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Mar

HEADACHES: SUB-ARACHNOID HAEMORRHAGE

The arachnoid is one of three layers of tissue which cover the brain in much the same way that a loaf of bread is wrapped in thin cellophane. The main blood vessels supplying the brain itself run between the arachnoid and the next layer of covering. In a sub-arachnoid haemorrhage, one of these blood vessels bursts, releasing blood into the gap between the arachnoid and the brain. In essence, it’s a burst blood vessel on the outside surface of the brain. Blood squirts out from the artery under high pressure, and because the skull is a closed cavity with soft tissue inside, as the blood cannot expand outwards, instead it pushes on the delicate brain substance, raising the pressure inside the head. The brain is squeezed in the process and the pressure inside the brain rises progressively until it stops functioning properly.

Genetically, certain people are prone to having little balloon-like blebs, called berry aneurysms, on the arteries supplying the brain, and because these blebs have thinner walls than the arteries, they are more prone to rupture. Arteries supply blood, so the blebs are under high pressure, making them more likely to rupture, especially in someone who has high blood pressure. When an aneurysm ruptures, the blood spurts out with some force into the surrounding space overlying the brain tissue itself. Often the haemorrhage occurs at a time of straining; for example, on trying to open the bowels; during exercise; or during intercourse. On these occasions the blood pressure has (naturally) risen slightly, but this extra rise in pressure is the proverbial straw that breaks the camel’s back: an already weakened and thinned artery wall finally gives way under the strain, and the high-pressure blood inside forces its way out.

The results can be dramatic. A sub-arachnoid haemorrhage can strike down a previously healthy man or woman in seconds: typically, the patient feels as though she has been hit on the back of the head with a lump of wood. Exactly what happens after this depends on where the leakage has occurred, and how big it is. A sudden big leak can produce an explosive headache, then render the victim unconscious within seconds; she collapses as though pole-axed. In less dramatic bleeds, the patient may well not lose consciousness completely, but may be drowsy and irritable, with altered consciousness – and a massive headache. Others, who have just a tiny tear with a continuous but small leak of blood, may feel unwell in a vague sort of way, without any obvious pointers to what is going on, except for the feeling that they have had a knock on the back of the head, and, of course, a headache.

Often the patient develops a stiff neck, just as in meningitis, but unlike meningitis, the stiff neck will have come on very quickly (within minutes) and the patient won’t have a temperature or any other signs of infection. On the other hand, a small leak from an aneurysm may produce little in the way of a stiff neck -or, indeed, any other symptoms, other than a headache.

Sometimes the amount of bleeding can be very small, and in a number of cases there are several small ‘warning’ bleeds before the big one strikes. It’s only on looking back that these warning bleeds can be recognised for what they are, rather than being dismissed as minor headaches from viral infection or stress.

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