Effects of poisons on the brain

 Many chemicals, including drugs, environmental toxins and high levels of metabolites (see above) are damaging to the central nervous system. Neurone function may be disturbed either by damage to the neurone itself or be secondary to dysfunction of other organs, e.g. liver, kidneys. The outcome depends on the toxicity of the sub-stance, the dose and the duration of exposure. This may: range from minor short-term neurological disturbance,, e.g. hypoglycaemia in diabetes mellitus  to, encephalopathy, which may cause coma and death as a consequence of liver failure . 

The brain and spinal cord are relatively well protected from microbial infection by the blood-brain barrier. The micro-organisms usually involved are bacteria and viruses, occasionally protozoa and fungi. The infection may originate in the meninges (meningitis) or in the brain (encephalitis), then spread from one site to the other. 

Bacterial infections 
Entry of bacteria into the CNS may be: 
• direct - through a compound skull fracture or through the skull bones from, e.g., middle  ear or paranasal sinus infections, niastoiditis 
• blood-borne - from infection elsewhere in the body, e.g. septicaemia, bacterial endocarditis 
• iatrogenic - introduced during an invasive procedure, e.g. lumbar puncture.
Bacterial meningitis The term 'meningitis' usually refers to inflammation of the subarachnoid space and is most commonly transmit-ted through contact with an infected individual. Bacte-rial meningitis is usually preceded by a mild upper respiratory tract infection during which a few bacteria enter the bloodstream and are carried to the meninges. Common microbes include: 
• Haemophilus influenzae in children between the ages of 2 and 5 years 
• Neisseria meningitidis in those between 5 and 30 years, the most common type 
• Streptococcus pneumoniae in people over 30 years. 

Viral infections 
Entry of viruses into the CNS is usually blood-borne from viral infection elsewhere in the body and, less com-monly, through the nervous system. In the latter situa-tion, neurotropic viruses, i.e. those with an affinity for nervous tissue, travel along peripheral nerve from a site elsewhere, e.g. poliovirus. They enter the body via: 
• the alimentary tract, e.g. poliomyelitis 
• the respiratory tract, e.g. shingles 
• skin abrasions, e.g. rabies. The effects of viral infections vary according to the site and the amount of tissue destroyed. Viruses may damage neurones by: 
• multiplying within them 
• stimulating an immune reaction which may explain why signs of some infections do not appear until there is a high antibody titre, 1 to 2 weeks after infection.
Viral meningitis 
This is the most common form of meningitis and is usually a relatively mild infection followed by complete recovery.
Viral encephalitis Viral encephalitis is rare and usually associated with a recent viral infection. Most cases are mild and recovery is usually complete. More serious cases are usually asso-ciated with rabies or Herpes simplex viruses. A wide vari-ety of sites can be affected and, as neurones cannot be replaced, loss of function reflects the extent of damage. In severe infections neurones and neuroglia may be affected, followed by necrosis and gliosis. If the individ-ual survives the initial acute phase there may be residual dysfunction, e.g. behavioural disturbances and demen-tia. If vital centres in the medulla are involved the condi-tion can be fatal.
Herpes zoster (shingles)
Herpes zoster viruses cause chickenpox (varicella) mainly in children and shingles (zoster) in adults. Sus-ceptible children may contract chickenpox from a per-son with shingles but not the reverse. Adults infected with the viruses may show no immediate signs of dis-ease. The viruses may remain dormant in posterior root ganglia of the spinal nerves then become active years later, causing shingles. Reactivation may be either spon-taneous or associated with intercurrent illness or depression of the immune system, e.g. by drugs, old age, AIDS. The posterior root ganglion becomes acutely inflamed. From there the viruses travel along the sensory nerve to the surface tissues supplied, e.g. skin, cornea. The infection is usually unilateral and the most common sites are: