Scott et al1 describe the modulation of cerebral and ocular hemodynamics and pressures in a model of spaceflight-associated neuro-ocular syndrome (SANS). Astronauts serving on long spaceflight missions on the International Space Station (ISS) missions have a number of interesting ocular findings (termed SANS), including choroidal folds, optic disc edema, cotton-wool spots, globe flattening, and a refractive (hyperopic) shift. The subject of SANS is fascinating, and I would direct the reader who is interested in more information to several recently published review articles on the topic.2,3 Loss of the hydrostatic pressure gradient in microgravity and a cephalad fluid shift can affect the eye, orbit, and optic nerve sheath during long spaceflight and is one of the major proposed mechanisms for SANS. Scott et al1 hypothesized that reversing a negative translaminar pressure gradient between intracranial pressure (ICP) and intraocular pressure (IOP) might be a relatively safe, inexpensive, and convenient countermeasure for SANS. Some potential modifiable variables in the translaminar pressure gradient include modulating cerebral blood flow, decreasing ICP (eg, with acetazolamide), or increasing IOP. The authors in this study examined whether exercise (eg, resistance, moderate-intensity aerobic, and high-intensity aerobic) or artificially increasing IOP (with commercially available swimming goggles) is associated with modulated cerebro-ocular hemodynamic and pressure changes during −15° head-down tilt. The head-down tilt is a terrestrial analog for the cephalad shift seen during and after long spaceflight and possibly SANS. As expected, the addition of the swimming goggles to these healthy adults on Earth was associated with increased IOP and with modulated translaminar pressure gradient in head-down tilt. The authors note that long spaceflight may be associated with an increase in ICP that is greater than IOP, resulting in a negative and anteriorly directed translaminar pressure gradient; thus, increasing the IOP with swimming goggles could be used to prevent or treat SANS. Intraocular pressure has been measured during long spaceflight, and low IOP (hypotony) is not present in astronauts on the ISS. The terrestrial analog, however, of elevated IOP (ocular hypertension) with secondary compression of the lamina cribrosa and retinal ganglion cell layer loss can result in glaucoma; thus, there is some risk to this proposed SANS countermeasure. In addition, although some limited ICP measurements have been reported in SANS after return to Earth, there remains no practical method to directly measure ICP on the ISS and there is no proven, valid, reliable, reproducible, and effective noninvasive method to indirectly measure ICP or the translaminar pressure gradient on the ISS.
Lee AG. Modulation of Cerebro-ocular Hemodynamics and Pressures in a Model of SANS Using Swimming Goggles. JAMA Ophthalmol. Published online April 18, 2019. doi:10.1001/jamaophthalmol.2019.0414
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