Patient Management
Refractive Surgery and Glaucoma
Altering the eye can make diagnosing and monitoring glaucoma a real challenge. Here's some helpful advice.
By Joseph Ducharme, M.D.

LASIK, PRK, LASEK and corneal rings all produce immediate, measurable results, often touted as "miraculous." Unfortunately, changes caused by some of these procedures can make detecting and managing glaucoma -- already a challenging task -- even more challenging.

In today's rapidly expanding refractive surgery market, this is becoming a more common problem, and the difficulties we encounter in this situation are made worse by the lack of long-term research on this issue. Here, I'd like to share some of what I've learned about dealing with these challenges.

Measuring IOP post-surgery

A primary concern in this situation is that refractive surgery can affect the measurement of IOP. Goldmann applanation tonometry (GAT), probably the most common means of measuring IOP, is most accurate in a healthy subject when the central corneal thickness (CCT) is about 520 microns. Refractive surgery that involves ablation of the central corneal stroma changes the CCT; numerous studies have shown that IOP readings made using GAT decrease after refractive surgery.

Ideally, we'd have a standard correction factor to compensate for this. Unfortunately, the ratio of microns ablated to change in IOP hasn't yet been shown to be consistent.

To compensate for this problem:

• If you plan to use GAT, take diurnal IOP measurements prior to the refractive procedure. Post-laser GAT diurnal readings can then be compared to the preoperative values to clarify the difference in measurement at each time period.
• If a patient didn't have preoperative diurnal values recorded, other options for checking post-surgical IOP may be appropriate, such as peripheral use of a Tono-pen (see Garzozi et al., Cornea, January 2001) or contact pneumotonometry (Dutch et al., Journal of Glaucoma, August, 2001). Measurements taken using these methodologies are less affected by changes to the central cornea.

Monitoring the optic nerve

Because IOP measurements are affected by corneal ablation, monitoring optic nerve changes in these glaucoma patients is essential. When it comes to detecting factors such as focal rim loss, notching, excavation, disc hemorrhage, nerve fiber layer defects, baring of the circumlinear vessels and optic disc asymmetry, careful visual scrutiny of the optic nerve -- preferably using direct stereoscopic viewing, in person or via photographs -- is still the gold standard. Unfortunately, these defects are often subtle in the early stages of the disease, and are typically missed by a less experienced examiner.

Instruments such as the Heidelberg Retina Tomograph, GDx Nerve Fiber Analyzer and Optical Coherence Tomog-rapher can provide additional information to help assess the optic nerve and nerve fiber layer. However, all of these devices produce a significant number of false negatives and false positives; none of them, used alone, is a perfect screening device.

Also, quantitative assessment of visual field defects by achromatic perimetry often fails to reflect subtle nerve changes. However, other devices, such as blue on yellow or short-wavelength perimetry (SWAP) and frequency doubling perimetry (FDP), can help identify patients with suspicious optic nerves and early visual field loss.

The myopia connection

The majority of refractive procedures today are done on young myopic patients. In terms of glaucoma, this is cause for concern for several reasons:

• Optic nerve appearance, which in general is highly variable, may be even more variable among myopic patients.
• Compared to nonmyopes, myopic patients are estimated to have two to three times the risk of developing primary open-angle glaucoma.
• Other glaucomatous syndromes, such as pigment dispersion syndrome, are more common in young myopic patients.

These concerns make it even more important to use careful optic nerve evaluation, including computerized instrumentation, to determine which optic nerves are pathologic.

Can LASIK trigger glaucoma?

Some reports indicate that a few patients who were glaucoma suspects, or who had a family history of glaucoma, have developed typical glaucoma visual field defects following LASIK. Some surgeons have attributed this to the patient's IOP being briefly elevated above 60 mm Hg by a suction device before creating the corneal flap during LASIK.

In young, healthy eyes, this transient elevation of IOP hasn't been proven to be detrimental to the optic nerve or retina. (See "Does LASIK Affect the RNFL?," right.) However, in experimental animal models of glaucoma, mediators of apoptosis have been released when individual ganglion cells were exposed to a pressure stimulus in vitro.

At this point, we don't know whether an apoptosis cascade could be triggered by the LASIK procedure in patients who are predisposed to developing glaucoma.

Glaucoma management strategies

Given these concerns, how should you manage a glaucoma suspect who has had, or is planning to have, refractive surgery?

• Before surgery, examine the patient thoroughly. Carefully assess IOP, anterior chamber structures and the optic nerve.
• Make sure that any nonophthalmologist performing screenings focuses on optic nerve appearance, not just IOP. (About one third of all glaucoma patients have normal IOPs.)
• If the patient has no clear visual field damage but a suspicious optic nerve, consider recommending PRK (which doesn't involve transient IOP elevation) instead of LASIK.
• If the patient still wishes to proceed with LASIK, measure diurnal IOP before surgery.
• If little is known about the past ocular status of the glaucoma suspect, ask the patient to put off the procedure for 6 to 12 months or more so you can monitor for progression.
• If the patient definitely has glaucoma, recommend that he or she not undergo a refractive procedure. Depending on the severity of the glaucoma, the decision to have refractive surgery can always be made at some future time.

It's important to caution patients in this setting that we don't know the long-term consequences of refractive surgery on eyes that have glaucomatous damage.

An ounce of prevention

The challenges caused by the juxtaposition of laser vision correction and glaucoma will become more significant as many of these patients develop glaucoma during the coming decades. In the meantime, taking extra care to identify glaucoma suspects before refractive surgery may go a long way toward preventing future vision loss.

Dr. Ducharme fellowship trained in glaucoma at the Bascom Palmer Eye Institute in Miami, Fla., and is director of the Glaucoma Service at Rhode Island Hospital/Brown Medical School in Providence, R.I.