Examination of subepithelial scar formation after photorefractive keratectomy with the ultrasound biomicroscope

The subepithelial haze and the concomittant refractive regression are the most important complications of photorefractive keratectomies (PRK). The authors examined the subepithelial haze characteristics by ultrasound biomicroscopy (UBM). Twenty previously photokeratectomized myopic eyes, which showed a certain level of subepithelial scarring have been examined (age: 34.22 +/- 3.57 years, 12 women, 8 men). The range of PRK treatments varied between -4.0 D and -12.0 D. The subepithelial haze graded according to Hanna et al, between 1.0 and 4.0. The UBM examinations were carried out between 1-3 months following PRK with the 50-80 MHz transducer of a Model 840 Zeiss-Humphrey Ultrasound Biomicroscope. The severity of subepithelial haze correlated with the previous photoablation depth. Up till haze graded 2.0 according to Hanna et al. UBM showed loss of Bowman membrane and a slight thinning of the central cornea. Between grade 2.0 and 3.0 the reflectivity of anterior stroma increased, above grade 3.0 a hyperreflective 1/3 anterior stroma with irregular borders were observed. Haze phenomen graded below 2.0 is not observable with UBM. Haze, graded more than 2.0 caused an increase of anterior stormal reflectivity. In more serious cases the 1/3 of the anterior stroma might be affected. The UBM-method with a 50-80 MHz probe is suitable to document and follow the serious haze phenomen after photoablations.