INTRODUCTION

The concept of “quality of vision” in refractive surgery is broad and should encompass not only the quality of retinal imaging formed by the new optics induced in the global optical system of the eye after surgery, but also the absence of changes in vision quality reported by patients subjectively.

In order to meet the need to dispense of visual compensating means, treatments such as corneal refractive surgery or intraocular lens (IOL), toric, multifocal or multifocal toric implantation, are increasingly used solutions.

In addition, limitations in corneal surgery procedures such as high ametropia in young presbyopes also lead to an increase in the number of surgeries in which the transparent lens is replaced by a multifocal IOL that allows functional vision over several distances without having to resort to any other visual aids. However, when combining these more complex optical designs with highly demanding patients, there is a greater need to measure results beyond the usual metrics that measure visual acuity or even contrast sensitivity, under different circumstances from those in which the patient develops his activity.

Dysphotopsies associated with implantation of bifocal and multifocal lenses with more complex optical designs or incorrect placement of implanted devices have also been the subject of scientific and clinical interest. The diffractive designs, due to the concentric rings on the surface of the IOL that allow for multifocality, inevitably create light distortion phenomena especially in low light conditions. In fact, current IOLs are designed to minimize these effects.

With the growing number of new devices marketed and implemented, it is necessary to use new visual quality assessment metrics that consider not only the subjective component (complaints) but also the objective component of the physical phenomenon (dysphotopsia), which can be evaluated with different instruments currently available commercially or in the experimental phase.

This chapter will discuss the results obtained by measuring light distortion as a metric that allows quantifying dysphotopsies with different IOLs, aspheric monofocal, bifocal, trifocal and extended depth of focus (EDoF), with or without toric geometry, either in patients with cataract or in presbyopia subjects with clear lens. Special emphasis will be given to the results obtained in refractive interventions using Premium intraocular devices.

POSITIVE DYSPHOTOPSIA

GLARE, STARBURST AND HALO

The term dysphotopsia referring to undesirable images/collateral effects after refractive surgery was introduced in the year 2000 by Tester1. In low light conditions, when the visual system is subjected to physiological mydriasis, light distortions can impair visual performance and the way objects are perceived. Manifestations of these distortions can be perceived by patients in the form of halos, starbursts and/or glare/dazzle. All of these photic phenomena can interact together and make descriptive analysis both objective and subjective by the patient difficult. In the literature, different designations for these light distortion phenomena can be found. Some examples:

• Photic phenomena2;
• Night Vision Disorders (NVD)3-7;
• Dysphotopic symptoms2;
• Image degradations6;
• Phenomena of brightness and halo8.

One of the main problems today is the lack of knowledge as to the origin of some of these phenomena which, unlike being attributed to only one of several factors such as eye aberrations, light scattering or diffraction, are assumed to be due to joint interaction of all these factors, leading to a too subjective description of complaints referred by patients. Although it is not possible to accurately classify the etiology of each of these light distortion phenomena, methods have been developed that attempt to quantify them. However, there is a lack of scientific standardization and validation of some of these methodologies and some are difficult to interpret for both the physician and the patient. On the other hand, most tests cannot correlate results with symptoms. Although there are different methods to quantify ocular dispersion and other night vision distortions, there is little clinical information to validate these systems for clinical practice9.

It is necessary to define several terms related to this topic, since there is a great variety of definitions and words mentioned incorrectly in several articles (Figure 1).

• Glare / Glitter / Dazzling: it is subdivided into: a) “brightness of discomfort” that causes subjective visual discomfort and eye fatigue without necessarily interfering with visual performance (discrimination and contrast)10 and explains some difficulties manifested by patients in night vision conditions; b) “incapacitating brightness” that causes a significant reduction in visual performance and poor visibility of the objects6,11. In this case considerable degradation of the image occurs when the luminance within the visual field is higher than the luminance to which the eyes are adapted11. Other authors also define it as the temporary loss of visual function in the presence of a bright adjacent light source11. Another typology of this phenomenon would be the “veil glow” caused by light scattering within the eye reducing the contrast of the retinal image10.
• Starburst: a common phenomenon and referred to as "star" light distortion also often referred to by patients who, despite not having undergone refractive surgery, are users of optical compensation (glasses), especially if the correction is out of date12.
• Halo: may occur with or without starburst11. Halos may occur when the pupillary diameter is greater than the optic zone after refractive surgery, or in multifocal devices by the overlap in the retinal plane of the focus of the light in different zones along or near of the optical axis.

As shown in Figure 2, subjective visual experience rarely focuses on a single form of light phenomenon and many patients may refer to them in real-life situations. For this reason, the term light distortion and light distortion analysis is used to refer to the visual phenomenon instead of classifying it individually as a halo, starburst or dazzling phenomenon.

Currently, although high levels of satisfaction are achieved in refractive surgery procedures, many patients have subjective complaints that do not correlate with the clinical results obtained with classical visual analysis metrics. After IOL implantation, patients frequently report complaints related to light distortion, with the most frequent visual complaints being brightness and halos13. In the particular case of phakic IOLs, the incidence of halos (34%) and brightness (26%)14 is directly related to the increase of the pupil diameter under low light conditions (night)15 and factors such as the optical zone diameter of the IOL, the difference between the size of the mesopic pupil and the diameter of the optic zone and the horizontal diameter of the visible iris are more related to the halos phenomenon, whereas the IOL toricity is more related to the incidence of brightness. The incidence of dysphotopsies including disabling brightness, halos and loss of contrast sensitivity in low illumination are important factors of dissatisfaction in patients undergoing refractive lens surgery with IOL implantation16.

There have been some studies in which the performance of different IOLs (trifocals, bifocals, monofocals, among others) has been evaluated with respect to light distortion phenomena, although there is no gold standard instrument to compare studies and results with each other. In most of the studies performed only the subjective responses of the patients are evaluated through the use of surveys17-19, which makes it even more difficult to compare the results. The most recent meta-analysis comparing the performance of bilateral implantation of trifocal and bifocal devices in cataract surgery or transparent lens replacement surgery shows the need for more scientific evidence to compare the results obtained with regard to the degree of independence from additional optical aids, patient satisfaction and analysis of dysphotopsies20. Multifocal IOLs have been in place since the 1980s and are designed to provide simultaneous vision for near and far vision. Depending on the optics used they can produce two or more spotlights ensuring functional vision in near and intermediate vision. However, due to the distribution of light energy in each focus, depending on pupillary size or not, patients experience a decrease in contrast sensitivity and the onset of dysphotopsic phenomena especially in low light conditions20-23.

Patients implanted with multifocal IOLs report a higher incidence (3.5 X more frequent) of dysphotopsies than patients implanted with monofocal lenses24, this phenomenon being one of the main reasons for dissatisfaction reported by patients implanted with these multifocal devices25 and are identified by surgeons as a common cause of IOL explanations, together with an excessive reduction of contrast sensitivity and failure of the neuroadaptation process26. Lenses of diffractive design are mostly identified as lenses with a higher number of explants compared with refractive lenses (84% vs 16% respectively).

Despite the research already done in this area there are many differences in the protocols used and in the periods in which the evaluations were established. All this, together with the lack of homogenization in the analysis techniques used, makes it impossible to establish a comparison between the obtained results. There is currently no consensus as to the length of time within which preoperative visual quality recovery is restored or stabilized.

Some authors suggest that if complaints about light distortion phenomena (halo and glare), arise in the immediate postoperative period and do not diminish one or two months after surgery and do not justify their presence (a residual refractive error for example), explantation of the multifocal lens should be considered and exchanged for a pseudo-accommodative or a monofocal lens. If, on the contrary, complaints about dysphotopsies appear within a few months of the intervention, the opacity of the posterior capsule should be considered as the cause of them, and appropriate treatment should be performed27.

Refractive surgery experts consider that improvements in patients' subjective perception are expected during the first year after surgery. However, there is interindividual variability between patients in time to recovery of visual function, depending on various factors such as age, biological issues or simply expectations about surgical outcomes. In a study evaluating visual acuity and contrast sensitivity comparing five decades of age (30 to 70 years) after diffractive IOL implantation, results showed that corrected visual acuity and contrast sensitivity were worse in elderly patients, but in the group with monofocal lenses, no differences were found. Such findings reveal an inversely proportional relationship between age and neuroadaptation28. According to other studies, there is a reduction in contrast sensitivity with multifocal IOLs compared with monofocal IOLs one month after implantation29, but contrast sensitivity improves with time after implantation of multifocal IOLs, stabilizing between 3 and 6 months after surgery. To understand how patient tolerance or neuroscience works in improving visual performance, it would be ideal to monitor outcomes more frequently, together with their impact on daily living tasks.

Postoperative follow-up of patients implanted with multifocal IOLs is the main difficulty in performing longitudinal studies. However, such an analysis would be convenient to assess long-term performance and possible changes that may appear in visual function. In a long-term follow-up with multifocal apodised IOLs, no differences were found between 6 months and 3 years after surgery in visual acuity and dysphotopsia rates (for glare and halo phenomena)30, suggesting apodised diffractive optics and neuroadaptation as contributing factors in these results.

METHODS OF MEASUREMENT

Several methods are currently available for the assessment of visual quality related to positive dysphotopsies.

1. Halo V1.0 (University of Granada, Spain)

It is a software that displays a dazzle stimulus in the center of a computer screen and then projects smaller peripheral stimuli. It has been used in several studies related to the measurement of visual quality after refractive surgery7,31 to assess the impact of macular pathology32, to evaluate corneal pathology that changes corneal transparency or more recently to assess the impact of alcohol consumption33. The advantage of this method is that it is easy to implement as it is available for download on the investigators page. The main limitations lie in the limited brightness range that can be presented on a computer screen, which does not allow for replicating real situations such as the observation of intense light sources in the dark, or the limitation in the parameters that only quantify the dimension of dysphotopsia, since the evaluations of asymmetry, and irregularity, among others, are of qualitative type.

2. ASTON halometer (ASTON University, UK)34:

It is a tablet application that incorporates a central physical LED as a dazzling source. Unlike the previous one, it includes letters (~ 0.3 LogMar) moving eccentrically instead of LEDs, as peripheral sensing stimuli, allowing quantifying radial brightness in eight meridians around a central LED. The main advantage is the portability of the device, a limitation being that it is performed at close range and includes stimuli that need to be read in order to be detected. It only presents an area value, making a geometric representation of dysphotopsia, but without providing quantitative parameters of its form or regularity.

3. Vision Monitor (Metrovision, France):

It consists of two off-axis light sources (LEDs) on opposite sides, acting as a dazzling source and using low-light letter optotypes that appear from the periphery towards the brightness source, in three radial lines forming 10 concentric rings35.

4. Light Disturbance Analyzer (LDA, CEORLab, Portugal):

It consists of a 5-mm diameter white central LED that acts as a dazzling source, surrounded by a matrix of 240 1-mm diameter white light source LEDs, distributed in 24 semi-meridians with a minimum angular separation of 15 degrees, and a linear separation of 10 mm to cover an angular field of 10° at the examination distance of 2 meters (see Figure 3).

Technical specifications of LED characteristics and examination procedures can be found in previously published papers36-38. In summary, in a dark room, the instrument displays the central dazzle source at maximum intensity, while the peripheral LEDs are displayed sequentially around the central light source using different sequences at random times of 250 to 750 ms and the different semi-meridians are explored in random order. The patient is instructed to always fix on the central LED and provide feedback on peripheral stimuli that can be seen by clicking on a remote actuator and the system automatically evaluates the next semi-meridian.

The luminous distortion index (LDI) is calculated as the proportion of the area of ​​points lost by the subject and the total area explored and is expressed as a percentage (%). Higher distortion values ​​are interpreted as the lowest ability to discriminate surrounding small stimuli that are hidden by distortion induced by the central light source.

APPLICATIONS IN CRYSTALLINE SURGERY OF LIGHT DISTORTION ANALYZER

Studies in a sample undergoing cataract surgery show an increase in LDI, measured with LDA, in patients implanted with either monofocal or multifocal IOLs, and this increase is significantly higher in multifocal IOLs. The binocular addition effect (calculated as the % decrease or increase in light distortion under binocular conditions compared to the mean monocular value: ((Monocular-Binocular) / Binocular) × 100) where positive values indicate a reduction of the disturbance under binocular conditions, while a negative value indicates an increase in disturbance under binocular conditions) was greater in the group implanted with multifocal IOLs (29% reduction) than in the group with monofocal IOLs (14% reduction). Age was identified as a factor that determines the value of the light distortion objectivized as LDA in patients implanted with multifocal IOLs having a moderate correlation of 40%.

In the short and medium term, after surgery, there is a continuous and nonlinear improvement in visual performance with multifocal IOL implantation after cataract extraction or transparent lens extraction. However, this improvement is not seen in common clinical exams, such as with the contrast sensitivity function (CSF). In fact, the blurring curves were stable over time. Other objective parameters, such as LDI, did not change significantly over time (Figure 5), suggesting that dysphotopsies resulting from multifocal IOL implantation remain present in the short and medium term after surgery39.

Despite stable performance over time, symptoms and objective metrics for assessing visual performance in low light conditions show an adaptive effect over time (1 to 6 months) that is more significant for subjective perception of the disturbance caused by the presence of the light disturbance (Figure 6). There is a negative correlation between LDI and CSF in the long term, which means that the greater the luminous disturbance, the lower the contrast sensitivity and vice versa, which can be interpreted as an improvement in patients' visual perception in the medium term, because the light disorders when subjectively assessed by questionnaire, patients report that the frequency and severity of symptoms remain quite stable. However, the uncomfortable subcategory was the one with the greatest reduction over the follow-up period. This suggests that while physical phenomena are still present and quite stable as objectively demonstrated by the LDA instrument, patients are beginning to alleviate their concerns in the medium term, suggesting a perceptual change over time. Dysphotopsies measured with LDA in the latest generation of multifocal IOLs, the so-called “Premium” ones, also show an increase in LDI. Nonetheless, unlike what is observed with trifocal lenses, with EDoF there is no reduction over time, either measured objectively by the LDA or when subjectively assessed by the completion of a questionnaire.

The binocular addition effect is observed for the LDA results, demonstrating that the binocular disorder is reduced compared to the monocular values obtained, especially in the immediate postoperative period, where higher levels are observed, attenuating monocular disturbance throughout of time.

In toric multifocal IOLs, despite their more complex optical design, an increase in LDI is observed. However, the results are similar to those obtained for spherical IOLs.