There are some pre-existing conditions that might affect the efficiency and efficacy of laser treatment. Potential patients will often ask if the laser treatment can be performed on someone who has had LASIK, or cataract surgery, etc. The answer is almost always “yes”, but there some things to know. A critical feature of the laser to understand is the energy profile. The lasts the energy in the shape of a cone. The energy is only delivered to one point in space, at the apex (or tip) of the cone of energy. It looks like this:
fig.1 (a) is the laser aperture, (b) the cone-shaped energy profile of the energy, (c) one of the two always-on red focusing aids, (d) the treatment contact lens applied to the eye, (e) is the apex or tip of the cone, the only point in space where energy is delivered
As shown, the broader part of the cone must pass through the eye’s cornea and crystalline lens. It must also pass through the dilated pupil. The treatment contact lens (d) will keep the eye lids open, stabilize the eye, and prevent any surface tear film fluctuation. It also magnifies the image for the doctor. Ideally, the energy “cone” will pass through unobstructed and undistorted. Any anatomical or optical feature that will limit or distort that cone of energy may decrease the efficiency of the laser treatment.
fig.2 examples of cornea or lens procedures that may affect the quality of the laser energy delivered into the vitreous space
Example 1 Radial Keratotomy (RK)
Radial K involves placing 4-16 radial incisions in the cornea approximately 90% of the thickness of the cornea. The purpose is the change the shape of the cornea: flattening the curvature of the center with a resultant steepening of the periphery of the cornea. There are a host of optical problems in doing so and well as short-term and long-term stability issues. This procedure is no longer perform (or shouldn’t be!) with the advent of laser vision correction. As the “cone” of energy passes through the peripheral parts of the cornea, is becomes distorted by the irregularly irregular shape of the cornea. It is the same type of distortion that causes the intractable glare and starburst that is known to plague RK patients, especially at night when the larger pupil allows for more peripheral light to pass through.
Example 2 LASIK and PRK Laser Vision Correction
LASIK and PRK are both automated laser corneal refractive procedures which replaced the more crude and manual RK. Optically, they are much improved especially with fast, 4th generation lasers using gee-whiz wavefront analysis software and correction. With all laser treatment, the goal is to change the shape of the central optical cornea to either flatten the curvature (myopia), steepen the curvature (hyperopia or farsightedness), and/or make the curvature more spherical to correct astigmatism. In all examples, there is the central optical axis “sweet spot”, a mid-peripheral transition zone, and a peripheral untouched and unchanged original curvature zone. The laser cone of energy must pass through one or more zones depending on where the floater is located. These zones have different optical curvature and characteristics which may distort and diminish the laser energy to various degrees. Sometime it is not noticeable to the laser surgeon, and sometimes it is quite difficult to get any energy on a posterior or peripherally located floater.
Example 3 Cataract Surgery with Artificial Implant Lens
With cataract extraction surgery, the entire crystalline lens is removed from its capsule sac, and an artificial lens is placed into the same location. A common diameter for implants lenses is between 5-6mm. A widely dilated pupil in a young person may be 8-9mm. This implant lens creates a smaller aperture to work though. It can make visualization by the surgeon and illumination of the back of the eye very difficult and challenging. If these patients have also had the posterior capsulotomy procedure, it may limit the aperture to 3.5-4mm further aggravating the situation.
These examples illustrate some of the challenges the laser surgeon must contend with. They can decrease the energy delivered to the floater by the laser, but THEY DO NOT INCREASE ANY RISK TO THE PATIENT. It just makes the surgeon work harder and/or may translate into less efficient treatments requiring more overall treatment sessions to deliver the same results.
Please do no use the comments section to leave personal contact information or detailed personal medical information. Keep it to comments, questions, or stories that will benefit other readers and floater sufferers. If you do have personal questions or requests, please go to the Email Dr. Johnson Page.
