Orthostatic intolerance with tachycardia (postural tachycardia syndrome) and without (hypocapnic cerebral hypoperfusion) represent …,2024, Novak etc

Because of the selection criteria, the patient groups already had orthostatic tachycardia or reduced cerebral perfusion. So I suppose the main finding is that these patient groups have only relatively mild autonomic dysfunction in other domains.

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The discussion also includes some speculation about why one group had tachycardia, while showing no other differences to the hypocapnic cerebral hypoperfusion (HYCH) group.
It was proposed that the orthostatic tachycardia in POTS compensates for the low stroke volume to maintain cardiac output (22) since cardiac output is defined as stroke volume multiplied by heart rate. This concept cannot explain the lack of orthostatic tachycardia in HYCH because the stroke volume, neuropathic, and cardiovascular peripheral variables influencing heart rate are equal in HYCH and POTS. Therefore, we infer that orthostatic tachycardia in POTS results from the central nervous system/brain driven overcompensation of the orthostatic challenge.

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Opposite scenario, e.g., wheatear the tachycardia in POTS is an adequate compensatory response, and the lack of tachycardia in HYCH is a sign of undercompensation, is unlikely for the following reasons: (1) there was no evidence of chronotropic incompetence, e.g., inability to increase the heart rate as needed during exercise (40), as HYCH patients were able to increase the heart rate during exercise to similar level as POTS; (2) HYCH patients had normal increase of the orthostatic heart rate comparable to healthy controls during the tilt test; (3) Portion of HYCH patients were on antitachycardic mediation, which should worsen the HYCH if the lack of tachycardia is undercompesation.
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Full title: Orthostatic intolerance with tachycardia (postural tachycardia syndrome) and without (hypocapnic cerebral hypoperfusion) represent a spectrum of the same disorder

Background: Spectrum of chronic orthostatic intolerance without orthostatic hypotension includes postural tachycardia syndrome (POTS), with orthostatic tachycardia and hypocapnic cerebral hypoperfusion (HYCH), without orthostatic tachycardia. This study compared autonomic, cerebrovascular, and neuropathic features of POTS and HYCH.

Methods: This retrospective study evaluated patients with orthostatic intolerance referred for autonomic testing. Analyzed data included surveys (Survey of Autonomic Symptoms, Compass-31, Neuropathy Total Symptom Score-6, Central Sensitization Inventory) and autonomic tests (Valsalva maneuver, deep breathing, sudomotor and tilt tests), cerebrovascular (cerebral blood flow velocity (CBFv) monitoring in the middle cerebral artery), respiratory (capnography), neuropathic (skin biopsies for assessment of small fiber neuropathy) and invasive cardiopulmonary exercise testing (iCPET).

Results: A total of 127 HYCH, 125 POTS, and 42 healthy controls were analyzed. Compared HYCH to POTS patients, there was no difference in the duration of symptoms, the prevalence of younger women, comorbidities, sensory and autonomic complaints, central sensitization syndrome, supine/standing norepinephrine levels, inflammatory markers and medical therapy except for gastrointestinal medication. Autonomic testing showed widespread but similar abnormalities in POTS and HYCH that included: reduced orthostatic CBFv and end-tidal CO2, preload failure (assessed in 16/19 POTS/HYCH), mild autonomic failure, and frequent small fiber neuropathy.

Conclusion: HYCH and POTS are syndromes of orthostatic intolerance with cerebral hypoperfusion associated with reduced orthostatic cerebral blood flow, hypocapnia, mild autonomic failure and small fiber neuropathy of a similar degree and distribution; except for tachycardia in POTS. Similarities in peripheral domain abnormalities that affect heart rate suggest that orthostatic tachycardia in POTS is driven by the central nervous system overcompensation of orthostatic challenge. These findings provide additional evidence that HYCH and POTS represent a spectrum of the same disorder. Reduced orthostatic cerebral blood flow is a key unifying feature of HYCH and POTS.

Open access (online version), https://www.frontiersin.org/journals/neurology/articles/10.3389/fneur.2024.1476918/full
 
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