IOL power calculation in keratoconus


In this article we will discuss about IOL power calculation in non progressive keratoconus patients. keratoconus is a relatively common corneal ectatic disorder. The reported incidence of keratoconus varies widely depending on geographic location, race, diagnostic criteria used, cohort of patients selected, etc. In one of the first population based studies, Hofstteter et al (1) reported 600 per 1,00,000. Prevalence in various studies however range from .3 per 1,00,000 in Russia (2) to 2300 per 1,00,000 in Central India (3). Some of the values published in recent papers are 5 to 10 times higher than previously reported or believed. Thus IOL power calculation in such eyes may need to be done more commonly. The true prevalence of keratoconus may be be even higher as we move from Placido based anterior topography to now more sophisticated tomography or even biomechanical based devices.


Calculating IOL power in patients with keratoconus is difficult due to the following reasons:


a. In normal patients, the visual axis is thought to be close to the corneal vertex and happens to be the thinnest part of cornea, thus corresponding to its steepest location. In keratoconus patients the visual axis does not pass through the steepest part of the cornea as the later is quite inferior to the visual axis.


b. When the apex of the Keratoconus is located around the entrance pupil, it can lead to discrepancies in reading between the central and paracentral corneal ring. The paracentral region may be a lot flatter than the area beyond, where the keratoconus apex falls.


c. High standard deviation of corneal readings is common in such patients.


d. Astigmatism is often non orthogonal/assymetrical, and the difference between steep and flat meridians or axis may be huge.


e. The anterior to posterior ratio of cornea (A/P ratio) is not the same. The normal A/P ratio is around 82%, which is much lower in case of keratoconus patients. Thus the common K index (corneal refractive endex or keratometry index) of 1.3375 used in most keratometers may not be perfect. The use of a standard K index in keratoconus patients can lead to an overestimation of corneal power, and an underestimation of IOL power, thus leading to hyperopic outcomes.


f. All vergence formula use the corneal power for the prediction of Effective Lens Position (ELP) in addition to using the corneal reading for IOL power . The use of corneal power for prediction of ELP by such formulas in keratoconus patients may lead to an erroneous outcome as a steep keratoconus cornea may fool the IOL formula to assume a deeper anterior chamber depth and ELP and therefore a stronger IOL power.


Thus many of the assumptions that are made in normal IOL power calculations are not valid for such patients.



The use of a standard K index in keratoconus patients can lead to an overestimation of corneal power, and an underestimation of IOL power, thus leading to hyperopic outcomes

Measuring the Posterior Cornea :

In keratoconus patients, measurement of the posterior cornea is important for the reasons mentioned before. The IOL Master 700 (with TK) based on swept source OCT, as well as all Scheimpflug based devices would help us have the posterior corneal values and thus the net corneal power. In the Scheimpflug based devices like the Pentacam, the True Net Power (TNP) map or the Total Corneal Refractive Power (TCRP) map will be useful to understand the total/net power of cornea taking into account posterior cornea anomalies. For a detailed understanding of these maps you can refer to the article https://www.quickguide.org/post/the-keratometry-or-corneal-reading-calculation-dilemma-for-post-refractive-lasik-patients . Figure I explains the difference between these maps in a tabular form.



Figure 1 - difference between simK TNP and TCRP maps in Pentacam

However, the values of neither the TNP or the TCRP should be used in a standard two variable IOL calculation formulae like the SRK T, Hoffer Q, or Holladay I, as doing so would lead to a mis match in K index used by these formulae (1.3375) and the K index used by the devices (1.376) to calculate the net power of cornea.


Most devices would use the Gaussian optics formula to derive the total or net corneal power.


where, Ka is the anterior corneal power

Kp is the posterior corneal power

d is the central corneal thickness

1.376 is the true indices of the cornea


An alternate way could also be using the EKR (Equivalent K Reading) in the Pentacam the values of which can be used in any standard two variable IOL calculation formulae like the SRKT/Hollday I or the Hoffer Q. This has been explained in detail in the article https://www.quickguide.org/post/holladay-ekr-report


However, since keratoconus patients have a higher standard deviation between readings, special attention should be paid to see that the standard deviation is within bounds. A very high standard deviation of K reading (more than .20 diopter) may call for repeat measurements. If, even after repeated measurements a high standard deviation persists, take the lowest K reading values from multiple measurements to avoid selecting a lower IOL power and thus avoiding a hyperopic outcome.





...a high standard deviation persists, take the lowest K reading values from multiple measurements to avoid selecting a lower IOL power and thus avoiding a hyperopic outcome

Choice of IOL formula in keratoconus:


The next step is now to choose the right IOL formulae for these challenging eyes. The standard two variables formulae like SRK T, Hoffer Q or Holladay I may not be very helpful in such eyes because

a. The anterior posterior ratio of cornea is no longer normal and hence the standard K index of 1.3375 that many of these formulae use may is not correct.


b. If you have measured the net power or total power of cornea with the IOL Master 700 TK or with Pentacam TNP or TCRP maps, then putting these values into standard IOL power calculation formulae will amount to a double correction.


c. As explained earlier the standard two variable formulae use the corneal power not only for IOL power calculation but also for ELP. Such assumptions of ELP based on corneal power may not be correct as keratoconus patients have unusually steep cornea leading to higher anterior chamber depthe (ACD) and thus ELP. In a study by Savini et al (4) all two variable formulae failed to give satisfactory outcome in Keratoconus patients. However in the study by Savinni et al, SRK T was seen to have the highest number of eyes (61 % of patients with stageI) to have landed within +/- .50 D of target refraction. The SRK T was compared to other formulae like the Barrett Universal, Haigis, Holladay I, Hoffer Q. . The Barrett Universal II, although the preferred formula in virgin eyes, proved to be less successful compared to SRK T in keratoconus eyes according to the study by Savini All formulae showed worsening of prediction error as the stage of Keratoconus advanced. In another study by Vicente Camps et al (5) SRK T and Holladay I had higher prediction accuracy than Haigis and Hoffer Q.

If you have measured the net power or total power of cornea with the IOL Master 700 TK or with Pentacam TNP or TCRP maps, then putting these values into standard IOL power calculation formulae will amount to a double correction.

While it is not advisable to use standard two variable formulae like Hoffer Q, Holladay I, etc, it is quite interesting to see that the SRK T has fairly perfomed in IOL prediction in such patients. It appears that in healthy eyes with steep cornea the SRK T may still underestimate the corneal power and over estimate the IOL power leading to myopic outcome. This tendency to recommend a higher IOL power even in steep corneas may work well in keratoconus eyes wherein all other standard IOL formulae lead to hyperopic outcome.



....in a study by Savinni et al SRK T was seen to have the highest number of eyes (61 % of patients with stage I ) in keratoconus patients to have landed within +/- .50 D of target refraction.

Since then, there has been newer IOL calculation formulae specially developed for keratoconus eyes - Kane keratoconus formula and Barrett keratoconus formula. The Kane keratoconus formula aims to reduce the influence of corneal power on effective lens position prediction. This formula is available at www.iolformula.com and was developed by theoretically modifying the original kane formula. The Kane keratoconus formula was tested against other formulae including the Holladay 2 keratoconus formula available with the Holladay IOL consultant software package. The Kane keratoconus formula was the most accurate in this study (figure 2). The Kane keratoconus formula aims to provide

a more appropriate corneal power measurement and reduce the influence of corneal power on ELP prediction. It is worthwhile to note that even in this study, the SRK T proved to be fairly accurate when compared to other two variable formulae.


Kane Keratoconus Formula - Key points to remember

  1. Do not input the total cornea values in Kane Keratoconus formula. Only input the anterior corneal values as the Kane formula theoretically derives the net corneal power from the anterior corneal values for a keratoconu patient.

  2. The formula minimizes the effect of corneal power on ELP prediction to enable more accurate IOL power calculation.

Barrett True K formula for Keratoconus - Key points to remember


1. Select the Keratoconus option from the drop down menu in Barrett True K formula

2. You have the option to input anterior corneal values thereby letting the Barrett True K formula to predict the total corneal values, or, input measured posterior corneal values if you have one.

3. If you have a measured posterior corneal value either from the IOL Master or Pentacam, then you must select the device from the drop down menu


Fig 2 Source: Ophthalmology 2020; American Academy of Ophthalmology



Kane Keratoconus formula available at www.iolformula.com

TORIC IOL in keratoconus:

The choice of toric IOL in keratoconus patients is debatable and should be considered on a case to case basis depending on the following:

a. Non progressive keratoconus and patients older than atleast 55 years

b. Stage I or Stage II keratoconus, that is, mild to moderate keratoconus

c. Satisfactory vision with glasses prior to developing cataract

d. No central corneal scarring.



Figure 3- pretty symmetric keratoconus

The key here is to have a good look at the topography map. Look at the central map within the pupil diameter. As this is the area through which the patient will look through mostly, it is important to consider the symmetry/assymmetry of the keratoconus in this area. In figure 3, one such topography picture is included. Notice the steep and flat meridian is pretty symmetrical and orthogonal in the central 3mm zone of the map.

Additionally,patients whose steep meridian is largely outside the pupil diameter may be better candidates for toric IOLs as the irregularity of cornea may be well away from the visual axis.


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In Summary:

  1. Account for the posterior corneal power and input the total power of cornea that uses true refractive indices of cornea in the IOL power formula

  2. Use a formula that is dedicated for IOL power calculation in keratoconus patients like the Kane Keratoconus or Barrett True K for Keratoconus.

  3. Look at the corneal power map from topography to understand the suitability of toric for such patient.



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Sources:

  1. Hofstetter HW. A keratoscopic survey of 13,395 eyes Am J Optom Arch Am Acad Optom. 1959;36:3–11

  2. Gorskova EN, Sevost’ianov EN. Epidemiology of keratoconus in the Urals Vestn Oftalmol. 1998;114:38–40

  3. Jonas JB, Nangia V, Matin A, Kulkarni M, Bhojwani K. Prevalence and associations of keratoconus in rural Maharashtra in central India: The central India Eye Medical Study Am J Ophthalmol. 2009;148:760–5

  4. Savinni et al : Intra Ocular lens power calculation in eyes with keratoconus; J Cataract Refract Surg 2019; 45(5): 576-81.

  5. Comparison of Four Intraocular Power Calculation Formulas in Keratoconus Eyes,Vicente J. Camps, DOI: 10.2174/1874364102115010096