Disorders of Peripheral nerves
Lesions of peripheral nerves
Lesions of one or more peripheral nerves cause a characteristic motor and sensory loss. In some cases there is a pure motor or pure sensory deficit, but in most there is a combination of both.
A neuropathy means a pathological process affecting a peripheral nerve. Damage to the peripheral nerves are caused by a number of mechanisms, principally
· demyelination, axonal loss,
· compression or traumatic sectioning of a nerve,
· ischaemia and
Mononeuropathies: Involvement of a single nerve. Traumatic peripheral nerve injuries may result from compression, penetrating trauma or closed fractures and dislocations. Traumatic nerve damage may result in:
· Neuropraxia, a transient loss of physiological function with no loss in continuity and no degeneration. Acute compression of the nerve causes focal (segmental) demyelination, but once the compression is relieved, recovery is usual within 6 weeks.
· Axonotmesis, which follows more severe compression or traction damage, with Wallerian degeneration of the nerve distal to the injury. The time taken to recover depends on the length of nerve needed to regrow down the nerve sheath. Excessive fibrosis (scarring) hinders growth.
· Neurotmesis is division of a nerve, following which there is distal Wallerian degeneration. The nerve bundle is interrupted, ingrowth of fibrous tissue prevents reinnervation, so that surgical repair is needed if function is to be restored. Ideally, immediate repair with end to end suture is undertaken with a reasonable prognosis. However if there is contamination the nerve ends are marked with non-absorbable sutures and after 2–3 weeks the nerve is surgically repaired – good recovery of function is rare.
Any cause of mononeuritis multiplex may also present initially as a mononeuropathy.
Mononeuritis multiplex: An uncommon form of neu-ropathy where two or more peripheral nerves are affected either together or sequentially. If symmetrical nerves are affected it may mimic a polyneuropathy. The main causes are diabetes mellitus, malignancy, amyloidosis, polyarteritis nodosa, connective tissue disorders, HIV infection and leprosy (commonest cause worldwide).
Peripheral neuropathy: A symmetrical disorder of peripheral nerves, usually distal more than proximal. It excludes cranial nerve palsies, mononeuropathies, mononeuritis multiplex and bilateral single nerve lesions. The commonest causes are
· Diabetes mellitus.
· Malignancy (e.g. lung, leukaemia, lymphoma, myeloma).
· Vitamin B deficiency (Thiamine (B1) deficiency in alcoholics, Vitamin B12 deficiency).
· Drugs (e.g. isoniazid, phenytoin, nitrofurantoin, vincristine, cisplatin).
Other rare causes include uraemia; hypothyroidism; systemic diseases and vasculitis, e.g. sarcoid, systemic lupus erythematosus, amyloidosis, polyarteritis nodosa; toxins such as lead (motor), arsenic & thallium (initially sensory); infections such as leprosy, diphtheria; Guillain– Barre´ syndrome (acute inflammatory or postinfective polyneuropathy).
Radiculopathy: Damage to one or more nerve roots or a nerve plexus. The most important causes are trauma, compression (e.g. prolpased intervertebral disc, cervical or lumbar spondylosis or neurofibroma), malignant infiltration and herpes zoster.