eMedicine Specialties > Ophthalmology > Refractive Disorders

Myopia, Radial Keratotomy: Workup

Author: Mounir Bashour, MD, CM, FRCS(C), PhD, FACS, Assistant Professor of Ophthalmology, McGill University; Clinical Assistant Professor of Ophthalmology, Sherbrooke University; Medical Director, Cornea Laser and Lasik MD
Coauthor(s): Mirelle Benchimol, MD, Consulting Staff, Benchimol Eye Clinic
Contributor Information and Disclosures

Updated: Jun 8, 2009

Workup

Imaging Studies

  • Pachymetry was designed to measure corneal thickness and to enhance the understanding and management of disorders of the corneal endothelium. It has become an integral part of the clinical practice, initially with RK and then with other types of refractive surgery, since making an accurate measurement of corneal thickness is a necessity.
  • An ultrasound pachymeter uses the principles of A-scan ultrasonography. It provides a convenient means of measuring corneal thickness. To determine corneal thickness, the ultrasonic beam is aligned precisely perpendicular to the corneal central surface. Ultrasonic echoes are obtained from the anterior and posterior surfaces of the cornea. The time interval between the echoes can be used to determine the corneal thickness if the ultrasonic speed of propagation in the cornea is known. The cornea thickness is the speed of sound in the cornea multiplied by the time interval between corneal echoes divided by 2. Mean corneal thickness values are 0.51-0.58 mm.

Other Tests

  • Refraction: Manifest and cycloplegic refractions should be measured, but cycloplegic refraction should be the basis for calculating the surgical plan and for comparing preoperative and postoperative results. The manifest refraction tends to overestimate the amount of myopia because of accommodation.
  • Slit lamp microscopy: Used to establish the normal corneal anatomy, a careful slit lamp microscope examination can reveal early keratoconus, corneal scars, mild lens opacities, subluxation of the lens, and syneretic cavities in the vitreous, suggesting pathological myopia or vitreous degeneration.
  • Tonometry: The major objective of measuring the intraocular pressure (IOP) before surgery is to identify individuals who have elevated pressures and to exclude them from keratotomy surgery. This screening is particularly important because myopes are more likely to develop elevated IOP and glaucoma than emmetropes, and patients with fluctuating IOPs have unstable vision. Eyes with IOPs in the reference range of approximately 10-20 mm Hg are acceptable for keratotomy surgery.
  • Central keratometry: This is the measure of the central cornea curvature. Little correlation exists between preoperative central keratometric power and the effect of keratotomy. Some contend that steeper corneas achieve more change in refraction; others find more change in refraction in flatter corneas. Still others contend that it is the overall corneal topography that affects the outcome, not just the central keratometric power. Even though many formulas and nomograms include keratometric power, the preoperative keratometric power plays little role in designing the surgical plan.
  • Keratography: Qualitative keratography has a minor role in evaluating patients with myopia for RK, but qualitative keratography does identify individuals who may have irregular astigmatism associated with keratoconus or warpage caused by contact lens wear. It plays a major role in planning surgery for patients with astigmatism, particularly for those with penetrating keratoplasty or ocular trauma, where asymmetric, irregular astigmatism may be present.
  • Fundus examination: Indirect ophthalmoscopy with visualization of the ora serrata is important because of the increased propensity of myopes, particularly intermediate and pathologic myopes, to develop lattice degeneration of the retina, retinal holes, and retinal detachment.
  • Ocular dominance: Many surgeons prefer to operate on a patient's nondominant eye first; if complications occur, the presumably more valuable dominant eye can be left unoperated.
  • Specular microscopy of the endothelium: Endothelial morphology does not play a role in patient selection for RK. Specular microscopy of the endothelium is limited to studies in which careful preoperative and postoperative examinations are performed in the same locations in the central cornea and in the areas of incisions.

More on Myopia, Radial Keratotomy

Overview: Myopia, Radial Keratotomy
Workup: Myopia, Radial Keratotomy
Treatment: Myopia, Radial Keratotomy
Follow-up: Myopia, Radial Keratotomy
Multimedia: Myopia, Radial Keratotomy
References
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References

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Further Reading

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Keywords

RK, refractive keratotomy, myopia, myopic, nearsightedness, nearsighted, shortsighted, corneal curvature, vision loss, visual deficit

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Contributor Information and Disclosures

Author

Mounir Bashour, MD, CM, FRCS(C), PhD, FACS, Assistant Professor of Ophthalmology, McGill University; Clinical Assistant Professor of Ophthalmology, Sherbrooke University; Medical Director, Cornea Laser and Lasik MD
Mounir Bashour, MD, CM, FRCS(C), PhD, FACS is a member of the following medical societies: American Academy of Ophthalmology, American Association for Pediatric Ophthalmology and Strabismus, American College of International Physicians, American College of Surgeons, American Medical Association, American Society of Cataract and Refractive Surgery, American Society of Mechanical Engineers, American Society of Ophthalmic Plastic and Reconstructive Surgery, Biomedical Engineering Society, Canadian Medical Association, Canadian Ophthalmological Society, Contact Lens Association of Ophthalmologists, International College of Surgeons US Section, Ontario Medical Association, Quebec Medical Association, and Royal College of Physicians and Surgeons of Canada
Disclosure: Nothing to disclose.

Coauthor(s)

Mirelle Benchimol, MD, Consulting Staff, Benchimol Eye Clinic
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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