How LASIK, HOA and LOA Affect Your Vision

Filed in archive Wavefront LASIK on February 26, 2010

When you're considering LASIK or any other refractive eye surgery to correct your vision, it's a good idea to understand what vision problems you're actually correcting.

The basic vision problems that glasses correct for are known as lower order aberrations, or LOA, and are easily recognizable by their common names: nearsightedness, farsightedness, and astigmatism.

Higher order aberrations, or HOA, are more complex vision errors caused by a distortion in the wavefront of light passing through irregularities in the eye's refractive system-cornea, aqueous humor, crystalline lens, and vitreous humor. When the cornea or crystalline lens is abnormally curved, it distorts the wavefront of light passing through to the retina. The results of the distortion are various visual phenomena: glare, blurriness, starbursts, ghosts, loss of contrast, double vision, etc. All people have HOA of one sort or another, though often they're not noticeable.

Tracking HOA Down:
HOA can only be detected by a wavefront diagnostic analysis, a newer technology whereby measurements are taken of the wavefront of the light reflected out of the eye. These measurements create a 3D topographical representation of the eye. Flaws (HOA) can be identified based on the shape of the light reflected, with different wavefront shapes indicating different flaws (HOA). Many of these flaws (HOA) are expressed mathematically, since they are measurements of a specific individual's unique aberrations; however there are basic terms (HOA) for basic aberrant wavefront shapes.

Some common HOA are:

• Coma
• Trefoil
• Spherical aberration

Double vision and light trails are symptoms associated with coma. Trefoil usually results in a person seeing light smeared in three directions, similar to a Mercedes Benz symbol. Spherical aberration is often associated with halos, starbursts, ghosts and night vision problems.

Currently, only wavefront LASIK can correct for HOA. Because conventional LASIK surgery is based solely on refraction and measurements of LOA-the same information used to create your prescription for glasses or contacts-it corrects solely for nearsightedness, farsightedness, and astigmatism. Wavefront data can be used to program the excimer laser in wavefront-guided or custom LASIK operations, allowing for correction of an individual's specific visual flaws (HOA).

One of the drawbacks of all refractive surgery is that it tends to cause an increase in HOA. Wavefront LASIK is no exception, however it has been shown that the wavefront procedures CustomVue, CustomCornea, and Zyoptix increase HOA less than conventional LASIK. Additionally, some studies have shown there to be less chance overall of increased HOA with wavefront LASIK than with conventional LASIK. Other forms of LASIK like Intralase or All-Laser LASIK, where the corneal flap is made with a laser instead of a microkeratome (blade) are proving to have less chance of increased HOA as well.