The enigma of syndrome X.

Syndrome X is characterised by chest pain suggestive of angina pectoris, normal epicardial coronary arteries on coronary angiography, and an exercise test with the characteristics (ST depression on the electrocardiogram) usually interpreted as myocardial ischaemia. The literature is confused because the last requirement, namely a positive exercise test, is not always present in groups of patients studied by a variety of techniques. The prognosis of patients with syndrome X is good. The cause is not known although many authors have argued that myocardial ischaemia due to small vessel disease is present. Coronary angiography cannot detect abnormalities of vessels less than 400 microns in diameter whilst electron-microscopy and light histology of biopsies visualise vessels less than 150 microns in diameter. Vessels of intermediate size could be abnormal and the abnormality be undetected. In many patients ischaemia cannot be the cause because the coronary sinus oxygen saturation is normal during an atrial pacing test when the patient complains of chest pain and changes are present on the surface electrocardiogram. Syndrome X describes an inhomogeneous group of patients with symptoms due to differing pathologies.