Letters to the Editor
We previously reported a 57-year-old male astronaut who flew on his first 6-month space mission in 2003 and his most recent flight in 2011–2012 (1). His first premission fundus examination was normal bilaterally, but his postmission examination revealed choroidal folds and a cotton wool spot in his right eye. The left fundus was normal. In 2011, his preflight optical coherence tomography (OCT) documented subtle choroidal folds in the right eye and magnetic resonance imaging (MRI) showed bilateral optic nerve (ON) sheath distention with mild right globe flattening. On ophthalmoscopy, spontaneous venous pulsations (SVPs) were present bilaterally. Five months into the second mission, remotely guided video funduscopy on the International Space Station documented mild disc edema in the right eye without SVPs. The left fundus was normal with prominent SVPs.
Two days postmission there was Frisen Grade 1 disc edema with barely detectable SVPs and choroidal folds in the right eye. The left fundus was normal with prominent SVPs. Six days postflight, MRI documented a moderate increase in both ON sheath diameters (left > right), compared with preflight, and there was bilateral globe flattening. Eight days postmission, the opening pressure was 18 cm H2O on lumbar puncture. Fifty-two days postmission, the right optic disc swelling had resolved, and SVPs remained unchanged from the previous examination. At 21 months after the mission, there was normal peripapillary retinal nerve fiber layer thickness in each eye but persistent right choroidal folds. High resolution Heidelberg spectralis OCT video and 78 D lens assessment revealed SVPs to be absent in the right eye and present in the left eye. Magnetic resonance imaging performed 7 months postflight was unchanged compared with the previous study.
SVPs are pulsatile changes in the diameter of the retinal veins just before their exit from the eye to form the central retinal vein (CRV) (2). The disappearance of previously documented SVPs is a well-known, usually bilateral, clinical sign associated with elevated intracranial pressure. SVPs are generally thought to arise from normal cyclical venous pressure changes that occur within the eye and retrolaminar region. Blood flows from the retinal capillaries into the CRV because there is a relatively high pressure in the retinal capillary bed compared with the low pressure CRV (2). The sudden transient increase in flow at the point of venous outflow during systole temporarily decreases the volume of blood in a small segment of the vein on the optic disc, resulting in venous collapse. During diastole, the flow of venous blood exiting the eye ceases because the retrolaminar venous pressure is higher than the intraocular venous pressure, and the venous blood volume quickly increases in the previously collapsed segment. SVPs are eliminated when the downstream pressure in the CRV at the lamina cribrosa rises above that of the maximum intraocular venous pressure produced during systole. From the lamina cribrosa, the CRV drains within the central portion of the ON but exits the nerve approximately 10 mm posterior to the globe. At this point, the CRV traverses the subarachnoid space, and the stage is set for an elevated subarachnoid space (SAS) pressure to directly impact the pressure within the CRV (3,4).
Before his second space mission, we confirmed that the astronaut had bilateral SVPs suggesting normal SAS pressures within the ON sheaths. However, 5 months into the mission, we documented mild optic disc swelling with absent SVPs in the right eye, despite prominent SVPs and a normal disc in the left eye. The disappearance of right SVPs associated with optic disc swelling and the normal appearance of the left disc during a space mission suggests that microgravity exposure led to increased right perioptic SAS pressure. Asymmetric postmission optic disc changes have been documented in other astronauts (1,5). It has been suggested that these findings are due to a microgravity-induced sequestration of CSF within the ON sheath compartment (1,5). This hypothesis is supported by the testing results in our case since the postmission lumbar puncture opening pressure was within normal limits in the presence of asymmetric findings. Absent right SVPs 21 months postmission suggest a continued pressure elevation within the ON sheath long after the mission ended.
1. Mader TH, Gibson CR, Pass AF, Lee AG, Killer HE, Hansen H-C, Dervay JP, Barratt MR, Tarver WJ, Sargsyan AE, Kramer LA, Riascos R, Bedi DG, Pettit DR. Optic disc edema in an astronaut after repeat long-duration space flight. J Neuroophthalmol. 2013;33:249–255.
2. Jacks AS, Miller NR. Spontaneous retinal venous pulsation: aetiology and significance. J Neurol Neurosurg Psychiatry. 2003;74:7–9.
3. Fry WE, Variations in the intraneural course of the central retinal vein. Arch Ophthalmol. 1930;4:180–187.
4. Fry WE. The pathology of papilloedema: an examination of forty eyes with special reference to compression of the central vein of the retina. Am J Ophthalmol. 1931;14:874–883.
5. Mader TH, Gibson CR, Pass AF, Kramer LA, Lee AG, Fogarty J, Tarver WJ, Dervay JP, Hamilton DR, Sargsyan A, Phillips JL, Tran D, Lipsky W, Choi J, Stern C, Kuyumjian R, Polk JD. Optic disc edema, globe flattening, choroidal fold, and hyperopic shifts observed in astronauts after long-duration space flight. Ophthalmology. 2011;118:2058–2069.