Radial Keratotomy for Myopia Correction Workup

Updated: Apr 18, 2017
  • Author: Mounir Bashour, MD, PhD, CM, FRCSC, FACS; Chief Editor: Michael Taravella, MD  more...
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Workup

Imaging Studies

Pachymetry

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 radial keratotomy (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. [2, 3]

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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.

Corneal topography

Corneal topography is considered a mandatory test in all refractive patients to rule out conditions such as keratoconus or other corneal ectatic disorders that would contraindicate incisional keratotomy.

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.

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