Intraocular Lens Calculations in Patients With Keratoectatic Disorders

Ryan G. Smith; Alexander Knezevic; Sumit Garg

Disclosures

Curr Opin Ophthalmol. 2020;31(4):284-287. 

In This Article

Abstract and Introduction

Abstract

Purpose of Review: Intraocular lens (IOL) calculations in patients with keratoconus and other keratoectatic disorders continues to be a challenge for today's cataract surgeon. In this article, we review data published over the past 18 months (June 2018 to January 2020).

Recent Findings: Cataract surgery in keratoconus patients has the potential to greatly improve patients' vision. However, keratoconic eyes are notorious for unpredictable outcomes because of difficulty in obtaining proper preoperative biometry and lack of data and consensus on IOL calculation formulas that can reliable in providing the desired outcome. Recent studies suggest the Barrett II Universal calculation is the most accurate in mild-to-moderate keratoconic eyes. All studies note the level of predictability decreases with the steepness of keratometric readings. Historically, the SRK/T has been shown to provide the most reliable calculations.

Summary: There is still no consensus on which formula is best for IOL calculation in keratoconic eyes. On the basis of the most recent literature, we recommend using the Barrett II Universal in conjunction with the SRK/T formula for mild-to-moderate eyes. Preoperative counseling of expectations with the patient is the key to achieving a satisfied patient and avoiding an unpleasant situation in the result of refractive surprise.

Introduction

Keratoconus (KCN) is a bilateral corneal ectasia characterized by progressive thinning and conical protrusion of the cornea.[1] These changes may result in irregular astigmatism and high myopia leading to decreased visual acuity. Patients with keratoconus are also more likely to develop cataracts and do so at a younger age than the general population. Histopathologically, epithelial basement membrane deposition, stromal thinning, and defects in Bowman's layer are noted.

Keratoconus staging is controversial, with debate continuing over proper staging guidelines. Thebpatiphat classification has been cited in the literature and defines KCN severity as mild (maximum keratometry less than 48 D) moderate (maximum keratometry equal to or more than 48 D but less than 52), and severe (maximum keratometry equal to or more than 52 D). Alternatively, the Krumeich staging criteria define stage I KCN as having lower than 48 D corneal power. Stage II contains eyes with corneal power between 48.01 and 53 D, stage III encompasses all eyes greater than 53 D, and stage IV describes eyes with central scarring and unmeasurable refraction.[2]

Census data estimates that the population over 65 will increase from 15 to 24% between 2014 and 2060.[3] Thus, cataract surgeons can expect to see an increase in KCN patients who will require cataract surgery. Keratoconus presents unique challenges to intraocular lens (IOL) calculations, as the disease process fundamentally affects the keratometry (K) readings in an unreliable manner, most often with asymmetric and irregular astigmatism. Traditional equipment can struggle to generate K values as well as have difficulty measuring axial length. Axial length may be normal or patients may also have axial myopia. The effective lens position is more difficult to predict in keratoconic eyes. Additionally, the decentered cone may impact the precision of keratometry readings. Keratoconus patients often have concurrent ocular surface abnormalities, and an irregular tear film will further complicate biometric readings. Lastly, keratoconus can manifest with posterior corneal astigmatism. Although some algorithms do account for posterior corneal astigmatism, the irregular and inconsistent changes caused by keratoconus can further complicate these measurements.

Previous studies disagree as to the most accurate and reliable formula to use for intraocular lens implantation. A trend in literature over the past 10 years, however, points to considering lens formulas based upon severity of KCN. A review article by Ghiasian et al.,[1] for example, concludes that the SRK II formula may provide the most accurate IOL power in mild KCN, with mild disease defined as K readings less than 48 D, but no consensus for moderate or severe disease (moderate, or stage II, disease defined as corneal power between 48.01 and 53 D, and severe, or stage III, defined as corneal power greater than 53).

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