[The enigma of pulmonary hypertension of undetermined origin]

The purpose of this review paper is to discuss: A) The differences in the nosological characteristics of this condition. Some authors only accept as primary pulmonary hypertension those patients without any other possible cause. They even exclude those with familiar pulmonary hypertension since its genetical etiology is now well established. It is more generally accepted that the very low incidence of pulmonary hypertension in conditions such as anorexigen use, portal hypertension or others, suggest the coincidence of a permissive genotype, susceptible phenotype (endothelial dysfunction) and a triggering factor. In such a way, pulmonary hypertension may be associated with apparently dissimilar conditions. B) The current interpretation of histologic lesions and their relationship with recent histochemical and immunological findings. The previously proposed hypothesis that some lesions are final and inactive results of prolonged hypertension is difficult to uphold since they were found only months after the clinical beginning of the disease. Moreover cells at the center of the plexiform lesion show histochemical activity patterns. It is also proposed that the anatomically inapparent endothelial dysfunction may be the original event. C) Proposed causal mechanisms such as down-regulation or even absence of K+ voltage channels of the pulmonary vascular smooth muscle cell. This finding would include primary pulmonary hypertension among the simultaneous "channel diseases". The data that justify the possible influence of serotonin plasma levels are also commented.