four and five, which is not seen in wrist compression.
To conclude the ulnar nerve screen, look for wasting of the intrinsics, particularly the first dorsal interosseous, at the back of the hand in the first web space as well as wasting of the thenar eminence (Figure 1). The motor screen also examines the finger abductors and adductors, which can be tested by the patient squeezing a piece of paper between the fingers (Figure 2), and Froment’s sign (Figure 3).
Lower trunk or medial cord brachial plexus injuries and C8 radiculopathies are the main differential diagnoses. EMG studies are very useful in localizing ulnar nerve lesions.
Treatment and protection of the nerve involves avoiding repetitive flexion, direct pressure and hyperflexion. If conservative measures fail, the patient may need surgical release and/or anterior transposition of the nerve.
Suprascapular nerve entrapment
The suprascapular nerve gives rise off the upper trunk of the brachial plexus and supplies the supraspinatus and infraspinatus muscles. Injury is most commonly seen in volleyball players and in throwing sports, with the patient reporting pain in the shoulder blade region. Both the supraspinatus and infraspinatus muscles may show apparent wasting if the nerve is entrapped at the supragleniod notch. If it’s trapped at the transscapular ligament, only the infraspinatus is affected. Nerve conduction studies can quite readily show slowing or loss of conduction compared to the unaffected side and the two muscles are superficial and quite easily needled. An MRI may show a ganglion as cause.
Peroneal nerve injury
Peroneal mononeuropathy is the most common lower extremity entrapment. The nerve is most frequently injured at the fibular neck where it’s most vulnerable.
The peroneal nerve, a branch of the sciatic nerve, divides into the deep branch, which supplies the ankle and toe extensors; and the superficial branch, which supplies the ankle evertors. Patients will present with a foot slap and a tendency towards ankle sprains. Sensory disturbances will be seen over the lower lateral calf and dorsum of the foot.
The main differential diagnosis is L5 radiculopathy. With this condition, the patient will have preferential weakness of great toe extension (L5-S1) whereas peroneal neuropathy will more likely show weakness of ankle dorsiflexion, (L4-5), back and leg pain, depression of the medial hamstrings reflex, and most importantly, weakness of foot inversion.
Lumbosacral plexus injuries may also mimic a peroneal neuropathy and present with a foot drop. It’s vital to look for more proximal nerve injuries of the superior and inferior gluteal nerves, which would cause weakness of hip abductors, hip extensors and internal rotators. Compartment syndrome is another mimicker and can present with pain, parasethesias and foot drop.
EMG and nerve conduction studies are key in localizing the lesion and predicting outcomes. Demyelinating injuries have an excellent prognosis, whereas injury through the myelin into the nerve axon will have a protracted and sometimes incomplete recovery.2
Tarsal tunnel syndrome
The distal tibial nerve can be compressed under the flexor retinaculum at the medial ankle causing sensory complaints and weakness of the foot intrinsics. This condition is quite rare, easily confused with plantar fasciitis, and often overdiagnosed. Patients with tarsal tunnel syndrome present with pain around the medial malleolus and burning of the sole of the foot often at night and with weight bearing.
Use of the EMG in entrapment syndromes
Nerve conduction studies are extremely helpful in showing focal entrapments. If only demyelination has taken place, there may be slowing of the nerve conduction and prolongation of the latency of the nerve action potentials. If further degradation of the nerve occurs, there will be loss of action potential amplitudes as well. The needle EMG is extremely important as it may demonstrate exactly which muscles are affected by a lesion of the nerve and therefore help to localize the portion of the nerve that’s been damaged. The EMG is also useful to identify if denervation of the muscle is ongoing. Even if there is complete loss of function, for example in a wrist drop, you may see the re-emergence of motor units. In partial lesions, polyphasic motor units will also show re-innervation.
Case revisited
Our patient underwent nerve conduction studies. The peroneal motor nerve studies, stimulating the nerve at the ankle, fibular head and popliteal fossa, were normal. The superficial peroneal nerve study was also normal, but this is stimulated at the distal tibia only. The patient was sent for an ultrasound and a focal hypoechoic area was seen just distal to the fibular head in the vicinity of the superficial peroneal nerve. A differential of a cyst or ganglion was suggested, but it wasn’t of typical density. An MRI was therefore ordered which showed a hyperintense 1 cm density at the same level, highly suggestive of a benign lesion, a neural sheath tumour.
Figure 1. Wasting of the intrinsics in the left hand is most noticeable in the first web space, due to loss of the first dorsal interosseous muscle.
Figure 2. The paper slips easily through the adducted fingers, due to the weak ulnar innervated palmar interossei.
Figure 3. Froment’s sign, the patient uses his intact median innervated flexor pollicis longus to grip the paper being pulled away by the examiner. His ulnar-innervated adductor pollicis is too weak to grip the paper.
Figure 4. Axial (left) and coronal T2 weighted MRI showing the location of the neural sheath tumour in the left superficial peroneal nerve. Tapping at that level caused a positive Tinel’s test.
References:
- Krivickas L, Wilbourn A. Sports and Peripheral Nerve Injuries: Report of 190 injuries evaluated in a single electromyography laboratory. Muscle and Nerve 1998 21:1092-4.
- Preston D, Shapiro B. Electromyography and Neuromuscular Disorders. 2005 Elsiever. Philadelphia.
- Toth C, McNeil S, Feasby T. Peripheral Nervous System Injuries. Sports Med 2005;35(8) 717-38.
