The different nerve fibre types within a peripheral nerveare shown in Table 18.49. All are myelinated except the C fibres, which carry impulses from painful stimuli. Two pathological processes affect peripheral nervesaxonal (the axon itself) degeneration and demyelination(the myelin sheath). Neuropathies are classified broadly into which of the two processes predominate. Wallerian degeneration refers to the changes seen after section of a nerve.
Axonal degeneration occurs after a nerve has been sectioned or its axon severely damaged. Within 7-10 days the axon and the myelin sheath distal to the injur degenerate and ar in excitable electrically. When a motor nerve is damaged there is atrophy of the muscle fibres inthe motor units it supplies. Denervation may be detected by recording fibrillation potentials on electromyography. Regeneration occurs by axonal growth down the nerve sheath and axonal sprouting from the stump. Growth takes place at a rate of up to 1 mm daily. In a chronic neurop athic process (e.g. MND or polyneuropathy), sprouts from the terminal axons of normal motor axons reinnervate the denervated muscle fibres; giant polyphasic units are then seen on electromyography. Demyelination
Here damage to the myelin sheath (the axon itself is nitially preserved) causes conduction block or marked lowing of conduction; this can be used to distinguish demyelination from axonal degeneration. Local demyelination is caused by pressure (compression and entrapment neuropathies) or by inflammation (e.g. Guillain-
NEUROPATHY means a pathological process affecting a peripheral nerve or nerves.
MONONEUROPATHY is a process affecting a single nerve, and multiple mononeuropathy (or mononeuritis multiplex) is a process affecting several or multiple nerves.
POLYNEUROPATHY is a diffuse, symmetrical disease process, usually progressing proximally. It is either acute, subacute or chronic. Its course may be progressive, relapsing or towards recovery. Polyneuropathy may be motor, sensory, sensorimotor (mixed) or autonomic.RADIe ULOPATHY means a disease process affecting the nerve roots.
Peripheral nerve compression and
entrapment (Table 18.50)
Damage to a nerve by compression is either acute (e.g. due to a tourniquet or other sustained pressure) or chronic (entrapment neuropathy). In both, 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 occurs where a nerve passes through relatively tight anatomical passages (e.g. the carpal tunnel). These conditions are diagnosed largely from the clinical features. Diagnosis is confirmed by nerve conduction studies and electromyography. The commoner conditions are mentioned below.
Median nerve compression at the wrist (carpal tunnel syndrome)
This common syndrome is sometimes seen in:
• Diabetes mellitus
• Pregnancy and obesity
• Rheumatoid arthritis
The condition is, however, usually idiopathic. It causes nocturnal tingling and pain in the hand (and sometimes forearm) followed by weakness of the thenar muscles. Wasting of abductor pollicis brevis develops, with sensory loss of the palm and radial three-and-a-half fingers. Tinel’s sign may be positive, i.e. tapping on the carpaltunnel will reproduce the pain. Treatment with a splint at night or a local steroid injection in the wrist gives temporary relief. When the condition occurs in pregnancy (due to fluid retention) it is often self-limiting. Surgical decompression of the carpel tunnel is a simple and definitive treatment.
Ulnar nerve compression This typically occurs at the elbow, where the nerve is compressed in the cubital tunnel. It follows fracture ofthe ulna or prolonged or recurrent pressure on the nerve at this site.
Wasting of the ulnar-innervated muscles develops (hypothenar muscles and interossei) together with sensory loss in the ulnar one-and-a-half fingers. Decompression and transposition of the nerve at theelbow may be necessary. The deep (solely motor) branch of the ulnar nerve maybe damaged in the palm by recurrent pressure from tools (e.g. screwdrivers), crutches or cycle handlebars. Radial nerve compression
The radial nerve is compressed acutely against the humerus, e.g. when the arm is draped over a hard chair for several hours (‘Saturday night palsy’). Wrist drop and weakness of finger extension and of the brachioradialis muscle follow. Recovery is usual within 1-3 months. Meralgia paraesthetica
Burning, tingling and numbness on the anterolateral aspect of the thigh is caused by entrapment of the lateral cutaneous nerve of the thigh beneath the inguinal ligament.
Many of the patients are obese; weight reduction helps to relieve the symptoms. Division of the inguinal ligament is not usually effective.
Common peroneal nerve palsy When the common peroneal nerve is compressed against the head of the fibula (owing to prolonged squatting,
wearing plaster casts, prolonged bed rest or coma) there is foot drop and weakness of eversion. A patch of numbness on the anterolateral border of the shin or dorsum of the foot may be found. Recovery is usual (but not invariable) within several months. Multiple mononeuropathy
(mononeuritis multiplex) Multiple mononeuropathy occurs in:
• Diabetes mellitus
• Leprosy (still the commonest worldwide)
• Connective tissue disease (polyarteritis nodosa, SLE, giant-cell arteritis, rheumatoid arthritis)
Diagnosis is largely clinical, supported by electrical studies.
Treatment is that of the underlying disease.
Although many toxins and disease processes are known to be associated with polyneuropathy (see below), the cause of the majority of cases remains undetermined. The commonest presentation is a chronic or subacute sensorimotor neuropathy. A classification of polyneuropathy. Idiopathic chronic sensorimotor neu ropath ies
The patient complains of a progressive symmetrical umbness and tingling in the hands and feet, which spreads proximally in a ‘glove and stocking’ distribution. There is distal weakness, which also ascends. Rarely the cranial nerves may be affected. Tendon reflexes involving affected nerves are lost. The symptoms may progress over many months, remain static or remit at any stage. Autonomicfeatures are sometimes seen.
Investigati on (nerve conduction studies, electromyography) of the neuropathy shows either axonal degeneration or demyelination, or features of both of these processes. Some cases of demyelinating (not axonal) chronic sensorimotor neuropathy respond to steroid or immunosuppressive therapy.
Idiopathic chronic sensorimotor neuropathies Postinfective neuropathy (Guillain-Barre syndrome) Drugsltoxic, metabolic and vitamin-deficiency neuropathies
Neuropathies in connective tissue diseases Autonomic neuropathies
Hereditary sensorimotor neuropathies ostinfective polyneu ropathy
(Guillain-Barre syndrome, acute inflammatory neuropathy)
This demyelinating neuropathy, which is the commonestrecognizable acute neuropathy has an autoallergic basis. It follows 1-3 weeks after a viral infection that is often trivial. The patient complains of weakness of distal limb muscles or distal numbness. This ascends over several days or weeks. In mild cases there is little disability, but in some 20% of cases the respiratory and facial muscles are affected and the patient may become paralysed. Weakness, areflexia and sensory loss are found (i.e. an ascending weakness).
Autonomic features (see below) are sometimes seen. There is a rare proximal form of the condition that initially affects the ocular muscles and in which ataxia is found (Miller-Fisher syndrome).
DIAGNOSIS This is established on clinical grounds and is confirmed by nerve conduction studies, which show the slowing of conduction or conduction block seen in demyelinating neuropathies. The CSF contains a normal cell count and sugar level but the protein is frequently raised to 1-3 g litre “,
The differential diagnosis includes other paralytic illnesses such as poliomyelitis, botulism or primary muscle disease.
TREATMENT AND COURSE
High dosage intravenous y-globulin reduces the duration and severity and should be given to all patients. Supportive treatment is essential, with particular attention being paid to compensating for weakness of the
respiratory and bulbar muscles. Ventilation may be necessary.
Corticosteroids are sometimes used to treat the Guillain-Barre syndrome but are not of proven value. Plasmapheresis has been shown to be of proven benefit in shortening the period of disability.
Recovery, though gradual over many months, is usual but may be incomplete.
Demyelinating neuropathy is sometimes caused by the exotoxin of Corynebacterium diphtheriae. Palatal weakness followed by pupillary paralysis and a sensorimotor neuropathy occur several weeks after faucial infection. The condition is now rare in countries where immunization against diphtheria is practised.