David P.S. O'Brart

Corneal Optical Aberrations Induced by Photorefractive Keratectomy

BACKGROUND Photorefractive keratectomy causes marked alteration to anterior corneal topography, and is likely to induce major changes to the optical aberrations of the eye. METHODS Six diopters (D) of myopia correction was attempted on one eye of 50 patients, randomly allocated to one of three different treatments: 5-mm or 6-mm single ablation zone, or a double ablation (multizone; -5.00 D correction over 4.6 mm and -1.00 D over 6 mm). Topographic data was used to estimate corneal aberration coefficients. These were compared for effect of ablation zone, before and 1 year after photorefractive keratectomy. The coefficients were used to derive modulation transfer functions for the anterior corneal surface. RESULTS Corneal spherical aberrations and coma-like aberrations both increased significantly following photorefractive keratectomy (p < 0.001). The mean spherical aberration coefficient increased from 0.36 +/- 0.11 before, to 0.91 +/- 0.37 after treatment, while the mean coma-like aberration coefficient changed from 0.28 +/- 0.16 before, to 0.60 +/- 0.31 after treatment. Ablation zone form had a significant effect on spherical aberration (p = 0.030), but not for coma (p = 0.96). The spherical aberration coefficient increased least for the 6-mm ablation (by 0.38 +/- 0.17), compared with the 5-mm ablation (0.69 +/- 0.45) and the multizone (0.62 +/- 0.38). Corneal modulation transfer functions were reduced significantly following the photorefractive procedure. The effect was greatest for large pupil diameters and for spatial frequencies between 2 and 15 cycles per degree. CONCLUSIONS Corneal modulation transfer function calculations suggest that a significant loss of visual performance should be anticipated following photorefractive keratectomy, the effect being greatest for large pupil diameters. Results for three ablation zones show that induced aberrations are least for the largest (6 mm) ablation zone.

Effect of complete epithelial debridement before riboflavin–ultraviolet-A corneal collagen crosslinking therapy

PURPOSE: To evaluate the importance of complete epithelial removal before riboflavin–ultraviolet‐A (UVA) corneal collagen crosslinking therapy. SETTING: School of Optometry and Vision Sciences, Cardiff University, Wales, United Kingdom. METHODS: Riboflavin eyedrops were applied at 5‐minute intervals for 35 minutes to the anterior corneal surface of 36 porcine eyes (12 with no epithelial trauma but treated with tetracaine eyedrops, 12 with superficial epithelial trauma but with an intact basal epithelium, and 12 with a fully removed epithelium). The corneal surface of 6 tetracaine‐treated eyes, 6 eyes with superficial epithelial trauma, and 6 eyes with a fully removed epithelium was exposed to UVA light for 30 minutes during riboflavin administration. The light transmission spectra of the enucleated corneas were analyzed with a spectrophotometer and compared with those of 9 untreated porcine corneas. RESULTS: Corneas with a fully removed epithelium treated with riboflavin showed an abnormal dip in the transmission spectrum between 400 nm and 510 nm (P<.01). This was attributed to the presence of riboflavin in the corneal stroma. The spectra of riboflavin‐treated corneas with no epithelial trauma but tetracaine administration and those with superficial epithelial trauma did not differ from those of the non–riboflavin‐treated controls. Exposure to UVA following riboflavin administration did not alter corneal light transmission. CONCLUSIONS: Complete removal of the corneal epithelium is an essential component of riboflavin–UVA crosslinking therapy as superficial epithelial trauma and tetracaine administration alone are not sufficient to permit the penetration of riboflavin into the corneal stroma. Failure to achieve adequate stromal absorption of riboflavin may impair the efficacy of the crosslinking process.

Corneal Light Scattering and Visual Performance in Myopic Individuals With Spectacles, Contact Lenses, or Excimer Laser Photorefractive Keratectomy

Corneal transparency and visual performance are currently receiving much attention after excimer laser surgery. To date, emphasis has been on comparison of eyes on which laser surgery has been performed to eyes with emmetropia. A more appropriate comparison should be between eyes on which laser surgery has been performed and eyes with other forms of correction for myopia. Forward-scattered light, back-scattered light, and visual acuity were investigated and data were collected from 35 myopic individuals with various types of correction for myopia (spectacles, hard and soft contact lenses, and excimer laser surgery). Forward-scattered light was measured by using a new computerized technique, back-scattered light was measured with a charge coupled device-camera system, and visual acuity was measured with a computerized system at various levels of contrast. Spectacles, hard contact lenses, and excimer laser surgery are all superior to soft contact lenses in terms of light scatter and low-contrast visual acuity and excimer laser photorefractive keratectomy produces comparable results to spectacles one year postoperatively. At low-contrast visual acuity, mean visual acuity was 2.45 minutes of arc for the spectacle wearers, 3.21 minutes of arc for the hard contact-lens wearers, and 5.04 minutes of arc for the soft contact-lens wearers. Excimer laser patients had a mean visual acuity of 9.04 minutes of arc three months postoperatively, and 2.53 minutes of arc after one year. A mean value of 2.4% contrast for forward light scatter was obtained for spectacle wearers compared with a level of 3.84% contrast for hard contact-lens wearers and 16.1% contrast for soft contact-lens wearers. The mean value for excimer laser patients was 20% contrast three months postoperatively and 2.1% contrast one year postoperatively.

The Effects of Topical Corticosteroids and Plasmin Inhibitors on Refractive Outcome, Haze, and Visual Performance after Photorefractive Keratectomy: A Prospective, Randomized, Observer-masked Study

BACKGROUND This study of 86 patients with 12 months of follow-up was designed to determine whether topical corticosteroids or plasmin inhibitors have an effect on the outcome of photorefractive keratectomy. METHODS Patients were allocated randomly to either steroid (0.1% fluorometholone for 6 months), plasmin-inhibitor (aprotinin 40 IU/ml for 3 weeks), or control (no treatment) groups and underwent either -3.00- or -6.00-diopter (D) corrections. RESULTS With -3.00-D corrections, the mean refractive change was significantly greater at 3 and 6 months (P < 0.05) in the steroid group compared with the control group. When steroids were discontinued, the difference became insignificant within 3 months. Similarly, with -6.00-D procedures the mean refractive change was greater at 6 weeks and 3 and 6 months (P < 0.01), but the refractive change again became insignificant 3 months after stopping steroid treatment. Four patients treated with steroids had a hyperopic shift greater than +2.00 D of that intended at 12 months. Similar overcorrections were not noted in the other treatment groups. There were no differences in refractive outcome between the aprotinin and control groups at any stage. With -6.00-D procedures, objective measurements of haze were significantly greater in the aprotinin group compared with the control group at 9 and 12 months (P < 0.05). With this exception, there were no differences in haze, forward or backward scatter of light, best-corrected visual acuity, or halo measurements between the groups. CONCLUSIONS Corticosteroids can maintain a hyperopic shift during their administration, but this effect is reversed on cessation of treatment. Objective tests have shown that steroids have no effect on corneal haze or visual performance after PRK. There is no justification for routinely submitting all patients to long-term steroid regimens and their associated side effects. Treatment with aprotinin produced no beneficial effect on refractive outcome, and haze was greater in the -6.00-D procedures. The concept of modulating the plasminogen activator/plasmin system to regulate wound healing after PRK is discussed.

A randomised, prospective study to investigate the efficacy of riboflavin/ultraviolet A (370 nm) corneal collagen cross-linkage to halt the progression of keratoconus

Aims A blind, randomised, prospective, bilateral study to investigate the efficacy of riboflavin/ultraviolet A corneal collagen cross-linkage to halt the progression of keratoconus. Methods 24 patients with early/moderate bilateral keratoconus with recent progression were recruited. One eye was randomly assigned to undergo collagen cross-linkage following epithelial removal with riboflavin 0.1% and ultraviolet A (370 nm at 3 mW/cm2). The other remained untreated as a control. The follow-up was 18 months in 22 patients. Results At 18 months, Orbscan II 3 mm, 5 mm keratometry and simulated astigmatism and cone apex power and wave-front measurements (Keraton Scout), including root mean square, coma and pentafoil showed significant reductions from baseline in treated compared with untreated eyes (p=0.04). In treated eyes at 18 months, the best spectacle-corrected acuity improved (p=0.01), and Orbscan II-simulated keratometry (p<0.001), 3 mm keratometry (p=0.008), simulated astigmatism (p=0.007), cone apex power (p=0.002), root mean square, coma, spherical aberration, secondary astigmatism and pentafoil (p=0.05) decreased from baseline. One treated eye experienced transient recurrent corneal erosions; otherwise there were no complications attributable to the treatment. Conclusions Corneal collagen cross-linkage appears to be an effective and safe modality to halt the progression of keratoconus. Improvements in visual and topographic parameters are seen in some eyes.

Effect of Epithelial Retention and Removal on Riboflavin Absorption in Porcine Corneas

PURPOSE To compare stromal riboflavin absorption after 20% alcohol application and partial or complete epithelial removal by analyzing light transmission properties of porcine corneas after riboflavin/ultraviolet A (UVA) corneal collagen cross-linking. METHODS Riboflavin 0.13% eye drops were applied to 18 porcine eyes (6 in which 20% alcohol solution had been applied for 40 seconds, 6 eyes with a grid pattern of full-thickness epithelial trauma, and 6 with the central epithelium fully removed) at 5-minute intervals for 35 minutes. In all eyes, the corneal surface was exposed to UVA light for 30 minutes during riboflavin administration. The light transmission spectra of the corneas were analyzed with a spectrophotometer and compared to those of 9 untreated controls (4 corneas with epithelium and 5 without) and to the spectra of riboflavin 0.13% solution. RESULTS The spectra of riboflavin-treated corneas in the alcohol group were similar to controls. Those with grid-pattern epithelial trauma showed a dip in light transmission between 400 and 490 nm, but this was significantly less than that in eyes for which epithelial removal was complete, where the spectrum was similar to that of riboflavin 0.13% solution. CONCLUSIONS Complete removal of the corneal epithelium appears to be necessary to allow sufficient riboflavin absorption into the stroma to alter the normal light transmission properties of the porcine cornea. Although partial grid-pattern epithelial removal allows some riboflavin penetration, uptake is limited and non-homogeneous, which may affect the efficacy of the cross-linking process.

Phototherapeutic keratectomy for recurrent corneal erosions

Recurrent corneal erosion may be a difficult disorder to treat, with a number of patients suffering persistent symptoms despite conventional therapy. We present a series of 15 patients (17 eyes) who underwent excimer laser phototherapeutic keratectomy (PTK) for recurrent corneal erosion. In 9 patients a previous episode of corneal trauma could be identified, while in 6 (8 eyes) the problem had occurred spontaneously. The mean duration of symptoms prior to PTK was 13 months (range 6-60 months). All patients had received lubricant ointments, 7 had tried bandage contact lenses and 4 had undergone epithelial debridement with no amelioration of their symptoms. The affect area of epithelium was removed and a 20-30 pulse (5-7 µm) ablation was performed to Bowmans membrane. Great care was taken to avoid the edges of treatment zones from encroaching on the axial cornea. In 11 patients (13 eyes) there was a marked improvement in symptoms post-operatively with no recurrences. The mean follow-up was 11 months (range 6-24 months). Four patients experienced recurrent episodes 3-6 months after PTK and 2 have subsequently been retreated, 1 of whom has been symptom-free for 12 months. Post-operative best corrected visual acuity was unaltered in 9 eyes and improved by at least one Snellen line in 8 eyes. Excimer laser PTK appears to be a safe and promising procedure for recurrent corneal erosion in cases refractory to medical treatment. Further studies are indicated to compare its effectiveness with established surgical procedures.

A randomised, prospective study comparing trabeculectomy with viscocanalostomy with adjunctive antimetabolite usage for the management of open angle glaucoma uncontrolled by medical therapy

Aims: To compare trabeculectomy with viscocanalostomy augmented with adjunctive antimetabolite use for the control of intraocular pressure (IOP) in open angle glaucoma (OAG). Methods: 45 patients (50 eyes) with uncontrolled OAG were randomised to either trabeculectomy (25 eyes) or a viscocanalostomy technique (25 eyes). Preoperatively, all eyes were graded in terms of risk factors for drainage failure and were given intraoperative antimetabolites (5-fluorouracil 25 mg/ml (5-FU), mitomycin C (MMC) 0.2 mg/ml and 0.4 mg/ml) according to a standard protocol. Results: There were no significant differences between the groups in age, sex, type of OAG, preoperative medications, risk factors for drainage failure, and preoperative IOP. Mean follow up was 20 months (range 3–24 months). It was 12 months or longer in all eyes, except two lost to follow up at 3 months. At 12 months, complete success (IOP<21 mm Hg without antiglaucoma medications) was seen in 91% of eyes undergoing trabeculectomy, but in only 60% of eyes undergoing viscocanalostomy (p<0.02). Similarly, at the last follow up visit (mean 20 months) complete success was seen in 68% of eyes undergoing trabeculectomy and 34% with viscocanalostomy (p<0.05). In terms of qualified success (IOP<21 mm Hg with or without glaucoma medications) and mean IOP measurements postoperatively there were no difference between the groups, although the mean number of antiglaucomatous medications required postoperatively was less with trabeculectomy (0.39) than viscocanalostomy (1.04) (p<0.05). Needling procedures were more commonly required after trabeculectomy (p<0.02). YAG goniotomy was required in three eyes (13%) after viscocanalostomy. Early transient complications such as anterior chamber shallowing and encysted blebs were more common in the trabeculectomy group (p<0.05). Late postoperative cataract formation was similar between the two groups. Conclusion: In terms of complete success and number of antiglaucomatous medications required postoperatively, IOP control appears to be better with trabeculectomy. Viscocanalostomy is associated with fewer early transient postoperative complications.

Night vision after excimer laser photorefractive keratectomy: haze and halos

A series of 85 patients with myopia, up to −6.00D, was treated by photorefractive keratectomy (PRK), using a 5 mm diameter ablation zone. At six months, 38 patients (45%) reported slight disturbances of night vision, nine (11%) of whom had significant problems. Perturbations of night vision after PRK are seen as starbursts and halos around lights. Corneal haze produces the starbursts, which are usually transient. In contrast, halos are myopic blur circles and may be persistent. Using a computer program, halos after PRK were found to be significantly larger than those in emmetropes and myopes corrected with spectacles (p < 0.01). The halos were diminished by using either artificial pupils or negative lens over-correction. In patients with identical bilateral PRK corrections, except for the ablation zone size, the magnitude of the halo was less with 5 mm than 4 mm zones (p < 0.01). Patients treated with 5 mm reported fewer problems attributable to halo than with the 4 mm ablation diameters (p < 0.01). Halos and pupil diameters were measured in nine patients with significant impairment of night vision haze. Those with starbursts had small hyperopic shifts, minimal halos and high haze and light scatter measurements, whilst patients with halos had large hyperopic shifts, little haze and large pupil diameters. Patients with persistent halo problems benefited from either negative lens over-correction or miotics at night.

Long-term follow-up of riboflavin/ultraviolet A (370 nm) corneal collagen cross-linking to halt the progression of keratoconus

Aims To determine long-term efficacy of riboflavin/ultraviolet A corneal cross-linking (CXL). Methods Thirty patients (30 eyes) who had undergone CXL following epithelial removal 4–6 years previously were examined. Results At 1-year mean, spherical equivalent error (SEQ) increased by +0.72 dioptres (D) (p<0.002), corrected distance visual acuity (CDVA) improved (p<0.005), mean simulated keratometry (Sim K) reduced by 0.27D (p<0.04), cone apex power (CAP) reduced by 0.4D (p<0.02), and secondary astigmatism improved (p<0.03) compared with preoperative values. At 4–6 years, mean SEQ increased by +0.82D (p<0.001), CDVA improved (p<0.03), mean Sim K reduced by 0.84D (p<0.00001), CAP reduced by 1.16D (p<0.0005), and root mean square (RMS) (p<0.0001), coma (p<0.0001), secondary astigmatism (p<0.005) and pentafoil (p<0.05) decreased compared with preoperative values. At 4–6 years, mean Sim K reduced by 0.59D (p<0.0005), CAP reduced by 0.76D (p<0.02), RMS (p<0.001), coma (p<0.002) and secondary astigmatism (p<0.02) reduced and central pachymetry increased (p<0.05) compared with 1 year. No treated eyes progressed. None lost >1 line of CDVA. Seven untreated fellow eyes progressed. Conclusions CXL is an effective and safe treatment with up to 4–6 years follow-up. Improvements in topographic and wave-front parameters evident at 1 year continue to improve at 4–6 years.