Lifting the Fog Over
Post-PRK Haze

BY DAVID I. GEFFEN, O.D.
APR. 1996

Stromal haze following photorefractive keratectomy usually commences three to four weeks after laser surgery and peaks by the three-month mark.

There are three types of corneal healing following PRK -- minimal (1-2% of the population), normal (96% of the population) and aggressive (2-3% of the population).

Minimal healers exhibit almost no haze. They take longer to achieve emmetropia from the postsurgical hyperopic shift, and occasionally, they remain slightly hyperopic.

Between four and six weeks posttreatment, normal healers exhibit trace haze which increases and peaks at about three to four months. They usually progress from the immediate hyperopic shift to emmetropia in about three months. The haze decreases after three months and is usually gone by six months.

Aggressive healers begin to exhibit haze fairly rapidly, or mild haze at four to six weeks progressing to moderate or severe at three to four months. These patients regress quickly and become myopic again. Because of this aggressive healing, the FDA has mandated a three-month waiting period between eyes for PRK.

GRADING HAZE

We grade haze as follows:

• Clear: totally clear; cannot tell the eye has been treated.
• Trace: barely detectable, observed only with oblique illumination (Fig. 1).
• Mild: observed with direct illumination (Fig. 2).
• Moderate: easily observed with direct illumination, beginning to obscure the iris detail (Fig. 3).
• Marked: dense and obscuring iris detail (Fig. 4).

FIG. 1: TRACE HAZE.	FIG. 2: MILD HAZE.
FIG. 3: MODERATE HAZE.	FIG. 4: MARKED HAZE.

Patients with haze usually see better than you would expect because haze causes light scatter in all directions. At the slit lamp, you view forward and back scatter, while the patient views only forward scatter.

In the Phase III PRK data submitted to the FDA by Summit Technology, 48.4 percent of eyes were clear at the six-month visit and 62.6 percent were clear by the 12-month visit. Only 4.1 percent of eyes showed moderate haze at the 12-month visit. With the current technique that uses larger optical zones, the incidence of haze is less than that reported in Phase III data.

CAUSES OF HAZE

Most researchers feel that keratocytes cause postoperative haze. However, vacuoles may influence the amount of haze, and new collagen formation and discontinuity in inter-lamellar alignment may also be factors. Haze seems to occur between two and four weeks after PRK. This is a time of high cellular activity, so most of the active cellular repair is complete by the third month. All of these theories probably factor into haze formation.

Steroids after surgery may reduce haze formation and regression, however, recent studies show that steroids may not have a long-term effect on the healing process.

Although haze is not usually a serious problem during the post-PRK months, it's important to educate patients so they won't be surprised if there is a slight decrease in acuity during healing. CLS

Dr. Geffen is in a joint refractive surgery practice in San Diego.