When a man stands up, his brain is about 15 inches (38 cm) higher than his heart; when he lies down, the two organs are at the same level. When he stands up, therefore, the arterial pressure has to increase so that blood flow to the brain remains unchanged. Normally, this is accompained smoothly by a combination of increased heart rate and by constriction of the blood vessels in the abdomen and legs. Experience informs us of examples of a breakdown in this mechanism. The most familiar is the extreme slowing of the heart-rate produced in some sensitive people by the sight of blood or in response to pain. This cardiac slowing is mediated through the vagal nerve, and the name vasovagal attack is often given to such an episode.

The contraction of leg and thigh muscles during walking normally drives venous blood back to the heart. If venous return is insufficient because of immobility—for example, a soldier at attention on parade, or a young girl in assembly at school—then syncope may occur. Such syncope seems to be socially infectious—once a girl or soldier has slumped, others may follow in the next few minutes.

Normally blood returns to the heart from the legs smoothly through the chest and abdomen. During prolonged coughing, or straining while trying to pass a stool, the pressure within the chest is greatly increased, preventing venous return to the heart. What the heart is not getting back, it cannot put out, so this sequence of events again may result in impaired blood-flow to the brain, and syncope.

If the blood vessels in trunk and legs are pleasantly dilated in a hot bath or nice warm bed, suddenly getting up—for example, to answer the telephone—may cause syncope. This may also happen in older people, when they get out of bed at night to pass urine. The situation is more complex in this case because we know that, at the onset of urination, there is a reflex dilatation of blood vessels in the legs. This so-called ‘micturition syncope’ affects men more than women, not only because they more often have to pass urine at night (because of prostatic enlargement) but because they pass urine standing up.

Syncope may occur in association with certain diseases. For example, in diabetes the nerve fibres controlling the heart rate and the diameter of blood vessels may be diseased, and the normal adjustments to blood pressure to posture may fail to occur. There are other rare diseases of the brain in which a similar failure to control blood pressure occurs. One, which bears some similarity to Parkinson’s disease, is known as the Shy-Drager syndrome after the two American neurologists who first described it.

A much more common cause of syncope, however, is medication. Many people take tablets to control high blood pressure. One effect of some of these drugs is to cause syncope on standing up. Some antidepressants, such as imipramine (Tofranil), do the same.

How does the neurologist or paediatrician decide that his patient’s blackouts are due to syncope rather than epilepsy? Again, all depends upon the story. The first clue is the circumstances in which the blackout, occurred. If it happened at the scene of a road accident, or during a horror movie, syncope is very likely. A common story is for a man to faint while attending his wife’s delivery. Syncope virtually never occurs lying down, so if loss of consciousness happens then, a seizure is more likely. Very occasionally, vagal slowing of the heart can be so profound that syncope does happen lying down. For example, one of our patients was a woman who was so terrified of dental treatment that she lost consciousness due to syncope even if the dentist started treating her with the chair tilted back almost to the horizontal position.

The next point is the occurrence of pre-syncopal symptoms. Blood flow to the brain is reduced in syncope often for many seconds before consciousness is lost. During that time, the nervous system makes desperate attempts to constrict other blood vessels in order to elevate the central pressure. The constriction of blood vessels in the skin results in pallor, and the associated discharge of the vegetative (non-voluntary) nervous system causes nausea and sweating. The person therefore feels and looks cold, pale, and clammy.

Other points which help distinguish syncope from seizures include limpness, rather than rigidity and/or convulsions during the period of unconsciousness, and usually no incontinence during the event. Recovery of full consciousness and orientation is much more rapid after syncope than after a seizure, following which there is usually a period of confusion. Recovery after syncope often rapidly follows assumption of the horizontal position, whether the person falls, or is placed like this, so that the head is on the same level as the heart. This is nature’s safety mechanism whereby cerebral blood flow is restored. Occasionally the safety mechanism cannot operate—the position of a hand-basin or lavatory may prevent the limp body falling to the floor. Sometimes the sufferer is supported in a vertical position by well-meaning but ill-advised friends or bystanders. In these cases, cerebral blood flow may fall to such extremely low levels that incontinence, twitching, or a full-blown seizure may occur. This should be regarded as an ‘anoxic seizure’ rather than a seizure caused by epilepsy.

As an example of the difficulties that this unusual sequence of events can cause, one of us was asked to see a young nurse. Three days after a straightforward appendicectomy, she got up for the first time to go to the ward lavatory. She felt faint as she walked there, and therefore left the door ajar. She felt fainter still as she was sitting on the seat, straining to open her bowels. Before losing consciousness she called another nurse for help. This girl seeing her colleague about to tumble off the seat, held her up to prevent injury. The resulting cerebral anoxia caused an anoxic seizure. An incorrect diagnosis of epilepsy had been made, and her continued employment as a nurse was under threat.

Syncope in adolescents—usually girls—can be very troublesome, and occasionally injury occurs. Physique and life-style seem irrelevant, so the usual advice to take plenty of fresh air and exercise is probably useless. Much more important is to tell the young person to lie down at once if she feels the onset of typical pre-syncopal symptoms. Fortunately recurrent episodes are rarely troublesome for more than a year.

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