When you have syncope (loss of consciousness), your doctor’s main concern should be to determine whether you are at high risk for sudden death. Most syncope is benign in nature, but when it is caused by a cardiac problem, sudden death is a real possibility. In Syncope, Part I, we reviewed the cardiac conditions that cause syncope, and discussed your doctor’s first obligation – to rule out these conditions.
Fortunately, most syncope is not cardiac in nature, and thus is not life-threatening. In this article, we will review the non-cardiac causes of syncope, then outline what you should expect from your doctor when you or a loved one have had a syncopal episode.
The non-cardiac causes of syncope
Neurological causes. For practical purposes there are only four neurological conditions that cause syncope, and none of them are particularly common.The first two - vertebrobasilar TIAs (transient ischemic attacks) and subclavian steal syndrome – are actually vascular problems. The vertebrobasilar arteries are located in the back of the neck near the spinal column, and supply blood to the consciousness center of the brainstem. If blood flow to these arteries is interrupted, syncope occurs. Both vertebrobasilar TIAs and subclavian steal are caused by such an interruption in blood flow. (Notably, many doctors immediately worry over the carotid arteries when a patient has syncope. But the carotid arteries – while vitally important – do not supply the consciousness center, and thus interruption in carotid blood flow does not cause syncope.) The doctor should suspect vertebrobasilar TIAs, if that is the cause, after taking a careful medical history. Subclavian steal syndrome reveals itself when the doctor takes the patient’s blood pressure in both arms, and finds the blood pressure in the right arm to be markedly lower than in the left.
Normal pressure hydrocephalus is caused by an accumulation of excess fluid in the brain. It too has a characteristic clinical pattern – it affects elderly patients, who present with a symptom complex of urinary incontinence, dementia, and syncope.
Because of their characteristic presentations, seizure disorders should only rarely be confused with other causes of syncope.
The bottom line is that if your syncope was caused by any of these neurological disorders, your doctor ought to strongly suspect it after doing a careful medical history and physical examination. In the vast majority of patients with syncope, neurological causes can be ruled out before the doctor leaves the bedside.
Metabolic causes of syncope. Similarly, there are only a few metabolic causes of syncope. Hypoxia is reduced oxygen concentration in the blood. Hypoxia severe enough to cause syncope is virtually never seen unless obvious and severe lung or heart disease is present. Thus, the diagnosis of hypoxia is not a problem. Hyperventilation is seen in severe anxiety reactions, and most times the medical history reveals the problem. Hypoglycemia rarely causes syncope, and once again a good medical history – with emphasis on the relationship of syncope to meals or to diabetic medications – should point to this problem.
