Cerebral Blood Flow Is Reduced in ME/CFS and Orthostatic Intolerance — and Having Both Makes It Worse

The question of whether reduced cerebral blood flow is a core physiological feature of ME/CFS has been debated across individual studies for years. Some studies find it; others don't. The methodological variation — different measurement tools, different patient populations, different posture protocols — has made it difficult to synthesize the evidence into a clear clinical conclusion. A 2025 systematic review by Christopoulos and colleagues resolves much of that ambiguity by synthesizing 118 studies and 9,185 participants in a single PRISMA-compliant analysis. The conclusions are not ambiguous: cerebral blood flow is significantly reduced in ME/CFS and orthostatic intolerance compared to healthy controls, and patients with both conditions carry a greater cerebral perfusion deficit than patients with either alone. The mechanism driving symptoms in these overlapping conditions is orthostatic cerebral hypoperfusion — and it is additive when both are present.

The Systematic Review: Scale and Scope

Christopoulos and colleagues searched PubMed, MEDLINE, EMBASE, and Cochrane from inception through February 2025, identifying 14,928 articles. Of these, 118 met inclusion criteria for analysis. The studies were divided by condition: 26 examined CBF in ME/CFS only, 81 examined CBF in orthostatic intolerance only, and 11 examined CBF in populations with both conditions. The total participant pool across all included studies was 9,185, with a mean age of 39 years and 73.8% female — a demographic profile consistent with the known sex and age distribution of ME/CFS and OI populations.

Measurement methods varied across studies and included transcranial Doppler ultrasound, extracranial Doppler echography, MRI-based perfusion imaging, SPECT, and near-infrared spectroscopy. Protocols included supine rest measurements, active stand tests, and head-up tilt table testing at various angles. This methodological breadth is both a strength of the review — it captures findings across multiple approaches — and a complicating factor in synthesis, since different tools measure different aspects of cerebral perfusion and are not directly interchangeable.

The Core Finding: CBF Is Consistently Reduced in Both Conditions

The findings were consistent across the evidence base. Cerebral blood flow was significantly reduced compared to healthy controls in 12 of 26 ME/CFS-only studies, in 57 of 81 OI-only studies, and in 7 of 11 studies examining both conditions. The consistency of the CBF reduction across conditions, measurement methods, and study populations establishes it as a robust finding rather than a methodological artifact of any particular measurement approach.

The CBF reductions were more consistently detected during orthostatic challenge — tilt testing and active stand protocols — than at supine rest. This is expected given the role of gravitational stress in precipitating the perfusion deficits that drive orthostatic symptoms. Resting CBF differences were present in a subset of studies but were less consistent, suggesting that the orthostatic component of CBF impairment is the more clinically significant and reliably detectable feature of these conditions. A supine measurement alone — without an orthostatic challenge — will miss the bulk of the perfusion deficit in most patients.

The Additive Effect: Having Both ME/CFS and OI Makes It Worse

The most clinically significant finding in the Christopoulos review is the additive effect of comorbid ME/CFS and orthostatic intolerance on cerebral blood flow. Of the 11 studies that examined CBF in patients with both conditions, 4 directly compared ME/CFS patients with and without OI. All 4 found that having both conditions produced greater CBF reductions than having ME/CFS alone. OI was not simply a secondary label that co-occurred with ME/CFS as an independent parallel problem. Its presence amplified the cerebral perfusion deficit in a measurable, additive way.

This additive finding has a mechanistic explanation. ME/CFS involves impaired cerebral blood flow regulation through multiple pathways — reduced baroreflex sensitivity, elevated resting sympathetic tone, impaired autonomic calibration of cerebrovascular resistance. Orthostatic intolerance adds a specific gravitational challenge to this already-compromised regulatory system: when upright, venous pooling reduces cardiac output and threatens cerebral perfusion, and the OI cardiovascular system cannot fully compensate. When both mechanisms are operating simultaneously, the CBF deficit they produce is larger than either would produce alone.

A patient with ME/CFS and concurrent OI is carrying a cerebral perfusion burden composed of two additive contributors: the baseline CBF impairment attributable to ME/CFS pathophysiology, and the positional CBF impairment attributable to their OI. Van Campen and Rowe's research on severe ME/CFS established that sitting alone can reduce CBF by nearly 25% — even without standard POTS diagnostic criteria being met. In a patient carrying both conditions simultaneously, these deficits compound.

The Methodological Contamination Problem in Prior ME/CFS Research

The Christopoulos review raises a methodological issue with significant implications for the prior ME/CFS CBF literature. Most studies examining CBF in ME/CFS alone did not screen participants for OI before measuring. A substantial proportion of ME/CFS patients experience OI symptoms even when they do not meet formal diagnostic criteria for POTS or orthostatic hypotension — the sub-threshold OI population documented in the Stewart 1999 research and described in the van Campen body of work. If those patients with undiagnosed sub-threshold OI were included in ME/CFS-only studies without identification, the CBF values attributed to ME/CFS alone were contaminated by an uncontrolled OI contribution.

The practical consequence is that the studies finding the largest CBF reductions in ME/CFS may have been inadvertently capturing ME/CFS-plus-OI patients within their ME/CFS-only samples. Studies finding smaller or no CBF reductions may have had lower OI prevalence in their samples by chance or by recruitment setting. The heterogeneity in ME/CFS CBF findings across the literature may partly reflect uncontrolled OI variation rather than genuine variability in ME/CFS pathophysiology itself.

This means the true CBF profile of ME/CFS without any orthostatic component remains poorly characterized. Future research needs to explicitly screen for and stratify by OI status — including sub-threshold OI — before characterizing CBF in ME/CFS to prevent the contamination that has distorted the prior literature.

What This Means for Clinical Evaluation

For patients and clinicians, the practical implications of the Christopoulos review run in two directions. First, measuring cerebral blood flow during orthostatic challenge is likely essential for understanding disease severity in ME/CFS and OI. The CBF reductions during tilt were more consistently detected than resting CBF differences, and the patients most severely affected by OI showed the largest deficits. A clinical evaluation that assesses CBF only at rest, or that uses heart rate and blood pressure as proxies for cerebral perfusion, will systematically underdetect the primary mechanism driving symptoms in most patients.

Transcranial Doppler ultrasound provides real-time cerebral blood flow velocity during orthostatic challenge and is the measurement tool that has generated much of the most clinically informative CBF data in ME/CFS and OI research. It is not standard in clinical autonomic evaluation. It should be, for this population.

Second, the additive finding supports treating OI as a clinically significant comorbidity in ME/CFS that deserves its own systematic evaluation and management — not as a coincidental observation or a secondary complaint behind the primary ME/CFS diagnosis. For ME/CFS patients who have not had their OI status formally evaluated, this review provides the evidence basis for why that evaluation matters: the presence or absence of OI has measurable consequences for the magnitude of the cerebral perfusion deficit they are carrying, and addressing it is not a peripheral intervention. It is addressing a quantifiable contributor to the central physiological burden of the condition.