Blur From Under- or OvercorrectionThis is a common postoperative complaint. It’s not a true optical aberration, per se, but it does create blur in fine details.
That’s not necessarily a problem when viewing images of low spatial frequency—those with rounded edges and less detail—such as people, trees and other large objects. But residual refractive error can be troublesome when patients read or view images with sharp edges and much critical detail. This is when patient will complain the images are out of focus.
Treatment is straightforward. It involves correcting the residual refractive error either with spectacles or contact lenses, or with a LASIK retreatment.
Fog From ScatterIn a normal corneal stroma, there is little light scatter. That’s because the lamellar stromal fibers are well organized, spaced 1/4 of a wavelength apart and surrounded by a transparent ground substance. But the light waves scatter upon disruption of the alignment and spacing of the lamellar fibers. The result is the sort of foggy vision familiar to PMMA contact lens wearers who experience stromal edema or those who develop acute corneal swelling.
LASIK likewise alters the lamellar spacing and alignment when the microkeratome dissects the lamellae during flap formation. The misaligned lamellar fibers, no longer 1/4 wavelength apart, cause incoming light to scatter. The patient then complains of fog around bright lights. The extent of scattering varies with the wavelength of light hitting the stromal interface. The longer the wavelength, the more scatter. A sodium vapor lamp with a longer wavelength (orange), for example, will cause more fog than a mercury vapor lamp (blue). The patient will describe this phenomenon as a beaten-metal fog or an orange-peel effect around lights at night.
Treatment strategies for fog are palliative, since time is in your favor. Within 6-12 months there’s a natural healing and reorganization of the stromal lamellae. Tell the patient not to worry. The scatter and fog will diminish.
Flare From DiffractionWhen you introduce an edge into an optical system, you cause diffraction of the light waves at the edge. Patients who wear small-diameter contact lenses sometimes may notice this visual aberration. When the edge of the lens encroaches inside the pupil margins under dark conditions, points of light will diffract off the edge of the contact lens and radiate out from the light source.
Take a patient who has with-the-rule astigmatism and who undergoes LASIK. A 4mm band of ablation will create a stromal edge that falls within the central corneal optic zone at the top and bottom when the pupil exceeds 4mm in diameter. This creates flare above and below bright light sources. Patients observe rays of light that radiate out along a horizontal line rather than a curved line, as in the case of a hard contact lens edge. You can observe this effect around LED readouts in a dark room.
You can eliminate flare temporarily by enlarging the pupil either with additional ambient light or a topical miotic agent. But the flare will return once the pupil dilates. Another approach is to restore the cornea to a normal post-refractive topography, without straight edges inside the optical zone. Surgeons are developing two ways of retreating the corneal surface in such cases: flattening the steep edge using custom ablation masks and topography-assisted excimer laser treatment.
In the masking technique, the surgeon covers the flattest part of the cornea with silicone, plastic or a biomaterial. He then applies the ablation over the steep edges on the surface of the cornea to produce a smooth surface free of diffracting edges or junctions. With a topography-assisted excimer, the laser customizes an ablation pattern to correct for the aberrant corneal contour. Here too the aim is to produce a smooth and optically pure surface.
Soft Focus From Spherical AberrationBecause the normal cornea is aspheric, the eye experiences no increase in spherical aberration as the pupil expands. Following LASIK, the refracting surface is no longer aspheric. It’s shaped more like a Fresnel lens, more spherical than aspheric. When the pupil expands and light rays from the peripheral cornea add to the retinal image, the rays do not focus in the same plane. The result is soft focus in dim lighting conditions.
For example, in a church or store where lighting is subdued the vision will seem soft. This effect is much like misty soft-focus portrait photos that downplay unflattering details. The effect dissipates with time as the patient adapts to ambient light levels and the pupil contracts.
Monocular Diplopia/Bi-refractionWhen the pupil size exceeds the optical zone diameter of the ablation, it creates a condition known as negative clearance. This results in two retinal images. The more prominent image is the clearly focused one resulting from the new central curve. A second image of the same object, formed by the peripheral cornea of the original steeper myopic curve, will appear slightly larger and blurred. This phenomenon occurs most noticeably at night with a quarter moon in a dark sky. The patient will see a clear sharp sliver of the moon with a blurred ghost moon next to it. At other times bright signs with fine details may appear ghosted.
The treatment: create a condition of positive clearance. This can be
accomplished by increasing the ablation optic zone to a size equal to or
larger than the pupil. Unfortunately, doing so also increases the depth
of the ablation, which in turn limits the amount of myopic ablation due
to corneal thinning. The use of a multi-zone ablation, minimal transition
zones and aspheric profiles can reduce the depth of the ablation and maximize
the optic zone. Scanning lasers can now expand the optic zone to 9-10mm,
which helps reduce the potential for bi-refraction.
Minimizing RisksMany postoperative visual symptoms are associated with large pupils. Yet, large pupils themselves usually are not the direct cause of these symptoms. They arise most often due to mild, surgically induced defects such as dry eye, interface debris or microstriae. But the consequences of these defects may be worse in a patient with large pupils.
One way to reduce the risk of post-LASIK optical aberrations: Don’t recommend the procedure for patients with large pupils. Yet, many of these patients are happy with their vision after LASIK. The best approach is to educate patients thoroughly about potential symptoms and to work with an experienced surgeon, preferably one who uses an excimer laser with an adjustable ablation pattern. That way, you’ll be confident that your patients are likely to achieve satisfying results.