Higher Order Aberrations

Higher order aberrations have relatively unfamiliar names — such as coma, spherical aberration, and trefoil. These aberrations can cause difficulties when viewing at night, glare, halos, blurriness, starburst patterns, or double vision (diplopia).

No eye is perfect, meaning all eyes have at least some degree of higher order aberration. If you are diagnosed with higher order aberration, you do not need to worry unless they are severe enough to cause visual symptoms.

So what exactly are higher order aberrations?

Higher order aberrations are distortions caused by the wavefront of light as it passes through an eye with uneven refractive components (tear film, cornea, vitreous humor, lens, and aqueous humor).

Abnormal curvature of the cornea and lens can contribute to the distortion caused by the wavefront of light. Severe higher order aberrations can also occur due to corneal scarring from eye surgery, trauma, or disease.

Cataracts that cloud the natural lens of the eye can also cause higher order aberrations. Aberrations can also occur when dry eyes reduce the tear film of the eye, which helps bend or refract light rays to achieve focus.

Common Wavefront Shapes (Aberrations)

How are higher order aberrations diagnosed?

Higher order aberrations are identified by the types of distortions obtained by the wavefront of light as it passes through your eye.

Since no eye is optically perfect, the wavefront of light passing through the eye will have some three-dimensional distorted shapes. So far, more than 60 different wavefront shapes, or aberrations, have been identified.

There are two types of aberrations commonly used to describe the refractive errors of the eye:

• Lower order aberrations primarily include myopia and hyperopia (blur), as well as astigmatism. They account for about 85 percent of all aberrations in the eye.
• Higher order aberrations include various types of aberrations. Some of them have names like coma, trefoil, and spherical aberration, but many others are only defined by mathematical expressions (Zernike polynomials). They account for about 15 percent of the total aberrations in the eye.

The order refers to the complexity of the wavefront shape appearing through the pupil — the more complex the shape, the higher the order of the aberration.

This chart shows the more common aberration shapes created when the wavefront of light passes through an eye with imperfect vision. A theoretically perfect eye (top) is represented by a flat plane with no aberration, referred to as a piston for reference.

How do higher order aberrations affect vision quality?

The impact of higher order aberrations on vision quality depends on many factors, including the underlying cause of the aberration.

People with larger pupils may often experience more vision symptoms due to higher order aberrations, especially in low-light conditions when the pupils are more dilated.

But even those with small or medium-sized pupils can experience significant vision issues when higher order aberrations are due to conditions like surface scarring of the eye (cornea) or cataracts that cloud the natural lens of the eye. Additionally, some studies have found that specific types and directions of higher order aberrations affect the vision quality of eyes with smaller pupils.

A large amount of certain higher order aberrations can cause severe, even debilitating, impacts on vision quality.

What symptoms are associated with higher order aberrations?

One eye often has several different higher order aberrations interacting with each other. Therefore, it is not easy to draw a correlation between a specific higher order aberration and a specific symptom. However, higher order aberrations are often associated with double vision, blurriness, ghosting, halos, glare, loss of contrast, and poor night vision.

Can higher order aberrations be corrected?

Today, quite a bit of attention is focused on higher order aberrations because they can finally be diagnosed using wavefront technology (aberrations) and because they have recently been identified as a serious side effect of refractive surgery.

Currently, many forms of adaptive optics have been developed or are being developed to customize the correction of higher order aberrations. This includes new types of glasses, contact lenses, intraocular lenses, and refractive surgery that help change the shape of the eye's surface or cornea.

The goal of adaptive optics is to achieve a type of vision correction that can flatten the shape of the wavefront appearing on the plane of the pupil by compensating for its distortion.

However, adaptive optics may not be able to identify the specific physical defects of the refractive components in the eye that cause these distortions in the first place.