To investigate the magnitudes and the axis orientations of anterior, posterior, and total central corneal astigmatism in eyes with keratoconus.
Retrospective case series.
This study comprised 137 eyes of 137 keratoconic patients (97 men and 40 women; mean age ± standard deviation, 36.9 ± 12.2 years). The magnitude and the axis orientation of each corneal astigmatism were determined with a rotating Scheimpflug system.
The mean magnitudes of anterior, posterior, and total central corneal astigmatism were 3.93 ± 2.74 diopters (D), 0.93 ± 0.64 D, and 3.90 ± 2.75 D, respectively. With-the-rule (WTR), against-the-rule (ATR), and oblique astigmatism of the anterior corneal surface was found in 90 eyes (65.7%), 33 eyes (24.1%), and 14 eyes (10.2%), respectively, whereas the corresponding astigmatism of the posterior corneal surface was found in 14 eyes (10.2%), 15 eyes (10.9%), and 108 eyes (78.8%), respectively. We found a significant correlation between the magnitudes of anterior and posterior corneal astigmatism (Pearson correlation coefficient r = 0.769, P < .001).
The mean magnitudes of anterior and posterior corneal astigmatism were approximately 4 D and 1 D, respectively, in eyes with keratoconus. Approximately 65% and 80% of eyes showed that WTR anterior astigmatism and ATR posterior astigmatism, respectively. The presence of posterior corneal astigmatism is not necessarily negligible for the accurate astigmatic correction of toric intraocular lens implantation or rigid gas-permeable contact lens wear for keratoconus.
Keratoconus is a progressive noninflammatory disorder characterized by ectasia and thinning of the cornea. The progressive thinning and subsequent anterior protrusion of the cornea can result in not only severe myopic astigmatism but also asymmetrical irregular astigmatism, leading to distorted vision. Placido disk–based corneal topography in combination with biomicroscopic examination is widely used in the diagnosis of keratoconus in daily practice. Although Placido disk–based corneal topography is known to be a highly sensitive and specific diagnostic tool, it only examines the anterior corneal surface, and does not evaluate the curvature and elevation of the posterior corneal surface, which is considered to be significant especially in early-stage keratoconus detection. The development of new technologies, such as slit-scanning technologies, rotating Scheimpflug devices, and optical coherence tomography, makes it now possible to quantitatively measure the posterior corneal curvature, and to provide useful diagnostic information for the detection of keratoconus in a clinical setting. However, there have been so far no detailed studies on the magnitude and the axis orientation of posterior corneal astigmatism for keratoconus with respect to each clinical stage of the disease. Although the magnitude and the axis orientation of anterior corneal astigmatism are easier to determine by the use of Placido disk–based corneal topography, an autokeratometer, or a manual keratometer, in daily practice this evaluation of posterior corneal astigmatism may give us intrinsic insights on optical performance in keratoconic patients, especially when toric intraocular lens (IOL) implantation is planned, and when rigid gas permeable (RGP) lenses are worn. The purpose of this study is to retrospectively assess the magnitudes and the axis orientations of anterior, posterior, and total corneal astigmatism with respect to clinical stage of the disease in a large cohort of keratoconic subjects.
This retrospective study comprised 137 eyes in 137 keratoconic patients (97 men and 40 women, mean age ± standard deviation [SD]: 36.9 ± 12.2 years) with good-quality scans of corneal tomography measured with a rotating Scheimpflug imaging instrument (Pentacam HR; Oculus, Wetzlar, Germany). The patients were recruited in a continuous cohort. Only 1 eye per subject was selected randomly for statistical analysis. Some of the subjects were those in our preceding reports on anterior and posterior corneal elevation in eyes with keratoconus. The sample size in this study offered >99% statistical power at the 5% level in order to detect a correlation of 0.700. Keratoconus was diagnosed by 1 experienced clinician (K.K.) with evident findings characteristic of keratoconus (eg, corneal topography with asymmetric bow-tie pattern with or without skewed axes), and at least 1 keratoconus sign (eg, stromal thinning, conical protrusion of the cornea at the apex, Fleischer ring, Vogt striae, or anterior stromal scar) on slit-lamp examination. Pellucid marginal degeneration having inferior corneal thinning with ectasia above the area of thinning and no inflammatory signs on slit-lamp biomicroscopy was excluded from the study. Eyes with other corneal diseases and eyes with previous ocular trauma or surgery were also excluded from the study. The study population was divided into 4 subgroups, grade 1 (44 eyes), grade 2 (38 eyes), grade 3 (16 eyes), and grade 4 (39 eyes) keratoconus groups, according to the Amsler-Krumeich classification, based on astigmatism, corneal power, corneal transparency, and corneal thickness, obtained using the rotating Scheimpflug imaging instrument and slit-lamp biomicroscopy. The patients who wore RGP and soft contact lenses were asked to stop using them for 3 and 2 weeks before this assessment, respectively. This retrospective review of the data was approved by the Institutional Review Board at Kitasato University and followed the tenets of the Declaration of Helsinki. Our Institutional Review Board waived the requirement for informed consent for this retrospective study.
Assessment of Corneal Astigmatism
The magnitude and the axis orientation of anterior, posterior, and total corneal astigmatism on the central 15-degree ring (equal to the 3.0-mm ring) around the corneal apex were automatically measured with the Scheimpflug imaging system (Pentacam HR, software version 1.20; Oculus) by experienced examiners, who were masked to the clinical condition of the subjects. The readings were taken as recommended in the instruction manual of the instrument. In brief, the patient’s chin was placed on the chin rest and the forehead against the forehead strap. The patient was asked to open both eyes and stare at the fixation target on the black background in the center of the blue fixation beam. After attaining perfect alignment, the instrument automatically took 25 Scheimpflug images within 2 seconds. Image quality was checked, and for each eye only 1 examination with a high quality factor was recorded.
Anterior corneal astigmatism is defined as the difference in simulated keratometry between the flattest and steepest meridians, calculated by using the standard keratometric index (1.3375) and the radius of anterior corneal curvature. Posterior corneal astigmatism is defined as the difference in keratometry between the flattest and steepest meridians, calculated by using the refractive index of the cornea (1.376), and the aqueous humor (1.336), and the radius of posterior corneal curvature. Total corneal astigmatism is defined as the difference in total corneal refractive power between the steepest and flattest meridians, calculated by ray tracing through the anterior and posterior corneal surfaces according to Snell’s law.
Anterior and total corneal astigmatism was classified as “with-the-rule” (WTR) when the steep meridian was within 60–120 degrees, and as “against-the-rule” (ATR) when the steep meridian was within 0–30 degrees or 150–180 degrees. Otherwise, the remaining astigmatism was classified as oblique astigmatism. Since the dioptric power of the posterior corneal surface was negative, posterior corneal astigmatism was classified as WTR when the steep meridian was within 0–30 degrees or 150–180 degrees, and as ATR when the steep meridian was within 60–120 degrees. Otherwise, the remaining astigmatism was classified as oblique astigmatism, as described previously.
In addition, in order to confirm the repeatability of the measurements, anterior, posterior, and total corneal astigmatism measurements were made at the same time of day on 2 consecutive days in 20 keratoconic eyes. The repeatability of the 2 measurements was evaluated using Bland-Altman plots, as described previously.
All statistical analyses were performed using a commercially available statistical software (Ekuseru-Toukei 2015; Social Survey Research Information Co, Ltd, Tokyo, Japan). After normal distribution of the data was confirmed with the Kolmogorov-Smirnov test, the Pearson correlation coefficient was calculated in order to assess the relationship of the 2 variables. One-way analysis of variance (ANOVA) was used to assess the relationship of each corneal astigmatism with the Amsler-Krumeich classification. The Cohen kappa coefficient was used to assess the agreement between the axis orientations (WTR, ATR, and oblique astigmatism) of anterior and posterior corneal astigmatism. The results are expressed as mean ± SD, and a value of P < .05 was considered statistically significant.
The demographics of the study population are summarized in Table 1 . In the entire population, the mean magnitudes of anterior, posterior, and total central corneal astigmatism were 3.93 ± 2.74 diopters (D) (range, 0.10–12.60 D), 0.93 ± 0.64 D (range, 0.00–2.90 D), and 3.90 ± 2.75 D (range, 0.10–12.70 D), respectively. Figure 1 shows the magnitudes of anterior, posterior, and total corneal astigmatisms in each stage of the disease, respectively. Anterior, posterior, and total corneal astigmatism showed no statistically significant increase with the progressive stages of keratoconus ( P = .056 for anterior corneal astigmatism, P = .051 for posterior corneal astigmatism, P = .058 for total corneal astigmatism, 1-way ANOVA). Although there was considerable variability in each corneal astigmatism, eyes graded as stages 3 and 4 tended to show larger anterior, posterior, and total corneal astigmatism than those graded as stages 1 and 2. WTR, ATR, and oblique astigmatism of the anterior corneal surface was found in 90 eyes (65.7%), 33 eyes (24.1%), and 14 eyes (10.2%), respectively, whereas WTR, ATR, and oblique astigmatism of the posterior corneal surface was found in 14 eyes (10.2%), 15 eyes (10.9%), and 108 eyes (78.8%), respectively. Table 2 shows the axis orientations of anterior and posterior corneal astigmatism in the entire population. We found no significant agreement between the axis orientations of anterior and posterior corneal astigmatism (Cohen’s kappa coefficient κ = −0.016, P = .558). Figure 2 shows the distributions of anterior, posterior, and total corneal astigmatisms in each stage of the disease, respectively. We found a trend for a high prevalence of WTR anterior corneal astigmatism, but it was gradually decreased with the progressive stages of keratoconus. In contrast, most eyes showed ATR posterior corneal astigmatism in each stage of the disease.
|Age (y)||36.9 ± 12.2 years (range, 15–66 years)|
|Sex (male : female)||97 : 40|
|Manifest spherical equivalent (D)||−4.74 ± 4.40 D (range, −18.00 to 3.00 D)|
|Manifest cylinder (D)||−1.47 ± 2.20 D (range, −10.00 to 0.00 D)|
|LogMAR CDVA||0.07 ± 0.22 (range, −0.08 to 1.30)|
|Anterior corneal astigmatism (D)||3.93 ± 2.74 D (range, 0.10–12.60 D)|
|Posterior corneal astigmatism (D)||0.93 ± 0.64 D (range, 0.00–2.90 D)|
|Total corneal astigmatism (D)||3.90 ± 2.75 D (range, 0.10–12.70 D)|
|Grade 1||44 eyes|
|Grade 2||38 eyes|
|Grade 3||16 eyes|
|Grade 4||39 eyes|
|Posterior Corneal Astigmatism|
|WTR Astigmatism||Oblique Astigmatism||ATR Astigmatism||Total|
|Anterior Corneal Astigmatism|
|WTR Astigmatism||4 eyes (2.9%)||3 eyes (2.2%)||83 eyes (60.6%)||90 eyes (65.7%)|
|Oblique Astigmatism||2 eyes (1.5%)||12 eyes (8.8%)||19 eyes (13.9%)||33 eyes (24.1%)|
|ATR Astigmatism||8 eyes (5.8%)||0 eye (0%)||6 eyes (4.4%)||14 eyes (10.2%)|
|Total||14 eyes (10.2%)||15 eyes (10.9%)||108 eyes (78.8%)||137 eyes (100%)|