Spaceflight-Associated Neuro-Ocular Syndrome and Idiopathic Intracranial Hypertension: Can Anemia and Hyperemia Underlie Both?

We read with interest the article in the AJNR by Tidwell et al¹ which describes a head-down tilt study with increased inspired carbon dioxide (CO₂) levels, which was undertaken as an analog to long-duration spaceflight. The purpose of this study was to investigate the alterations occurring in brain physiology in long-term spaceflight. This study may shed some light on the spaceflight-associated neuro-ocular syndrome (SANS). SANS affects the vision of individuals undergoing spaceflight.² The authors found that there was a decrease in CBF of 18%, an increase in ventricular size of 7%, and an increase in perivascular space size of 20% in their study.¹ They acknowledged that although their study replicated some of the physiology of spaceflight, the CBF may actually be increased in long-term spaceflight rather than decreased as they found, and they referenced our review on the subject.² In our review article, the literature indicated that there was evidence of a 41%–56% increase in cardiac output, an 83% increase in straight sinus blood flow velocity, and an 84% increase in the middle cerebral vein flow velocity in long-term spaceflight.² We wish to discuss the relevance of blood flow and SANS in light of our own most recent findings in idiopathic intracranial hypertension (IIH).

The ocular findings in SANS and IIH are similar.² While it has long been thought that IIH is associated with either a normal or reduced CBF, it has recently been shown that there is a subset of patients with IIH who have an increased CBF, ie, cerebral hyperemia.³ Approximately 20% of patients with IIH are anemic.³ A review of the hemoglobin concentration in a previously published cohort of children being investigated for IIH showed that 17% were anemic, with these individuals’ hemoglobin concentrations being 16% below the mean reference levels.³ These anemic children had their average CBF increased by 60% compared with controls. Cerebral hyperemia is an expected result of anemia, and elevated blood flow can increase the venous pressure, which is the presumed underlying cause of IIH.³ Most interesting, chronic hemolytic anemia and a reduction in the hemoglobin concentration of about 10% are found in long-term spaceflight.⁴ Thus, anemia in both IIH and spaceflight would be expected to be associated with cerebral hyperemia and not a reduced blood flow. Perhaps the head-down-tilt experiment could be supplemented with hypoxia as well as an increased inspired CO₂ level to increase the CBF and better match the effects of spaceflight.

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