Choroidal Folds in Astronauts

Abstract

We read with interest and enthusiasm the recent article by Sibony and colleagues 1 which accurately described the biomechanical etiology of peripapillary wrinkles, retinal folds, and choroidal folds caused by idiopathic intracranial hypertension (IIH). The authors correctly state that chorioretinal folds have been documented in a variety of ophthalmic disorders including the microgravity environment of prolonged space flight. We would like to emphasize two main theories that might explain the optic disc edema, globe flattening, and choroidal folds observed in astronauts during and after longduration space flight. The first theory proposes that these changes result from a rise in intracranial pressure (ICP) similar to that seen with terrestrial IIH.2 In this case, the rise in ICP would presumably be propagated from the cerebral spinal fluid (CSF) surrounding the brain down the optic nerve (ON) sheaths to the posterior globe. This could result in a posterior pressure within the optic nerve sheath as well as an anteriorly directed pressure on the posterior globe and optic nerve head. As nicely described by the authors, this would result in biomechanical stress on the ON head and load bearing structures and ultimately produce retinal and choroidal folds. The second possible explanation, in some astronauts, is that these changes are the end result of localized events occurring at the level of the intraorbital ON with or without a rise in intracranial pressure. 2,3 In this scenario, prolonged microgravity fluid shifts may cause alterations in CSF flow dynamics in the intraorbital portion of the subarachnoid space (SAS) such that CSF enters the SAS but outflow may become impeded. Thus, CSF within the SAS of the ON may gradually become sequestered with a resulting local elevation of optic nerve sheath (ONS) pressures capable of producing the anatomic changes observed. In a previously affected astronaut, following long-duration space flight, we documented asymmetric disc edema in conjunction with a normal lumbar puncture opening pressure 1 week after space flight 4 as well as the unilateral loss of previously visible spontaneous venous pulsations during space flight in the eye with disc edema. 5 These continued to be absent 21 months following his return to Earth with an otherwise normal disc.5 Also, a more recent report of lumbar puncture opening pressures of 22 cm and 16 cm H 2 O, obtained at 1 week and 1 year post mission respectively, in the presence of asymmetric disc edema still present 6 months following longduration flight, suggests optic nerve sheath compartmentation as the etiology (Lee A, unpublished observations, 2016).We hypothesize that, although the retinal and choroidal folds in papilledema could result from elevated ICP, these changes could also be consistent with a rise in ON sheath pressure with or without a commensurate elevation in ICP. This may be the case in some of the affected astronauts as described above.It is interesting to note that there was only a 1% and 10% frequency of choroidal folds, by...

Spaceflight associated neuro-ocular syndrome (SANS) and the neuro-ophthalmologic effects of microgravity: a review and an update

“…Visual acuity, Amsler grid, ophthalmoscopy, tonometry, fundus photography, orbital ultrasound and OCT are available on the International Space Station (ISS) and have been essential in documenting the development of the in-flight changes of SANS. [1][2][3] We recognize and acknowledge the limitations of the current SANS definition and that the clinical syndrome has been re-defined over time as new information has emerged over the last decade. Mader et al in 2011 originally described the clinical findings of SANS in astronauts after LDSF.…”

“…In the terrestrial environment, CSF is largely produced in the choroid plexus and drains into the lower pressure cervical venous vasculature. [1][2][3][4][5][6][7][8][9] Although vascular autoregulation stabilizes the cerebral and ONH arterial diameter, 10,11 jugular venous distention has been well documented during both head down and microgravity (MG) studies 12 suggesting that cerebral and jugular venous congestion may be present in the MG environment. Jugular venous distention alone does not necessarily prove that venous pressure is increased during spaceflight and some studies have suggested that the venous pressure may in fact be decreased.…”

Optic Nerve Sheath Distention as a Protective Mechanism Against the Visual Impairment and Intracranial Pressure Syndrome in Astronauts

“…As nicely reported by Mader et al, 1,2 a significant proportion of the astronauts who spend extended periods in microgravity develop ophthalmic abnormalities, including optic nerve sheath (ONS) distention, optic disc swelling, globe flattening, choroidal folds, and hyperopic shifts, which is also designated as visual impairment and intracranial pressure (VIIP) syndrome. Importantly, astronauts with VIIP can experience decreases in visual acuity that remain unresolved years after flight.…”

“…As noted above, the rise in ICP, resulting from microgravity-induced cephalad fluid shifts, would presumably be propagated from the CSF surrounding the brain down the ONSs to the posterior globe, ultimately resulting in the ophthalmic abnormalities of the VIIP syndrome. 1 However, we believe that the ONS response to the rise in ICP during extended microgravity exposure may play a prominent role in determining whether or not an astronaut is susceptible to developing ophthalmic changes of VIIP. It is important to note that elevated CSF pressure results in dilation of the ONS before papilledema appears.…”

Author Response: Choroidal Folds in Astronauts

“…We would like thank Mader and colleagues 1 for their comments on our study 2 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.…”