Many of us know about common lower-order aberrations such as nearsightedness or farsightedness, but higher-order aberrations are what make up the remaining 15 per cent of total aberrations in the eye and are often little understood.
What are Higher Order Aberrations?
Existing outside the realms of common visual deficiencies such as astigmatism, higher order aberrations are varied and complex in nature, with terms such as coma or trefoil being terms a doctor would use to describe them, while mathematical expressions can sometimes be the only way of describing the nature of such eye disorders.
In general terms, an aberration deviates from normal vision due to refractive errors which cause symptoms such as blurring of an image due to lighting distortion from misshaped eyes as an example. It is believed that every living human has some form of higher order aberration, with advancements in optical technology being the only reason why we can now correctly diagnose such deformations.
Imperfections in the cornea lead to flawed retinal focusing, impacting on a person’s visual quality dependant on a myriad of factors such as pupil size and whether the condition is caused by physical scarring or as a sub condition of a cataract.
Symptoms and Causes
Higher-order aberrations are often reported to include image ‘ghosting’, glare or a starburst effect which distorts a person’s view of the world, while it is thought that the prevalence of several higher-order aberrations can be present in the eyes as they interact with each other, making it hard to attribute certain symptoms with a specific diagnosis. Poor night vision and a loss of general contrast in certain lighting conditions are also often reported in this type form of eye aberration.
A wavefront of light used by an optical specialist to determine any significant distortion as the light passes through the eye, which is termed as the ‘Shack-Hartmann aberrometer’ method, and is likely how you would be tested in modern day eye examinations. Comparisons are drawn between a theoretical working eye’s natural plane and the results seen in the test of each person’s unique planes, working as a base map to determine any abnormalities, with necessary measurements now taking only seconds to obtain.
It is thought that people with larger pupils are more predisposed to noticing visual symptoms resulting from higher-order aberrations, with the widening of the pupils in darkened environments making the apparent imperfections all the more obvious. Traditionally, curvature of the cornea causes refraction errors of all kinds, while dry eyes and general eye trauma can also impact on prevalence of eye aberrations.
Due to the fact that diagnosis of high order aberrations has only recently been possible, treatment of such errors is only likely to improve within the coming years, with standard contact lenses, eyeglasses and refractive surgery looking to be developed in new ways to allow for vision correction of such aberrations, while the ultimate goal in optometry is to make it possible for flattening the wavefront on the pupil plane to remove any distortion and lessen symptoms of refractive error.
Jamie blogs for DirectSight – glasses online experts.