The following changes are developed in the heart and vessels as the result of mitral stenosis: The left ventricle becomes smaller, and sometimes its walls are thinner than normal. The aorta is also small and thin-walled. An almost necessary result of mitral stenosis is dilatation, with subsequent hypertrophy of the left auricle. Sometimes the auricular cavity is enormously dilated—so much so that fifty years ago Thurman described it as true aneurism of the left auricle.17 Not infrequently the left auricular walls are from one-eighth to one-fourth of an inch in thickness. Its appendix is elongated, assuming a peculiar curved form, the aperture between it and the auricle becoming wider than normal. Moxon records a case of extensive mitral stenosis where the appendix was two and three-quarter inches long.
17 Med.-Chir. Trans., vol. iii., Ser. 2, p. 244.
As soon as the auricular hypertrophy ceases to be compensatory and dilatation begins, the pulmonary circulation becomes obstructed, causing increased tension in and distension of the pulmonary vessels. The walls of the pulmonary vessels, especially those of the main trunk, are thickened and hypertrophied; in rare cases they have been found twice the thickness of those of the aorta.
Although mitral stenosis is a disease of youth, and atheroma one of old age, yet it not infrequently happens that even before the age of puberty atheromatous degeneration occurs in the pulmonary vessels, especially in the small branches, as a result of the increased blood-tension in the pulmonary system.18
18 Trans. Path. Society, vol. xvii. p. 90.
The passive pulmonary hyperæmia which results from the obstructed pulmonary circulation may lead to those changes which collectively constitute brown induration of the lung. Another occasional occurrence, directly due to extensive mitral stenosis, is nodular hemorrhagic infarction. Hemorrhagic infarction of the lungs is in nearly every case preceded by thrombosis of the right side of the heart.
In some instances the enormously dilated left auricle may, by pressing on a bronchus, reduce its calibre one-half, and thus interfere with the functional activity of the left lung. When the pulmonary hyperæmia is extensive violent physical exertion or violent coughing may cause a rupture of one of the larger pulmonary vessels, and true pulmonary apoplexy result.
Bronchorrhoeal expectoration of large quantities of glairy mucus is a very frequent result of the intense hyperæmia of the mucous membrane of the bronchial tubes which sometimes occurs in mitral stenosis. The secretion is increased with every increase in the passive hyperæmia. The lungs are at all times so liable to congestion and oedema that any sudden or violent exercise may lead to a rapidly fatal result. Again, when the conditions enumerated have existed for some time, mitral stenosis may lead to hypertrophy of the right heart. In some rare cases the tricuspid orifice has become slightly insufficient.
ETIOLOGY.—Mitral disease is especially met with in the young, and in the child it is almost invariably a stenosis. The average age is about thirty-one; it is very rare to find it occurring after the fiftieth year of life. It seems from statistics that it is nearly twice as frequent in females as in males.
It is not infrequently of congenital origin. Acute rheumatic endocarditis is its most frequent cause. The mitral valves are more frequently affected in chorea than the aortic. In some few instances stenosis results from extension of the inflammatory process from the aortic semi-lunar valves, or prolonged aortic regurgitation and stenosis may lead mechanically to mitral disease, but not to stenosis. Niemeyer regards atheroma as an exceptional cause of mitral stenosis. No other authority regards it as a possible cause.