Mitral Regurgitation Part 1: Etiology, Epidemiology, Pathophysiology, and Clinical Presentation

Introduction and Etiology

• The mitral valve has two cusps, anterior and posterior. The cusps form a saddle-shaped annulus (Figure 1).
• Each cusp is anchored by tendinous cords, also known as chordae tendineae, which insert into the papillary muscles of the LV. This anchoring allows the cusps to resist opening during systole, in response to the pressure developed in the LV during left ventricular contraction, under normal conditions.
• There are two papillary muscles: the anterolateral and posteromedial papillary muscles. The blood supply to the antero-lateral papillary muscle arises from the left anterior descending artery and the marginal branch of the circumflex artery. Blood supply to the posteromedial papillary muscle is derived from the right coronary artery, assuming right dominance.
• The Carpentier classification delineates mitral valve regurgitation into three types, based on leaflet motion-types 1, 2 and 3 (Table 1, Figure 2).^1,2
• Based on clinical presentation, MR can be divided into two main subtypes: acute MR and chronic MR (Table 2).
• MR can also be clinically classified as primary or secondary (Table 3).
  • Primary MR involves the pathology of the mitral leaflets. Types of leaflet pathology include prolapse of the leaflets, rheumatic disease changes, leaflet thickening (e.g., radiation), and damage due to infectious endocarditis. Degenerative MR falls under this category.⁴
  • Secondary MR involves pathology of the mitral apparatus other than the leaflets.  This includes regional wall motion abnormalities and/or LV dilation and annular dilation with loss of central coaptation of mitral leaflets. Ischemic MR falls under this category.⁴

Epidemiology

• MR is a common valvular disorder affecting about 10% of the population.⁴
• With color Doppler echocardiography, mild MR can be detected in up to 20% of middle-aged and older adults.⁵
• Myxomatous degeneration is the leading cause of mitral valvular abnormalities.
• Outside the Western world, rheumatic heart disease is the leading cause of MR.⁵

Pathophysiology

• Acute MR is characterized by a sudden increase in preload and a decrease in afterload, leading to an increase in end-diastolic volume (EDV) and a decrease in end-systolic volume (ESV). This leads to an increase in total stroke volume (SV). Forward SV is diminished because much of the total SV regurgitates back into the LA, which, in turn, increases the left atrial pressure.⁵
• In chronic decompensated MR, cardiac dysfunction occurs, which impairs both the total SV and the forward SV. This results in a higher ESV and EDV (Table 4).⁵

Clinical Presentation

• The presentation of MR can vary, depending on the timing and progression of the disease.
• Symptoms of MR can range from subtle, such as fatigue manifested by limitation in daily physical activity, on to acute and life-threatening symptoms, such as severe dyspnea due to pulmonary edema.
• Patients with chronic MR can be asymptomatic for years and may have normal exercise tolerance until systolic dysfunction develops.⁵
• Subsequently, they can have symptoms of progressive DOE, orthopnea, and PND.
• In acute MR, severe pulmonary edema is often the initial manifestation due to sudden volume overload of the LA and pulmonary venous system. This is often seen as a result of an acute MI and papillary muscle rupture (Table 5).⁴