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

MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 6.34: Zywave raw data

FIGURE 6.35: Well-centered Zy wave scan

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sharp, and that the lines from other centroids connect them all. If the lines are broken around the extreme edges it is still ok, but any break in the central areas denotes poor quality and needs to be repeated.

Lastly, the Zywave will display all five sets of Predicted Phoropter Refractions (PPRs). Three will be highlighted and checked as the best three chosen by the computer for analysis (Fig. 6.36). It is our practice to not only verify the tight deviations among the chosen three, but to verify that all five PPRs correlate tightly with each other and with the subjective refraction. A range greater than 0.75 diopters on sphere, 0.50 diopters on cylinder and 015 degrees on axis is considered unacceptable. An examination summary

FIGURE 6.36: Predicted phoropter refractions (PPRs)

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screen will display the subjective refraction, the PPR at the 3.5 mm zone, and the differences between them. It will highlight in red any deviation larger than the amounts mentioned.

If the undilated pupil size is large enough to allow for the desired optical zone for the individual patient, then no further testing is required. If, however, the pupil size is not large enough, then dilation should be performed with 2.5% phenylephrine and 0.5% mydriacyl. A full 20 minutes should be allowed for dilation to ensure that there is no asymmetrical shift to the pupil during mid dilation that might then alter the centration of the captured Zywave over the actual physiologic pupil center during treatment.

After successful data capture the surgeon has the ability to review the wavefront data at three different pupil sizes (Fig. 6.37). The data can be viewed at a 5 mm zone, a more standard 6 mm zone, or at the size of the captured pupil. This can be view in a higher-order point spread function showing a graphic splay of how light is scattered based on HOAs only. It can be viewed graphically broken down by each individual type of HOA at the 5 mm or 6 mm zones and have the graphs displayed over a normal amount of each of these aberrations for the given zone size. Or it can be displayed as a color map of the HOAs similar to the color topographic maps we are accustom to viewing. One of the most useful features of these displays is the ability to

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CHAPTER 6: THE ORBSCAN IIZ DIAGNOSTIC SYSTEM AND SWAGE

FIGURE 6.37: Examination summary

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choose a standard viewing zone (not related to treatment zone) for examining the three mentioned displays of HOAs. This allows the surgeon, over time, to be able to use pattern recognition for different types and amount of HOAs similar to how pattern recognition is used in topographic maps today. It would be impossible to do this if all HOA maps were displayed at different zone sizes as opposed to displaying them all at the same zone size. Again as a reminder the 6mm zone chosen to display the HOA data does not then mandate that the treatment be at a 6 mm zone. The surgeon may still choose any zone desired for treatment from 5.5 mm to 7.0 mm.

After capturing and reviewing the Zywave data the surgeon may then comfortably discuss the benefits of wavefront treatment with each patient. Individual markers for custom treatment vary from surgeon to surgeon, but generally patients with larger pupil sizes, higher prescriptions, residual bed issues, higher amounts of HOAs to begin with, and higher quality of vision expectations are good candidates for wavefront treatments.

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INTRODUCTION

Irregular astigmatism represents one of the problems that are very difficult to manage and frustrating in results to refractive surgeons. It is also one of the worst sequelae of corneal injuries. It can also complicate certain corneal diseases as keratoconus. With the recent evolution of refractive surgery techniques and diagnostic tools, new types of irregular astigmatism are being observed.1,2

Astigmatism is defined as irregular if the principal meridians are not 90 degrees apart, usually because of an irregularity of the corneal curvature. It cannot be completely corrected with a sphero-cylindrical lens.3 Duke– Elder defines irregular astigmatism as a refractive state in which the refraction in different meridians conforms to no geometric plane and the refracted rays have no planes of symmetry.4

The alternatives for correction of irregular astigmatism are very scarce and with very limited expectations. Spectacle correction is usually not useful in the correction of corneal irregular astigmatism as it is difficult to define principal meridians. Hard contact lenses represent a good alternative in which the tear fluid layer under the contact lens evens out the irregularity. We should consider that adaptation and stability of contact lenses is limited by irregularity corneal of surface and the patient’s comfort.

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We also must remember that our patients consented to undergo refractive surgery because they did not want to use more the contact lens.

Lamellar and full thickness corneal grafting are surgical alternatives. The limited availability of corneal donor as well as the biological and refractive complications of allografic corneal graft limit the clinical applicability of these procedures.

Many surgeons have made great efforts in finding a solution to this problem.5-7 To this date, we believe there should be safe, efficient and predictable methods to resolve this problem. Accordingly, the approach to new surgical methods for the correction of irregular astigmatism is one of the greatest expectations in today’s refractive surgery, especially when the very near future is supposed to bring generalization of corneal refractive surgical techniques.

ETIOLOGY OF IRREGULAR ASTIGMATISM

Primary Idiopathic

There is a general prevalence of low levels of irregular astigmatism of unknown cause within the population. This might explain the mildly reduced best corrected visual acuity (BCVA) in patients presenting for laser vision correction.1

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Secondary

Dystrophic

In the cornea, keratoconus, which, in optical terms, is primarily an irregularity of the anterior corneal surface, is the best example. Pellucid degeneration and keratoglobus may also be associated with posterior corneal surface irregularity causing irregular astigmatism. In the lens, lenticonus may cause irregular astigmatism; and in the retina, posterior staphyloma.1

Traumatic

Corneal irregularity is caused commonly by corneal wounds (incision or excision) or burns (chemical, thermal or electrical).1

Postinfective

Postherpetic keratitis is the most common form of postkeratitic healing and scarring that may lead to an irregular surface.1

Postsurgical

Irregular corneal astigmatism can complicate any if the following refractive surgical procedures: keratoplasty, photorefractive keratectomy (PRK), laser in situ keratomileusis (LASIK), radial keratotomy (RK), arcuate kerato-

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tomy (AK), and cataract incisions. Scleral encirclement or external plombage may also contribute.1

DIAGNOSIS OF IRREGULAR ASTIGMATISM

Clinically, irregular astigmatism will present with one of those typical retinoscopy patterns, the most common being spinning and scissoring of the red reflex. On attempting kertaometry the mires will appear distorted. Corneal topography shows certain patterns for irregular astigmatism that will be discussed in detail later. The most recent and sophisticated technique is the application of wavefront analysis (aberrometers).8 This emerging method measures the refractive status of the whole internal ocular light path at selected corneal intercepts of incident light pencils. By comparing the wavefront of a pattern of several small beams of coherent light projected through to the retina with the emerging reflected light wavefront, it is possible to measure the refractive path taken by each beam and to infer the specific spatial correction required on each path.

CLINICAL CLASSIFICATION OF IRREGULAR ASTIGMATISM FOLLOWING CORNEAL REFRACTIVE SURGERY

In corneal refractive surgery using laser in situ keratomileusis (LASIK) the surgeon uses a microkeratome,

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