If one reads about LASIK patient complaints from a few years ago, many of them relate to night vision-complaints of glare and haloes at night, especially while driving. However, these days, this does not seem to as big an issue for patients. Patients are increasingly satisfied with night vision after LASIK. So what’s going on? What changed in the last few years?
Indeed, this is one of the biggest advances in the LASIK field since the initial use of LASIK. This change has come about with the advent of wavefront technology, and with a better understanding amongst surgeons and laser companies about the kind of tissue removal profile shapes that allow patients to retain and even improve the quality of their night vision.
Let us get a little technical here. The eye functions like a camera. It has a sensor (called the retina) and a set of two lenses (the cornea and the “lens”) which focus light from a distance on the retina. It has a shutter (the “iris”) which regulates the amount of light incident on the retina, by varying the size of the aperture (the “pupil”) depending on the amount of ambient light. During day time, the pupil is constricted (i.e., it is a very small aperture), while during night, the pupil is dilated, i.e. there is a larger aperture to capture more light.
It is important to understand, for the purpose of this discussion, that light rays which travel from the center of the cornea through the pupil to the retina travel a shorter distance than light rays travelling from the periphery of the cornea through the pupil to the retina. This difference in the length of the optical path is small during daylight. Since the pupil is small, only light rays travelling around the center of the cornea reach the retina. Peripheral rays get blocked. In the dark, on the other hand, this difference is accentuated. Since the pupil is very wide, rays incident from the peripheral cornea also reach retina.
If the cornea was a perfect sphere, which bent all rays of light by a similar amount, whether they originated in the centre or the periphery, the all the light rays would not focus on the retina. There would be a zone of focus, rather than a point of focus. This effect would be especially true at night, and there would be symptoms of glare and haloes. Obviously, this is not a happy situation. Therefore, our cornea has evolved into an “aspheric” lens, where it is slightly steeper in the center, and flatter towards the periphery. Light rays from the center (which travel the shortest distance to the retina) are focused by a steeper lens, while light rays from the periphery (which travel longer distances to the retina), are focused by a flatter lens. In such a manner, nature has ensured that all light rays, whether from the center or the periphery of the cornea, reach the retina at the same point. This is why we have naturally good night vision.
So how did LASIK change this situation? Excimer lasers remove tissue from the cornea to correct focusing errors in the eye. To correct myopia, or near sightedness, one must flatten the cornea, removing more tissue in the center of the cornea, and less tissue in the periphery of the cornea. The amount of tissue removal with an excimer laser is more or less proportional to its fluence, or the energy per area of tissue. What laser companies and eye surgeons realized in early part of this century (several years after LASIK was first performed, and after several million eyes were already treated), is that excimer lasers lose fluence as the spot moves from the center to the periphery of the cornea. In the center, the incident excimer laser spot is the smallest. In the periphery, the incident excimer laser spot is larger. This leads to a reduction in fluence. Another reason for the decrease in fluence towards the periphery is reflection losses. On earth, as we move from the equator to the poles, the amount of incident solar energy becomes less, because light rays which hit at an angle to the earth’s surface reflect more than light rays which are perpendicular to the earth’s surface. Similarly, excimer laser spots which are incident on the center of the cornea reflect less energy than excimer laser spots which are incident on the periphery of the cornea.
As a result of such fluence losses in the cornea, excimer lasers remove less tissue in the periphery than intended and remove more tissue from the center than intended. Then the cornea becomes the reverse of what nature intended. It becomes flatter in the center, and steeper in the periphery. This leads to all kinds of night vision difficulties, because now instead of a point of focus of all light rays on the retina, we have a zone of focus. In technical terms, this is known as an increase in spherical aberration, or the Zernike (4,0) higher order aberration.
Once laser companies and eye surgeons realized this (partly by using wavefront technology), this was relatively easy to correct. Laser companies started programming the lasers to remove (i.e., ablate) additional tissue from the periphery of the cornea. Such tissue removal profiles are known variously as Aspheric Profiles, Aberration Neutral or Aberration Smart Profiles, or Wavefront-Optimized Profiles. These profiles do not induce spherical aberration into the LASIK treated eyes, and prevent night vision difficulties after LASIK. Such profiles are now more or less the standard way of doing LASIK.
Often, Aspheric profiles are preferable to using wavefront-customized treatment profiles. Most people, especially in India, do not have significant higher order aberrations. Treating them with wavefront-customized profiles does not benefit these patients, and in fact, leads to the removal of additional tissue, which is undesirable. Treating them with aspheric profiles, on the other hand, would suffice, and lead to excellent day time and night vision.