The more we comanage refractive surgery, the more we’re discovering that not all patients are LASIK candidates. We need to evaluate a choice of procedures for each individual. This is one of the next trends in refractive surgery, as more doctors begin to realize that LASIK is not a one-size-fits-all proposition for patients who want laser refractive surgery.
Pupil size, for example, is but one important determinant. Even though the largest optic zone laser has allowed LASIK treatment of many patients with large pupils who otherwise could not have the procedure, it still does not mean that all patients can have LASIK. In the future, corneal power or steepness, pachymetry and anterior chamber depth measurements will be critical in determining who can have LASIK. Even dry eye, anterior membrane dystrophy and recurrent erosion are forcing doctors to choose alternative procedures.
‘E’ Equals Modified PRK
One such alternative is LASEK. This is essentially a modified photorefractive keratectomy (PRK). Rather than the surgeon scraping away the epithelium with a Beaver blade, as he does in PRK, he first separates the epithelium from Bowman’s layer with diluted alcohol, then pushes back the epithelium to expose the ablation bed.
LASEK involves placing an 8mm trephine, or well, over the cornea. The surgeon instills diluted alcohol (20%) into the well for up to 30 seconds, then uses cotton swabs to absorb the excess. This concentration of alcohol is sufficient to break the bonds of the epithelium from Bowman’s layer.
The surgeon then pushes aside the epithelium and ablates the exposed cornea with the laser. Once the ablation is complete, the surgeon replaces the epithelium onto the cornea and inserts a bandage contact lens.
LASEK Profiler
Although LASEK will probably not surpass LASIK as the refractive procedure of choice, there are times when LASEK will be the better choice. Patients at risk of complications related to the microkeratome would be better suited for LASEK vs. LASIK. Those with very steep corneas risk the potential of a buttonhole when the microkeratome makes its pass in LASIK.
Others best indicated for LASEK are patients with deep-set eyes or prominent brow, features that make it difficult for the standard microkeratome plate to fit and may cause loss of suction during the blade pass. Conjunctival pathology such as a pronounced pinguecula may also cause the microkeratome to lose suction in LASIK. Patients with such conditions would have less risk of flap complications with LASEK.
During the microkeratome pass in LASIK, intraocular pressure rises above 65mm Hg. Although this transient pressure rise has never been associated with glaucoma, patients at risk for glaucoma and even glaucoma suspects may be better suited for LASEK.
Limited corneal thickness in some individuals may also make them better suited for LASEK. It has become standard for surgeons to leave a minimum of 250 microns in the stromal bed after LASIK; some surgeons advocate 260-300 microns at a minimum. For those cases in which the remaining LASIK bed thickness is close to the minimum or predicted to exceed the minimum, LASEK leaves an additional 160 microns of corneal tissue because it does not involve a stromal flap.
Recent studies suggest that LASIK exacerbates pre-existing dry eye. Theory suggests this is secondary to neurotrophic changes that occur until the corneal nerves regenerate. LASEK does not sever the nerves, and although the surgeon may apply ablations to the nerve endings, they are likely to regenerate more quickly than with LASIK. Although studies have yet to confirm this, patients with moderate dry eye may likely be better candidates for LASEK.
Patients who experience a sloughing of their epithelium during LASIK will often heal three times more slowly than if they had a PRK or LASEK. The slow healing of the epithelium may also result in an irregular corneal surface that may require phototherapeutic keratectomy (PTK) 6-12 months later. Any patient with this level of trauma may be at greater risk for developing diffuse lamellar keratitis (DLK) and epithelial ingrowth.
Patients could avoid post-LASIK complications if during the preoperative examination their doctors identify whether they’re prone to anterior membrane dystrophy, have a history of recurrent erosion or even had a previous corneal abrasion. In the case of recurrent corneal erosion and anterior membrane dystrophy, you would not only avoid a potential complication, but the laser ablations applied during LASEK would also actually treat the diseased tissue.
Military personnel may also be best suited for LASEK. The military will not allow personnel who have had LASIK flaps in many of its disciplines because extreme G-forces may cause the flap to separate. There is no flap involved in LASEK, so the military may very well accept this procedure.
LASEK may become the procedure of choice for future custom ablation. The corneal flap in LASIK has been shown to induce higher-order aberrations, making wavefront treatment difficult to predict.
Putting more options such as LASEK at your disposal, and understanding them, gives you more opportunities to select the best possible treatment for each patient and avoid complications afterward.
Dr. Karpecki is a consultant with Bausch & Lomb, which manufactures a laser used in refractive surgery procedures.
