Table 1 The Heath and Edwards pathological grading method

The medial thickness of the muscular pulmonary arteries and muscularization of the pulmonary arterioles without intimal alterations.

Together with the medial hypertrophy, intimal thickness with cell proliferation in the smaller muscular pulmonary arteries.

Intimal obstruction with concentric or eccentric masses of less cellular fibrous tissue in the arterioles and small muscular arteries. Large elastic arteries show atherosclerosis.

Progressive dilatation of the small arteries with plexiform lesions and muscle hypertrophy is less apparent.

Chronic dilatation of the small arteries with plexiform and angiomatoid lesions.

Necrotizing arteritis with thrombosis.