Causes of Severe Pulmonary Arterial Hypertension
Severe PAH arises from multiple distinct etiologies classified into WHO Group 1, with idiopathic PAH being most common, followed by heritable forms (particularly BMPR2 mutations), connective tissue diseases (especially systemic sclerosis), congenital heart disease with systemic-to-pulmonary shunts, drugs and toxins, HIV infection, portal hypertension, and schistosomiasis. 1
Primary Classification Framework
The 2015 ESC/ERS guidelines provide the definitive classification structure for understanding PAH causes 1:
Group 1: Pulmonary Arterial Hypertension (PAH)
Idiopathic PAH (IPAH)
- Most common form in registries
- Female predominance (1.7:1 ratio)
- Prevalence approximately 6 per million population
- Mean age at diagnosis 37 years, though increasingly diagnosed in patients >70 years 2
Heritable PAH (HPAH)
- BMPR2 mutations are the primary genetic cause:
- Found in 50-90% of familial cases
- Present in up to 25% of "idiopathic" cases
- Characterized by genetic anticipation and incomplete penetrance
- Earlier onset and more severe disease when expressed 2
- EIF2AK4 mutations (associated with pulmonary veno-occlusive disease subtype) 1
Associated PAH (APAH) - Key Causes:
Connective Tissue Diseases:
- Systemic sclerosis (limited cutaneous form/CREST) is the most common association
- 65-80% show histopathological changes at autopsy, though <10% develop clinically apparent disease
- Also occurs with systemic lupus erythematosus, mixed connective tissue disease, rheumatoid arthritis
- Carries particularly poor prognosis 2
Congenital Heart Disease:
- Develops with uncorrected systemic-to-pulmonary shunts
- Eisenmenger syndrome represents the most severe form with shunt reversal
- More frequent with high-flow, high-pressure defects (VSD, PDA, truncus arteriosus)
- Can occur even with low-pressure shunts (ASD) 2
HIV Infection:
- Incidence approximately 0.5% (6-12 times general population)
- Independent of CD4 count or opportunistic infections
- Related to duration of HIV infection
- Has not declined significantly with antiretroviral therapy 2
Portal Hypertension:
- Well-established association requiring screening in liver transplant candidates 2
Drugs and Toxins:
- Definite causes: Aminorex, fenfluramine, dexfenfluramine, benfluorex (anorexigens affecting serotonin metabolism)
- Likely causes: Dasatinib (strongest evidence among tyrosine kinase inhibitors), amphetamines, methamphetamines
- Possible causes: Cocaine, phenylpropanolamine, selective serotonin reuptake inhibitors, interferon-alpha, alkylating chemotherapy agents
- Drug-induced PAH accounts for 10.5% of cases in large registries 1, 3
Schistosomiasis:
- Added to 2015 classification as recognized cause in endemic areas 1
Other associations:
- Thyroid disorders
- Glycogen storage disease
- Gaucher disease
- Hereditary hemorrhagic telangiectasia
- Hemoglobinopathies (sickle cell disease)
- Chronic myeloproliferative disorders
- Post-splenectomy 2, 1
Critical Pathophysiologic Insight
The development of severe PAH likely requires individual genetic predisposition combined with environmental triggers 4. The BMPR2 pathway disruption appears central, with additional abnormalities including:
- Endothelial dysfunction (excess endothelin, reduced prostacyclin/nitric oxide)
- Smooth muscle cell proliferation (serotonin transporter upregulation)
- Imbalance favoring vasoconstriction and proliferation over vasodilation and apoptosis 2, 5
Important Clinical Caveats
Distinguish PAH from other PH groups: Severe PH can also result from left heart disease (Group 2), lung diseases/hypoxemia (Group 3), chronic thromboembolic disease (Group 4), or multifactorial mechanisms (Group 5). These require different management approaches and are NOT classified as PAH 1.
Poor prognostic indicators regardless of etiology include: advanced functional class, poor exercise capacity, high right atrial pressure, significant RV dysfunction, low cardiac index, elevated BNP, and underlying scleroderma spectrum disease 2.
Screening is warranted in high-risk populations: known BMPR2 mutation carriers, scleroderma spectrum patients, and those with portal hypertension undergoing liver transplant evaluation 2.