Group 5 Pulmonary Hypertension: The Orphan's Orphan Disease.

Pulmonary hypertension is a complex disorder with multiple etiologies; the World Health Organization classification system divides pulmonary hypertension patients into 5 groups based on the underlying cause and mechanism. Group 5 pulmonary hypertension is a heterogeneous group of diseases that encompasses pulmonary hypertension secondary to multifactorial mechanisms. For many of the diseases, the true incidence, etiology, and treatment remain uncertain. This article reviews the epidemiology, pathogenesis, and management of many of the group 5 pulmonary hypertension disease states.