Ординатура / Офтальмология / Английские материалы / Step by Step Reading Pentacam Topography (Basics and Case Study Series)_Sinjab_2010

22 Step by Step Reading Pentacam Topography

Fig. 2.16: The useless map. This map should not be relied upon when taking the decision because there are neither values nor the other important maps to be compared with. The colors are not sufficient to decide because they can be changed according to the chosen scale.

Let us start defining the abbreviations of the page and their interpretations:

Cornea Front Surface (Fig. 2.17)

K1: Horizontal curvature power of the cornea in the central 3 mm circle expressed in diopters.

Quick Review of Pentacam Main Maps 23

Fig. 2.17: Cornea front and cornea back surfaces in the main page of the Pentacam.

K2: Vertical curvature power of the cornea in the central 3 mm expressed in diopters.

Km: Mean curvature power of the cornea in the central 3 mm expressed in diopters.

Rh: Horizontal curvature radius of the central 3 mm expressed in mm.

Rv: Vertical curvature radius of the central 3 mm expressed in mm.

Rm: Mean curvature radius of the central 3 mm expressed in mm.

Qs: “Quality specification”. It specifies the quality of the topographic capture and should be displayed “OK”. Otherwise, there is missed information which was virtually

24 Step by Step Reading Pentacam Topography

produced by the computer and the capture should preferably be repeated.

Q-val (6 mm): Value of Q in the central 6 mm as shown between two brackets. Any other central circle can be chosen through the program settings.

Astig: Amount of corneal astigmatism on the front cornea surface, i.e. the amount of differentiation between the two curvature radii of the central 3 mm.

Axis: The axis of corneal astigmatism in the central 3 mm.

Rmin: Minimum radius of curvature expressed in mm. It is shown as a symbol (Fig. 2.18) not necessarily central.

Rper: Radius of corneal curvature in the peripheral 9 mm of the cornea expressed in mm.

Cornea Back Surface

As mentioned before, the cornea back surface plays the role of a concave lens; therefore, the curvature power is displayed on the back surface as negative values (see Figs 2.3A and B).

Coordinates of the Corneal Thickness (see Fig. 2.14)

Pachy apex: Corneal thickness at the apex. The computer considers the apex as the origin of the coordinates, X for the horizontal and Y for the vertical. Therefore, zero is displayed in both squares of pachy apex coordinates.

Pupil center: Corneal thickness in the pupil center. The X- and Y-coordinates show the position of the pupil center from the apex. The two coordinates differ according to pupil medriasis or miosis, because the pupil center is often

Quick Review of Pentacam Main Maps 25

Fig. 2.18: Minimal radius of curvature, a very important item to consider when treating hypermetropia.

shifted superotemporally when dilated. This is very important in the process of decentration, or what is known as offset pupil.

Thinnest location: Thinnest point in the cornea. It is the most important in the decision procedure. The X- and Y- coordinates indicate the position of the thinnest location from the apex.

Quick Review of Pentacam Main Maps 27

plane. It is important for phakic IOLs. It should not be less than 2.8 mm to keep the corneal endothelium intact. By entering the settings, we can choose (Ext.) instead of (Int.) to have the depth with the corneal thickness added.

IOP: Click the icon and input the IOP to have the modified IOP according to corneal thickness. For the same purpose, the nearby square displays the amount that should be added algebraically to IOP to be modified.

KPD: The average value of keratometric power deviation of corneal points located within a circle of a diameter between 0.8 mm and 1.6 mm and centered on the corneal apex. The normal value is less than +0.75. When the value is more than +1.5, it is abnormal indicating an abnormal cornea, such as: post-lasik, post-PRK, keratoconus or after a corneal graft.

Angle: The average value of anterior chamber angle. An angle less than 25° should alert us to check the patient for angle closure glaucoma.

Pupil diameter: It is important to measure the pupil diameter neither in high illumination (photopic pupil) nor in the dark (scotopic pupil) but in medium illumination (mesopic pupil).

Lens thickness: Central thickness of the lens. If the pupil is not dilated enough, the slit light of the camera may not reach the posterior surface of the lens, so the computer cannot measure its thickness.

30 Step by Step Reading Pentacam Topography

INTRODUCTION

Reading corneal topography is a very important step in taking the right decision before performing the refractive surgery. The decision is changed sometimes completely into other modality of treatment (such as switching into phakic IOLs instead of Lasik because of thin cornea), or we may need changing some parameters within the same modality (such as reducing the treatment zone in 0.1 mm steps to gain some more correction when corneal thickness is critical).

On the other hand, reading corneal topography should be regularized and arranged in a logical manner to rule out all possible risk factors, such as critical K-readings, corneal thickness, Q-value and keratoconus indices. I am trying in the following paragraph to simplify the procedure, but still we have to refer to my book “Corneal Topography in Clinical Practice” published by Jaypee Brothers 2009, for more details; because this book is concerned in clinical examples to clarify and explain the ideas.

STEPS OF READING THE TOPOGRAPHY

Displaying the Four Refractive Maps

This option consist of 4 main maps: the anterior sagittal map, the anterior and posterior elevation maps and the corneal thickness map. These maps give us a thorough overview of the case, but we should then display each map separately as will be discussed later.

Performing Main Page Analysis

We have to look at cornea front and cornea back parameters, then we have to follow the following rules: