Metabolic, toxic and vitamindeficiency neuropathies Medical Assignment Help

The commoner of these neuropathies . All are due to impairment of normal metabolism of the axon, myelin or both.
METABOLIC NEUROPATHIES Diabetes mellitus
Several varieties of neuropathy occur in diabetes mellitus:
1 Symmetrical sensory polyneuropathy
2 Acute painful neuropathy
3 Mononeuropathy and ultiple mononeuropathy
(a) Cranial nerve lesions
(b) Isolated peripheral nerve lesions, e.g. median
4 Diabetic amyotrophy
5 Autonomic neuropathy
.
Uraemia

Progressive sensorimotor neuropathy occurs in chronic uraemia. The response to dialysis is variable but the neuropathy usually improves after renal transplantation. Thyroid disease
A mild chronic sensorimotor neuropathy is sometimes seen in both hyperthyroidism and hypothyroidism. Porphyria
Acute intermittent porphyria is a rare metabolic disorder (see p. 866) in which there are episodes of a severe, mainly proximal, neuropathy, sometimes associated with abdominal pain, confusion and later coma. Alcohol and barbiturates may precipitate attacks.Amyloidosis.

Refsum’s disease

This is a rare condition inherited as an autosomal recessive trait. There is a sensorimotor polyneuropathy with ataxia, retinal damage and deafness. It is due to a defect in the metabolism of phytanic acid.

TOXIC NEUROPATHIES

Alcohol

A polyneuropathy, mainly in the lower limbs, occurs in chronic alcoholics. Calf pain is common. The response to abstention is variable. Thiamine should be given.

Drugs

Many drugs have been reported to be associated with apolyneuropathy. The more important ones.

Industrial toxins

A wide variety of industrial toxins have been shown tocause polyneuropathy:  LEAD POISONING causes a motor neuropathy. ACRYLAMIDE (PLASTICS INDUSTRY), TRICHLORETHYLENE (A SOLVENT), HEXANE AND OTHER FAT-SOLUBLE HYDROCARBONS (e.g. those inhaled in glue-sniffing, see p. 985) cause a progressive polyneuropathy. ARSENIC AND THALLIUM cause a polyneuropathy. VITAMIN-DEFICIENCY NEUROPATHIES Vitamin deficiencies are an important cause of disease of the nervous system because they are potentially reversible if treated early (and progressive if not). They occur in malnutrition, when they are commonly multiple. Thiamine (vitamin B\)
Deficiency causes the clinical syndrome of beri-beri (see p. 162). The principal features are polyneuropathy, an amnesic syndrome (Wernicke-Korsakoff psychosis) and cardiac failure. Alcohol abuse is the commonest cause in Western countries. Other neurological  onsequences of alcohol are summarized.

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WERNICKE-KoRSAKOFF SYNDROME. This important syndrome is an acute or gradual encephalopathy associated with alcohol abuse and other causes of thiamine deficiency. The typical triad comprises ocular signs, ataxia and a confusional state, but the condition occurs in partialforms. It is due to ischaemic damage to the brain stem and its connections. Clinical features include: OCULAR SIGNS. Nystagmus, bilateral lateral rectus palsies, conjugate gaze palsies, fixed pupils and, rarely,papilloedema are found. ATAX IA. There is a broad-based gait, cerebellar signs in the limbs and vestibular paralysis (absent response to
caloric stimulation). CONFUSION. Apathy, decreased awareness or restlessness, amnesia, stupor and coma occur. ypothermia and hypotension due to hypothalamic damage are rare findings.
The condition is underdiagnosed. Erythrocyte transketolase activity is reduced but is of limited practical value because the measurement is rarely available. Thiamine (i.m. or i.v.) should be given immediately if the diagnosis is in question: it is harmless, the condition is not. Untreated Wernicke-Korsakoff syndrome commonly leads to a severe irreversible amnesic syndrome and residual brain stem signs. Pyridoxine (vitamin B6) Deficiency causes a mainly sensory neuropathy (seep. 164). It may be precipitated during isoniazid therapy  for tuberculosis in those who acetylate the drug slowly.

Acute intoxication

Disturbance of balance, gait and speech Coma
Head injury and its sequelae
Alcohol withdrawal
‘Morning shakes’
Tremor of arms and legs
Delirium tremens
Thiamine deficiency
Polyneuropathy
Wernicke-Korsakoff syndrome
Epilepsy
Acute intoxication
Alcohol withdrawal
Hypoglycaemia
Cerebellar degeneration
Cerebral infarction
Cerebra I atrophy
Dementia
Central pontine myelinolysis
Marchiafava-Bignami syndrome (a rare degeneration of the corpus callosum)

Vitamin B12

Deficiency causes disease of the brain, spinal cord and peripheral nerves.
SUBACUTE COMBINED DEGENERATION OF THE CORD. This syndrome of the spinal cord is a sequel of Addisonian pernicious anaemia and rarely other causes of vitamin B12 deficiency (see p. 306). It is frequently associated with a polyneuropathy. The patient complains initially of numbness and tingling of the extremities. The signs are of distal sensory loss (particularly posterior column) with absent ankle jerks (due to the neuropathy) combined with evidence of cord disease (exaggerated knee-jerk reflexes, extensor plantar responses). Optic atrophy and retinal haemorrhage may occur. In the later stages sphincter disturbance, weakness and dementia are seen.
Macrocytosis and megaloblastic changes in the bone arrow are invariable in subacute combined degeneration of the cord. The cause of the B’2 deficiency should be established (see p. 305). Treatment with parenteral B12 reverses the peripheral nerve damage but has little effecton the eN5 (cord and brain) signs.  Neoplastic neuropathy
Polyneuropathy is sometimes seen as a non-metastatic manifestation of malignancy (see p. 950). In myeloma and other dysproteinaemic states polyneuropathy occurs, probably owing to impaired perfusion of nerve trunks or to demyelination associated with allergic reactions within peripheral nerves. P0 EM5 s YN D ROM E is characterized by a chronic
inflammatory demelinating Polyneuropathy, Organomegaly (hepatomegaly 50%), Endocrinopathy (gynaecomastia and atrophic testes), an M protein band on electrophoresis with less than 5% of plasma cells in the bone marrow and Skin hyperpigmentation. Chemotherapy is used.
Neuropathies associated with connective tissue diseases Neuropathy occurs in 5LE, polyarteritis nodosa, rheumatoid disease and giant-cell arteritis owing to microinfarction of peripheral nerves. This presents either as a multiple mononeuropathy or a symmetrical sensorimotor neuropathy.

Autonomic neuropathy

Autonomic neuropathy causes postural hypotension, retention of urine, impotence, diarrhoea (or occasionally constipation), diminished sweating, impaired pupillary responses and cardiac arrhythmia. Many varieties of neuropathy affect autonomic function to a mild, and often subclinical, degree. Occasionally, when there is damage to small myelinated and nonmyelinated B and C fibres, the clinical features of the
autonomic neuropathy predominate. This situation may occur in diabetes mellitus, in amyloidosis and in the Guillain- Barre syndrome.

Cervical rib (thoracic outlet syndrome)

A fibrous band or cervical rib extending from the tip of the transverse process of C7 to the first rib stretches the lower roots of the brachial plexus (C8 and Tl). There is pain along the ulnar border of the forearm, and sensory loss initially in the distribution of Tl with wasting of thethenar muscles, principally the abductor pollicis brevis mu scle. Horner’s syndrome may occur. The rib or band can be excised.
In other patients the rib or band causes subclavan artery or venous occlusion. The neurological and vascular problems rarely occur together.
Neuralgic amyotrophy This is a condition in which severe pain in the muscles of the shoulder is followed by wasting, usually of theinfraspinatus, supraspinatus, deltoid and serratus anterior
mu cles (a ‘brachial plexus neuropathy’). The cause is unknown but, since the condition follows viral infection or immunization in some cases, an allergic basis is postulated.
Recovery of the wasted muscles occurs over some months.

Malignant infiltration

Metastatic disease of nerve roots of the brachial or lumbosacral plexus causes a painful radiculopathy.

Plexus

Trauma
Malignant infiltration
Cervical rib
Neuralgic amyotrophy
Nerve root
Trauma
Herpes zoster
Meningeal inflammation (e.g. syphilis)
Tumours (neurofibroma, metastases)
Cervical and lumbar spondylosis

A common example is an apical bronchial neoplasm (Pancoast’s tumour) that causes a T1 lesion and involves the sympathetic outflow. There is wasting of the small muscles of the hand, pain and sensory loss in areas supplied by Tl and ipsilateral Horner’s syndrome. This condition also occasionally occurs in apical tuberculosis.

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