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Refractive
Surgery Update
The Optical Aberrations of LASIK
Paul M. Dowd, O.D.
Jeffrey M. Augustine, O.D.
LASIK has resulted in growing legions of happy patients. Yet some
individuals experience optical aberrations following the procedure. There
are five such aberrations: blur, fog, flare, soft focus and monocular diplopia.
Many factors may contribute to these symptoms, even in cases where the
procedure has no complications. In order to detect these optical aberrations
and manage the patient appropriately, you need to understand the basic
optical principles associated with LASIK. Proper management of these symptoms
can help assure better surgical outcomes.
Blur From Under- or Overcorrection
This 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 Scatter
In 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 Diffraction
When 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 Aberration
Because 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-refraction
When 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.
When you measure the pupil size preoperatively in dim illumination,
an infrared pupillometer will help detect patients at risk of negative
clearance.
Minimizing Risks
Many 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.
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© Review of Optometry OnLine
February 15, 2000
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