Two subtypes of this anomaly have been described, a "supramitral" variant and an "intramitral" variant. The supramitral type is a fibrous shelflike membrane located just above the mitral valve annulus but inferior to the left atrial appendage. The membrane is distinct from and not adherent to the mitral valve leaflets. This variant typically has an otherwise normal mitral valve apparatus. The intramitral variant is a membrane located within the mitral tunnel, closely adherent to the valve leaflets. This subtype is associated with a high incidence of mitral valve abnormalities, including restricted mobility of the mitral leaflets, reduced chordal length, reduced interpapillary muscle distance, single papillary muscle, or hypoplastic mitral annulus.
The intramitral type is also frequently part of the Shone complex, in which multiple levels of left heart obstruction are present, including aortic arch hypoplasia or coarctation, aortic valve stenosis, or subaortic stenosis.
Supravalvar mitral stenosis can develop as an acquired lesion late after mitral annuloplasty to repair mitral regurgitation.