Laser-induced fluorescence during photorefractive keratectomy: a method for controlling epithelial removal

Laser-induced autofluorescence assisted by multivariate techniques discriminates a cataractous lens from healthy lens tissues of Sprague-Dawley rats

Laser-induced autofluorescence (LIAF), combined with multivariate techniques, has been used to discriminate a cataractous lens from healthy lens tissues. In this study, 405 nm and 445 nm were used as excitation sources to induce the autofluorescence. Results show higher autofluorescence intensity in cataractous lens tissues than in healthy ones. Cataractous lens tissues show a red shift of 0.9 nm and 1.2 nm at 405 nm and 445 nm excitations, respectively. Using principal component analysis (PCA), three principal components (PCs) gave more than 99% variability for both 405 nm and 445 nm excitation sources. Based on the three PCs, Fisher's linear discriminant model was developed. An accuracy of 100% was obtained in classifying the lens tissues using Fisher's linear discriminant analysis (FLDA). The LIAF technique assisted by PCA and FLDA may be used for objective discrimination of cataractous lens from healthy lens tissues of Sprague-Dawley rats.

One-step transepithelial topography-guided ablation in the treatment of myopic astigmatism

PURPOSE

To evaluate one-step topography-guided transepithelial ablation in the treatment of low to moderate myopic astigmatism using a 1KHz excimer laser.

METHODS

Retrospective study of 117 consecutive eyes available for evaluation 12 months after surgery. Pre- and post-operative visual and refractive data as well as post-operative pain and haze were analyzed. A novel technique integrating custom refractive- and epithelial- ablation in a single uninterrupted procedure was used.

RESULTS

The mean pre-operative spherical equivalent (SE) and the mean cylinder were: -3.22 diopters (D) ±1.54 (SD) (range -0.63 to -7.25 D) and -0.77 D ±0.65 (range 0 to -4.50 D), respectively. At 12 months after surgery: no eyes lost ≥2 lines of corrected distant visual acuity (CDVA). Safety and efficacy indexes were 1.27 and 1.09, respectively. Uncorrected distant visual acuity (UDVA) was ≥20/20 in 96.6% of the eyes. Manifest refraction spherical equivalent was within ±0.5 D of the desired refraction in 93.2% of the eyes. Average root mean square (RMS) wavefront error measured at central 6 mm, increased from 0.38 pre-operatively to 0.47 µm post-operatively. Refractive stability was achieved and sustained 1 month after surgery. No visually significant haze was registered during the observation period. Post-operative pain was reported in 4.5% of patients.

CONCLUSIONS

One-step transepithelial topography-guided treatment for low to moderate myopia and astigmatism performed with a 1 KHz laser, provided safe, effective, predictable and stable results with low pain and no visually significant haze.

Fluorescence-guided laser removal of chemically damaged cornea

PURPOSE

To correlate the observed fluorescence spectrum with the depth of ablation during 193 nm argon-fluoride excimer laser ablation of chemically damaged corneas.

SETTING

Laser facility, Cedars-Sinai Medical Center, Los Angeles, California, USA.

METHODS

Three cadaver New Zealand white rabbit corneas were exposed to 1 N hydrogen chloride for 10 seconds. The resultant opaque corneas were ablated to perforation using the excimer laser. Laser-induced fluorescence was collected at 45 degrees from incidence and channeled into an ultraviolet-visible spectrometer coupled to an optical multichannel analyzer reading a diode array detector. The detector recorded single-shot fluorescence spectra. The data were examined by principal component analysis, and the evolution of eigenvectors and their weighting coefficients were used to compare data among corneas. The results were correlated with histopathological sections.

RESULTS

The eigenvalues of 3 principal components corresponded to 88.9%, 10.0%, and 0.4% of the data in acid-burned corneas. Compared to that in undamaged corneas, more information was stored in the first principal component and the third eigenvector was distinctly altered. Acid-scarred tissue blue shifted the dominant fluorescence peak compared to that in normal corneal tissue.

CONCLUSIONS

After severe hydrogen chloride burn to the rabbit corneal surface, monitoring the dominant peak wavelength shift of excimer-laser-induced fluorescence can detect the transition between severely acid-damaged and underlying tissue.

Ablation spectra of the human cornea

Ablation of human corneal tissue with 193 nm excimer laser energy generates fluorescence in the near ultraviolet and visible regions of the spectrum. The fluorescence spectra from five human corneas were collected during ablation in vitro. We find that the fluorescence spectrum changes continuously as the cornea is ablated from the epithelial surface towards the endothelium. We reduced the dimensionality of the large data set resulting from each cornea by a principal components analysis. The three most significant principal component eigenvectors suffice to describe the observed spectral evolution, and independent analysis of each tissue sample produces a similar set of eigenvectors. The evolution of the calculated eigenvector weighting factors during ablation then corresponds to the observed spectral evolution. In fact, this evolution is qualitatively consistent between corneas. We suggest that this spectral evolution offers promise as a real-time surgical feedback tool.

Detection of transplant vasculopathy in a rat aortic allograft model by fluorescence spectroscopic optical analysis

BACKGROUND AND OBJECTIVE

Transplant vasculopathy is a leading cause of late cardiac graft loss. We have examined laser-induced fluorescence (LIF) spectroscopy as an optical diagnostic tool for detection of intimal plaque development and inflammatory cellular invasion in a rat model of aortic allograft transplant.

STUDY DESIGN/MATERIALS AND METHODS

Infrarenal aortic segments were transplanted from Lewis to Sprague Dawley rats. A range of vasculopathy development was produced by treatment with a viral anti-inflammatory protein. LIF spectra were recorded from the intima of aortic implants at 28 days. Fluorescence intensity was analyzed for correlation with vasculopathy development.

RESULTS

Significant differences in LIF intensity at 400-450 nm (P < or = 0.05 by ANOVA) were detected. LIF emission was correlated with plaque growth (R2 = 0.980), vessel narrowing (R2 = 0.964), and cellular invasion (R2 = 0.971) by regression analysis.

CONCLUSION

LIF optical analysis provides a nontraumatic diagnostic approach for detection of atherosclerosis prior to cardiac transplant or during development of vasculopathy after transplant.

Epithelial removal with the excimer laser (laser-scrape) in photorefractive keratectomy retreatment

OBJECTIVES

To retrospectively evaluate refractive and topographic outcomes after excimer laser photorefractive keratectomy (PRK) retreatment using the laser-scrape technique for epithelial removal.

DESIGN

Retrospective clinical study.

PARTICIPANTS

Thirty eyes of patients who had PRK retreatment for undercorrection after primary PRK were examined.

INTERVENTION

The PRK retreatment was performed using a laser-scrape technique in which the excimer laser was used to remove the majority of the epithelium overlying the anterior stromal surface before additional PRK ablation.

MAIN OUTCOME MEASURES

Uncorrected visual acuity, best-corrected visual acuity (BCVA), surface regularity index (SRI), and surface asymmetry index were measured.

RESULTS

Primary PRK was performed for myopia of -5.1 +/- 1.7 diopters (range, -1.1 to -7.5 diopters). Mean spherical equivalent (SE) was -1.3 +/- 0.4 diopters (range, -0.6 to -2.0 diopters) before retreatment. Mean SE 6 months after retreatment decreased to +0.1 +/- 0.4 diopter (range, +1.25 to -0.75 diopters; P < 0.0001). Four eyes (15%) were more than +0.5 diopter overcorrected at 6 months. Ninety-six percent of eyes achieved mean SE within +/- 1 diopter and 77% within +/- 0.5 diopter of emmetropia after retreatment. Visual acuity improved significantly 6 months after reablation (P < 0.0001) with 100% 20/40 or better and 73% 20/25 or better without correction. Final BCVA also improved compared with before retreatment (P = 0.02). Twelve eyes gained 1 line of BCVA and no eye lost more than 1 line of BCVA. The SRI before retreatment was 0.6 +/- 0.3 (range, 0.0-0.9) and remained the same 0.6 +/- 0.2 (range, 0.1-1.0; P = 0.8), 6 months after retreatment.

CONCLUSION

Excimer laser PRK retreatment using the laser-scrape technique for epithelial removal is an accurate and safe procedure for treating undercorrection of eyes after PRK for low-to-moderate myopia.

Differences in Keratocyte Apoptosis Following Transepithelial and Laser-scrape Photorefractive Keratectomy in Rabbits

BACKGROUND

Anterior stromal keratocyte cells undergo programmed cell death (apoptosis) in response to corneal epithelial injury. Keratocyte apoptosis may be an initiator of the corneal wound healing response that includes keratocyte proliferation and activation, as well as changes to the overlying epithelium, occurring following refractive surgical procedures such as photorefractive keratectomy (PRK). This study compared the effect of laser-scrape and transepithelial PRK on keratocyte apoptosis.

METHODS

Photorefractive keratectomy was performed in both eyes of 10 New Zealand white rabbits using the Summit Apex excimer laser. Surgery was performed using transepithelial PRK in one eye and laser-scrape PRK in the other. The central cornea was analyzed at 4 hours after surgery using a quantitative TUNEL assay that detects fragmented DNA characteristic of apoptosis. Hepatocyte growth factor (HGF) production by keratocytes was detected by immunocytochemistry.

RESULTS

Average apoptotic cells per 400X microscopic field determined by 2 independent masked observers were 0.9 +/- 0.5 (scanning electron microscopy) and 0.2 +/- 0.2 in the transepithelial PRK group compared with 5.1 +/- 2.9 and 4.1 +/- 3.2 in the laser-scrape group. The difference between the two groups was statistically significant for both observers (P < .05, ANOVA). HGF was detected within keratocytes throughout the corneal stroma. Less HGF was detected in the anterior stroma in the laser-scrape group at 4 hours after surgery, consistent with more anterior keratocyte apoptosis in this group.

CONCLUSIONS

Transepithelial PRK induced less anterior keratocyte apoptosis in rabbits than laser-scrape PRK. This suggests that transepithelial PRK could be useful in preventing or minimizing refractive regression and subepithelial scarring.

Retreatment after photorefractive keratectomy for low myopia

OBJECTIVE

The purpose of the study was to evaluate the results of retreatment for low myopia after primary photorefractive keratectomy (PRK).

DESIGN

A prospective study.

PARTICIPANTS

A total of 48 eyes of 37 patients from 566 eyes of 331 patients originally treated for myopia of up to -6 diopters (D) were studied.

INTERVENTION

Photorefractive keratectomy by the Coherent Schwind Keratom II excimer laser was performed.

MAIN OUTCOME MEASURES

The parameters evaluated were visual acuity, refraction, and corneal clarity.

RESULTS

Of the 566 eyes with myopia up to -6 D, 48 eyes (8.5%) required retreatment. The sphere (mean +/- standard deviation) was -0.88 +/- 1.24 D before second surgery. At 6 months, the mean was -0.04 +/- 0.91; at 1 year, it was -0.33 +/- 0.75 D. At 6 months, 75% of sphere value was within +/- 1 D. The preoperative uncorrected visual acuity (UCVA) was 20/200 or worse in 12.8% of eyes and 20/40 or better in 40.4% of eyes. Six months after reoperation, 20/60 UCVA was obtained in 17.4% of eyes. All others (82.6%) had UCVA of 20/40 or better, 26.1% better than 20/20. The proportion of eyes with best spectacle-corrected visual acuity better than 20/20 decreased from 60.0% to 47.1% 6 months after reoperation. Six months after reoperation, haze 1 was present in 42.9% and haze 2 in 4.7% of eyes, and in no eye was haze worse.

CONCLUSION

Retreatment after PRK for low myopia can significantly increase UCVA and decrease residual refractive error without significant complications.