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

MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

The “keratoconus screening” display incorporates the keratometry, pachymetry, anterior chamber information, corneal volume, and thickness assessment relative to the thinnest point. “Rings” with diameters of 1 mm, 2 mm, 3 mm, 4 mm, and 5mm around the thinnest point are created, and mean corneal thickness and corneal thickness progression are illustrated in graphical displays, as show in Figure 2.14.

Various indices, are used to determine the level of KC risk, and suspect values appear in red, as shown in Figure 2.15. Note these indices are calculated using only the anterior surface. The KC index describes the risk of keratoconus using a 4 stage classification based on those of Amsler and Muckenhirm. Selecting “Keratoconus Level Topography” opens the table which describes classical signs of keratoconus used by the KC index. Clinical assessment must be incorporated as these should not be used for diagnosis alone.

Parameters listed on the KC display are based on studies indicating differences exist among those with KC and healthy corneas. Emre et al found that with progression of disease, total corneal thickness and anterior chamber depth were statistically different and there were statistically significant differences in anterior chamber angle and corneal volume measurements between the mild keratoconus and severe keratoconus groups.

(64)

CHAPTER 2: PENTACAM

FIGURE 2.14: Keratoconus display. Note suspicious indices appear yellow and red

(65)

MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 2.15: Various indices, are used to determine the level of KC risk, and suspect values appear in red

Ambrosio et al studied mild to moderate keratoconic eyes compared with normal eyes using the Pentacam. Corneal-thickness spatial profile, corneal-volume distribution, percentage increase in thickness, and percentage increase in volume were different between keratoconic corneas and normal corneas.

Evaluation of cataracts can be objectively performed using densometry, as shown in Figure 2.16.

The tomography display (Fig. 2.17) can also be used to educate patients on the level of cataract development, and is tremendously valuable for patients presenting for elective

(66)

CHAPTER 2: PENTACAM

vision correction with mild cataracts. Pentacam imaging has been reported to be helpful in the evaluation of penetrating eye injury and intralenticular foreign body, as well as diagnosis and management of CBDS, posterior capsular haze, and subclinical manifestations of electric cataract injuries. Note that when the goal of the examination is densitometry, the “enhanced dynamic” examination scanning mode should be selected. This lengthens the exposure time per Scheimpflug image during the scan. Progress may be monitored when the same number of images during the scans is selected upon the patient’s subsequent visits.

The Holladay report (Fig. 2.18) was developed to assist surgeons in calculation of IOL power in patient with a history of refractive surgery. Changes in the central topography may produce “refractive surprise”, an unwanted result in this age of high patient expectation. Typical keratometry fails to estimate the true corneal power in patients with surgically altered topography. Errors result because keratometers assume the ratio of power between the anterior and posterior cornea is 82%, as in a virgin eye. They calculate corneal power using an index of 1.3375. Because excimer treatment alters this ratio, the calculation is inaccurate, and the lens implant power fails to properly correct the refractive error. The Pentacam calculates the front and back surface power, and produces the

(69)

MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

(70)

CHAPTER 2: PENTACAM

“Equivalent K readings” or EKR’s. The EKR’s are used with IOL calculation formulas to obtain correct IOL powers for patients with surgically altered corneas.

The calculation of EKR’s is related to the pupil center rather then the corneal apex as in keratometry, and utilizes the refractive power map rather than the curvature map. The calculation is based on the ratio of the anterior and posterior surface radii within the 4.5 mm zone, shown to be the most pertinent for patients with a history of excimer treatment.

The detailed report (Fig. 1.19) shows the EKR’s for manually selected zone measurements, and may be of value when other zones may be better options, such as in patients with a history of radial keratotomy (RK), trauma, or severe irregular astigmatism.

The Pentacam presents data regarding anterior chamber depth and volume, as well as assessment of the chamber angle. The anterior chamber display is show in Figure 2.20. The chamber volume is calculated directly, while the distance between the cornea’s posterior surface and the iris are integrated in a 12 mm diameter around the corneal apex. The angle should not be evaluated while the pupil is dilated, and may be used to evaluate patients for glaucoma risk due to narrow anglea. Anterior chamber information can also be used when evaluating patients for phakic lens implants.

(71)