eMedicine Specialties > Ophthalmology > Refractive Disorders

Myopia, PRK: Workup

Author: Fernando H Murillo-Lopez, MD, Senior Surgeon, Unidad Privada de Oftalmologia CEMES
Contributor Information and Disclosures

Updated: Apr 18, 2006

Workup

Other Tests

  • All patients who are surgical candidates for PRK should undergo preoperative screening with videokeratography (corneal topography).
    • A high percentage of candidates for keratorefractive surgery have clinical or subclinical keratoconus.
    • Preoperative corneal topography must be radially symmetric and free of irregular astigmatism.
    • Patients suspected of having keratoconus are detected most easily with videokeratography using a 1.5-D interval scale by looking for a local area of corneal steepening.
  • Pseudokeratoconus is a form of localized corneal steepening caused by an artifact. The most common cause of pseudokeratoconus is found in contact lens wearers who have worn decentered contact lenses.
  • Contact lens warpage is a phenomenon resulting in destabilization of a patient's refraction. This effect may last for several weeks after contact lens use has been discontinued.
  • Quantitative descriptors of corneal optical performance, such as the Surface Regularity Index (SRI), can be helpful to quantify the smoothness of the ablation postoperatively.

Histologic Findings

The following histologic excimer-induced changes in corneal morphology have been reported:

  • Epithelium: Initial reepithelialization occurs within the first 3-5 days. Over the following 6-18 months, the epithelium thickens primarily at the deepest part of the ablation site. No clear explanation exists for this phenomenon of epithelial hyperplasia. It is most likely secondary to the epithelium "smoothing out" the ocular surface in conjunction with the mechanical smoothing from the eyelids with each blink. Morphologic and immunohistochemical studies demonstrate normal epithelial attachment complexes as evidenced by the presence of type III collagen (anchoring fibrils), beta 4-integrin (epithelial hemidesmosomes), and type IV collagen (basement membrane).
  • Stroma: Within the first 24 hours after excimer PRK, stromal wound healing begins, as inflammatory cells invade the corneal stroma from the tear film. For at least 3 weeks following PRK, activated fibrocytes repopulate the treated area. These cells are responsible for the formation of new collagen and proteoglycan matrix.
  • No significant changes have been found in the corneal endothelium or the Descemet membrane following excimer PRK as long as 250 µm of stroma remain.

More on Myopia, PRK

Overview: Myopia, PRK
Workup: Myopia, PRK
Treatment: Myopia, PRK
Follow-up: Myopia, PRK
References
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References

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  2. Erie JC, McLaren JW, Hodge DO. Recovery of corneal subbasal nerve density after PRK and LASIK. Am J Ophthalmol. Dec 2005;140(6):1059-1064. [Medline].

  3. Gambato C, Ghirlando A, Moretto E. Mitomycin C modulation of corneal wound healing after photorefractive keratectomy in highly myopic eyes. Ophthalmology. Feb 2005;112(2):208-18; discussion 219. [Medline].

  4. Krueger RR, Trokel SL. Quantitation of corneal ablation by ultraviolet laser light. Arch Ophthalmol. Nov 1985;103(11):1741-2. [Medline].

  5. McDonald MB, Kaufman HE, Frantz JM, et al. Excimer laser ablation in a human eye. Case report. Arch Ophthalmol. May 1989;107(5):641-2. [Medline].

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  7. O'Brart DP, Patsoura E, Jaycock P. Excimer laser photorefractive keratectomy for hyperopia: 7.5-year follow-up. J Cataract Refract Surg. Jun 2005;31(6):1104-13. [Medline].

  8. Paysse EA, Coats DK, Hussein MA. Long-term outcomes of photorefractive keratectomy for anisometropic amblyopia in children. Ophthalmology. Feb 2006;113(2):169-76. [Medline].

  9. Rajan MS, Jaycock P, O'Brart D. A long-term study of photorefractive keratectomy; 12-year follow-up. Ophthalmology. Oct 2004;111(10):1813-24. [Medline].

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  12. Taboada J, Archibald CJ. An extreme sensitivity in the corneal epithelium to far UV ArF excimer laser pulses. Proceedings of the Aerospace Medical Association. San Antonio;1981.

  13. Tanzer DJ, Isfahani A, Schallhorn SC. Photorefractive keratectomy in African Americans including those with known dermatologic keloid formation. Am J Ophthalmol. Nov 1998;126(5):625-9. [Medline].

  14. Taylor HR, Guest CS, Kelly P, Alpins NA. Comparison of excimer laser treatment of astigmatism and myopia. The Excimer Laser and Research Group. Arch Ophthalmol. Dec 1993;111(12):1621-6. [Medline].

  15. Uozato H, Guyton DL. Centering corneal surgical procedures. Am J Ophthalmol. Mar 15 1987;103(3 Pt 1):264-75. [Medline].

  16. Wang Q, Klein BE, Klein R, Moss SE. Refractive status in the Beaver Dam Eye Study. Invest Ophthalmol Vis Sci. Dec 1994;35(13):4344-7. [Medline].

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Further Reading

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Keywords

photorefractive keratectomy, shortsighted, vision loss, visual deficit, laser surgery

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Contributor Information and Disclosures

Author

Fernando H Murillo-Lopez, MD, Senior Surgeon, Unidad Privada de Oftalmologia CEMES
Fernando H Murillo-Lopez, MD is a member of the following medical societies: American Academy of Ophthalmology
Disclosure: Nothing to disclose.

Medical Editor

Daniel S Durrie, MD, Director, Department of Ophthalmology, Division of Refractive Surgery, University of Kansas Medical Center
Daniel S Durrie, MD is a member of the following medical societies: American Academy of Ophthalmology and Association for Research in Vision and Ophthalmology
Disclosure: Nothing to disclose.

Pharmacy Editor

Simon K Law, MD, PharmD, Assistant Professor of Ophthalmology, Jules Stein Eye Institute; Chief of Section of Ophthalmology Surgical Services, Department of Veterans Affairs Healthcare Center, West Los Angeles
Simon K Law, MD, PharmD is a member of the following medical societies: American Academy of Ophthalmology, American Glaucoma Society, and Association for Research in Vision and Ophthalmology
Disclosure: Nothing to disclose.

Managing Editor

Louis E Probst, MD, Medical Director of Refractive Surgery, Chicago, Madison, Milwaukee, and Windsor Centers, TLC the Laser Eye Centers
Louis E Probst, MD is a member of the following medical societies: American Academy of Ophthalmology, American Society of Cataract and Refractive Surgery, and International Society of Refractive Surgery
Disclosure: Nothing to disclose.

CME Editor

Lance L Brown, OD, MD, Ophthalmologist, Affiliated With Freeman Hospital and St John's Hospital, Regional Eye Center, Joplin, Missouri
Disclosure: Nothing to disclose.

Chief Editor

Hampton Roy Sr, MD, Associate Clinical Professor, Department of Ophthalmology, University of Arkansas for Medical Sciences
Hampton Roy Sr, MD is a member of the following medical societies: American Academy of Ophthalmology, American College of Surgeons, and Pan-American Association of Ophthalmology
Disclosure: Nothing to disclose.

 
 
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