This name is applied to the condition in which the mitral orifice is not large enough to allow of the usual quantity of blood passing from the auricle to the ventricle. The normal width of the mitral orifice may be roughly estimated with the fingers; in the adult it should allow the index and middle fingers to pass freely through as far as the first joint.
The contraction may be very slight or it may be to such an extent that hardly a crow-quill can be admitted into the orifice. In the case of stenosis of the mitral, as well as in that of the aortic orifice, the obstruction is usually caused by the curtains of the valves becoming thickened and rigid, but especially by their coalescence. The thickened curtains grow together by their edges, and so the valve is converted into a funnel with its apex turned down into the ventricle - the so-called Funnel-shaped deformity. The normal orifice is at the base of the curtains, but when the curtains coalesce, the orifice while becoming contracted is moved downwards, and comes to have its site at the apex of the funnel. This will be understood from the accompanying diagram (Fig. 231), in which black lines represent the orifice and curtains in their normal condition during the diastole of the ventricle, the curtains lying back against the wall of the ventricle, and the orifice at their base. The dotted lines represent the coalesced funnel-shaped valve; the contraction of the orifice and its removal downwards being shown. The chordae tendineae are commonly thickened, and often partly incorporated in the funnel (see Fig 223, p. 457). On laying open the ventricle this thick, rigid, funnel-shaped deformity is often strikingly prominent. These conditions result from chronic •endocarditis, and it will be readily understood that the rigid valve is frequently incompetent, so that this condition is often combined with the one before mentioned. There are also not infrequently changes in the aortic valve.
Fig. 231. - Diagram of funnel-shaped deformity of mitral valve. The dotted lines indicate the coalesced curtains forming a funnel projecting into the ventricle with a reduced aperture at the apex.
Obstruction is occasionally produced by thrombi growing on the valve, or having their seat in the auricle and projecting into the orifice. This is a rare cause of obstruction, arid still a rarer is the presence of tumours growing in such a way as to obstruct the orifice.
It might be supposed that the vegetations occurring in acute endocarditis would obstruct the orifice, but although these rough projections undoubtedly interrupt the even flow of blood, and may produce during life a murmur of mitral obstruction, yet their actual influence on the function of the orifice must be very slight, and we are not to look for any definite evidences of their influence on the circulation.
We have now to consider the results to the circulation of mitral obstruction. The most direct effect will be dilatation of the left auricle, as the blood is to a certain extent hindered in its passage into the ventricle. As a consequence, the whole pulmonary vessels will be loaded and the right ventricle distended with the accumulated blood. On the principles already laid down there will be increased action and consequent Hypertrophy of the right ventricle, and this is commonly more extreme than in mitral insufficiency. The contraction of the orifice interferes with the passage of blood into the left ventricle, which, in extreme cases, is, as it were, starved of blood. The increased force of the right ventricle may in great part make up for the deficiency, and sometimes there is also aortic insufficiency, so that the ventricle is fed from the aorta as well. According to these various circumstances will be the state of the left ventricle. It may be actually atrophied and appear as a small appendage to the enlarged right ventricle, or it may be normal in size or ■even hypertrophied. In any case the hypertrophy of the right ventricle is the predominating condition. The shape of the heart is more quadrilateral, the apex is blunt and formed by the right ventricle. During life instead of the defined apex beat of the left ventricle, there is the more diffused heaving of the right.
As a further consequence, we have a permanent Passive hyperemia of the pulmonary circulation, with consequent brown induration of the lungs. There is also a tendency to slight haemorrhages, the blood showing itself in the sputum. The dilatation of the right ventricle, when followed by thrombosis in the auricle or ventricle, also frequently leads to embolism of the pulmonary artery and the haemorrhagic infarction. The hyperemia is reflected to the systemic venous circulation, especially if the dilatation of the right ventricle lead to relative insufficiency of the tricuspid valve, and we find evidences of passive hyperemia of the liver (nutmeg liver), kidneys, and other organs. Not infrequently serious oedema of the skin and dropsy of the serous cavities develop. Thrombosis in the veins of the legs often complicates the condition, and .this again may be a source of pulmonary embolism.