Excimer laser effectively treats corneal pathology

Issue: August 1997

ByPaul M. Karpecki, OD, FAAO

With all the discussion about refractive surgery and the emphasis on laser in situ keratomileusis and photorefractive keratectomy (PRK), we often forget that the excimer laser is an effective tool for treating corneal pathology. When the excimer laser is used to treat anterior corneal pathology, the treatment is termed phototherapeutic keratectomy (PTK).

The excimer laser has two unique properties that make it a terrific tool for corneal pathology: the ability to create a smooth corneal surface and provide excellent epithelial adhesion.

There are three primary treatment categories of PTK: corneal opacity, surface irregularity and epithelial breakdown. A condition can fit into one, two or all three categories.

PTK's goals


Salzmann's nodular degeneration falls under the PTK category of irregular corneal surface disease. This patient had a best-corrected visual acuity (BCVA) of 20/60-1 before PTK (left), and a BCVA of 20/25 after PTK (right).

The goal of PTK in the corneal opacity group is to increase transmission of light through the cornea by using the excimer laser to cut off the pathology portion of the cornea, thus allowing a clearer image. Conditions that fall into the corneal opacity category include dystrophies, such as granular and lattice, Schnyder's crystalline dystrophy and Reis-Buckler's. This category also includes patients with corneal scars from previous infections or trauma.

The goal of PTK with respect to the irregular surfaces category is to improve the corneal topography. Conditions fitting into this category include Salzmann's nodular degeneration, post-pterygium trauma, irregular astigmatism, trauma and infections.

The goal for recurrent epithelial breakdown conditions is to improve comfort. Conditions in this category include granular dystrophy, lattice dystrophy, band keratopathy and anterior membrane dystrophy, to name a few.

Patient selection


Granular dystrophy falls under the PTK category of corneal opacity and epithelial breakdown. This patient had a BCVA of 20/70 before PTK (left), and of 20/20-2 after PTK (right), with no further recurrent erosion symptoms.

Choosing the proper patient for PTK is extremely important to the results. The ideal patient has an elevated corneal scar, a homogenous anterior stromal scar and pathology limited to the anterior 100 µm of the cornea. Patients not treatable with PTK include those with significant loss of corneal tissue or scars beyond 100 µm on the cornea, as well individuals with extreme corneal thinning and active inflammation, such as ongoing keratitis.

In evaluating a patient with a corneal opacity for PTK, three criteria should be considered. The first is the depth of the opacity. The ideal way to measure this depth is by looking to see how deep into the optic section the opacity transverses. It should not exceed one-quarter of the entire optic section.

The second criterion is the location of the opacity. A central corneal opacity, once removed, will induce a hyperopic shift. This is how PRK works to correct myopia — flattening the central portion of the cornea. However, a peripheral opacity will induce a myopic shift.

The third is the refractive error. Induced anisometropia is undesirable, of course, and treatment of a central opacity may be contraindicated if the patient is already significantly hyperopic. A guideline to remember is that any removal of corneal tissue up to Bowman's layer induces a 2-D hyperopic shift in the refractive error, and once the first 100 µm of the cornea is removed, the result is a 5-D change in refractive error.

Evaluating candidates

For evaluation of patients with surface irregularity, it is best to use a corneascope or any placido disc-type image that reflects off the cornea to determine the amount of irregularity. Many topography systems also have rings available that can be used to monitor surface irregularity.

In patients with surface breakdown, the key piece of information is the history. Recurrent erosion has very characteristic symptoms, including sharp pain upon waking, with photophobia, tearing and foreign body sensation. It is a chronic condition and usually has a history of previous trauma or dystrophy.

Traditional treatments include hyperosmotic ointments, such as 5% NaCl, ocular lubricants and punctal occlusion, bandage contact lenses, stromal puncture (provided the condition is not in the visual axis) and, perhaps the most effective for recalcitrant conditions, PTK.

Surgical technique

The surgical technique involves a polishing motion; however, different conditions, such as elevated scars and recurrent erosion, require a separate surgical technique.

A large elevated scar must first be mechanically removed using a 69 or 64 Beaver blade and forceps. Once the majority of the scar has been removed, 5% methylcellulose solution is applied to the corneal surface. This solution accumulates in the valleys of the scar tissue and allows the peaks to be exposed to the laser. A circular motion of the laser is used, and the peaks are ablated to the point where there is very little scar tissue remaining. A lighter concentration, such as 2% or 1% methylcellulose, is then applied and the same technique is used.

In the case of recurrent erosion, the area of erosion is isolated using a cellulose sponge, the epithelium is debrided and then a spray of less than 50 photons is applied to the area. This does not remove tissue, but rather "spray paints" a series of photons onto the cornea and allows the new epithelial cells to adhere better. All of these procedures are performed under topical anesthesia.

Postoperative care

The postoperative care of the PTK patient is very similar to that of the PRK patient. A bandage contact lens is inserted after the procedure for pain management. The patient is usually given a combination of antibiotic/steroid drops to use four times daily until re-epithelialization. Patients are often given topical nonsteroidal anti-inflammatory drugs for pain and oral pain medications.

The comanaging doctor follows the patient the day after surgery; the 1-day visit is to check the fit of the bandage contact lens, to ensure compliance with the medications, to check for infiltrates or other complications and to measure the exact size of the epithelial defect. On day 3, the bandage contact lens is removed using copious amounts of artificial tears for ease in separation of the lens from the fresh epithelium.

Once the patient is epithelialized, all previous medications are discontinued. Fluorometholone 0.1% solution is begun four times daily until the 1-month visit. An exception to this is a patient undergoing PTK for recurrent erosion. Because very little corneal tissue is removed in treating this condition, there is less likelihood of hazing of the cornea, and thus fluorometholone is not indicated. Instead, the patient is placed on a hyperosmotic ointment, such as Muro 128 (sodium chloride hypertonicity 5%, Bausch & Lomb), at night, as well as ample artificial tears throughout the day for the following 6 weeks.

At 1 month, the refraction is measured, the haze is monitored and the fluorometholone is tapered as necessary. Routine follow-ups include 3 months, 6 months and 1 year.

PTK is proving to be a terrific procedure in anterior corneal pathology. It has, in many cases, delayed or even prevented the need for a corneal transplant. It is a very exciting technology that is often overlooked, and it is a wonderful tool for treating superficial corneal disease.

For Your Information:

Paul M. Karpecki, OD, is director of research for the Novamed/Hunkeler Eye Study Center and the clinical director of refractive surgery for the Hunkeler Eye Center. He is also the residency director of the Cornea and Refractive Surgery Residency fellowship program affiliated with the Pennsylvania College of Optometry and a faculty member of the Kansas University Department of Ophthalmology, where he heads the refractive surgery clinic program for residents. Dr. Karpecki can be reached at 4321 Washington, Suite 6000, Kansas City, MO 64111; (816) 931-4733; fax: (816) 931-9498. Dr. Karpecki has no direct financial interest in any products mentioned in this article nor is he a paid consultant for any companies mentioned.