In Reply We thank you for the opportunity to respond to the interesting and eloquent letter of Wostyn and De Deyn. In their letter, they stress the notion that increased intracranial pressure (ICP) may not be the sole or even the primary cause of disc swelling in astronauts. They go on to describe a possible explanation for why space flight-associated neuro-ocular syndrome (SANS) may differ from (terrestrial) idiopathic intracranial hypertension (IIH).
As Wostyn and De Deyn mention, 2 general mechanisms for SANS that may occur independently or in combination have been proposed. Either is thought to result in elevated cerebrospinal (CSF) pressure within the subarachnoid space (SAS) surrounding the optic nerve and might be associated with the magnitude of the pressure difference across the lamina cribrosa.1-3 In the first mechanism, the ocular changes are thought to result from ICP similar to that which occurs in IIH. Cerebrospinal fluid normally drains from the brain into the veins of the head and neck as a result of a pressure gradient. It is thought that venous stasis in the head and neck produced by cephalad fluid shifts in the microgravity setting may cause impairment of brain CSF drainage and venous congestion within the brain. Both of these could cause an elevation of ICP that would be transmitted down the optic nerve sheaths into the orbit. Moderately elevated lumber puncture opening pressures in astronauts following long-duration space flight, which might have been higher during the mission, support this notion. So does the recent article by Roberts et al4 documenting narrowing of the central sulcus, upward shift of the brain, and narrowing of the CSF spaces at the vertex after long-duration space flights. Lack of headaches and other findings commonly associated with IIH suggest that increased ICP might not be the sole cause and that other etiologies should be considered.
Lee AG, Mader TH, Gibson CR. Why Space Flight-Associated Neuro-ocular Syndrome May Differ From Idiopathic Intracranial Hypertension—Reply. JAMA Ophthalmol. 2018;136(4):452. doi:10.1001/jamaophthalmol.2018.0319
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