The onset may be rapid or very gradual and not become obvious for months or years, although most masses eventually cause varying combinations of headache and vomiting – symptoms of raised pressure within the inexpansible bony box formed by the skull; general or localised epileptic fits; weakness of limbs or disordered speech; and varied mental changes. Tumours may be primary, arising in the brain, or secondary deposits which have spread from tumours arising in the lung, breast or other organs. Some brain tumours are benign and curable by surgery: examples include meningiomas and pituitary tumours. The symptoms depend on the size and situation of the mass. Abscesses or blood clots (HAEMATOMAS) on the surface or within the brain may cause the same symptoms as tumours; some are removable. GLIOMAS are primary malignant tumours arising in the glial tissue (see GLIA) which despite surgery, chemotherapy and radiotherapy usually have a poor PROGNOSIS, though some astrocytomas and oligodendronogliomas are of low-grade malignancy.

Clinical examination and brain scanning (CT, or COMPUTED TOMOGRAPHY; Magnetic Resonance Imaging (MRI) and functional MRI) are safe, accurate methods of demonstrating the tumour – its size, position and treatability.

When a blood vessel, usually an artery, is blocked by a clot, formed at the site (thrombus) or washed there from some other part of the body (embolus), the area of the brain fed by that artery is damaged (see STROKE). The resulting infarct (death of cells) causes a stroke as the cells die in that part of the brain tissue. The obstruction in the blood vessel may be in a small artery in the brain, or in a larger artery in the neck. Aspirin and other anti-clotting drugs reduce recurrent attacks, and some people benefit if a narrowed neck artery is cleaned out by an operation – endarterectomy. Similar symptoms develop abruptly if a blood vessel bursts, causing a cerebral haemorrhage. The symptoms of a stroke are sudden weakness or paralysis of the arm and leg of the opposite side to the damaged area of brain (HEMIPARESIS), and sometimes loss of half of the field of vision to one side (HEMIANOPIA). The speech area is in the left side of the brain, controlling language in right-handed people. In 60 per cent of left-handers the speech area is on the left side, and in 40 per cent on the right side. If the speech area is damaged, difficulties both in understanding words, and in saying them, develop (see DYSPHASIA).

For reasons often unknown, various groups of nerve cells may degenerate prematurely. The results are determined by which groups of nerve cells are affected. If those in the deep basal ganglia are affected, a movement disorder occurs, such as Parkinson's disease, hereditary Huntington's chorea, or, in children with CEREBRAL PALSY, athetosis and dystonias. Modern drugs, such as DOPAMINE drugs in PARKINSONISM, and other treatments can improve the symptoms and reduce the disabilities caused by some of these conditions.

Alcohol in excess, the abuse of many sedative drugs and artificial brain stimulants – such as cocaine, LSD and heroin (see DEPENDENCE) – can damage the brain; the effects can be reversible in early cases. Severe head injury can cause localised or diffuse brain damage (see HEAD INJURY).

Damage to the brain can occur before or during birth or can result in infancy from rare hereditary and genetic diseases. Severe neurological illness such as MENINGITIS can also cause this condition, in which there is abnormal muscle tone and mobility and speech and language defects are common. Some of these children are learning-disabled.

In older people a diffuse loss of cells, mainly at the front of the brain, causes ALZHEIMER'S DISEASE – the main feature being loss of memory, attention and reasoned judgement. This affects about 5 per cent of the over-80s, but is not simply due to ageing processes. Most patients require routine tests and brain scanning to indicate other, treatable causes of dementia.

Like PARKINSONISM, Alzheimer's disease progresses slowly over many years. It is uncommon for these diseases to run in families. Multiple strokes can cause dementia, as can some organic disorders such as cirrhosis of the liver.

in the brain are uncommon. Viruses such as measles, mumps, herpes, human immunodeficiency virus and enteroviruses may cause ENCEPHALITIS – a diffuse inflammation (see also AIDS/HIV).

Bacteria or viruses may infect the membrane covering the brain, causing MENINGITIS. Viral meningitis is normally a mild infection lasting only a few days, although there can be serious side-effects – for example, mumps meningitis may lead to severe deafness. However, bacterial meningitis – caused by meningococcal groups B and C, pneumococcus (less commonly because of immunisation) and (rarely) haemophilus – is potentially life-threatening. With efficient supportive care and high dose intravenous antibiotics, nearly all should survive unless they have co-existing, and more dangerous, SEPTICAEMIA. Severe headaches, fever, vomiting and increasing confusion, sleepiness or unrousability are the principal symptoms which demand urgent advice from the doctor, and usually admission to hospital. Group B meningococcus is the commonest of the bacterial infections, but Group C causes more deaths. A vaccine against the latter has been developed and has reduced the incidence of cases by 75 per cent.

If infection spreads from an unusually serious sinusitis or from a chronically infected middle ear, or from a penetrating injury of the skull, an abscess may slowly develop. Brain abscesses cause insidious drowsiness, headaches, and at a late stage, weakness of the limbs or loss of speech; a high temperature is seldom present. Early diagnosis, confirmed by brain scanning, is followed by antibiotics and surgery in hospital, but the outcome is good in only half of affected patients.

Swelling of the brain can occur after injury, due to engorgement of blood vessels or an increase in the volume of the extravascular brain tissue due to abnormal uptake of water by the damaged grey (neurons) and white (nerve fibres) matter. It can seriously affect the functioning of the brain and is a particularly dangerous complication following injury because sometimes an unconscious person whose brain is damaged may seem to be recovering after a few hours, only to have a major relapse. This may be the result of a slow haemorrhage from damaged blood vessels raising intracranial pressure, or because of oedema of the brain tissue in the area surrounding the injury. Such a development is potentially lethal and requires urgent specialist treatment to alleviate the rising intracranial pressure: osmotic agents (see OSMOSIS) such as mannitol or frusemide are given intravenously to remove the excess water from the brain and to lower intracranial pressure, buying time for definitive investigation of the cranial damage. Cerebral oedema can also occur rarely as a complication of diabetic coma or of water intoxication, for example due to excessive dilute fluids being given in an intravenous drip.