A bacterial brain abscess usually now results from blood-borne bacteria which are deposited in the cerebral hemispheres in a patient who is acutely ill with septicaemia. However, most blood-borne bacteria from an infection are filtered out from venous blood as this passes through the capillaries of the lungs. An exception occurs if there is a hole between right and left sides of the heart. Some bacteria may then pass directly from the venous circulation into the left ventricle and into the cerebral circulation. This accounts for the high incidence of cerebral abscess in those with these types of congenital heart disease.

An abscess may also form by direct extension into the brain from a local infection—for example, severe middle ear suppuration or frontal sinusitis may cause abscesses respectively in the temporal or frontal lobes of the brain.

Acute abscesses can certainly cause epileptic seizures, but, even if successfully treated by drainage and by antibiotics, further seizures may arise from the scar. In an attempt to avoid this, many surgeons now excise totally the capsule of the abscess rather than simply aspirate the pus.

Viral meningitis is a self-limiting illness, and epilepsy does not occur after this. Sometimes, however, the viruses are present within the substance of the brain, rather than remaining confined to the surface. This is called encephalitis, and seizures may result. Two of the more common viruses causing seizures in this way are the herpes and cytomegaloviruses. Cytomegalovirus usually affects the fetal (unborn baby’s) brain, and the herpes virus usually affects infants, young children, and adults. The HIV virus can also cause seizures, either by itself or, through depressing the immune system, allowing invasion of the brain by other viruses, and often by a small organism known as Toxoplasma.

Some viruses behave in a very strange way in the brain. Measles, for example, is an illness which affects nearly all children without significant late effects. The illness is terminated by the production of antibodies. A tiny number of children, however, do not succeed in eradicating the virus from their brains, and, some years later a new measles-related illness begins—sub-acute sclerosing pan-encephalitis—in which seizures and mental deterioration are prominent. Fortunately this is now becoming very rare with the wider use of measles vaccine.

Parasites can also cause epilepsy. The pork tapeworm Taenia solium may cause epilepsy if the cystic stage of the tapeworm, usually found in pigs, occurs in the muscles and brain of man. In developing countries, calcified cysts are found in the brains of many of the rural population and this disorder, cysticercosis, and tuberculosis accounts for a lot of the greater incidence of epilepsy in such populations. The dog tapeworm Toxocara has also been incriminated in the development of epilepsy, though with less certain evidence. Toxoplasmosis, possibly acquired through infection in utero from domestic animals is certainly associated with seizures.

Creutzfeldt-Jakob disease is a rare disorder caused by an infectious agent which is not a bacteria or a virus. One route of transmission is through surgical instruments (ordinary sterilization does not kill it) or through tissue transplantation (for example, corneal transplantation). It is related to ‘mad-cow, disease (BSE, bovine spongiform encephalopathy). Affected adults may have seizures as part of the serious neurological illness.

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