Table 2. Potential reasons for poorer outcome in SSc-PAH than in IPAH.

Factor	Evidence
Age	Mean age of diagnosis of SSc-PAH ≈10 yrs > IPAH [25]. Age was independent prognostic factor in combined group of SSc-PAH and IPAH [50].
Pulmonary vasculopathy	Higher proportion of pulmonary venule involvement in SSc-PAH [10,51].
Right ventricle (RV)	Reduced RV contractility assessed by pump function graph and pressure-volume loops in SSc-PAH [54,55]. Higher NT-proBNP in SSc-PAH despite less severe pulmonary haemodynamics [53].
Left ventricle (LV)	High prevalence of LV systolic and diastolic dysfunction in SSc [50,57]. ≈1/3 of patients fulfilling haemodynamic criteria for SSc-PAH may have occult left heart disease based on response to fluid challenge and clinico-radiological characteristics [58].
Interstitial lung disease (ILD)	ILD common in SSc. Different registries have included varying degrees of ILD in SSc-PAH cohorts [29]. Response to PH therapy in SSc in the presence of ILD appears poor [75].
Multisystem disease	Coexisting renovascular and gastrointestinal disease including iron deficiency and malnutrition more common in SSc than in general population.
Antibodies	Exact role of SSc-associated autoantibodies (e.g. anticentromere) in pathogenesis of PAH and poorer prognosis compared with idiopathic PAH not clear [76]. Elevated stimulating anti-endothelin receptor type A antibodies and anti-angiotensin receptor type-1 antibodies in SSc-PAH compared with idiopathic PAH [17].

Abbreviations: IPAH, idiopathic pulmonary arterial hypertension; PAH, pulmonary arterial hypertension; PH, pulmonary hypertension; NT-proBNP, N-terminal pro-brain natriuretic peptide; SSc, systemic sclerosis