Some investigators consider MCTD to be an overlap syndrome. The term Undifferentiated connective tissue disease has been suggested as a designation for patients who do not have diagnostic criteria for any one connective tissue disease. Approximately 85 percent of patients with MCTD have pulmonary involvement, which is often asymptomatic 3 . Diffusing capacity for carbon monoxide may be the only abnormality. Pleurisy commonly occurs but is seldom associated with large pleural effusions.
Some patients develop interstitial lung disease.
PH is usually caused by intimal proliferation and medial hypertrophy of pulmonary arterioles. It resembles the histopathology of primary pulmonary hypertension. The pathophysiology of PH in MCTD is believed to be activation of endothelial cells, with perivascular mononuclear cell infiltrates. The release of angiogenic cytokines is believed to induce vascular damage with proliferative repair, resulting in irreversible proliferative angiopathy. In addition, pericarditis occurs in 30 percent of patients.