There are actually two conditions known as syndrome X: cardiac syndrome X and metabolic syndrome X (more commonly known as simply metabolic syndrome).

Cardiac syndrome X is a term used to describe a condition in which patients experience chest pain that resembles angina – pain, pressure or discomfort in the chest usually associated with coronary artery disease (CAD) – without actually having CAD. This diagnosis is usually confirmed by cardiac catheterization, and patients who suffer cardiac syndrome X have excellent long–term health prospects. Although the causes of cardiac syndrome X are unknown, some researchers have speculated it is caused by ischemia, or lack of oxygen in the heart, increased sensitivity to cardiac pain, or possibly a combination of both.

Metabolic syndrome also has important implications for the heart. According to the American Heart Association, metabolic syndrome is a term used to describe a cluster of cardiovascular risk factors, including:

• Elevated glucose levels and insulin resistance
  
• Elevated triglyceride levels
  
• Low HDL levels
  
• High blood pressure (hypertension)
  
• Abdominal obesity

Although there is some debate over whether this cluster of conditions constitutes a true syndrome—and if the presence of metabolic syndrome should have any impact on clinical treatment—there is no doubt that people who have these overlapping conditions are at greatly elevated risk of heart disease and diabetes. According to a study done on the Framingham Heart Study population, people with the metabolic syndrome are 10 times more likely to develop diabetes and twice as likely to develop heart disease as a person of normal weight.

Cardiac syndrome X and metabolic syndrome are distinctly different conditions, despite the similarity in their names. Though cardiac syndrome X is not associated with increased risk of atherosclerosis or heart attack, there is a clear link between metabolic syndrome and heart attack. Research has shown no link as of yet between the two syndromes.