AUSTIN, TEX. – Postural orthostatic tachycardia syndrome (POTS) is not a single disorder, but rather includes multiple overlapping subtypes, according to , a professor of neurology at the University of Texas, Dallas.
“It’s pretty well established that there’s a heterogeneous spectrum of disorders that can present this way,” Dr. Vernino told attendees at the annual meeting of the American Association for Neuromuscular and Electrodiagnostic Medicine. “Investigation is somewhat difficult because we have limited tools.”
In his overview of POTS, Dr. Vernino defined it as a chronic condition with an “inappropriate orthostatic increase in heart rate” and symptoms that persist for at least 6 months. The heart rate increase should be at least 30 beats per minute – or 40 bpm in those aged 12-19 years – within 5-10 minutes of quiet standing or an upright tilt, but the patient lacks orthostatic hypotension. Often, however, other symptoms continue even if the tachycardia is not always present.
These symptoms range widely, including fainting, shortness of breath, headaches, fatigue, fibromyalgia, dizziness, brain fog, chest tightens, sensitivity to light or sound, tingling, heat intolerance, and gastrointestinal problems. Pain is particularly common.
Though peak incidence occurs around age 14 years, the average age of patients with POTS is 30 years. Women comprise 86% of those with POTS and 93% of patients are white, though this last figure may result from multiple reporting biases. A quarter of patients are disabled to a degree similar to heart failure or chronic obstructive pulmonary disease, he said.
Prevalence estimates are all over the map, ranging in academic literature from “up to 1% of teens” to “millions of Americans,” Dr. Vernino said. A commonly accepted range puts the estimate at 500,000 to 3 million Americans, the number.
Key to treatment of POTS is assessing possible underlying causes and individualizing treatment based on likely contributing etiologies, such as hypovolemia, deconditioning, and autoimmunity, Dr. Vernino said.
Classifications and etiologies of POTS
With its various possible etiologies, “it’s our job as physicians to try to understand, if you can, what the underlying the etiology is and try to address that,” Dr. Vernino said. About 11% of patients have a family history of POTS, and some research has suggested genes that may be involved, including the one that encodes the norepinephrine transporter and alpha tryptase.
Patients with neuropathic POTS have a mild or partial peripheral autonomic neuropathy “that causes a problem with the vasomotor function so that when patients stand, they don’t have an adequate increase in vascular tone, blood pools in the feet and they develop relative hypovolemia, and the autonomic nervous system compensates with tachycardia,” he said. The Quantitative Sudomotor Axon Reflex Test may show distal sweating, and a skin biopsy can be done to assess intraepidermal nerve fiber density.
Hyperadrenergic POTS involves “the presence of a dramatic, excessive rise of norepinephrine” and can involve tremor, nausea, sweating, and headache when patients are upright, Dr. Vernino said.
“These are patients who appear, clinically and in laboratory testing, to have inappropriate sympathetic response to standing up,” he said, and they may have orthostatic hypertension along with an increased heart rate.
Other subtypes of POTS can overlap neuropathic and hyperadrenergic types, which can also overlap one another. About 30% of patients appear hypovolemic, with a 13%-17% volume deficit, even with copious intake of water and sodium, he said. Despite this deficit, renin levels are typically normal in these patients, and aldosterone levels may be paradoxically low. Reduced red blood cell mass may be present, too ().
“What causes that and how that’s related to the other features is a bit unclear, and then, either as a primary or as a secondary component of POTS, there can be cardiac deconditioning,” Dr. Vernino said, requiring quantitative ECG. “It’s unclear whether that deconditioning happens as a consequence of disability from POTS or as a primary part of it.”
Questions still exist regarding whether autoimmunity is one of the underpinnings of POTS, Dr. Vernino said. It’s associated with elevated inflammatory biomarker levels and systemic autoimmune disorders such as Sjögren’s syndrome, as well as with antiphospholipid antibodies.
“More recently there’s been evidence on specific autoantibodies that have been found in POTS patients, and we’re still working through what all that means,” he said. “The real question is whether these antibodies are the cause of POTS” versus an effect or an epiphenomenon.
These antibodies include some G protein–coupled receptor antibodies, such as adrenergic receptor autoantibodies, angiotensin II type 1 receptor antibodies, and muscarinic acetylcholine receptor M3 antibodies. Others include thyroid autoantibodies, ganglionic acetylcholine receptor antibodies, and IgG antibodies, as well as several dozen cardiac membrane proteins.