"Myxomatous" mitral valves: collagen dissolution as the primary defect.

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Abstract
The morphologic and histologic characteristics of redundant prolapsing (myxomatous) mitral valves from 12 symptomatic patients with severe mitral regurgitation who required mitral valve replacement were studied and compared with those in 13 control valves. The mean surface area of the myxomatous mitral valves (MMV) was 1990 mm and of the control mitral valves (CMV) 760 mm2 (P < 0.001). The mean longest diameter of MMV was 56.4 mm and of the CMV 36.8 mm (P < 0.001). The mean shortest diameter of MMV was 44.8 mm and of CMV 22.7 mm (P < 0.001). The mean commissural diameter for MMV was 50.4 mm and for CMV 30.0 mm (P < 0.001). The mean coapting line distance of MMV was 34.1 mm and of CMV 22.5 mm (P < 0.001). The mean surface area of the anterior cusp of MMV was 910 mm2 and of CMV 560 mm2 (P < 0.01). The mean surface area of posterior leaflet was 927 mm2 in MMV and 534 mm2 in CMV (P < 0.02). CMV densities were nearly uniform (1.07 g.cm), while the mean density of MMV was 0.687 g/cm3 (P < 0.01). Dissolution of elastin was only slightly more frequent in MMV than in CMV. Myxomatous degeneration, noted in 1/2 of CMV, was found in chordae and pars fibrosa only in MMV (P < 0.05). Mucopolysaccharide infiltration was more severe in all sites except the annulus in MMV than in CMV (P < 0.05). Fragmentation of collagen was severe in either the pars fibrosa or chorda of all MMV but was not seen in any CMV in these areas (P < 0.05). Gross morphology (increased surface area, increased diameter and decreased density) and histologic characteristics (collagen dissolution with myxomatous degeneration) allowed clear separation between CMV and MMV that produced severe mitral regurgitation. Collagen dissolution in the pars fibrosa and chordae was present only in MMV, which suggests the primacy of collagen dissolution in mitral valves of patients with severe mitral regurgitation complicating the mitral valve prolapse syndrome. Apparently, this process is a disorder of collagen synthesis, content or organization (i.e., a dyscollagenosis).