Ocular Changes After Space Travel

Background on Space Flight and Ocular Changes

According to experts in space medicine and physiology, the real limitation to successful deep-space exploration and settlement lies in the intrinsic physiologic and psychological fragility of the human machine. Long-duration space flight has significant and measurable pathophysiologic effects on crew members. Accelerated loss of bone and muscle tissue, cardiovascular deconditioning, and immune system compromise are some of the more notable adverse physiologic changes that can occur with extended exposure to microgravity.^[1,2] The dangers of bodily exposure to elevated levels of ionizing and cosmic radiation during space missions are increasingly appreciated.^[3]

The technological limitations of exploration class missions (those beyond low earth orbit) are arguably readily surmountable with adequate time and investment. However, recent evidence suggests another potentially deleterious physiologic complication: intracranial hypertension, with potential eye damage and visual compromise.

Since 1989, NASA physicians have routinely queried astronauts on the subjective status of their vision after short- and long-duration missions. After persistent reports of vision changes by the astronauts, NASA undertook more rigorous investigations to explore the potential sources of these changes. Publications on this emerging phenomenon have begun to appear more extensively in the literature.

Chung and colleagues^[4] documented changes in directly measured intraocular pressure in space flight, reporting increases of up to 26.3%. In a study that included assessments related to visual compromise, Peters and colleagues^[5] showed reversible decrements in dynamic visual acuity (eg, visual changes during movement, such as walking) in crew members after missions lasting 6 months.

In one of the most detailed evaluations to date, Mader and colleagues^[6] conducted a retrospective observational examination of ophthalmic anomalies in 7 astronauts who had been on 6-month missions to the International Space Station. Each astronaut underwent pre- and postflight complete eye examinations, which included cycloplegic and/or manifest refraction and fundus photography. Six astronauts also underwent postmission optical coherence tomography (OCT) and MRI, and 4 had lumbar puncture.

Ophthalmologic findings were disc edema, globe flattening, and choroidal folds, each occurring in 5 astronauts; cotton-wool spots in 3 astronauts; and nerve fiber layer thickening (on OCT) and decreased near vision, each in 6 astronauts. Lumbar punctures performed in the 4 astronauts with disc edema revealed mildly elevated opening pressures of 22, 21, 28, and 28.5 cm H₂O at 6, 19, 12, and 57 days postmission, respectively, suggesting a chronic increase in intracranial pressure.

The researchers also analyzed postflight questionnaires on in-flight vision changes in approximately 300 additional astronauts who had flown in space for variable durations. The questionnaires documented that approximately 29% and 60% of astronauts on short- and long-duration missions, respectively, experienced a decrement in distant- and near-vision acuity. Some of these vision changes remain unresolved years after flight.^[6]

In a follow-up study using quantitative and qualitative MRI, Kramer and colleagues^[7] evaluated 27 astronauts who had previously flown space missions of various durations; 8 of these individuals underwent repeat MRI after an additional space flight. Optic nerve sheath diameter and optic nerve diameter were quantified at various sections of the nerve. Qualitative analyses of optic nerve sheath, optic disc, posterior globe, and pituitary gland morphology were also determined and were correlated for possible association with intracranial evidence of hydrocephalus, vasogenic edema, central venous thrombosis, and/or mass lesion.

This cohort of astronauts demonstrated various combinations of optic nerve sheath distention, posterior globe flattening, optic disc protrusion, and increased optic nerve diameter. The frequency and severity of such findings related to lifetime duration of microgravity exposure: The longer the duration of exposure, the greater the likelihood of positive eye findings).

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Cite this: Eleanor A. O'Rangers, Kieran T. Smart. Ocular Changes After Space Travel - Medscape - May 23, 2012.