Author Response: Choroidal Folds in Astronauts

We would like thank Mader and colleagues¹ for their comments on our study² and their invitation to compare the folds and disc edema that occur in microgravity environments with those that occur in idiopathic intracranial hypertension (IIH). Based on our recent reports,^2,3 it is unlikely that we are going to provide new insights into the mechanisms that the correspondents have not already meticulously and exhaustively considered in their original publications.^4,5 

We agree that retinal and choroidal findings in IIH appear to differ from the changes associated with microgravity environments; however, it may be premature to draw any definitive conclusions given the small number of reported cases. Nonetheless, the eye findings in microgravity may differ from IIH in several ways: relatively mild degree of disc edema, ostensible absence of peripapillary wrinkles and retinal folds, higher frequency of choroidal folds, mildly elevated or normal intracranial pressure, and cotton wools spots. If confirmed, these features may reflect a difference in pathophysiology or severity. 

The microgravity environment has widespread physiological effects that are not completely understood. In contrast to IIH, the redistribution of body fluids is an important factor that could, in addition to elevating intracranial pressure, also affect the intraorbital pressure, intraocular pressure, ocular blood flow, the material properties of the sclera, and loading force conditions at the scleral flange. Any or all of these effects may induce or modulate the development of folds and disc edema. We suspect that the ophthalmic manifestations of microgravity may have more in common with jugular vein obstruction or venous sinus thrombosis than IIH. 

Enhanced optical coherence tomography (OCT) imaging that quantifies shape deformation (i.e., anterior-posterior displacement) of the peripapillary Bruch's membrane layer^6,7 before, during, and after exposure to a microgravity environment may be able to identify differences among these entities and help clarify the underlying mechanisms in each. An analysis of the types and patterns of folds can also provide important information about the nature of the biomechanical forces acting on the optic nerve head. We found that high-resolution rasters and en face OCT are more sensitive than photos in detecting and characterizing the folds in patients with papilledema.^2,3