Microvascular angina ( previously known as Cardiac Syndrome X ) is a form of angina (chest pain) associated with insufficient blood flow to the heart (and some other organs/regions) but in patients who have normal coronary angiograms.

It's estimated that as many as 30% of patients with angina have this type, rather than 'standard' angina which is caused by narrowing of arteries - typically due to Arterial sclerosis (a.k.a Atherosclerosis)plugin-autotooltip__plain plugin-autotooltip_bigArterial sclerosis (a.k.a Atherosclerosis)

Arterial sclerosis (a.k.a. atherosclerosis) is an extremely common condition which can cause coronary thrombosis, myocardial infarction and other serious conditions.

"The exact cause of atherosclerosis isn't known.

(ref. Hospital Practice Journal Volume 35, 2000 - Issue 2)

There is currently no known specific cause of the condition.

While numerous physiological mechanisms have been proposed, none have been proven.

Source : Wikipedia