Anatomic and light microscopic characterisation of hypertrophic obstructive and non-obstructive cardiomyopathy.

Macroscopically asymmetric hypertrophy of the interventricular septum is the characteristic change, though mild degrees can be seen in normal hearts and in combination with congenital or acquired heart disease. If asymmetric hypertrophy is severe, this alone characterises the condition. Concentric forms have, however, been described. At light microscopic level disarray and often extreme hypertrophy of myocardial fibres together with degenerative changes and interstitial fibrosis are typical. Overlap of individual features with 'ordinary' hypertrophy exists but if all criteria are considered in combination a high degree of diagnostic reliability can be achieved, frequently aided by severe accumulation of glycogen. In support semiquantitative studies will also be presented. The patterns of abnormal fibre arrangement differ in the two clinical types. In cases with obstruction the abnormal fibres are predominantly confined to the interventricular septum, whereas in cases without obstruction they are scattered focally throughout the ventricular walls. Unfortunately, these disturbances are not always present. Subdivisions based on histological changes by other workers will be mentioned. Brief reference will be made to experimental work that has shown that a possible endogenous mechanism may be operative in some of the patients with hypertrophic cardiomyopathy. It is concluded that the morphology of hypertrophic cardiomyopathy is sufficiently characteristic to separate it as a distinct entity and to ensure reliable diagnosis.