Orthostatic intolerance: orthostatic hypotension and postural orthostatic tachycardia syndrome

The impairment of adaptive mechanisms during orthostatic challenge may evoke orthostatic intolerance, a heterogeneous condition, in which the standing position elicits a fall in blood pressure and/or excessive tachycardia, accompanied by a wide spectrum of subjective symptoms such as dizziness, discomfort, nausea, and palpitations. Apart from chronic and potentially debilitating symptoms, orthostatic intolerance may occasionally lead to sudden loss of consciousness and fall injuries. Consequently, orthostatic intolerance should be considered as a possible cause of unexplained syncope. Two main forms of orthostatic intolerance are orthostatic hypotension (OH) and postural orthostatic tachycardia syndrome (POTS). Clinical variants of OH include initial, classical, and delayed forms. The prevalence of OH increases with age, ranging from less than 5% under 40 years to about 20% above 70 years of age, and is higher in chronic diseases, such as hypertension and diabetes, reaching above 35% in Parkinson’s disease and advanced kidney failure. The presence of OH is associated with a higher mortality and an increased incidence of cardiovascular disease, with the majority of patients being asymptomatic in normal conditions. In contrast, POTS affects predominantly young women (70–80%) within an age range of 15–40 years and is usually accompanied by non-specific symptoms: deconditioning, headache, cognitive impairment, and gastrointestinal dysfunction. Management of orthostatic intolerance includes both non-pharmacological and pharmacological methods with limited efficacy in the severe cases. Empirical treatment with vasoactive and volume expanding drugs for OH and POTS, and rhythm controlling therapy for POTS are recommended. Future studies on syndromes of orthostatic intolerance should focus on mechanisms leading to OH and POTS, novel diagnostic methods, and more effective therapeutic options.