Managing Complications in Refractive Surgery
Two articles focus on strategies for preventing common and uncommon side effects of LASIK.

One of the major challenges ophthalmologists face is patient complications from undergoing refractive procedures. Often, these complications can be avoided by screening candidates who are likely to develop complications, and treating other pre-existing conditions preoperatively. In the first of two articles dealing with such issues, Barrie Soloway, M.D., discusses strategies to deal with the risk of ectasia in laser vision correction surgery. In the second piece, Renee Solomon, M.D., and Eric Donnenfeld, M.D., discuss the prevention of dry eye and other corneal complications before LASIK.

Managing the Risk of Ectasia in LASIK
An ounce of prevention is worth a pound of cure.
BY BARRIE SOLOWAY, M.D., F.A.C.S.

ILLUSTRATION BY JOEL & SHARON HARRIS

Reducing a patient's risk of ever developing a LASIK complication may be the easiest way of dealing with a LASIK complication, but as every surgeon knows, the only way to completely avoid complications is to stop operating altogether. Some potential LASIK-related problems, such as ectasia, can be more predictable than others, which can make them easier to avoid.

While rare, ectasia is one of the more severe complications associated with LASIK and thus is the focus of the article. Other complications range from simpler intraoperative flap issues, such as thin flaps and buttonholes, to postoperative epithelial ingrowth, under/overcorrection, diffuse lamellar keratis and dry eye (which is addressed in an article beginning on page 33 of this issue). Learning how to predict where the patient will be after the surgery in order to avoid ectasia and other complications requires meticulous and detailed evaluation and a critical eye toward performing LASIK surgery.

In this article, I'll discuss dealing with — and minimizing — the risk of ectasia in laser vision correction surgery.

Ectasia: A Complication to Avoid

With the advent of wavefront-guided and larger optical zone excimer treatments, deeper ablations are now being routinely performed, rather than the more conventional 6.0 or 6.5 mm ablations. Ectasia after LASIK surgery can be a difficult complication to manage. A first line of treatment might be rigid contact lenses, which may be difficult to fit due to the shape of the cornea or changes in the patient's tear film after lamellar surgery.

Surgery, such as implantation of Intacs (Addition Technology, Sunnyvale, Calif.) corneal implants might stabilize the cornea and allow for spectacle or soft lens use. Corneal transplantation and its incumbent after-care requirements is a drastic option for ectasia, and certainly not what the patient and the surgeon were hoping for as a result of the LASIK procedure. In order to reduce the risk of this occurrence, it is important to understand that there are a number of different reasons why patients can develop ectasia after LASIK surgery. It is critical to be on the lookout for some of these signs.

One of the original tenets of lamellar refractive surgery and LASIK surgery was that in order to preserve the tectonic integrity of the cornea, a minimum of 250 μm of corneal tissue must remain after the creation of a flap and removal of stromal tissue by the excimer ablation.

In the decade since the excimer laser's approval in the United States for refractive surgery, the 250 μm number has undergone some modification. There have been suggestions that this number should be no less than 50% of the original corneal thickness, or that we should not go below 300 μm. Although there are no studies to show that there is a magic number that is safe to operate down to, using these higher numbers as guidelines helps to reduce the risk of ectasia by preserving additional corneal tissue for possible retreatment. More corneal tissue also provides a buffer should the original corneal measurements overstate its minimum thickness, or if the LASIK flap is thicker than planned.

Figure 1. Measuring the thickness of the microkeratome cut can allow for aborting the procedure if the depth proves to be too great. Post-op UHF immersion B-scan showed flap thickness to be 224 μm, leaving less than 250 μm in the stromal bed and resulting in ectasia. Pre-op Rx: -3.50 – 1.25 x 175 Pre-op Pachymetry: 503 μm Ablation depth: 52.5 μm Calculated residual with 160 flap: 290.5 μm Calculated residual with 224 flap: 226.5 μm

When the residual corneal stromal thickness is calculated to be close to or less than these minimums, I counsel the patient that I will not be able to perform LASIK surgery and may recommend a surface treatment such as epi-LASIK with the epi-k (Moria, Antony, France). With surface excimer surgery, I start more anteriorly in the cornea and leave a greater thickness of undisturbed stroma intact for stability. To be able to reliably calculate these residual stromal thicknesses, it is imperative that we have knowledge of the depth of the laser treatment at any specific power and diameter. Of equal importance is the measurement of your keratome's range of cutting thickness by a method such as subtraction pachymetry at the time of all of your surgeries. An example of this problem is shown in figure 1.

Evaluating Borderline Patients

It is important to recognize that the issue of remaining corneal thickness is useful only in the patient with a normal cornea. A second type of risk of ectasia can occur in patients who have a predilection toward ectasia before they have surgery. As these corneas are already showing signs of reduced tectonic rigidity, any surgery that further reduces the thickness could be the straw that breaks the camel's back and spirals these patients to a poor result of surgery.

In these patients, a careful review of the topography can usually be helpful, as many topography units have special features to evaluate the possibility of keratoconus — such as the inferior to superior curvature ratio, elevated posterior float and other proprietary indexes of keratoconus.

Visually, looking at the topographic color maps can also be illuminating as long they are set to appropriate scaling. Setting the scale too far apart may mask the color difference of the problems, while setting the midpoint of the scale too high will cause the colors to all be in the blue and green range. I set my topography defaults to use an 0.5 D change in power to correspond to one color unit change and the midpoint of the scale to be the patient's midpoint of curvature. While this will overemphasize any increase in steepness in one meridian over 180Þ away, I can accept 0.5 units of difference as normal and more easily remember to run any tests of keratoconus when there is asymmetry.

Figure 2. Preoperative topography shows significant asymmetry with greater than 1.5 D steeper inferior giving this patient a higher risk of ectasia with any ablative surgery. This steepness is masked in the color image due to the color scale being set for greater than twice the 0.5 μm difference between colors.

It is common to see some eccentricity in the astigmatism of contact lens wearers, but with sufficient time out of lenses (minimum 2 weeks) a normal cornea will become symmetric. However, in this group of potential problem patients, the cornea will remain asymmetric. For patients in whom we see these signs of pre-existing corneal strength problems, we counsel against any corneal-thinning procedure in favor of other additive procedures such as Intacs or Verisyse (AMO, Santa Ana, Calif.) phakic lens implant if appropriate. An example of this problem is shown in figure 2.

Unanticipated Ectasia

Another group of patients may fall into this category of patients who can develop ectasia — and we must be aware of them. These are patients who have normal corneal topography and appear to have adequate corneal thickness for the procedure but respond to LASIK or a retreatment in a different way than we expect.

There are two common scenarios with these patients. Typical of some of these patients is the need for a retreatment due to the result of the original surgery not maintaining emmetropia over time. Every patient who develops progressive myopia after LASIK surgery needs to be re-evaluated for the development of nuclear sclerosis and/or the development of ectasia. We counsel those patients who have a progressive increase in their myopia after surgery that, just as it was important for their prescription to be stable for 12 to 18 months preoperatively, we need to establish a stable prescription before rushing into retreatment.

Figure 3.This patient underwent three retreatments. The result of his third retreatment was significantly different than the treatment predicted without any system entry errors. This result and his current low pachymetry put him at a higher potential risk for ectasia if future treatments are performed. Pre-third retreat Rx   Plano – 1.00 x 130 Post-third retreat Rx   -.50 – 1.75 x 120 Current minimum pachymetry   413 μm

As these patients are observed over the course of their changing prescription, it will become clearer to the surgeon whether they were undercorrected originally or simply became more myopic.

Further excimer surgery can be helpful in some cases. However, if some of the other problems that can also cause progressive myopia, such as the aforementioned nuclear sclerosis or ectasia are occurring, further excimer surgery would be contraindicated. Another scenario occurs when a patient undergoes retreatment and the result is quite different than that expected. Here, shifts in cylinder axis or changes in spherical or cylindrical power that do not follow the treatment must be treated with suspicion. An example and the history of this problem are shown in figure 3.

When to Say No to LASIK

At times, knowing when to say no to excimer ablative refractive surgery is the best method of avoiding complications. When LASIK is indicated, knowing the depth of the laser ablation and the thickness range that the keratome cut in the past will help to avoid ectasia in patients who have normal topography. Casting a critical eye to any asymmetry in corneal topography and repeating testing to confirm if it is due to artifact, contact lens use or a forme fruste kerato-ectasia will also be helpful. Lastly, knowing when to say no to a patient's request for a retreatment because the risk of his or her problem becoming worse is high can also improve results by decreasing risks. Given the vision correction options now available, we can provide solutions while also minimizing risk.

Barrie D. Soloway, M.D., F.A.C.S., is director of Vision Correction, The New York Eye and Ear Infirmary, and assistant professor of ophthalmology, The New York Medical College.