POTS: a syndrome with multiple upstream drivers
A 2019 review by Bryarly and colleagues, published in the Journal of the American College of Cardiology, is one of the most thorough mainstream syntheses of POTS pathophysiology available in the cardiology literature. Its central argument is that POTS is a syndrome defined by a measurable pattern — orthostatic tachycardia — that can emerge from mechanistically distinct upstream failures. The tachycardic pattern at the end is the same. The physiological problems producing it are not. And if treatment is aimed at the pattern rather than the problem producing it, results will be variable by design.
The Five Subtypes and What Makes Each Distinct
The Bryarly review organizes POTS pathophysiology into five primary mechanistic subtypes. Each requires a different treatment logic, and each requires different testing to identify.
Hypovolemic POTS is characterized by reduced circulating blood volume, often measurably so by plasma volume assessment. When total blood volume is low, the upright posture produces more extreme reductions in venous return to the heart. The heart accelerates to compensate. Volume-expanding interventions — high sodium and fluid intake, fludrocortisone, desmopressin — address the actual deficit. In a patient whose POTS is hypovolemic, these interventions can produce substantial improvement. In a patient whose POTS has a different driver, they add volume to a system that doesn't have a volume problem, with minimal therapeutic benefit and potential for side effects.
Neuropathic POTS involves small fiber neuropathy affecting the autonomic nerve fibers that regulate peripheral vasoconstriction. When lower extremity vasoconstriction is impaired, venous pooling in the legs increases upon standing. Cardiac filling falls. The heart rate rises to compensate. Skin punch biopsy demonstrating reduced intraepidermal nerve fiber density, combined with thermoregulatory sweat testing, can identify this subtype. The treatment target here is not volume alone — it is the failure of vasoconstriction that is allowing pooling to occur. Compression garments address the mechanical consequence. Some pharmacological approaches target the adrenergic signaling that drives vasoconstriction.
Hyperadrenergic POTS presents differently from the other subtypes. Standing norepinephrine levels of 600 pg/mL or greater are characteristic. These patients often experience a worsening blood pressure response on standing rather than a fall, and symptoms frequently include not just fatigue and cognitive impairment but palpitations, tremor, and anxiety-like activation. In this subtype, the sympathetic nervous system is not failing — it is overactivating. Rate-reducing medications that target sympathetic output make physiological sense here in a way they may not in other subtypes.
Deconditioned POTS reflects reduced cardiovascular reserve from prolonged inactivity or bed rest. Orthostatic tolerance requires an adaptive cardiovascular system capable of redistributing blood volume and maintaining cardiac output against gravity. Deconditioning reduces cardiac stroke volume, reduces blood volume, and reduces the body's capacity to mount the vasoconstriction needed for upright posture. The tachycardia in these patients reflects a system operating near its limits. Structured, graduated exercise rehabilitation — particularly recumbent and swimming-based protocols that avoid the upright posture while building cardiovascular capacity — addresses this directly. It is one of the interventions with the strongest evidence base in POTS overall. The challenge is that it must be appropriately paced for patients with severe exercise intolerance.
Autoimmune POTS involves autoantibodies targeting adrenergic receptors, most commonly beta-1 and beta-2 adrenergic receptor autoantibodies, and in some cases alpha-1 adrenergic and muscarinic receptor autoantibodies. The immunological driver in these patients means their autonomic dysregulation has a different proximate cause than the structural or volume-based subtypes. This subtype is underdiagnosed because testing for adrenergic autoantibodies is not standard in most POTS workups. Cases have been documented following viral illness and, more recently, following SARS-CoV-2 infection, which may explain a subset of post-COVID dysautonomia presentations.
The Final Common Pathway and Why It Misleads Treatment
Across all five subtypes, the final common mechanism is the same: inadequate venous return to the heart in the upright position leads to reduced cardiac filling, which leads to compensatory tachycardia. This convergence is why POTS presents as a coherent clinical syndrome despite its mechanistic heterogeneity. The heart rate rise is real and measurable across all subtypes. It is also the signal that the system is compensating, not the problem the system is compensating for.
Current clinical practice frequently treats the heart rate rise as the primary target. Beta blockers are among the most commonly prescribed medications for POTS, and they do reduce heart rate across subtypes. But their effect on the underlying problem depends entirely on which subtype is present. In hyperadrenergic POTS, beta blockade addresses overactive sympathetic drive, which may be appropriate. In neuropathic POTS, it slows the compensatory response without correcting the vasoconstriction failure that is causing venous pooling. In hypovolemic POTS, it reduces cardiac output in a patient whose tachycardia was already the mechanism keeping cerebral perfusion adequate in the upright position. The same medication produces different physiological effects depending on the subtype, but the subtype is rarely identified before prescribing.
What Subtype Identification Requires
The Bryarly review is not an argument against treating POTS. It is an argument for identifying what kind of POTS is present before selecting the treatment. Doing that requires tests that are not universally performed in current POTS evaluations.
Plasma volume measurement can identify hypovolemia. Skin punch biopsy with quantification of intraepidermal nerve fiber density can identify neuropathic involvement. Standing plasma norepinephrine levels can identify the hyperadrenergic subtype — a value above 600 pg/mL is considered the diagnostic threshold. Thermoregulatory sweat testing adds another dimension to neuropathic assessment. Adrenergic receptor autoantibody panels can identify autoimmune involvement. These tests are available, and in combination they can narrow the subtype diagnosis with reasonable confidence.
The more complete the upstream characterization, the more the treatment plan can be targeted to the actual mechanism rather than the measurable pattern that mechanism produces. Fludrocortisone for confirmed hypovolemia. Compression and vasoconstriction support for confirmed neuropathic pooling. Careful sympatholytic therapy for confirmed hyperadrenergic drive. Graduated exercise rehabilitation for deconditioning. Immunological workup and appropriate referral for autoimmune involvement.
What the Reader Now Knows That Many Specialists Haven't Applied
The Bryarly 2019 review was published in JACC, one of the highest-circulation cardiology journals in the world. Its argument — that POTS subtypes require subtype-specific treatment — has been available to cardiologists for years. The gap between what the literature supports and what most patients experience in clinic is not a literature gap. It is an implementation gap.
If you have POTS and your treatment has not improved your standing symptoms, the question the Bryarly review asks is: has anyone determined which subtype you have? A POTS diagnosis identifies the pattern. It does not identify the mechanism. Without the mechanism, the treatment plan is built on a label that was designed for measurement, not for targeted intervention.
Understanding which upstream driver is producing your tachycardia is not a detail. It is the difference between a treatment that addresses what is happening in your physiology and one that manages its most visible downstream number.