Screening and prophylaxis of retinal degenerations prior to refractive surgery

The need and demand for surgical correction of refractive errors, particularly myopia, has been increasing. Degenerations involving the peripheral retina are common in myopes and can result in a rhegmatogenous retinal detachment. There are no clear guidelines for retinal screening and management of asymptomatic retinal degenerations prior to refractive surgery or for follow-up of these patients. This article aims to provide a set of guidelines for the management of retinal degenerations in eyes undergoing refractive surgeries.

Inter-observer agreement and sensitivity of Optomap images for screening peripheral retinal lesions in patients undergoing refractive surgery

Purpose:

The aim of this study was to compute the sensitivity, specificity and inter-reader variability of ultra-widefield retinal imaging (Optomap 200Tx) for screening retinal lesions before myopic refractive surgery.

Methods:

Two hundred and eight eyes of 109 consecutive refractive surgery candidates were included in this study. All subjects underwent Optomap 200Tx, mydriatic slit-lamp lens examination and dilated retinal examination with scleral indentation by a retinal specialist. Retinal findings by indirect dilated examination by retinal specialist was considered as the gold-standard. Sensitivity analyses for the readers were calculated between the Optomap images and the gold-standard retinal examination.

Results:

Seventy-three of the 208 eyes (35.1%) had peripheral retinal lesions diagnosed by the retinal specialist on dilated fundus examination. Peripheral lesions were seen on the Optomap images in 111 (53.4%) eyes. Compared to the dilated retinal examination, the detection rate with the Optomap 200Tx was 78.1% and specificity rate was 60%. The accuracy rate between the 3 readers ranged from 72% to 87%. The highest accuracy was noted with the reader post 1 year of retinal training (86.54%).

Conclusion:

The Optomap 200Tx showed a high sensitivity and moderate specificity for identifying peripheral retinal lesions in eyes undergoing refractive surgery. The Optomap examination is a convenient, fast and feasible method for detecting the pathological fundus changes in myopic eyes. The reliability of the examination improves when the images are interpreted by a reader with prior retinal training.

How the cornea heals: cornea-specific repair mechanisms affecting surgical outcomes

In mammals, penetrating injuries typically heal by deposition of fibrotic "repair tissue" that fills and seals wounds but does not restore normal function. Excessive deposition of fibrotic repair tissue can lead to pathologies involving excessive scarring and contracture. In the cornea, fibrotic repair presents special challenges affecting both clarity and shape of the cornea. With the increasing popularity of surgical techniques that alter corneal refractive errors, understanding of cornea repair mechanisms has acquired new significance. The cornea has unique anatomic, cellular, molecular, and functional features that lead to important mechanistic differences in the process of repair in comparison with what occurs in skin and other organs. Moreover, corneal function calls for special outcomes. This review addresses these features from the viewpoint of the authors' research on factors of importance to understanding and improving surgical outcomes.

Prospective Randomized Study of Corneal Aberrations 1 Year After Radial Keratotomy or Photorefractive Keratectomy

PURPOSE

To evaluate the optical properties of the cornea 1 year after either radial keratotomy (RK) or photorefractive keratectomy (PRK) in a randomized group of patients with low myopia.

METHODS

Ninety-six patients with myopia between -0.75 and -5.00 D were randomized to either radial keratotomy (n = 46) or photorefractive keratectomy (n = 50). Topography maps were obtained 1 year after surgery and analyzed by computation of total corneal wavefront aberration and Zernike polynomial coefficients for pupil sizes of 2, 4, and 6 mm. The 4-mm pupil size was used for optimization of the model.

RESULTS

The total corneal wavefront aberrations after RK and PRK were similar and not statistically different. Wavefront aberrations arising from astigmatism or defocus accounted for approximately 70% of the total wavefront error at all pupil sizes in both groups. All types of aberrations, and in particular spherical aberration, increased significantly with increasing pupil size. Higher-order wavefront aberrations were almost twice as high after RK than after PRK at pupil sizes of 4 and 6 mm. Spherical aberration and coma were slightly higher after PRK than after RK.

CONCLUSIONS

Pupil size had a major effect on corneal aberrations after RK and PRK. The most important aberrations were sphero-cylindrical, in which eyes became significantly more myopic with increasing pupil size. The image forming properties of the cornea are better after PRK compared with RK due to the lesser amount of higher-order aberrations.

Extracellular matrix changes in human corneas after radial keratotomy

Extracellular matrix and basement membrane alterations were identified in human corneas after radial keratotomy. Ten normal and five radial keratotomy autopsy corneas (two at 6 months post surgery, and three at 3 years post surgery) were studied by immunofluorescence with antibodies to 28 extracellular matrix and basement membrane components. Outside of radial keratotomy scars, all studied components had a normal distribution. Of stromal extracellular matrix, only type III collagen accumulated around the scars. The basement membrane around epithelial plugs had a normal composition except for type IV collagen. Its alpha1-alpha2 chains, normally present only in the limbal basement membrane, appeared around all plugs. alpha3 and alpha4 chains were very weak or absent in these areas, contrary to nonscarred areas. This basement membrane pattern was similar to the normal limbal but not to the central corneal pattern. Keratin 3 also had a limbal-like, suprabasal expression in the plug epithelium. The stroma around the scars accumulated tenascin-C, fibrillin-1, types VIII and XIV collagen, all of which were absent from normal corneal basement membrane and extracellular matrix. Only tenascin-C showed less staining in anterior scars 3 years post surgery than 6 months post surgery, but still persisted in posterior scars. Incomplete scar healing was evident even 3 years post radial keratotomy. It was manifested by the accumulation of abnormal extracellular matrix in the anterior and posterior scars and by the limbal-like pattern of type IV collagen isoforms in the basement membrane around epithelial plugs.

Ultrastructural and Immunohistochemical Findings after Linear Excimer Laser Keratectomy

BACKGROUND

One human pathology specimen has been studied previously following the use of the excimer laser for the correction of astigmatism. We report histopathologic findings following linear corneal excisions with the excimer laser.

METHODS

A 193 nm excimer laser was used to create symmetrical, transverse excisions in a human eye to correct astigmatism. Three months later, a full-thickness corneal transplant was performed due to unsatisfactory refractive results. The excised corneal button was examined with light microscopy, transmission electron microscopy and immunohistochemistry.

RESULTS

An area 10 to 20-micron wide was observed between the epithelial cells within the keratectomy and the sharply dissected stromal lamellae. This area stained positive for laminin and pro-collagen type III. Some epithelial cells showed processes reaching into this area. Descemet's membrane, immediately underneath the area of the keratectomy, contained atypically striated collagen fibers.

CONCLUSIONS

These findings demonstrate wound healing changes similar to those reported following diamond knife keratotomy and photorefractive keratectomy for myopia. The changes in the posterior cornea are similar to those previously reported when an excimer laser beam approached Descemet's membrane.

Comparison of the UniversalKeratome and the Automated Corneal Shaper

PURPOSE

To compare the morphologic appearance and measurements of in situ keratomileusis performed with the UniversalKeratome (UK) with those done with the Automated Corneal Shaper (ACS).

SETTING

Surgical suite within private practice.

METHODS

Procedures were performed the same day on mate eye-bank eyes. In situ keratomileusis was done using existing nomograms for each instrument to resect a cap thickness of 160 microns and a myopic resection of 100 microns. Intraocular pressures were increased by inflating the globes with balanced salt solution and were measured with the suction fixation rings in place. The excised caps and stromal resections were measured twice independently after surgery, again after tissue fixation, and then evaluated with light and scanning electron microscopy.

RESULTS

No complications were encountered. Compared with the ACS, the UK was easy to set up, use, clean, and take down. Its excised tissue dimensions were greater and more predictable, it resected a concave shaped lenticule (edges imperceptibly blending with the host stroma), and it created a smoother power resection surface and primary resection base.

CONCLUSIONS

Smoother, predictable tissue resection, and simple assembly/disassembly and use give the UK an apparent advantage over the ACS. The UK corrects astigmatism and hyperopia by changing the shape of the poly(methyl methacrylate) optical insert.

Measurement of Irregular Astigmatism Using Semimeridian Data from Videokeratographs

BACKGROUND

Irregular astigmatism is one of the most troublesome side effects of corneal surgery. We attempted to quantify irregular astigmatism in a simple way, using semimeridian data from videokeratographs of corneas with different amounts of irregular astigmatism.

METHODS

We studied the corneal topography of 30 normal corneas to be used as controls, 30 corneas from patients after radial keratotomy, 30 corneas from patients after nonfreeze myopic keratomileusis, and 30 corneas with clinically apparent keratoconus. We developed two diagnostic formulas, a refractive power symmetry index and an angle symmetry index, using the absolute-scale, color-coded map, and semimeridian data from the EyeSys Corneal Analysis System, and used them to compare the four groups of corneas.

RESULTS

In the corneas with irregular astigmatism, values for the refractive power symmetry index and angle symmetry index were significantly higher than in the normal corneas. The refractive power symmetry index confidence interval set at 95% was 0.58 diopters (D) (range 0.00 D to 0.58 D) in the control group. Using these limits, most values in the radial keratotomy, keratomileusis, and keratoconus groups were abnormal. Spectacle-corrected visual acuity correlated negatively with the refractive power symmetry index (rs = -0.58, p < .001). The refractive power symmetry index sensitivity and specificity were respectively 97% and 66%. Spectacle-corrected visual acuity did not correlate with the angle symmetry index.

CONCLUSIONS

Semimeridian data from videokeratographs can be used to quantify irregular astigmatism.

Mechanical Stability of the Cornea After Radial Keratotomy and Photorefractive Keratectomy

BACKGROUND

Corneal refractive surgical procedures alter the shape and structure of the tissue, possibly compromising its mechanical stability.

METHODS

One or both eyes of 87 subjects were studied: 26 of these 87 had been treated for myopia by excimer laser ablation and 36 by radial keratotomy; 25 without previous corneal surgery functioned as controls. Corneal topography was evaluated by TMS-1 videophotokeratography before and after 180 degrees tilting of the patient, and the pressure-induced ring-wise changes in spherical equivalent power (axial power) were calculated. Changes in corneal shape also were evaluated by computing the instantaneous radius of curvature. The intraocular pressure was measured before and after tilting by a hand-held applanation tonometer.

RESULTS

The mean intraocular pressure increased from 13.9 +/- 2.3 mm Hg before tilting to 30.0 +/- 3.8 mm Hg during tilting. In the control eyes, the mean power of the central cornea during tilting decreased 0.187 +/- 0.045 diopters (D) (p < .05); in the excimer-laser-ablated eyes, 0.038 +/- 0.056 D (not statistically significant); and in those treated with radial keratotomy, 0.523 +/- 0.054 D (p < .01). After radial keratotomy, the cornea steepened outside the clear zone in response to pressure loading, whereas it did not change significantly in the laser-ablated or control eyes.

CONCLUSIONS

Pressure-induced deformation of normal and excimer laser-ablated human cornea is small, whereas radially incised corneas have significantly decreased mechanical stability.

Specular microscopy of the corneal endothelium after excimer laser photorefractive keratectomy

PURPOSE

To evaluate endothelial cell morphology and density after excimer laser photorefractive keratectomy (PRK).

METHODS

We used a noncontact specular microscope to examine the central corneal endothelium of 50 eyes of 50 patients who had PRK for an attempted correction between -2.5 and -17.0 diopters (D) (mean -7.8 D) beginning 18 to 24 hours postoperatively.

RESULTS

After a follow-up of 11.4 +/- 6.1 months (mean +/- standard deviation), mean endothelial cell density was 2577.6 +/- 402.0 cells/mm2 with rare signs of polymegathism and pleomorphism. Preoperative and untreated fellow eye endothelial cell density values were used as a control. Paired Student's t-test and analysis of variance results were not significant (P > .05).

CONCLUSION

Excimer laser PRK did not significantly change cell density and morphology.

Scheimpflug anterior segment photography assessment of wound healing after myopic excimer laser photorefractive keratectomy

PURPOSE

To describe a method for analyzing Scheimpflug anterior segment images for a new measure of the cornea's response to excimer laser photoablation.

SETTING

Mericos Eye Institute, Scripps Memorial Hospital, La Jolla, California.

METHODS

Digitized Scheimpflug anterior segment photographs of operated and unoperated eyes were obtained in 17 patients 1 to 15 months after photorefractive keratectomy (PRK). The images were analyzed to determine their dimensions. Each imaged opacity was compared with corneal haze observed by slitlamp biomicroscopy, intended ablation depth, postoperative corneal thickness, and refractive error change.

RESULTS

All postoperative corneas displayed a nonhomogeneous, meniscus-shaped pattern in the ablated area that ranged from 17 to 40% of corneal thickness. This pattern correlated poorly with intended laser ablation depth.

CONCLUSION

This technique provides a new assessment of corneal response to PRK. Improvements in software analysis may facilitate quantitative assessment.

Photorefractive keratectomy to treat myopia and astigmatism after radial keratotomy and penetrating keratoplasty

Fifteen eyes with an initial myopia between -5.00 diopters (D) and -12.00 D were treated with radial keratotomy (RK) followed by photorefractive keratectomy (PRK) at least 6 months later and observed for 6 months to 24 months. Five eyes that had penetrating keratoplasty (PKP) were treated for residual ametropia by PRK and followed for up to two years. For the RK-treated eyes, mean pre-PRK refraction was -4.00 D sphere and + 1.25 D cylinder, which improved to -0.52 D sphere and + 0.73 D cylinder. Incidence of complications, including corneal haze, was extremely low in both the RK and PKP groups. In summary, PRK is a valuable method for correcting ametropia following RK and PKP, with risks similar to that for eyes having PRK as the initial refractive procedure.