Q: Are there any frank contraindications to LASIK following a contact lens-related protozoan infection? The patient has a very small anterior stroma mid-corneal scar. The diagnosis was made early two years ago and the patient responded well to topical agents.
A: Two distinct factors should be considered in a case such as this, says Christopher J. Rapuano, MD, a corneal specialist at Wills Eye Hospital in Philadelphia. First, optometrists must decide whether the eye is a good candidate for LASIK following a parasitic corneal infection and, second, determine whether LASIK is the best option for patients with corneal scars in the area of the flap.
Corneal infections resulting from Acanthamoeba can be difficult to treat. The protozoa’s life cycle comprises two distinct stages: an infective trophozoite and a dormant cyst that forms when the active organism is exposed to adverse conditions. Acanthamoeba cysts are highly durable and in some cases may survive through initial treatment and subsequently reactivate, perpetuating the infection and necessitating further aggressive therapy.
Dr. Rapuano recommends patients with significant corneal scarring from Acanthamoeba who wish to undergo corneal transplantation to improve vision be taken off all medications for a minimum of three months—and ideally, six to 12 months—before surgery. “I would wait for an eye to be off all Acanthamoeba treatment for at least one year before considering any refractive surgery,” he says. “I would also warn patients that the infection could recur, with or without refractive surgery.” Confocal microscopy can be performed to try and determine whether any presence of the protozoa remains.
|Corneal scars, like the one pictured here, can impede the success of LASIK. Photo: Elyse L. Chaglasian, OD, and Gregg Eric Russell, OD. |
Onset of corneal scarring typically involves some loss of stromal tissue, which is often filled in by epithelial cells. These cells are not nearly as strong as the corneal stroma itself, and this weakness increases risk of a LASIK flap buttonhole, which can result from an abnormal lamellar incision made with either a microkeratome or a femtosecond laser. If this happens, says Dr. Rapuano, the flap should be replaced immediately and allowed to heal. Excimer laser treatment should not be performed, as this can lead to an irregular corneal surface and possibly loss of correctable visual acuity from a contour mismatch between the flap and the stromal bed.
James Aquavella, MD, a professor in the department of ophthalmology at the University of Rochester, cautions that LASIK should not be performed if the scar is central, as visual acuity would remain affected following the procedure. LASIK is also contraindicated for patients with thin corneas (i.e., less than 500µm) to avoid problems associated with secondary ectasia. Dr. Aquavella recommends a minimum residual stromal bed of 300µm should remain following the incision.In patients like these, PRK is a better option, says Dr. Rapuano. The procedure, which removes layers of the epithelium to expose the underlying cornea, may decrease corneal scar depth or even remove the scar entirely, and obviates the flap complications characteristic of LASIK. Healing time and risk of infection following PRK, however, are slightly increased in comparison to LASIK.