Measurement of change in angle kappa and its correlation with ocular biometric parameters pre- and post-phacoemulsification

Abstract

Purpose:

Angle kappa has been considered to play a role in causing glare and haloes despite accurate centration during implantation of multifocal intraocular lenses following phacoemulsification. There is a lack of substantial data regarding whether angle kappa is a constant entity or changes following ocular surgical procedures. To answer this question, in this prospective observational study, we measured change in angle kappa following phacoemulsification, and studied the ocular biometric parameters correlating with this change.

Methods:

Angle kappa was measured objectively using synoptophore. Ocular Biometric parameters (Anterior Chamber Depth, Corneal White-to-White measurement, Lens Thickness, and Axial Length) using LenStar LS 900 Haag Streit Anterior Segment imaging system. outcome measures were a quantitative change in angle kappa from the preoperative value by one degree or more and observation of correlation between change in angle kappa and ocular biometric parameters. The Wilcoxin Signed Rank Test was used to determine the difference between pre-operative and post-operative measurements for angle kappa. A p-value of less than 0.05 was considered statistically significant. Pearson’s correlation coefficient was employed to find the relationship between preoperative ocular biometric parameters and a change in angle kappa. A linear regression model was used to derive an equation considering corneal white-to-white measurement as the predictor and change in angle kappa as the outcome measure.

Results:

A significant change in angle kappa was recorded, and a significant correlation was found with corneal white to white measurements. This change could be predicted preoperatively, for a known corneal white to white measurement using the standard equation y=mx+c.

Conclusion:

This study explains the possible cause of dissatisfaction among seemingly ideal patients who undergo multifocal IOL implantation and the potential for better decision-making during patient selection for multifocal IOL implantation.

A large angle kappa has been implicated in causing inaccurate centration in implantation of multifocal intraocular lens (IOL). Such alignment errors may cause many optical problems, like glare and haloes, induce astigmatism, and leave the patient’s visual deficits uncorrected. Studies evaluating the factors predicting these effects have considered angle kappa to play a possible role in causation of photic phenomena in patients.[1–8] According to Prakash et al.[1] severity of glare and haloes can be correlated to angle kappa. Mamalis et al.[9] described that the glare and optical aberrations were one of the main indications of multifocal IOL exchange.

Angle kappa is defined as the angle between the pupillary axis and the visual axis of the eye.[10,11] It measures the angular misalignment between these two axes. Angle kappa values obtained preoperatively have been used as a selection criterion as well as a guide to predict visual outcomes after refractive procedures. A recent study.[12] suggested that the magnitude of angle kappa changes following phacoemulsification surgery.

There is a lack of substantial data regarding whether angle kappa is a constant entity or changes following ocular surgical procedures, along with limited justification on reliability and reproducibility of anterior segment imaging devices in evaluating angle kappa measurements. This study attempts to address these lacunae.

The primary objective of this study was measuring the change in angle kappa before and after phacoemulsification procedure using a synoptophore.

The secondary objective was to observe significant correlation, if any, of ocular biometric parameters (anterior chamber depth (ACD), Corneal white-to-white measurement, lens thickness (LT), axial length (AL), and pupil size) with quantitative changes in angle kappa measurement.

Methods

This prospective observational study was conducted in a tertiary care eye hospital in south India. After obtaining a well-informed consent from the eligible patients, we examined those undergoing elective cataract extraction with monofocal IOL implantation using phacoemulsification technique. Preoperative measurements were recorded before the surgery and the same parameters were re-assessed at one week follow-up, postoperatively. Approval from institutional ethics committee was obtained for the study on 09.09.2020.

The sample size was estimated to be 50 eyes with a statistical power of 90% and a 5% level of significance. This sample included a 20% margin to account for attrition after inclusion into the study. Data collection was done over a period of nine months from September 2020 to May 2021.

Fig. 1 shows a flowchart summarizing the methodology of recruitment and assessment of patients enrolled in the study.

Figure 1.

Flowchart summarizing the methodology of recruitment and assessment of patients enrolled in the study

Inclusion and Exclusion criteria are shown in Table 1.

Table 1.

Inclusion and Exclusion Criteria

Inclusion Criteria:

Patients planned for elective cataract extraction using monofocal

intraocular lens implantation;

Patients having a best-corrected visual acuity (BCVA) of 6/18 or

better on Snellen chart when viewing monocularly;

Patients having provided verbal and written informed consent

Exclusion Criteria:

Patients with a history of previous intraocular surgery;

Patients diagnosed to have mature cataract;

Patients with BCVA not corresponding to 6/18 on Snellen chart;

Patients evaluated to have corneal or retinal pathology;

High myopes whose axial length could not be obtained by the

Lenstar LS 900;

Patients who manifest a squint on examination;

Patients with amblyopia on examination;

Patients with traumatically mydriatic pupil during preoperative

evaluation and slit-lamp examination;

Patients who experienced intraoperative or immediate

postoperative complication during or after the procedure.

The parameters recorded during the preoperative evaluation were the following:

1. Uncorrected visual acuity (UCVA) and best-corrected visual acuity (BCVA) for distance and near using Snellen chart

2. Cover-uncover test to rule out heterophoria and heterotropia

3. Slit-lamp biomicroscopic examination for evaluation of anterior segment

4. Pupil size, ACD, corneal WTW, LT, and AL

5. Angle kappa measurement using synoptophore

6. Intraocular pressure (IOP) measurement using Goldmann applanation tonometer

7. Dilated fundus examination by indirect ophthalmoscopy using 20 D lens

A change in angle kappa was defined as a change from the preoperative angle kappa value by 1° or more.

Grading of cataract was done using the Lens Opacities Classification System (LOCS) grading system.

Ocular biometric parameters were recorded using the Lenstar LS 900 by Haag-Streit, USA.

Methodology for Angle Kappa Measurement

Angle kappa measurement was done separately for each eye with monocular viewing using the angle kappa measuring slide on the synoptophore (Baliwalla and Homi Pvt. Ltd.), as described previously.[13–17]

This slide is a specialized slide having a row of alphabets and numbers on either side of a central fixation point marked “0”. The separation between adjacent alphabets and numbers corresponds to 1°.

After ensuring that they are in an undilated state, the patient placed their chin on the chin rest and their forehead in contact with the headrest. The arms of the synoptophore were adjusted at the zero mark to ensure there was no horizontal and vertical displacement, and cyclotorsion. Under binocular viewing conditions, the interpupillary distance was recorded.

The background illumination of the examination room where angle kappa measurement was recorded had the same light conditions as when the ocular biometric parameters, including pupil measurement, were taken.

With the patient fixating on the central “0” mark of the slide, the line of sight was localised clinically as the axis connecting the fovea with the fixation point. The center of the pupil marks the anatomic localization of the pupillary axis. Subsequently, a light was shown on the fixing eye, and the examiner viewed the corneal light reflex coaxially with the left eye (when viewing the right eye) and vice versa to make note of the corneal light reflection as being nasal or temporal to the pupil center. When the corneal light reflex was noted to be nasal to the pupil centre, a positive angle kappa was recorded, and when temporal, as negative.

The magnitude of positive or negative angle kappa was measured by instructing the patient to sequentially look at the letters or the numbers on the angle kappa slide while the examiner noted the location of the corneal light reflex in relation to the pupil center. The number or letter observed by the patient, when the examiner noted the corneal light reflex as coinciding with the apparent pupil center, was recorded as the magnitude of angle kappa. For example, when the corneal light reflex was observed to be centred when the patient looked at the letter B on the angle kappa measuring slide, the angle kappa was noted as 2° and so on.

Three consecutive measurements were taken for each eye. On follow-up, the interpupillary distance was maintained at the same measurement noted preoperatively to avoid subjective intersession variability.

Surgical procedure:

Cataract extraction and monofocal IOL implantation performed by phacoemulsification technique was carried out by a single experienced surgeon under topical anesthesia. All IOLs were implanted in the capsular bag. Routine postoperative antibiotic and steroid eye drops were prescribed and follow-up was done as per the standard protocol of the institution.

Sources of bias

Factors that contribute to a shift in the line of sight or the pupillary axis could directly lead to a change in angle kappa. The change in the size of the pupil due to changing luminance at different times of measurement is known to influence the location of the pupil center.[18–20] The change in pupil center will shift the line of sight and thus directly affect angle kappa. In our study, we measured the pupil size before and after cataract surgery while maintaining similar conditions of background illumination and observed if the change in pupil size was statistically significant to have caused a change in angle kappa measurements.

Another source of bias arises in the measurement of angle kappa using synoptophore which involves subjective observation of the corneal light reflex viewed coaxially and its relation to the center of the pupil. The corneal light reflex viewed coaxially will be seen differently depending on the observer’s eye dominance and eye balance.[21] In our study, this inter-subject variability was limited by maintaining a single observer for measurements in all subjects, preoperatively and postoperatively. Being a subjective measurement, the intersession variability in measuring the interpupillary distance could also affect the measured angle kappa on the synoptophore. On follow-up of patients, the interpupillary distance measured for a patient was kept the same as the reading recorded at the time of recruitment. Thus, this confounding factor was eliminated.

Sample size

The minimum required sample size for the study was calculated using the following formula:

n = (Zα + Zβ) 2/C2 where

C = 0.5 × ln (1 + r/1 − r), r = correlation coefficient

Zα =1.96 at 95% confidence interval (CI) and Zβ =1.28 at 90% power

r = 0.492 (angle kappa in magnitude was significant (r = 0.492, P =0.000) with 95% CI and 90% power.

Statistical analysis

The data were analyzed descriptively first, followed by the derivation of mean and standard deviation estimates.

The Wilcoxon signed-rank test was used to determine the difference between preoperative and postoperative measurements of angle kappa, pupil size, and interpupillary distance. A P value of less than 0.05 was considered statistically significant.

The Pearson correlation coefficient was employed to find the relationship between preoperative ocular biometric parameters and a change in angle kappa. A linear regression model was used for a significant correlation, considering corneal white-to-white measurement as the predictor and change in angle kappa as the outcome measure. A multivariate analysis of all the ocular biometric parameters was also done.

All statistical analyses were carried out using the Statistical Package for the Social Sciences (SPSS) version 22.0 (Chicago, USA) software for Windows.

There was no loss to follow-up.

Results

Fifty-four eyes of 42 patients undergoing cataract surgery were analyzed during the study period. Of the patients examined, 28 were males (51.9%), and 26 were females (48.1%). Mean age of the participants was 63.69 ± 7.42 years.

Our study group comprised of patients with nuclear sclerosis grade I (21%), posterior subcapsular cataract (PSC) (14%), nuclear sclerosis grade II (18.5%), nuclear sclerosis grade II with PSC (3.7%), nuclear sclerosis grade I with PSC (3.7%), and nuclear sclerosis with cortical cataract (5.6%)

The preoperative ocular biometric parameters as measured on the Lenstar LS 900 are shown in Table 2.

Table 2.

Preoperative ocular biometric parameters

Ocular Biometric Parameters	Unit	Mean	Standard Deviation	Range
White-to-white corneal diameter	mm	11.83	0.41	10.67-12.24
Anterior chamber depth	mm	3.04	0.40	2.64-3.44
Lens thickness	mm	4.43	0.42	4.01-4.85
Axial length	mm	23.06	1.16	21.9-24.22

The comparison of mean preoperative and postoperative values of angle kappa and interpupillary distance calculated by the synoptophore and pupil size measurement recorded on the Lenstar LS 900, preoperatively and at one-week follow-up, are shown in Table 3.

Table 3.

Comparison of parameters preoperatively and postoperatively

Parameter	Preoperative Mean (SD)	Postoperative Mean (SD)	P
Preoperative kappa	0.92 (1.85)	−0.11 (1.92)	0.001*
Pupil size	3.43 (0.47)	3.36 (0.51)	0.19
IPD	62.19 (3.24)	62.17 (3.22)	0.800

Wilcoxin Signed Rank Test – P *Significant. IPD=Interpupillary Distance

The change in angle kappa ranged from no change to a change of up to 5° postoperatively. The majority of the patients had a change in angle kappa of 1° postoperatively. There was no significant difference in the change in angle kappa between males and females.

A statistically significant difference in angle kappa before and after cataract surgery was noted (r = 0.283, P = 0.001).

The ocular parameters compared preoperatively and postoperatively are shown in Table 3. The mean preoperative pupil size was 3.43 ± 0.47mm and postoperative pupil size was 3.36 ± 0.5mm. An average difference of 0.07 mm was noted in pupil size before and after surgery, which was not statistically significant.

Similarly, intersession variability in measurement of interpupillary distance, which is subjective to the synoptophore, was also compared and found to be statistically not significant (P = 0.8).

Correlation of change in angle kappa with preoperative parameters was performed using the Pearson correlation coefficient and is shown in Table 4. A statistically significant negative correlation (r = −0.440, P < 0.001) was found between corneal WTW measurement on the Lenstar LS 900 and angle kappa measured on the synoptophore. A greater change in angle kappa was noted with decreasing corneal WTW values measured preoperatively.

Table 4.

Correlation between change in angle kappa with ocular biometric parameters

Parameter	Correlation Coefficient	P
ACD	−0.23	0.08
WTW corneal diameter	−0.44	<0.001*
Axial length	−0.003	0.87
Lens thickness	0.12	0.41

ACD: Anterior chamber depth; WTW: White-to-white; *Significant

Other ocular biometric parameters like AL and preoperative ACD did not correlate significantly with the change in angle kappa.

To understand if the postoperative angle kappa could be predicted based on the preoperative ocular biometric parameters, we performed a simple linear regression analysis between corneal WTW (as the predictor) and change in angle kappa (as the outcome variable), as shown in Table 5. Every 1 mm increment in WTW diameter led to a − 2.42 scale reduction in angle kappa.

Table 5.

Linear regression analysis of white-to-white corneal diameter with change in angle kappa

Results	Value	95% Confidence Interval	P
Beta coefficient	−2.42	−3.80 to-1.04	0.001
Constant	29.770	13.426-46.113	0.001

In multivariable analysis adjusting for preoperative ACD, WTW diameter continued to remain the only factor predicting the change in angle kappa.

The expected postoperative change in angle kappa could be calculated using the standard formula of linear regression analysis; that is, y = mx + c, where y is the expected change in angle kappa, x is the calculated corneal WTW diameter preoperatively, m is the beta coefficient, and c is the constant. A scatter plot with a locally weighted scatterplot smoothing (LOWESS) curve showing the change in angle kappa with change in values of corneal WTW diameter is shown in Fig. 2.

Figure 2.

Scatter plot with LOWESS curve showing a moderate negative correlation between preoperative WTW corneal diameter and change in angle kappa

Discussion

Available literature on consistency of magnitude of angle kappa and its correlating factors post phacoemulsification is scarce. In our study, we found a significant change (P = 0.001) in the magnitude of angle kappa following phacoemulsification. We also noted a significant negative correlation of the corneal WTW diamter with the magnitude of angle kappa change. With the help of linear regression analysis, we derived an equation that could help predict the expected change of angle kappa for a given preoperative corneal WTW value in patients undergoing phacoemulsification.

Most of the studies done previously utilized various methods for the measurement of angle kappa,[1,2,8,11–14,17,22–29] and though a change in angle kappa has been suggested post diffractive trifocal IOL implantation,[30] post photorefractive keratectomy,[27] post phacoemulsification[29] and with postural changes in LASIK,[8] existing literature suggests that a synoptophore,[17] as used in our study, continues to be the most accurate instrument for the measurement of angle kappa.[17]

Until recently, angle kappa has been described to have a contributory role in patients who experience glare and haloes following multifocal implantation. Prakash et al.[1] established a statistically significant association between larger angle kappa with glare and haloes. Interestingly, in their study many patients were asymptomatic despite having high values of angle kappa. The reason for this too could be hypothesized to be linked to a change in angle kappa values post phacoemulsification. We attempted to address these lacunae in our study.

Compared to our data, a similar study done by Wang et al.[12] studied the changes in angle alpha and angle kappa before and after cataract surgery. They made a similar observation, suggesting that angle kappa might change after cataract surgery. They also proposed that angle alpha—the angle between optical and visual axes—due to its more organized distribution horizontally around the corneal light reflex, may be more reliable for taking into consideration prior to cataract surgery with multifocal implantation.[12] However, angle alpha cannot be measured practically as the optical axis is an imaginary line passing through the nodal points of the eye, which cannot be determined clinically[31] using fixed anatomical landmarks.[30] Another recent study by Tutchenko et al.[29] also observed a change in angle kappa following phacoemulsification. However, they could neither explain the cause of this change nor a way to predict this change preoperatively. Our study found a significant correlation of change in angle kappa with corneal WTW diameter. With the help of the equation derived using linear regression analysis—that is, using the equation y = mx + c—the expected postoperative angle kappa could be predicted preoperatively for a known corneal WTW measurement.

A study conducted by Meng et al.[32] in Shanghai found a larger value of angle kappa to be associated with a shorter WTW diameter and a shallower ACD. Their study included a larger cohort of 15,127 patients compared to ours and found that angle kappa was predominantly located temporal to the visual axis. We obtained similar results; however, our study did not include very long and very short eyes. This along with a smaller sample size could have resulted in the lack of significant correlation in our study with respect to AL and ACD. The prediction of postoperative angle kappa, taking into consideration not only the corneal WTW diameter but also possibly other ocular biometric parameters affecting angle kappa, like ACD and AL, remains to be studied.

Garzón et al.[30] studied changes in pupil offset following trifocal diffractive IOL implantation and observed that a pupil offset did not impact visual outcomes negatively post IOL implantation. However, pupil offset was measured using Pentacam HR topographer (Oculus) which Does not record angle kappa. It measures a two-dimensional cartesian displacement which roughly correlates with the concept of angle kappa.[22,33] Thus, prediction of angle kappa based on preoperative values measured objectively could prove more useful for future predictions and better patient outcomes.

Although preoperative angle kappa assessment is not yet part of standard preoperative examination,[5] the importance of its consideration for multifocal IOL implantation has been studied by Bonaque-González et al.[34] They used an image quality (IQ) metric known to account well for changes in retinal image quality and observed that far vision IQ was affected negatively when optimal orientation of implantation was calculated without accounting for angle kappa, which reversed when the same were made including the value of angle kappa. Thus, understanding what causes angle kappa to change and factors affecting it would guide our decision-making during patient selection for multifocal IOL implantation, thereby leading to possibly better visual outcomes.

Although the exact cause of change in angle kappa is unknown, factors that contribute to a shift in the line of sight or the pupillary axis could contribute to a change in angle kappa.

The change in the size of the pupil due to changing luminance at different times of measurement is known to influence the location of the pupil center.[18] Our study documented a small change in pupil size despite similar conditions of background illumination; however, it was not statistically significant, and thus is unlikely to have contributed to the observed change in angle kappa.

Another contributing factor could be a change in corneal curvature following clear corneal incision in phacoemulsification surgery as there could be a shift in the position of the corneal light reflex in relation to the center of the pupil. With microincision surgery and minimal surgical astigmatism, the magnitude of change in corneal curvature contributing to a significant shift in corneal light reflex seems unlikely; yet further research is needed.

The mean angle kappa measurements in our study population were found to be lower than those in the study conducted by Basmak et al.[15] We believe this difference could be attributed to a difference in ethnicity, which is known to cause significant changes in measurements of interpupillary distance.[35] Although interpupillary distance does not correlate with angle kappa measurements,[36] it can affect subjective measurements of angle kappa when measured using the synoptophore.

Most studies done on optimal centration without accounting for angle kappa have included myopes. Hyperopes, unlike myopes, have smaller functional ablation zones and larger angle kappa that cause a greater deviation of the line of sight from the pupillary axis. Thus, a changing angle kappa becomes a more significant consideration for hyperopes undergoing refractive procedures. Thus, future directions of research in this arena require a better understanding of angle kappa.

While synoptophore is considered as the gold standard for measurement of angle kappa, it is limited in being based on subjective observation of the corneal light reflex. This emphasizes the need for studies involving more objective methods of angle kappa measurement, which would ensure repeatability and reliability despite multiple observers and would also enable a larger cohort to be studied in a limited time. With the help of further research, this elucidation can help in more objective screenings of seemingly fit patients.

Conclusion

Our study found a significant change in magnitude of angle kappa following phacoemulsification surgery. The magnitude of this change correlates inversely with the corneal WTW measurement, and for a given preoperative WTW we can help predict the estimated change with a simple equation.

More extensive prospective studies, including extremes of AL, and a larger sample size need to be considered to better predict changes in angle kappa and analyze the association of its change with all preoperative ocular biometric parameters.

The results of our study indicate the necessity to investigate the expected magnitude of change of angle kappa post phacoemulsification. This would contribute to better decision-making during patient selection for multifocal IOL implantation and help reduce incidents of optical problems in the same group post procedure.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.