Reduced Quality of Vision After LASIK
Reduced visual quality after LASIK is a difficult concept for individuals who have never experienced it to comprehend. The general perception is that vision can be measured by an eye chart. In a healthy, unoperated eye, this is generally true. However, after LASIK, an eye chart may be of little value because it fails to measure visual quality. For example, a LASIK patient may be able to read the 20/20 line on the eye chart, but the image they see may be grossly distorted by multiple vision, smearing, or other visual aberrations.
LASIK alters the shape of the cornea in an attempt to change the focusing power of the eye. LASIK-induced corneal irregularities vary widely. Consequently, visual aberrations reported by patients also vary. Patients may complain of ghost images, starbursts, halos, smeared vision, and loss of contrast sensitivity (difficulty seeing fine detail). Glasses and soft contact lenses cannot correct these visual aberrations. Rigid gas permeable (RGP) contact lenses may be required after LASIK as a means to restore a more optically-correct refractive surface to the eye.
Contrast sensitivity testing and wavefront aberrometry are methods that may be used to objectively quantify reduced visual quality after LASIK. Contrast sensitivity is a measure of how well a patient can distinguish detail under low contrast conditions. Wavefront aberrometry is performed by objectively measuring the shape and severity of defocus of light rays reaching the retina.
In an eye with excellent vision, light rays entering the eye will focus precisely on the retina. Refractive errors of the eye result in an unfocused retinal image, and include myopia (nearsightedness), hyperopia (farsightedness), and astigmatism. The eyeball is elongated in an eye with myopia, which causes the image to focus in front of the retina rather than on the retina. The opposite is true of an eye with hyperopia, where the plane of focus is behind the retina. In LASIK for myopia, the center of the cornea is flattened by the laser in an attempt to compensate for the elongation of the eyeball. In LASIK for hyperopia, more tissue is removed from the periphery of the cornea to steepen the cornea in an attempt to compensate for a shortened eyeball.
The diameter of the cornea that is fully treated by the laser is called the optical zone, which is generally 6.0 millimeters (mm) to 6.5 mm in diameter with current laser systems. Outside the optical zone there may be a blend zone, which is a tapered region created by the laser to prevent a sharp transition between treated and untreated areas of the cornea. The entire treated area, including the optical zone and the blend zone, is called the ablation zone. Due to the natural prolate shape of the cornea, the laser loses efficiency on the slope of the cornea, typically resulting in an optical zone that is smaller than intended. The greater the level of myopia, generally, the smaller the effective optical zone. Modern laser systems attempt to compensate for this problem by applying more laser pulses in the periphery of the optical zone, but there is also a limit on how much tissue can be safely removed. Understanding and consideration of corneal biomechanics is essential to an appreciation of emerging safety concerns surrounding LASIK eye surgery. If too much tissue is removed during LASIK, the cornea may no longer be able to withstand the constant outward force of intraocular pressure of the eye. A surgically weakened cornea may develop ectasia, which is a progressive forward bulging of the cornea characterized by increased myopia and irregular astigmatism with associated loss of best corrected vision. Patients who develop ectasia may require corneal transplantation.
Generally, visual aberrations increase with increasing pupil size. For this reason, patients with large pupils are likely to experience poor vision at night after LASIK. The iris muscle (colored part of the eye) controls the size of the pupil, which is the opening in the center of the eye through which light passes. In the daytime when light is abundant, the pupil shrinks in size, blocking light that passes through the outer area of the cornea not fully corrected by the laser. At night, the pupil opens widely to gather more light to form the retinal image. If a patient's pupils naturally dilate larger than the diameter of the cornea that is fully corrected, light passing through the uncorrected area of the cornea will enter the eye and degrade the retinal image. The unfocused light rays entering through the mid-periphery of the cornea create visual aberrations and distortions, resulting in poor image quality. The wavefront term for visual disturbances resulting from a pupil size/optical zone mismatch is spherical aberration. An eye with spherical aberration sees starbursts and halos, and experiences a loss of contrast sensitivity. Prescription glaucoma eye drops, which have a side effect of reducing the pupil size, are sometimes prescribed to LASIK patients with large pupils. This is an unapproved use of a prescription drug, and potential side effects on patients without glaucoma have not been studied.
A laser treatment that is not properly centered is called a decentered ablation, which can result in smeared or ghosted images, especially at night. The wavefront term for a decentered ablation is coma. Spherical aberration and coma are common types of irregular astigmatism, also known as ‘higher order aberrations’, after LASIK. Other forms of wavefront aberration may be induced by LASIK. RGP contact lenses may be the only effective means of reducing visual aberration at night after LASIK—ironically, LASIK-induced dry eye may render patients intolerant to RGP contact lenses.