Spaceflight eye syndrome

As India eagerly awaits the launch of Axiom Mission 4, which is set to carry its astronaut Shubhanshu Shukla into space, here’s a look at the adverse effects that most astronauts experience during their prolonged stay in the microgravity environment. Almost all of them get affected with spaceflight-associated neuro-ocular syndrome (SANS), causing some visual disturbances. These are, however, reversible.

Although more than half of the astronauts had been reporting visual disturbances in space, it was only in 2011, 50 years after manned space flights began, that Thomas Mader and colleagues from the Alaska Medical Centre, USA, discovered significant visual changes related to microgravity in space.

On earth, gravitational forces ensure a progressively increasing gradient of blood pressure from head to toe, but it diminishes during space flight. As bodily fluids shift towards the head, intracranial pressure (ICP) elevates, leading to a spaceflight-associated neuro-ocular syndrome. Fortunately, most of its effects are reversible.

The body fluids are mainly contained in blood vessels and body tissues. The eye maintains its shape at a pressure of less than 21 mmHg, while the brain floats in cerebrospinal fluid at a pressure of 5-8 mmHg. A rise in ICP results in swelling of the optic nerve head and flattening of the eyeball. Interestingly, the eye pressure rises in space but spontaneously normalises during extended stay. Measuring the ICP in space is a challenge, but the returning astronauts do show a somewhat raised ICP.