Spaceflight associated neuro-ocular syndrome

[3] The study of visual changes and ICP in astronauts on long-duration flights is a relatively recent topic of interest to space medicine professionals.

[5] Optic disc edema, globe flattening, choroidal folds, hyperopic shifts and an increased intracranial pressure have been documented in these astronauts.

It seems unlikely that resistive or aerobic exercise are contributing factors, but they may be potential countermeasures to reduce intraocular pressure (IOP) or ICP in-flight.

[5] Although a definitive cause (or set of causes) for the symptoms outlined in the Existing Long-Duration Flight Occurrences section is unknown, it is thought that venous congestion in the brain brought about by cephalad fluid shifts may be a unifying pathologic mechanism.

[8] As part of the effort to elucidate the cause(s), NASA has initiated an enhanced occupational monitoring program for all mission astronauts with special attention to signs and symptoms related to ICP.

[9] Animal research from the Russian Bion-M1 mission indicates duress of the cerebral arteries may induce reduced blood flow, thereby contributing to impaired vision.

[10] On 2 November 2017, scientists reported that significant changes in the position and structure of the brain have been found in astronauts who have taken trips in space, based on magnetic resonance imaging (MRI) studies.

Astronauts are well known to have orthostatic intolerance upon reentry to gravity after long-duration spaceflight, and the dietary sodium on orbit is also known to be in excess of 5 grams per day in some cases.

[21] Two other investigations using transcranial Doppler ultrasound techniques showed that resistive exercise without a Valsalva maneuver resulted in no change in peak systolic pressure or ICP.

While the common theories regarding vision issues during flight focus on cardiovascular factors (fluid shift, intracranial hypertension, CO2 exposure, etc.

Data identified as part of an ongoing nutrition experiment found biochemical evidence that the folate-dependent one-carbon metabolic pathway may be altered in those individuals who have vision issues.

Thus, data from the Nutrition SMO 016E provide evidence for an alternative hypothesis: that individuals with alterations in this metabolic pathway may be predisposed to anatomic and/or physiologic changes that render them susceptible to ophthalmologic damage during space flight.

An anatomic cause of the microgravity related intracranial hypertension and visual disturbances has been proposed and is termed Space Obstructive Syndrome or SOS.

This hypothesis has the possibility of linking the various symptoms and signs together through a common mechanism in a cascade phenomenon, and explaining the findings in one individual and not another due to specific anatomic variations in the structural placement of the internal jugular vein.

In a standing position, the main outflow from the head is through the vertebral venous system because the internal jugular veins, located primarily between the carotid artery and the sternocleidomastoid muscle are partially or completely occluded due to the pressure from these structures, and in a supine position, the main outflow is through the internal jugular veins as they have fallen laterally due to the weight of the contained blood, are no longer compressed and have greatly expanded in diameter, but the smaller vertebral system has lost the gravitational force for blood outflow.

The astronauts affected by long term visual changes and prolonged intracranial hypertension have all been male, and SOS may explain this because in men, the sternocleidomastoid muscle is typically thicker than in women and may contribute to more compression.

On the ground, lumbar puncture is the standard method of measuring cerebral spinal fluid pressure and ICP,[6][44] but this carries additional risk in-flight.

[4] NASA is determining how to correlate ground-based MRI with inflight ultrasound[4] and other methods of measuring ICP in space is currently being investigated.

[44] To date, NASA has measured intraocular pressure (IOP), visual acuity, cycloplegic refraction, Optical Coherence Tomography (OCT) and A-scan axial length changes in the eye before and after spaceflight.

[45] There are different approaches to non-invasive intracranial pressure measurement, which include ultrasound "time-of-flight" techniques, transcranial Doppler, methods based on acoustic properties of the cranial bones, EEG, MRI, tympanic membrane displacement, oto-acoustic emission, ophthalmodynamometry, ultrasound measurements of optic nerve sheath diameter, and Two-Depth Transorbital Doppler.

Such approaches can not measure an absolute ICP value in mmHg or other pressure units because of the need for individual patient specific calibration.

His postflight fundus examination (Figure 1) revealed choroidal folds below the optic disc and a single cotton-wool spot in the inferior arcade of the right eye.

[5] The second case of visual changes during long-duration spaceflight on board the ISS was reported approximately 3 months after launch when the astronaut noticed that he could now only see Earth clearly while looking through his reading glasses.

At 10 days post landing, an MRI of the brain and eyes was normal, but there appeared to be a mild increase in CSF signal around the right optic nerve.

[5] The fourth case of visual changes on orbit was significant for a history of transsphenoidal hypophysectomy for macroadenoma where postoperative imaging showed no residual or recurrent disease.

Approximately 2 months into the ISS mission, the astronaut noticed a progressive decrease in near-visual acuity in his right eye and a scotoma in his right temporal field of vision.

This case is interesting because the astronaut did not have disc edema or choroidal folds, but was documented to have nerve fiber layer (NFL) thickening, globe flattening, a hyperopic shift and subjective complaints of loss of near vision.

MRI of the brain and eyes days postflight revealed bilateral flattening of the posterior globe, right greater than left, and a mildly distended right optic nerve sheath.

[5] According to guidelines set forth by the Space Medicine Division, all long-duration astronauts with postflight vision changes should be considered a suspected case of VIIP syndrome.

Each case could then be further differentiated by definitive imaging studies establishing the postflight presence of optic-disc edema, increased ONSD and altered OCT findings.

STS-41 crewmembers conduct Detailed Supplementary Objective (DSO) 472, Intraocular Pressure on the middeck of Discovery, Orbiter Vehicle (OV) 103. Mission Specialist (MS) William M. Shepherd rests his head on the stowed treadmill while Pilot Robert D. Cabana , holding Shepherd's eye open, prepares to measure Shepherd's intraocular pressure using a tonometer (in his right hand).
Figure 1: Fundus examination of the first case of visual changes from long-duration spaceflight. Fundus examination revealed choroidal folds inferior to the optic disc and a single cotton-wool spot in the inferior arcade of the right eye (white arrow).
Figure 2: Fundus examination of second case of visual changes from long-duration spaceflight. Fundoscopic images showing choroidal folds (white arrows) in the papillomacular bundle area in the right eye and left eye and a cotton-wool spot (bottom arrow) at the inferior arcade in the left eye. Both optic discs show grade 1 disc edema.
Figure 5: On-orbit ultrasound of posterior orbit of the fourth case of visual changes from long-duration spaceflight. In-flight ultrasound image of the right eye showing posterior globe flattening and a raised optic disc consistent with optic-disc edema and raised ICP.
Figure 6: On-orbit ultrasound of optic nerves of the fourth case of visual changes from long-duration spaceflight. In-flight ultrasound shows proximal kinking and increased optic nerve sheath diameter (ONSD) of approximately 12 mm that is consistent with raised ICPs. Optic nerve shown in purple and the ONSD in green.
Figure 10: MRI (R+30 days) of the fourth case of visual changes from long-duration spaceflight. There is prominence of central T2-hyperintensity of the optic nerves bilaterally, right greater than left approximately 10 to 12 mm posterior to the globe (arrow) that represents an element of optic nerve congestion.
Figure 11: MRI (R+30 days) of the fourth case of visual changes from long-duration spaceflight. Tortuous optic nerve and kink on left (arrow). Control orbit on the right.
Figure 13: Fundus examination of the sixth case of visual changes from long-duration spaceflight. Preflight images of normal optic disc. Postflight right and left optic disc showing grade 1 (superior and nasal) edema at the right optic disc.
Figure 15: Preflight images of the right and left optic discs (upper). Postflight images of the ONH showing in more detail the extent of the edematous optic-disc margins and glutting of the superior and inferior nerve fiber layer axons OD and OS (arrows) (lower).