Usual suspect: Smoking can?t always be blamed for strokes

Quitting cigarettes, treating high blood pressure, and managing diabetes make up the traditional strategies to prevent stroke. But a new set of risk factors now explain stroke in young people below 40. Doctors at the National Institute of Mental Health and Neurosciences (NIMHANS) in Bangalore say genetics, infections, and even nutrition are among the emerging unconventional causes that need to be further investigated.

About a third of strokes in young people below the age of 40 remain unexplained through conventional risk factors. ?We need to pinpoint all the factors to develop effective prevention strategies,? said Dr Rita Christopher, additional professor in the department of neurochemistry. Stroke, a condition in which the blood flow to the brain is temporarily disrupted, is most often caused by a clot which blocks an artery or a blood vessel in the brain, although some strokes can also be caused by bleeding in the brain. The mechanisms that culminate in stroke appear to be a lot more complicated than previously thought.

?Multiple genetic as well as environmental factors act individually or in combination to cause stroke,? Christopher said at a scientific conference last week in New Delhi organized by the Ranbaxy Science Foundation. She said studies at the NIMHANS provided fresh evidence to link stroke in the young with the emerging risk factors. One study by researchers at the institute indicated that 43 per cent of stroke patients under 40 had recent infections prior to the stroke, the most common being respiratory infections.

?Some infections can trigger a cascade of biochemical events that might lead clot formation and stroke,? said Christopher. Foreign studies have hinted at connections between stroke and infections.

One study at Canada?s McMaster University had indicated that people who had been exposed to four common disease-causing bacteria or viruses have a higher risk of heart attack, stroke or cardiovascular death. The study found that people who have antibodies to the four microorganisms ? Chlamydia pneumoniae, cytomegalovirus, Helicobacter pylori, and Hepatitis A virus ? were 41 per cent more likely to suffer a heart attack, or a stroke or die from cardiovascular disease than people with antibodies to none or just one of these microorganisms.

Another study by a team at the University of Texas a few years ago had found that certain strains of the ulcer-causing germ, Helicobacter pylori, are common in the bloodstreams of people who have suffered a stroke. Some strains of Helicobacter pylori produce chemicals called cytotoxins that attack cells, cause tissue inflammation and lesions in the arteries leading to the brain. This leads to a restricting blood flow, increasing the chance of a stroke, the researchers reported in the journal Circulation.

The NIMHANS studies have corroborated a link between autoimmune, or self-attacking, antibodies called antiphospholipid antibodies and stroke (23 per cent of stroke patients below 40 had these antibodies). The body usually produces antibodies in response to infections, but sometimes antibodies turn on normal cells in the body. The antiphospholipid antibodies are one such class of antibodies. The primary trigger for these antibodies is still unknown, said Christopher, but they can attach themselves to the walls of blood vessels, leading to changes that favour clot formation. They can also bind to platelets in the blood, which can form the clots.

Doctors think the risk factors need to be investigated further to determine how they act collectively. ?More studies with larger samples of patients from different cities are needed before we can apply such risk factors for prevention strategies,? said Dr Prakash Tandon, former head, neurosurgery, All India Institute of Medical Sciences, and now president of the National Brain Research Centre, New Delhi.

Christopher said another emerging risk factor for stroke in the young is the high levels of the amino acid homocysteine. In one survey conducted by NIMHANS, doctors found that 40 per cent of stroke patients under 40 had high levels of homocysteine compared to only 17 per cent normal people found to have high levels of this amino acid.

High homocysteine levels may be a consequence of genetics or nutritional deficiencies. Deficiency in vitamin B12 can prevent appropriate processing of homocysteine in the body, leading to elevated levels of this molecule. Homocysteine can lead to the production of other molecules that can damage blood vessels and favour clotting mechanisms.

While some researchers have suggested that a vegetarian diet ? low in these vitamins ? may lead to high homocysteine levels, surveys indicate that homocysteine levels in normal people vary widely across India. ?The high ethnic, geographic, and dietary variations may be one reason why there is such high variations, although they may also be due to differences in the way the surveys were done,? said Dr Gulshan Ahuja, a senior neurologist at the Sitaram Bharatiya Institute in New Delhi.