Mononeuropathies

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

• Mechanical - e.g. trauma, compression, constriction
• Entrapment
• Diabetes mellitus
• Hypothyroidism
• Rheumatoid arthritis
• Vitamin deficiencies
• Vasculitis
• Sarcoidosis
• Amyloidosis
• Pregnancy - associated with carpal tunnel syndrome
• Other generalised neurological conditions e.g. hereditary neuropathy with liability to pressure palsies (HNPP), multifocal motor neuropathy and neuralgic amyotrophy¹

Carpal tunnel syndrome is the commonest form of mononeuropathy. It is more common than tarsal tunnel syndrome. Carpal tunnel syndrome is one of the most disabling work-related conditions in the US resulting from repetitive strain injury.

Carpal tunnel syndrome

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

Ulnar neuropathy

• Usually caused by a lesion at the ulnar groove or in the cubital tunnel or prolonged pressure at base of hand (affecting the deep branch)
• Claw hand deformity (fourth and fifth fingers) if complete paralysis (wasting and weakness of small muscles of the hand leading to hyperextension at MCP joints and flexion at IP joints)
• Sensory loss over fifth finger and outer half of fourth finger and palm
• Weakness of small muscles without sensory loss occurs if deep palmar branch of ulnar nerve affected

Thoracic outlet syndrome

• Caused by compression of nerves of the brachial plexus e.g. presence of fibrous band or cervical rib
• Pain in arm
• Paraesthesia in arm and hand (usually in C8 and T1 distribution)
• Atrophy of hand
• Weakness of 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 ankle and sole of foot
• Paraesthesia on walking
• Tibial nerve trunk is tender (posterior to medial malleolus)
• Sensory deficit on 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 dorsal aspect at root of thumb

Lateral femoral cutaneous nerve compression

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

Sciatic nerve damage

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

Common peroneal nerve

• Caused by trauma
• Leads to foot drop
• Weakness on everting foot
• Inability to extend toes
• Paraesthesia over dorsum of foot

Single neuropathies affecting thoracodorsal, dorsal scapular, suprascapular and medial pectoral nerves have been described in body builders.¹ Other mononeuropathies include Bell's palsy, interosseus nerve compression and femoral nerve entrapment.

• Patients should have a full neurological examination and systemic examination
• Nerve conduction studies - is the lesion axonal or demyelinating; any entrapment present?
• Electromyography
• Any evidence of systemic causes?
• MRI may help evaluate brachial and lumbosacral neuropathies and may have more use in the future²
• If any doubt then a nerve biopsy may be indicated

Conservative management

Conservative management if:

• No history of trauma
• Sudden onset
• No motor deficit
• Few or no sensory findings
• No axonal degeneration on electrophysiological studies

Conservative management begins with the use of simple analgesics e.g. NSAIDs. For carpal tunnel syndrome this also includes splinting and local steroid injection. However the evidence for the use of these measures has not been confirmed.¹ Low dose oral steroids have been used with better outcomes than other conservative measures.¹
However, this has been contradicted by a recent study that failed to find any difference between splinting and oral steroids.³ A more novel therapy is the injection of botulinum toxin to relieve pain in carpal tunnel syndrome with promising initial results - however no randomised controlled trials at present exist.⁴

Surgery

Decompression should be considered if:

• Chronic symptoms
• Neurologic deficit
• Worsening of deficit with time
• Wallerian degeneration on electrophysiological studies
• Commonly required in carpal tunnel syndrome and tarsal tunnel syndrome

Recent data from the Netherlands suggest that surgery is more cost-effective than splinting and associated with better outcomes.⁵ Surgical treatment of carpal tunnel syndrome can be performed endoscopically or open, both have low rates of complications.^6,7

Failure of procedure with recurrence of symptoms occurs in <3% of cases in Carpal tunnel syndrome.¹