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

MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

whether automated or manual, to fashion a corneal flap and a stromal bed. Once the flap is fashioned and lifted, the excimer laser is used to ablate tissue from the bed for the plannedcorrection,dependingonthecapabilitiesofthelaser.

In this clinical prespective, irregular astigmatism induced by LASIK can be classified according to its location as:

1.Superficial: due to flap irregularities.

2.Stromal: induced by bed irregularities.

3.Mixed: due to irregularities in both flap and stroma.

CORNEAL TOPOGRAPHY PATTERNS OF

IRREGULAR ASTIGMATISM

Topographic classification of irregular astigmatism patterns is very important in the following aspects:

1.To unify terms and concepts when we referring corneal topography images.

2.To determine the cause of the subjective symptoms referred by the patient (Haloes, glare, monocular diplopia, etc.).

3.Reaching a topographic basis for retreatment. The topographic approach for treatment patients with a previous unsuccessful excimer laser surgery should allow reshaping the cornea in the pattern appropriate for the specific patient.

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CHAPTER 7: IRREGULAR ASTIGMATISM: LASIK AS A CORRECTING TOOL

Based on the topography, we proposed the following classification for irregular astigmatism:7

•Irregular astigmatism with defined pattern, and

•Irregular astigmatism with undefined pattern.

Irregular Astigmatism with Defined Pattern

We define irregular astigmatism with defined pattern when there is a steep or flat area of at least 2 mm of diameter, at any location of the corneal topography, which is the main cause of the irregular astigmatism. It is divided into five groups:

A.Decentered Ablation: Shows a corneal topographic pattern with decentered myopic ablation in more than 1.5 mm in relation to the center of the cornea. The flattening area is not centered in the center of the cornea; the optical zone of the cornea has one flat and one steep area (Fig. 7.1A).

B.Decentered Steep: Shows a corneal hyperopic treatment decentered in more than 1.5 mm in relation to the center of the cornea (Fig. 7.1B).

C.Central island: Shows an image with an increase in the central power of the ablation zone for myopic treatment ablation at least 3.00D and 1.5 mm in diameter, surrounded by areas of lesser curvature (Fig. 7.1 C).

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

D.Central irregularity: Shows an irregular pattern with more than one area not larger than 1.0 mm and no more than 1.50 D in relationship with the flattest radius, located into the area of the myopic ablation treatment (Fig. 7.1D).

E.Peripheral Irregularity: It is a corneal topographic pattern, similar to Central Island, extending to the periphery. The myopic ablation is not homogeneous, there is a central zone measuring 1.5 mm in diameter and 3.00 D in relation to the flattest radius, connected with the periphery of the ablation zone in one meridian (Fig. 7.1E).

Irregular Astigmatism with Undefined Pattern

We consider irregular astigmatism with undefined pattern when the image shows a surface with multiples irregularities; big and small steep and flat areas, defined as more than one area measuring more than 3 mm in diameter in the central 6 mm (Fig. 7.1F). The differential between flat and steep areas were not possible to calculate in the Profile Map and Dk showed an irregular line or a plane line. Normally, Dk is the difference between the steep k and the flat k, given in diopters at the cross of the profile map. A plane line is produced when the Dk cannot recognize the difference between the steep k and the flat k in severe corneal surface irregularities.

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CHAPTER 7: IRREGULAR ASTIGMATISM: LASIK AS A CORRECTING TOOL

FIGURE 7.1A: Decentered ablation

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 7.1B: Decentered steep

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CHAPTER 7: IRREGULAR ASTIGMATISM: LASIK AS A CORRECTING TOOL

FIGURE 7.1C: Central Island

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 7.1D: Central irregularity

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CHAPTER 7: IRREGULAR ASTIGMATISM: LASIK AS A CORRECTING TOOL

FIGURE 7.1E: Peripheral irregularity

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 7.1F: Irregular astigmatism with undefined pattern

FIGURES 7.1A TO F: Topographic patterns of irregular astigmatism

(with ray tracing study)

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CHAPTER 7: IRREGULAR ASTIGMATISM: LASIK AS A CORRECTING TOOL

EVALUATION OF IRREGULAR ASTIGMATISM

In managing irregular astigmatism patients, a meticulous preoperative evaluation is necessary. We perform a complete preoperative ocular examination, including previous medical reports and complete ocular examination: uncorrected and best-corrected visual acuity, pinhole visual acuity and cycloplegic refraction, keratometry, contact ultrasonic pachymetry (Ophthasonic Pachymeter Teknar Inc. St. Louis, USA) and computerized corneal topography.

We perform the corneal topography with Eye Sys 2000 Corneal Analysis System (Eye Sys Co., Houston, Texas, USA). We also use the Ray Tracing mode of the C-SCAN Color-Ellipsoid-Topometer (Technomed GmbH, Germany) to determine the Superficial Corneal Surface Quality (SCSQ) and the Predicted Corneal Visual Acuity (PCVA), in addition to the topography. Recently, we have incorporated the elevation topography of the Orbscan system (Orbtek, Bausch & Lomb Surgical, Orbscan II corneal topography, Salt Lake City, Utah, USA) in our evaluation tools.

Follow up examinations after surgery were performed at 48 hours, and then at one, three and six months. Postoperative follow up included: uncorrected and bestcorrected visual acuity, pinhole visual acuity and cycloplegic refraction, biomicroscopy with slit-lamp and

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