Ординатура / Офтальмология / Английские материалы / Jaypee Gold Standard Mini Atlas Series CORNEALTOPOGRAPHY_Agarwal, Jacob_2009

MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

stromal depth, followed by trasplantation of a donor lamella of the same dimension on to the recipient bed.33 We have limited experience with this subject. We think it is a good option for patients with thin corneas, and with the preservation of the Descemet’s membrane, the complications of rejection should be extremely minimized if not eliminated. However, the subject is out of the scope of discussion in this chapter.

Intracorneal Ring Segments (Intacs)

These segments were originally designed to correct low to moderate myopia by inducing flattening of the central cornea through intralamellar insertion of 2 PMMA ring segments in the corneal midperiphery.34 Studies indicated that the range of corneal asphericity before and after surgery, provided good visual acuity and normal contrast sensitivity.10, 35 These segments could be used to modify the corneal surface in patients with irregular astigmatism whether natural as in keratoconus or surgically induced.

Contact Lens Management

Contact lenses are sometimes needed in the postoperative management of refractive surgery. This need arises as it has become evident to the refractive surgeon that an undesirable result has occurred. The decision of contact

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lens fitting has to be based on the impossibility of performing new surgeries, or the willing of the patient.36

SUMMARY

It is clear from the previous discussions that the subject of irregular astigmatism is still under investigation. In spite of the availability of various methods attempting to solve this problem, we are left with patients who are not satisfied with their vision and are in need for intervention. Penetrating keratoplasty is an ultimate solution that has to be undertaken only when the patient has no other alternative. More effort should be done to try to help these patients improving their corneal surface quality and BCVA. The evolution of newer techniques and the experience gained by refractive surgeons day after day represent a hope for irregular astigmatism patients.

REFERENCES

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3.Azar DT, Strauss I. Principles of applied optics. In: Albert DM, Jakobiec FA, (Eds): Principles and Practice of

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Ophthalmology, Philadelphia, PA, WB Saunders Co, 1994,5: 3603-21.

4.Duke-Elder S (Ed). Pathological refractive errors. In System of Ophthalmology. London: Publisher; 1970;363.

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6.Alpins NA. Treatment of irregular astigmatism. J Cataract Refract Surg 1998;24:634-46.

7.Alió JL, Artola A, Rodríguez-Mier FA. Selective Zonal Ablations with excimer laser for correction of irregular astigmatism induced by refractive surgery. Ophthalmology 2000;107:662-73.

8.Harris WF. Wavefronts and their propagation in astigmatic optical systems. Optom Vis Sci 1996;73:606-12.

9.Dick HB, Krummenauer F, Schwenn O, et al. Objective and subjective evaluation of photic phenomena after monofocal and multifocal intraocular lens implantation. Ophthalmology 1999;106:1878-86.

10.Holmes-Higgin DK, Burris TE, and The INTACS Study Group. Corneal surface topography and associated visual performance with INTACS for myopia. Phase III clinical trial results. Ophthalmology 2000;107:2061-71.

11.Alio JL, Belda JI, Shalaby AMM. Excimer Laser Assisted by Sodium Hyaluronate for correction of irregular astigmatism (ELASHY). Accepted for publication to Ophthalmology, September 2000.

12.Wiesinger-Jendritza B, Knorz M, Hugger P, Liermann A. Laser in situ keratomileusis assisted by corneal topography. J Cataract Surg 1998;24:166-74.

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13.Sher NA, Kreuger RR, Teal P, et al. Role of topical corticoids and nonsteroidal anti-inflammatory drugs in the etiology of stromal infiltrates after photorefractive keratectomy. J Refract Corneal Surg 1994;10:587-8.

14.Koch DD, Kohnen T, Obstbaum SA, Rosen ES. Format for reporting refractive surgical data. [letter]. J Cataract Refract Surg 1998;24:285-7.

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16.Buzard K, Fundingsland B. Treatment of irregular astigmatism with a broad beam excimer laser. Journal of refractive. J Refract Surg 1997;13:624-36.

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19.Kornmehl EW, Steinert RF, Puliafito CA, Reidy W. Morphology of an irregular corneal surface following 193 nm ArF excimer laser large area ablation with 0.3% hydroxypropyl methylcellulose 2910 and 0.1% dextran 70.1% carboxy-methylcellulose sodium or 0.9% saline (ARVO abstracts). Invest Ophthalmol Vis Sci 1990;31:245.

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22.Artola A, Alió JL, Bellot JL, Ruiz JM. Protective properties of viscoelastic substances (sodium hyaluronate and 2% hydroxymethyl cellulose) against experimental free radical damage to the corneal endothelium. Cornea 1993;12:109-14.

23.Kreuger RR, Trokel SL. Quantification of corneal ablation by ultraviolet light. Arch Ophthalmol 1986;103:1741-2.

24.Seiler T, Bendee T, Wollensak J. Ablation rate of human corneal epithelium and Bowman’s layer with the excimer laser (193 nm). J Refract Corneal Surg. 1990;6:99-102.

25.Klyce SD, Smolek MK. Corneal topography of excimer laser photorefractive keratectomy. J Cataract Refract Surg 1993;19: 122-30.

26.Bogan SJ, Waring GO III, Ibrahim O, et al. Classification of normal corneal topography based on computer-assisted videokeratography. Arch Ophthalmol 1990;108:945-9.

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28.Mierdel P, Kaemmerer M, Krinke HE, Seiler T. Effects of photorefractive keratectomy and cataract surgery on ocular optical errors of higher order. Graefe’s Arch Clin Exp Ophthalmol 1999;237:725-9.

29.Oshika T, Klyce SD, Applegate RA, et al. Comparison of corneal wavefront aberrations after photrefractive keratectomy and laser in situ keratomileusis. Am J Ophthalmol 1999;127:1-7.

30.Seiler T, Reckmann W, Maloney RK. Effective spherical aberration of the cornea as a quantitative descriptor in corneal topography. J Cataract Refract Surg 1993;19(Suppl):155-65.

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31.Applegate RA, Howard HC. Refractive surgery, optical aberrations and visual performance. J Refract Surg 1997; 13:295-9.

32.Sugita J, Kondo J. Deep lamellar keratoplasty with complete removal of pathological stroma for vision improvement. Br J Ophthalmol 1997;81:184-8.

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34.Ruckhofer J, Stoiber J, Alzner E, Grabner G. Intrastromal corneal ring segments (ICRS, KeraVision Ring, Intacs): clinical outcome after 2 years. Klin Monatsbl Augenheilkd 2000;216:133-42 (abstract).

35.Holmes-Higgin DK, Baker PC, Burris TE, Silvestrini TA. Characterization of the aspheric corneal surface with intrastromal corneal ring segments. J Refract Surg 1999;15:520-8.

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INTRODUCTION

Clinically significant decentered excimer ablations result in symptoms such as glare, shadows, dim lighting disturbances, distortion, and monocular diplopia, as well as loss of best-corrected visual acuity due to irregular astigmatism. Decentration can be a corollary of poor fixation due to poor patient instruction, anxiety, oversedation, or blurry vision due to high refractive error, or the exposed stromal bed causing difficulty seeing the laser’s target. It can also result from improper stabilization of the eye with a Thornton ring during excimer ablation.

Centration over the entrance pupil center has been accepted as the standard for many years and still remains the recommendation for many laser systems. However, variations in pupil size can affect the location of the entrance pupil center. Therefore, centration by this technique is not the most reliable estimation of the visual axis. This has led to the use of the coaxially sighted corneal light reflex for more accurate centration during refractive procedures. This reflex represents the corneal intercept of a line from the point of fixation to the center of corneal curvature.

In addition, hyperopic ablations may result in more clinically significant symptoms than myopic ablations following equal magnitude refractive treatments. The functional optical zone in hyperopic treatments is smaller,

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CHAPTER 8: DECENTERED ABLATION

emphasizing any possible decentration of the ablation zone. This is further accentuated by a greater average angle kappa in hyperopes as compared to myopes.

Proper centration of ablation requires careful preoperative and intraoperative instructions, especially with regard to the fixation target. It is important to keep both eyes open, preparing patients with regards to sounds and smells that may startle them. The head and body must remain still during surgery The advent of modern laser tracking systems, iris registration software, and wavefront-guided excimer laser technology has significantly reduced the incidence of decentration.

CLINICAL DEFINITION

The following clinical signs and symptoms often present in patients with a decentered ablation: (1) decentration of the ablation zone on corneal topography, (2) increased higher order aberrations as measured using wavefront aberrometry, predominantly coma, (3) the appearance of a tail on point spread functions, (4) reduced best spectaclecorrected visual acuity which improves only with gas permeable lenses, (5) a cylinder measurement on autorefraction and wavefront that differs from manifest refraction, and (6) a history of reduced vision immediately following surgery that fails to improve with time.

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