A problem well stated is a problem half solved.
The term dysphotopsia was propagated through the literature in the year 2000 to indicate any light-related visual phenomenon encountered by phakic and pseudophakic patients.1–3 The symptoms associated with pseudophakic dysphotopsia were further divided into positive and negative types.3,4 Dysphotopsia in association with monofocal intraocular lenses (IOLs) had already been described under other names, such as edge glare, photic phenomena, or undesired light images (terms used to describe the positive type), and has since been the object of numerous laboratory, clinical, and review articles.5–15
Patients generally describe positive dysphotopsia (PD) as unwanted bright images, such as light streaks, light arcs, central flashes, or starbursts within the visual field, which are usually induced by an oblique external light source. Its incidence in the immediate postoperative period was described to be as high as 49%, decreasing to 0.2% to 2.2% over the following 12 months.14 Factors that have been described as contributors to PD include the presence of a square, truncated IOL optic edge, IOL materials with high refractive index, and IOLs with peripheral nonimaging features that reduce the functional optic diameter.14,15 Typical negative dysphotopsia (ND), however, is usually reported by patients as a dark, temporal peripheral arc-shaped shadow or line. When specifically questioned about it, patients report an incidence of ND of 15% to 20% early postoperatively, which is reduced to approximately 3% at 1 year, seemingly due to neuroadaptation.15 Its origin is less understood and frequently described as multifactorial. Proposed etiologies have included design, smoothness, and thickness of the IOL edge, IOL material refractive index, IOL optic size, haptic configuration and orientation, pupil size, amount of functional nasal retina, edema from clear corneal incisions, distance between the iris and the IOL, and interaction between the anterior capsulorhexis and the IOL, among others.14,15 Laboratory analyses using ray-tracing optical modeling have greatly contributed to a better understanding of this phenomenon. The modeling shows that a space is created between the posterior surface of the iris and the anterior surface of the IOL when the IOL is fixated within the capsular bag, a space that is not present in the phakic eye. This provides the opportunity for the formation of a narrow nonilluminated band in the nasal retina, which is delimited on both sides by a brighter region. The anterior border of the illumination gap is composed by light that misses the optic and illuminate the retina in the far periphery and the posterior border by light that is refracted by the optic, representing the limit of the focused image.8,10
In their recent review on pseudophakic dysphotopsia, Masket and Fram appropriately noted that very little has been reported on optic size and PD or ND.15 Davison reported less dysphotopsia with a larger optic in a study describing the clinical performance of the Alcon AcrySof SA30AL (5.5 mm optic) and the SA60AT (6.0 mm optic) IOLs.4 PD occurred in 6 eyes, all with an SA30AL IOL; ND occurred in 20 eyes, 15 with an SA30AL IOL and 5 with an SA60AT IOL. The effect of the IOL optic diameter on dysphotopsia is featured in this issue, with a unique opportunity to have both a clinical and a laboratory study on the subject. The article by Bonsemeyer et al. (page 76) describes a prospective randomized patient-masked comparative clinical study including 120 eyes of 86 patients. Fifty-seven eyes received an IOL with an optic diameter of 7.0 mm and plate-haptics (Aspira-aXA, HumanOptics), and 63 eyes received an IOL with an optic diameter of 6.0 mm and C-loop haptics (Aspira-aA, HumanOptics). Both are single-piece, aspheric IOLs manufactured from the same hydrophilic acrylic material with a refractive index of 1.46. According to the manufacturer, both IOLs also have a 360-degree lens epithelial cell barrier on the posterior surface, which reduces the effective optic diameter to 6.5 mm for the Aspira-aXA IOL and to 5.5 mm for the Aspira-aA IOL. The overall design (plate vs C-loop) represents a second variable in the study, and the size of a single optic–haptic junction shoulder is 4.28 mm and 2.69 mm for Aspira-aXA and Aspira-aA IOLs, respectively.
Regarding PD, there was a statistically significant difference between both groups at 1 month postoperatively, with a lower incidence in the group with the 7.0 mm IOLs (18 [31.6%], vs 33 cases or 52.4%). The number of cases progressively decreased in both groups throughout the clinical follow-up, and the difference was no longer statistically significant at months 3 and 12. There was, however, a 2.4-fold reduction in PD in the group with the 7.0 mm IOL compared with the 6.0 mm IOL. Regarding ND, there was again a statistically significant difference between both groups at 1 month postoperatively, with a lower incidence in the group with the 7.0 mm IOL (3 cases or 5.4%, vs 13 cases or 20.6%). The difference was no longer statistically significant at months 3 and 12, but in the last follow-up, there were no cases with the 7.0 mm IOL and 2 cases with the 6.0 mm IOL.
The study by Bonsemeyer et al. prompted Erie et al. (page 96) to perform optical modeling using ray-tracing software to simulate retinal illumination from an extended light source for pseudophakic eyes with different IOLs to verify their effect on ND. The simulations involved biconvex IOLs with optic diameters of 6.0 mm and 7.0 mm, refractive indices of 1.46 and 1.55, and a 2.5 mm pupil. For the lower refractive index (1.46), the 6.0 mm IOL was modeled with a 5.5 mm optic surrounded by a 0.25 mm rim, and the 7.0 mm IOL was modeled with a 6.0 mm optic surrounded by a 0.5 mm rim. Similar results were found for IOLs with both refractive indices. The 7.0 mm optic expanded the image field. Furthermore, high-angle input light was found to miss a 7.0 mm optic at a larger visual angle than a 6.0 mm optic, shifting illumination of the peripheral retina by this light anteriorly. Consequently, an extended and more peripheral dark nasal region was created. The authors hypothesized that this would make a temporal shadow less bothersome, which may explain lower rates of ND in association with a larger optic diameter.
Of interest, studies suggest that orienting IOLs so that one optic–haptic junction is located inferotemporally, or the junctions are oriented horizontally, may minimize ND as the incident light would be internally reflected, removing the peripheral retina illumination that represents the anterior border of the illumination gap.10–12 The 7.0 mm optic IOL in the study by Bonsemeyer et al. was a plate-haptic IOL with a larger optic–haptic junction than the 6.0 mm IOL. In theory, this may be associated with a greater potential to prevent ND, provided the IOL is oriented accordingly. However, it is possible that the preventative effects observed were solely related to the larger optic diameter as no specific optic–haptic orientation was used in their study, although the authors did not specify how many IOLs ended up being oriented near or at the horizontal meridian.
Although PD currently is a relatively well-understood entity, there is still much to be learned about ND. Researchers and clinicians working on this subject are to be congratulated on their relentless efforts to provide a complete understanding of the root causes of dysphotopsia, especially the negative type. Only then will true eradication of this lingering issue be possible, and it will no longer represent such a source of frustration and dissatisfaction for surgeons and patients after uneventful cataract surgery.
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