Here I will try to explain the Holladay (Equivalent Keratometry Reading ) EKR report, and how this report could help in biometry for IOL implantation, especially for patients who particularly had prior refractive procedure, or have irregular cornea. To interpret the Holladay EKR report, do ensure that your settings are proper as a wrong setting will lead to a false display and improper clinical judgement. The Pentacam comes with a default Holladay setting and users are recommended not to edit or tamper this default setting. Thus for a proper interpretation of the Hollday EKR report, the Holladay default settings are set to a calculation zone of 8 mm while the eccentricity calculation zone ( angle kappa, alpha) is also set to 8mm. The default depiction of the shape factor (SF) of cornea is Asphericiy (Q).
At the outset I need to explain what is meant by EKR (equivalent K reading) and how is it useful in a clinical practice for IOL calculation. Well, consider this:
You take a K reading of the cornea for IOL calculation. Most keratometry machines, including optical biometry ( except IOL Master 700 with TK) would measure the anterior cornea only. This values are now used in a two variable formulae like SRK T or Holladay I to generate the IOL power. These third generation two variable formulae will now deduct a certain power from the anterior K reading values that you have input to compensate for the power of the posterior cornea. This is done by taking into account a fixed anterior to posterior corneal ratio (approximately 82 percent for normal cornea). If the patient's posterior cornea is not of average power, that is the anterior/posterior corneal ratio is abnormal, then the formula either overcompensates or undercompensates for the posterior corneal power while doing an IOL power calculation. This is what happens in Keratoconus or with patients with previous refractive laser surgery. That is why you will need a special formula for post lasik patients.
Explaining EKR - The average anterior and posterior corneal power are 48 D and -6.8 D (approximately). Let us assume that the patient in question has anterior/posterior corneal ratio lower than average. The Pentacam measures both anterior and posterior curvature of cornea, and measures posterior cornea as -6.5 D for this patient. Thus the patient's posterior cornea is -.3 D lower (less negative) than an average cornea. The EKR will therefore consider this and compensate for a weaker posterior corneal power and add .3D (adjust) to the anterior corneal values. The back surface of cornea has a negative power and therefore negates anterior corneal power. However, because the back surface of this patient's cornea is less negative (.3) than an average cornea, therefore the EKR will add to the anterior corneal power. The value derived of the anterior cornea ( K reading ) can then be directly input into any two variable IOL power calculation, regardless of the fact that this patient may have gone through a previous lasik or has an abnormal anterior posterior ratio.
Thus EKR is not net power of cornea. In net power, the topography/device measures both anterior and posterior corneal values and then gives the total power of cornea. This value cannot be used in any two variable IOL calculation formula ( SRK T, Holladay I, Hoffer Q) as in doing so may amount to a double correction (remember all these formulas have an inbuilt compensation for the back surface of the cornea). In EKR, the anterior corneal values are therefore reported with adjustments for any deviation of back surface power from normal.
Reports Keratometry value but adjusts for back surface power deviation from average
Interpretation of markers in the colour maps
The dashed circle in Figure 1 is the pupil margin. Notice that the pupillary margin is not an absolute circle, but takes the shape of the measured pupillary margin of the patient.
The Black and White plus sign signifies the pupil center, which helps to understand the eccentricity (angle kappa)
The Black bracket signifies the center of the limbus, which helps to understand eccentricity ( angle alpha)
Black dot in the middle of white circle signifies the vertex of the cornea. This is important as the patient's visual axis for a normal cornea is very close to this point. Thus the distance of number 2 and number 3 from number 4 signifies the eccentricity of pupil center and limbal center respectively from the visual axis/corneal vertex for a normal patient. This is however not applicable to abnormal corneas, like keratocconus.
The small black circle signifies the thinnest location of the cornea. For a normal patient, the thinnest location will be very close to corneal vertex (4). For patients with Keratoconnus, the thinnest location (5) will be located further away and inferior to corneal vertex (4).
The upper middle panel in Figure 2 gives us the EKR-65 K readings. In this panel the EKR-65 readings for the 4.5 mm zone is reflected. This value reflects the anterior corneal power that is already adjusted for any deviation in the posterior power of cornea from an average posterior corneal value.
Next comes EKR 65 mean. To explain this, let us consider a situation wherein we are doing an IOL calculation in a keratoconus patient. In a Keratoconus patient the steepest power of the cornea, that is the thinnest location of cornea, may not be in the central paraxial region. Such patients have a bifocal cornea. In such patients the paraxial region of the cornea is flatter than the steepest portion of the cornea which is inferior to this part. As a result the patient may read near objects (books) through the steep portion of cornea (very much like when you read through a bifocal glass) and looks at the distance through the paraxial region which is flatter.
However for all distance related activities, the patient looks through an area that covers the central 4.5 mm of cornea (4.5 mm cornea based on average pupil diameter in mesopic condition). The average corneal power in this zone is often flatter.
The EKR 65 Mean therefore takes a weighted mean of 65 per cent of the corneal power recorded in the central 4.5 mm zone. If the patient's pupil diameter is lower than 4.5 mm, then an appropriate corneal power for that pupil area for the patient may be selected alternatively (Figure 5). The EKR 65 Mean therefore is very much applicable to Keratoconus, post refractive surgery, any irregular astigmatism, etc. The asphericity/shape factor of cornea or Q factor is also recorded that gives an idea of the slope of the cornea that accounts for spherical aberration. In Figure 2, the patient has a Q factor .12 which may explain the slightly higher spherical aberration (Total SA) of .39 micron. Thus the cornea is oblate in shape thereby giving rise to a greater positive spherical aberration. In such a patient, the highest negative spherical aberration lens available as choice may be more applicable. For more understanding of Q factor and spherical aberration you can refer to https://www.quickguide.org/post/spherical-aberration-asphericity
Additionally in Figure 3 you can see the total RMS HOA value, which gives an understanding of the patient's higher order aberration in the 6 mm zone. A value which is lower than .3
micron is good, while a value between .3 micron to .6 micron may need a closer look for suitability for multifocal IOLs. A value above .6 micron may not help patients with multifocal IOL.
In the upper right panel (Figure 2) you will find in the EKR page what is represented here as Figure 4. The pupil diameter and the center of the pupil is located in the first row (Figure 4). In the corneal thickness map you will notice this as a dashed ring at the center of the map and as described earlier. The location of the pupil diameter in terms of x and y coordinates will help us determine
the chord mew or angle kappa. The chord mew value ( angle kappa) which is the distance between the visual axis and center of the pupil is thus derived from the x and y coordinates and reflected in the last row in Figure 4. Similarly the horizontal white to white (HWTW) in the second row gives us the total diameter of white to white and the center of the limbus in terms of x and y coordinates. This value helps us to have an understanding of the distance between the limbal or optical center of the cornea and the visual axis/corneal vertex (angle alpha). The thinnest location of the cornea which should be close to the corneal vertex or visual axis in a normal patient is represented through the x and y coordinates. For a Keratoconus patient, the y coordinate would be inferiorly displaced and a value of more than -.5 micron of y coordinate of the thinnest location is one of the indication of Keratoconus and a large deviation between visual axis and corneal vertex. In such cases, note, the steepest point of the cornea, that is, the thinnest location is far away from the paracentral area. The EKR 65 therefore takes a weighted mean of 65 per cent of corneal power in the central 4.5 mm zone (average pupil diameter in mesopic condition) to provide the EKR 65 mean power of cornea that can be considered for IOL power calculation.
The EKR 65 therefore takes a weighted mean of the corneal power in the central 4.5 mm zone (average pupil diameter in mesopic condition) to provide the EKR 65 mean power of cornea that can be considered for IOL power calculation.
However, not all eyes have the same mesopic pupil diameter, and for eyes which have lower or higher mesopic pupil diameter, an appropriate K value can be chosen from the table in Figure 5. Thus a patient who has a pupil diameter of 4.0 on an average, the corresponding EKR value of 4.0 may be considered.
The interpretation of the colour coded maps in the Holladay EKR report are similar to what has been already covered in https://www.quickguide.org/post/guide-to-pentacam-maps, and therefore excluded from discussion here. You may refer to the article for a brief guide in interpreting the four quadrant map.
The EKR 65 may help in special corneas like Keratoconus, prior lasik, etc.
The EKR 65 stands for equivalent K readings and accounts for any deviation of power of posterior cornea from average values and compensates such deviation by adding or subtracting to anterior corneal values.
The EKR 65 takes into account a weighted mean of 65 percent of corneal power within the 4.5 mm pupil diameter which could help avoid hyperopic surprise in special corneas like Keratoconus.
The EKR 65 can be used for IOL power calculation with any standard IOL formula like SRKT, Hoffer Q, Holladay I, etc.
Please note that above explanation is not definitive and is indicative and suggestive in nature. Please use your own judgment before application.