Orthostatic Cerebral Hypoperfusion Syndrome (OCHOS)

Novak's 2016 paper in Frontiers in Aging Neuroscience proposed Orthostatic Cerebral Hypoperfusion Syndrome as a distinct subtype of orthostatic intolerance, specifically to capture a patient population that the existing diagnostic framework was systematically missing. OCHOS is defined by a fall in cerebral blood flow velocity during tilt that cannot be explained by orthostatic hypotension, tachycardia, arrhythmia, or structural cardiac disease. The heart rate stays within normal range. The blood pressure stays within normal range. But cerebral blood flow velocity falls measurably and reproducibly.

The mechanism Novak proposes involves a failure of the cerebrovascular system to maintain adequate perfusion during the gravitational stress of upright posture, driven by some combination of impaired autoregulation, CO₂ dysregulation, and abnormal cerebrovascular reactivity. The brain is not receiving the pressure it needs to sustain normal function, but the peripheral circulation is compensating sufficiently to keep systemic vitals in the acceptable range. The compensation is successful at the periphery and insufficient at the brain.

Standard autonomic testing, which evaluates POTS criteria and OH criteria, has no mechanism for identifying OCHOS. A patient with OCHOS passes a standard tilt. They pass it every time. And every time they stand up in daily life, they are experiencing a fall in cerebral perfusion that produces their symptoms, with no abnormality appearing in the measurement that the clinician is watching.

The practical relevance of naming this subtype is that it gives patients and clinicians a framework for what is happening and a justification for requesting the additional testing, specifically cerebral blood flow velocity monitoring during tilt, that would make the problem visible. An invisible problem cannot be targeted. A named, measurable problem can.