To shed light on physiological determinants of postural orthostatic tachycardia (POT) among patients with Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS), with special reference to water homeostasis and joint hypermobility.

Postural orthostatic tachycardia syndrome is a frequent finding in patients with ME/CFS. Many patients also display signs suggesting abnormal water homeostasis as well joint hypermobility indicative of connective tissue abnormality. We conducted an observational series with a comprehensive assessment of patient characteristics, tilt table testing, osmolality measurements, and body composition analysis in patients with ME/CFS (according to The Canadian Consensus Criteria) to find out whether POT is predicted by these physiological factors.

A retrospective single-center study of a consecutive patient population (n = 244) referred to a specialist clinic for ME-CFS. Osmolality was measured after a 10-hour fasting. Joint hypermobility was determined using Beighton scale and 5PQ questionnaire. Body composition was analyzed with an advanced body composition monitor. Tilt table testing was performed with a rest period followed by suspension at an angle of 80 degrees. Heart rate increase of more than 30 beats per minute in combination with clinical symptoms were considered diagnostic of POT.   

Of 238 patients diagnosed, 65 (27.3%) displayed POT. Patients with POT were younger (mean±SD 39.8±11.0 vs. 48.1±12.4 years), had lower body mass index (25.9±5.5 vs. 27.9±6.1 kg/m²), higher relative total body water (50.1±6.5 vs. 47.7±6.7 kg) and a lower ratio of extracellular/intracellular water (0.77±0.07 vs. 0.82±0.08), higher prevalence of joint hypermobility (78.5% vs. 60.1%), higher supine heart rate (75.4±12.9 vs. 71.1±11.9), and lower systolic blood pressure (118±10.0 vs. 126±15.2). In logistic regression, the significant independent predictors of POT were younger age, ECW/ICW relationship, supine heart rate, and joint hypermobility.

POT in patients with ME-CFS has physiological predictors including younger age, hypermobile constitution, higher basal heart rate, and fluid distribution shifts between cellular compartments.