Mononeuropathies

Definition:

• A pathological process affecting a single peripheral nerve

Peripheral nerve compression or entrapment:

Damage to a nerve by compression can be:

• Acute (e.g. a tourniquet or other sustained pressure)
• Chronic (e.g. entrapment neuropathy)

In both the above, demyelination predominates (but some axonal degeneration occurs)

• Acute compression usually affects nerves which are exposed anatomically (e.g. the common peroneal nerve at the head of the fibula)
• Entrapment develops where a nerve passes through a relatively tight anatomical passage (e.g. the carpal tunnel)

Carpal tunnel syndrome:

This is caused by median nerve compression at the wrist. Many cases are idiopathic, but this syndrome is sometimes seen in:

• Hypothyroidism
• Diabetes mellitus
• Pregnancy
• Obesity
• RA
• Acromegaly

Clinical features:

• Nocturnal tingling/pain in the hand
• Weakness of the thenar muscles follows
• Wasting of abductor pollicis brevis develops
• Sensory loss of the palm and radial 3½ fingers
• Positive Tinel’s sign (tapping on the carpal tunnel reproduces the pain)

Treatment to cure is with surgical decompression

Ulnar nerve compression:

This typically occurs at the elbow, where the nerve is compressed in the cubital tunnel. It is usually caused by:

• Ulna fracture
• Prolonged/recurrent pressure on the nerve at this site

Clinical features:

• Wasting of hypothenar and interossei muscles
• Sensory loss in the ulnar 1½ fingers

Decompression and transposition of the nerve at the elbow may be necessary

Radial nerve compression (‘Saturday night palsy’):

The radial nerve is compressed acutely against the humerus, commonly when the arm is draped over a hard chair for several hours

Clinical features:

• Wrist-drop
• Weakness of finger extension and brachioradialis follow

Recovery is usual within 1-3 months