POTS and HYCH as a spectrum, not separate conditions

A 2024 paper by Novak and colleagues in Frontiers in Neurology is one of the most direct published challenges to the way POTS is currently diagnosed and categorized. By comparing POTS patients with patients who have hypocapnic cerebral hypoperfusion syndrome (HyCH/OCHOS) — a condition defined by orthostatic cerebral hypoperfusion without meeting tachycardia criteria — and healthy controls, the authors found something that reconfigures the diagnostic framework: the two patient groups are more alike than different. What separates them is not the underlying physiology. It is the compensatory response.

What the Study Measured and How

The researchers enrolled patients meeting established criteria for POTS (heart rate rise of 30 bpm or more within 10 minutes of standing, absent orthostatic hypotension, with chronic orthostatic symptoms) and patients meeting criteria for HyCH/OCHOS (symptomatic orthostatic cerebral hypoperfusion documented by transcranial Doppler, without the tachycardic threshold). Healthy age-matched controls were included for comparison. All subjects underwent comprehensive autonomic testing including quantitative sudomotor axon reflex testing (QSART), thermoregulatory sweat testing, heart rate variability analysis, and cerebrovascular monitoring with transcranial Doppler during head-up tilt. Neuropathic features were evaluated through skin punch biopsy for intraepidermal nerve fiber density.

This multi-domain approach is what makes the paper's findings so significant. Rather than comparing the two patient groups only on the heart rate metric that defines them, the authors looked across the full autonomic and cerebrovascular picture.

The Findings: Same Pathology, Different Compensation

Both POTS and HyCH/OCHOS patients showed comparable reductions in cerebral blood flow velocity during orthostatic challenge relative to healthy controls. Both groups showed similar profiles of peripheral autonomic dysfunction including sudomotor abnormalities and evidence of small fiber involvement. The rates of neuropathic pathology were statistically indistinguishable between the two groups.

The only feature that reliably separated the two groups was the magnitude of the tachycardic response. POTS patients mounted a heart rate increase large enough to cross the 30 bpm diagnostic threshold. HyCH/OCHOS patients experienced the same orthostatic cerebral hypoperfusion but did not produce a tachycardia of sufficient magnitude to qualify diagnostically. The cerebral perfusion failure — the thing that is actually producing the symptoms — was present in both.

The authors state this explicitly: the similarities in peripheral autonomic abnormalities between the two groups suggest that the orthostatic tachycardia in POTS patients represents central nervous system overcompensation to the orthostatic challenge rather than a primary failure of the heart or peripheral reflex. The tachycardia is the nervous system's attempt to increase cardiac output and restore brain perfusion. It is not the disease. It is the response to the disease.

Why This Reframes the POTS Diagnostic Framework

The standard POTS diagnostic criterion — a heart rate rise of 30 bpm within 10 minutes of standing — was designed as a practical threshold for identifying patients with significant orthostatic intolerance. It has been clinically useful. But taken literally, it produces a diagnostic category defined entirely by the magnitude of one compensatory response, not by the underlying physiological failure.

What the Novak 2024 data shows is that two patients can share the same cerebrovascular abnormality, the same autonomic neuropathic profile, and the same symptom burden, and end up with different diagnoses depending only on whether their compensatory tachycardia reached 30 bpm. One patient is told they have POTS. The other is told their testing is borderline or normal. Neither has received a characterization of the actual problem.

This matters clinically because current treatment protocols are organized around the POTS label. Volume expansion, beta blockers, and compression garments are prescribed for POTS patients. HyCH/OCHOS patients — who by this evidence have the same underlying physiology — often receive no comparable protocol, or are sent back through the diagnostic workup as if the problem hasn't been identified. The diagnostic asymmetry does not reflect a physiological asymmetry. It reflects a measurement choice made at the point of diagnosis.

The Implication for How Tachycardia Should Be Treated

If the tachycardia in POTS is the central nervous system compensating for inadequate cerebral perfusion when upright, then suppressing that tachycardia without addressing the perfusion deficit is physiologically backwards. Beta blockers, the most commonly prescribed rate-control agent in POTS, reduce heart rate and cardiac output. In a patient whose tachycardia is the mechanism keeping their brain perfused in the upright position, blunting that mechanism may reduce the heart rate on paper while worsening the very symptom — brain fog, cognitive impairment, fatigue on standing — that brought the patient to clinical attention.

This does not mean rate-control agents are always harmful in POTS. In hyperadrenergic subtypes where excessive sympathetic output is part of the problem, rate reduction may be appropriate. But the prescribing decision requires knowing whether the tachycardia is a driver of the problem or a compensation for it. That distinction is only visible if cerebral blood flow is being measured, and standard POTS workups do not routinely measure it.

The Novak paper provides a published rationale for adding transcranial Doppler monitoring to orthostatic assessment. Without it, both POTS and HyCH/OCHOS patients are being evaluated on the downstream metric — heart rate — while the upstream failure that is generating their symptoms goes uncharacterized.

What This Means If You Have Already Been Diagnosed

If you carry a POTS diagnosis, this paper tells you that your tachycardia is most likely a compensation mechanism, not a primary cardiac problem. The question your current treatment plan may not have asked is: what is your tachycardia compensating for, and does your treatment address that, or only the compensation?

If you have borderline POTS findings, or have been told your tilt table was inconclusive because your heart rate "only" rose 25 bpm, this paper tells you that the threshold is arbitrary relative to the underlying physiology. A patient with a 25 bpm rise and documented orthostatic cerebral hypoperfusion by transcranial Doppler has the same pathological profile as a POTS patient. The absence of the label does not mean the absence of the problem.

The Novak 2024 findings position POTS and HyCH/OCHOS as two expressions of a single spectrum — orthostatic cerebral hypoperfusion with variable compensatory responses. The spectrum framework is not academic. It changes what should be measured, what the measurement means, and what intervention logic follows from it.