Mononeuropathies

Nerve disorder resulting from focal involvement of a single nerve trunk and usually caused by local lesions.

Aetiology

• Mechanical - eg trauma, compression, constriction.
• Entrapment.
• Diabetes mellitus.
• Hypothyroidism.
• Rheumatoid arthritis.
• Vitamin deficiencies.
• Vasculitis.
• Sarcoidosis.
• Amyloidosis.
• Pregnancy - associated with carpal tunnel syndrome.^[1]
• Other generalised neurological conditions - eg, hereditary neuropathy with liability to pressure palsies (HNPP), multifocal motor neuropathy and neuralgic amyotrophy.

Epidemiology

Carpal tunnel syndrome is the most common form of mononeuropathy. It is more common than tarsal tunnel syndrome. Carpal tunnel syndrome is one of the most disabling work-related conditions.

Presentation

Carpal tunnel syndrome

• Causes include entrapment, excessive use of wrist, tenosynovitis, local infiltration - eg, acromegaly, amyloidosis.
• Patients complain of nocturnal paraesthesia affecting the thumb, index and middle fingers.
• Reduced sensation in the same area.
• Severe cases - weakness and wasting of the abductor pollicis brevis.

Ulnar neuropathy

• Usually caused by a lesion at the ulnar groove or in the cubital tunnel, or prolonged pressure at the base of the hand (affecting the deep branch).
• May be iatrogenic following humeral fracture treatment.^[2]
• Claw hand deformity may be seen if there is complete paralysis in the fourth and fifth fingers with wasting and weakness of small muscles of the hand. This leads to hyperextension at the metacarpophalangeal (MCP) joints and flexion at the interphalangeal (IP) joints.
• Sensory loss over the fifth finger and the outer half of the fourth finger and palm.
• Weakness of small muscles without sensory loss occurs if the deep palmar branch of the ulnar nerve is affected.

Thoracic outlet syndrome

• Caused by compression of nerves of the brachial plexus - eg, presence of a fibrous band or cervical rib. See the separate article on Cervical Ribs and Thoracic Outlet Syndrome.
• Pain in the arm.
• Paraesthesia in the arm and hand (usually in C8 and T1 distribution).
• Atrophy of the hand.
• Weakness of the arm.
• Rarely, there may be cyanosis or oedema of the arm.

Tarsal tunnel syndrome

• Caused by ankle sprains and fractures, ill-fitting footwear, cysts, ganglia, arthritis, tenosynovitis.
• Pain in the ankle and the sole of the foot.
• Paraesthesia on walking.
• Tibial nerve trunk is tender (posterior to the medial malleolus).
• Sensory deficit on the foot.
• Weakness of toe plantar flexion.

Radial nerve compression

• Radial neuropathy usually results from compression against the humerus.
• Leads to 'Saturday night palsy'.
• Wrist and finger drop.
• Variable paraesthesia - look for sensory loss in the dorsal aspect at the root of the thumb.

Lateral femoral cutaneous nerve compression

• Leads to meralgia paraesthetica.
• Numbness in the lateral aspect of the thigh.
• Severe pain which restricts activities (reproduced by palpation under the anterior superior iliac spine).
• Paraesthesia in the anterolateral aspect of the thigh.
• No motor weakness.

Sciatic nerve damage

• Sciatic nerve damage can result from fractures of the pelvis or femur or from pelvic tumours.
• Paraesthesia over the lateral aspect of the leg below the knee.
• Weakness of the hamstring and all muscles below the knee.

Common peroneal nerve

• Caused by trauma or surgery.^{[3, 4]}
• Leads to foot drop.
• Weakness on the everting foot.
• Inability to extend the toes.
• Paraesthesia over the dorsum of the foot.

Single neuropathies affecting the thoracodorsal, dorsal scapular, suprascapular and medial pectoral nerves have been described in bodybuilders. Other mononeuropathies include Bell's palsy, interosseous nerve compression and femoral nerve entrapment.

An apparent mononeuropathy may be the presenting feature of a peripheral neuropathy.^[5]

Investigations

• Patients should have a full neurological examination and systemic examination. Investigations for any possible underlying cause may be required.
• Nerve conduction studies - eg, to assess whether the lesion is axonal or demyelinating, or whether any entrapment is present.
• Electromyography.
• Ultrasound may be useful in some situations - eg, compression neuropathy.^[6]
• MRI scan may help to evaluate brachial and lumbosacral neuropathies and may have more use in the future.^[7]
• A nerve biopsy may be required.

Management

Conservative management

Conservative management if:

• There is no history of trauma.
• Onset is sudden.
• There is no motor deficit.
• There are few or no sensory findings.
• There is no axonal degeneration on electrophysiological studies.

Conservative management begins with the use of simple analgesics - eg, non-steroidal anti-inflammatory drugs (NSAIDs). For carpal tunnel syndrome this also includes splinting and local steroid injection. However, good evidence for the use of these measures is still awaited.^[8] Steroid injection is also used for the lateral cutaneous nerve of the thigh.^[9]

Surgery

Decompression should be considered for the following:

• Chronic symptoms.
• Neurological deficit.
• Worsening of deficit with time.
• Wallerian degeneration on electrophysiological studies.
• Carpal tunnel syndrome and tarsal tunnel syndrome - commonly required for these.

Data from the Netherlands suggest that surgery is more cost-effective than splinting and is associated with better outcomes.^[10] This is supported by a Lancet study finding modest differences in outcomes between surgical and conservative management, in favour of surgery.^[11] Surgical treatment of carpal tunnel syndrome can be performed endoscopically or open - both have low rates of complications.^{[12, 13]}