Higher-Order Aberrations

If your eyes have been diagnosed with higher-order aberrations, you may wonder exactly what this condition means and what impact, if any, it has on the quality of your vision. Higher-order aberrations or HOAs are more complex vision errors than lower-order aberrations, which have more familiar names such as nearsightedness, farsightedness and astigmatism.

Higher-order aberrations have relatively unfamiliar names such as coma, spherical aberration and trefoil. These types of aberrations can produce vision errors such as difficulty seeing at night, glare, halos, blurring, starburst patterns or double vision (diplopia).

No eye is perfect, which means that all eyes have at least some degree of higher-order aberrations. If you are diagnosed with higher-order aberrations, you need not be concerned unless they are significant enough to cause vision symptoms.

A higher-order aberration is a distortion acquired by a wavefront of light when it passes through an eye with irregularities of its refractive components (tear film, cornea, aqueous humor, crystalline lens and vitreous humor). [See also: How the Eye Refracts Light and Eye Anatomy.]

Abnormal curvature of the cornea and crystalline lens may contribute to the distortion acquired by a wavefront of light. Serious higher-order aberrations also can occur from scarring of the cornea from eye surgery, trauma or disease. Cataracts clouding the eye's natural lens also can cause higher-order aberrations. Aberrations also may result when dry eye diminishes your eye's tear film, which helps bend or refract light rays to achieve focus.

Higher-order aberrations are identified by the types of distortions acquired by a wavefront of light as it passes through your eye. Because light travels in bundles of rays, a common way of describing an individual wavefront involves picturing a bundle of light rays. The tip of each light ray in the bundle has its own point. You create the wavefront or wavefront map by drawing lines perpendicular to each point.

The shape of a wavefront passing through a theoretically perfect eye with no aberrations is a flat plane known, for reference, as piston (see chart). The measure of difference between the actual wavefront shape and the ideal flat shape represents the amount of aberration in the wavefront.

Because no eye is perfect (emmetropic), a wavefront passing through an eye acquires certain three-dimensional, distorted shapes. So far, more than 60 different shapes, or aberrations, have been identified.

Significant amounts of aberrations can pose vision problems because they interfere with the eye's ability to see clear and distinct images (focus). [Read more about how wavefront is used in eye examinations.]

Two categories of aberrations commonly are used to describe vision errors, including:

• Lower-order aberrations consist primarily of nearsightedness and farsightedness (defocus), as well as astigmatism. They make up about 85 percent of all aberrations in an eye.
   
• Higher-order aberrations comprise many varieties of aberrations. Some of them have names such as coma, trefoil and spherical aberration, but many more of them are identified only by mathematical expressions (Zernike polynomials). They make up about 15 percent of the total number of aberrations in an eye.

 

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

The impact of higher-order aberrations on vision quality depends on various factors, including the underlying cause of the aberration. People with larger pupil sizes generally may have more problems with vision symptoms caused by higher-order aberrations, particularly in low lighting conditions when the pupil opens even wider.

But even people with small or moderate pupils can have significant vision problems when higher-order aberrations are caused by conditions such as scarring of the eye's surface (cornea) or cataracts that cloud the eye's natural lens. Also, specific types and orientation of higher-order aberrations have been found in some studies to affect vision quality of eyes with smaller pupils.

Large amounts of certain higher-order aberrations can have a severe, even disabling, impact on vision quality.

An eye usually has several different higher-order aberrations interacting together. Therefore, a correlation between a particular higher-order aberration and a specific symptom cannot easily be drawn. Nevertheless, higher-order aberrations are generally associated with double vision, blurriness, ghosts, halos, starbursts, loss of contrast and poor night vision.

Quite a bit of attention is being focused on higher-order aberrations these days because they finally can be diagnosed by wavefront technology (aberrometry) and because they recently have been identified as sometimes serious side effects of refractive surgery.

At present, various forms of adaptive optics have been or are being developed to custom correct higher-order aberrations. These include new kinds of spectacles, contact lenses, intraocular lenses and refractive surgery, which modifies the shape of the eye's surface or cornea.

The aim of adaptive optics is to achieve the type of vision correction that can make flatter the shape of the wavefront emerging in the plane of the pupil by offsetting its distortion. Adaptive optics may not attempt to address specific physical imperfections of refractive components of the eye that cause the distortion, because these cannot always be individually distinguished.

[For more information about vision correction for higher-order aberrations, read about wavefront eyeglass lenses and wavefront or custom LASIK.]