Long duration spaceflight produces various physiological and pathologic changes in astronauts. Changes in the ocular structure were first noted to be occurring in these astronauts in 2011, and findings such as optic disc edema, cotton wool spots, choroidal folds, and globe flattening have been hallmarks of this newly discovered phenomenon that is now deemed Spaceflight Associated Neuro-ocular syndrome (SANS). Our recently accepted article was put together by some of the leading scientists and clinicians dedicated to studying SANS, and describes the current knowledge base regarding the proposed etiologies and pathogenesis, the latest structural descriptions from a variety of imaging technologies, and a discussion on proposed countermeasures that may one day help prevent these changes from occurring.
Scientists have been working to better understand the etiology of the ocular physiologic changes. However, one of the greatest hindrances is that there does not appear to be a reliable terrestrial analog. When increased intracranial pressure was believed to be a primary causative mechanism, idiopathic intracranial hypertension (IIH) was thought be such an analog. However, significant differences in the structural changes and symptoms that occur in these two syndromes lead us to believe that IIH is not a great model, and increased ICP may not solely explain the changes seen in SANS. Another proposed explanation includes compartmentalization of cerebrospinal fluid (CSF) within the orbital optic nerve sheath, creating a localized compartment syndrome secondary to interruption of both the CSF and/or glymphatic flow within the orbit. There also appears to be a potential contribution from venous pooling and vascular stagnation within the choroid that may lead to reactive oxygen species accumulation and inflammatory pathway activation.
A variety of imaging modalities have been used to better describe the ocular structural changes of SANS. Specialized orbital and cranial MRI can only be performed terrestrially and have been used to specifically characterize overall globe shape and orbital nerve kinking. Ultrasound, ocular coherence tomography and fundoscopy are used both in-flight and terrestrially. Ocular ultrasound is used to characterize globe shape and optic nerve width. Ocular coherence tomography is used to characterize the structure of the optic disc and retinal nerve fiber layer. Direct fundoscopy is used to detect the presence of retinal changes such as cotton wool spots, retinal hemorrhages, choroidal folds and optic disc edema.
Countermeasure development has focused on the most likely contributing factors. Reversal of the cephalic fluid shift has been attempted by the use of lower body negative pressure pants. Pressurized swim goggles have been hypothesized as a means to normalize the altered translaminar pressure gradient. Other countermeasures include attempts to counter the metabolic changes occurring in spaceflight and alter the makeup of both the atmosphere and food sources provided to astronauts in long duration spaceflight.
The etiology of the structural changes described in SANS is most likely multifactorial, and the development of successful countermeasures hinges on a better understanding of the contributing factors and disease progression.