Common eye movements may cause glaucoma

The research suggests that over time common, unavoidable eye movents cause strain on the optic nerve. It may also explain why the use of tension-lowering eye drops can improve the condition.Lead investigator and chief of paediatric ophthalmology and strabismus at UCLA Dr Joseph Der said that while elevated intraocular pressure (IOP) is a known risk factor for glaucoma, the fact that some people develop glaucoma despite having never had elevated IOP suggests “that there must be a mechanism that is independent of intraocular pressure that causes damage to the optic nerve.”To test the repetitive movent theory, the team used magnetic resonance imaging (MRI) to directly visualise the optic nerve as people gazed in several directions, such as straight ahead and in degrees of adduction and abduction. The results were then compared between healthy test volunteers and those with normal-tension glaucoma.{{quote-A:R-W:450-I:2-Q: It’s certainly plausible that some of these rapid eye movents would involve degrees of adduction that could have a physiological effect -WHO:Dr Joseph Der, Lead investigator and chief of paediatric ophthalmology and strabismus at UCLA}}The scans showed that the optic nerve acts as a tether that resists eye rotation from central gaze towards the nose. This causes the eyeball to retract when the eye rotates further into the tether effect.For both groups, the tether was slack in abduction and tight in adduction. However, for those with normal-tension glaucoma, the tethering in adduction caused the globe to retract significantly.The findings indicate that in people with normal-tension glaucoma, adduction strains the optic nerve and sheath, which mechanically loads the globe of the eye against the forces of the medial rectus. Conversely, in healthy participants the optic nerve and sheath stretched freely and absorbed the strain caused by adduction on the eye’s tissue.According to Der, everyday eye movents are likely to put even more strain on the optic nerve, as the ocular range of motion in the study is about half that achieved by people under normal conditions. Additionally, rapid eye movents called saccades occur approximately three times per second, or an estimated 183,000 times daily.“It’s certainly plausible that some of these rapid eye movents would involve degrees of adduction that could have a physiological effect,” Der said.A separate study conducted by the team sought to understand what effect such forces might have on various eye structures.Beginning with bovine specimens, the researchers conducted computational simulations of eye movents, in which they calculated biomechanical properties such as tensile strength and point of failure of the optic nerve and other eye tissues.{{quote-A:R-W:450-Q: Adduction causes stress in two regions of the eye commonly affected by glaucoma – the optic nerve sheath and sclera. }}A computer simulation using donated human eye models using finite elent analysis software shows deformation caused by the eye rotating towards the nose six degrees beyond the point where the optic nerve acts as a tether.The team found adduction causes stress in two regions of the eye commonly affected by glaucoma – the optic nerve sheath and sclera.“If these findings are confirmed by additional studies, the next step is to investigate therapies that could possibly relieve the strain produced by eye movent,” Der said.Der suggested that the prostaglandin eye drops routinely used to reduce IOP have a side effect that can benefit those with normal IOP. The eye drops tend to cause the fat around the eye to atrophy, which in turn, causes the globe of the eye to sink back slightly into the socket.Such an effect may create some slack in the optic nerve needed to help treat the disease, and if so could lead to more targeted ways of achieving the same outcome.