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Is visual field or electrophysiology a better functional assessment of vision in space?
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Electrophysiology provides objective functional metrics
Regardless of the pathophysiology of optic disc swelling present in spaceflight-associated neuro-ocular syndrome (SANS), the effect on the function of the optic nerve has not been established. OCT of the retinal nerve fiber layer provides a structural correlate for disc edema but does not provide functional data. The challenge in defining a functional metric is, in part, related to the inability of current clinical metrics, including best corrected visual acuity and visual fields, to detect subtle abnormalities in vision. Thus, while visual impairment has not been documented in SANS, it may be that central vision and visual fields are not sensitive enough to identify subclinical ocular compromise in astronauts.
Electrophysiology is a direct objective measure of the electrical activity in the eye. Broadly, different groups of cells can be tested using different functional measures. By recording the cornea potential timed to a light flash, distinct components of the electrical cascade that occur in the retina can be recorded. The inner plexiform and ganglion cell layer can be measured by using an adaptation of the full-field electroretinogram, which utilizes a pattern stimulus, or by altering the stimulus intensity and recording duration (PhNR). Both of these tests look at the spiking activity generated by retinal ganglion cells (and some amacrine cells) in the inner retina. Both have been found to be abnormal in patients with idiopathic intracranial hypertension and acquired and hereditary optic neuropathies and optic neuritis. Importantly, the PhNR has also been able to detect subclinical abnormalities in patients who are carriers of a hereditary optic neuropathy despite normal visual fields. This data suggest that electrophysiology may have a better dynamic range for detecting subclinical ocular impairment in SANS.
Ideally any investigation into visual impairment in SANS would rely on multiple metrics. Visual electrophysiology provides for an objective functional metric, with potentially a better dynamic range then visual fields, for enhancing our understanding of optic nerve dysfunction in SANS.
Rustum Karanjia, MD, PhD, is from Doheny Eye Institute and Doheny Eye Centers UCLA, Los Angeles, and Department of Ophthalmology, University of Ottawa. Disclosure: Karanjia reports no relevant financial disclosures.
Visual field test can follow visual function longitudinally
I believe the visual field test would be the better tool to assess astronauts’ visual function in space, given a few caveats. First, astronauts should have a series of three to five baseline examinations before flight so that we are confident that any changes seen on subsequent visual fields are real. Second, it would be helpful to have a skilled visual field examiner administer each test. This could be done remotely while the astronauts are in space. Use of a skilled examiner allows for immediate assessment of the astronaut’s concentration and alertness. The examiner serves as a coach and can pause the test or restart the test if any issues are identified. A skilled perimetrist is often key to obtaining interpretable tests. Third, it would be important to ensure a similar testing environment on the ground and in space, with a specific focus on matching the ambient lighting intensity. Finally, it would be important to be able to refract the patient accurately in space and correct any refractive errors before each testing session. If these few things can be accomplished, the visual field test is an excellent tool for following visual function longitudinally.
While the visual field test is considered a “subjective” test, we know from millions of visual field tests given each year to patients in our clinics that it can be a highly accurate diagnostic tool capable of picking up small changes in visual function. Luckily, the astronaut core is an enriched population of middle-aged, highly motivated adults who are chosen because of their ability to focus on and complete tasks when they are tired or stressed. This makes the astronaut core ideal candidates to use visual field testing. With coaching, they will likely perform well on the perimetry test (ie, have high repeatability and reproducibility on the test with minimal fixation losses and low false positive/negative responses), and eye care specialists should be able to confidently interpret their results.
Brian C. Samuels, MD, PhD, is an associate professor of ophthalmology at the University of Alabama at Birmingham. Disclosure: Samuels reports he has received consulting fees from NASA/Wyle in the last 36 months but none in the last 12 months and has received salary support from NASA grant NNX13AP91G.