Negative dysphotopsia (ND) is an unwanted shadow in the temporal field after cataract surgery. It is reported in approximately 12% of patients at 1 month postoperatively, decreasing to 3% at 1 year postoperatively.1,2 Importantly, ND is a major dissatisfier for patients after cataract surgery.3,4
In pseudophakia, the intraocular lens (IOL) is both smaller in diameter and thinner than the crystalline lens. Calculations show that when light enters the eye at very large angles (input visual angles 80 degrees or higher), it can no longer pass through the lens because of the reduced diameter and that this creates a limit to the focused image.5,6 When an IOL is placed in the bag, a space is created between the posterior iris and the anterior IOL surface that is not present in the phakic eye, and ray tracing shows that this space provides an opportunity for light to miss the IOL and illuminate the retina in the far periphery. These characteristics can create a narrow band of nonilluminated retina that is bounded on both sides by a brighter region: anteriorly by light that misses the IOL optic and posteriorly by the limit of the focused image.5–9 This distinct narrow shadow in the nasal retina is hypothesized by some to correspond to the temporal shadow described in ND (Figure 1).
It is believed that the perception of the ND shadow may be mitigated if the dark region of nasal retina can be illuminated by redirected light or if the illumination of the retina by light missing the IOL is shifted anteriorly off the retina or both.5–7,9 Giving credence to this theory is clinical evidence that ND is reduced by orienting the optic–haptic junction horizontally, shifting the IOL optic anteriorly, or by using a piggyback IOL; each is supported by ray-tracing modeling showing that these modifications alter illumination patterns in the far peripheral retina.1,9–14
A recent clinical study showed that enlarging an IOL hydrophilic optic from 6.0 to 7.0 mm significantly reduced ND symptoms at 1-month after cataract surgery.15 The purpose of this laboratory study was to use ray-tracing analysis and simulated retina illumination profiles to compare the influence of the 6.0 mm and 7.0 mm IOL optic diameters on illumination of the far peripheral retina.
Zemax OpticStudio optical design software (Zemax) was used to generate ray-tracing data and simulated retinal illumination profiles to evaluate illumination of the peripheral retina over a range of angles for an average eye.5,6 The eye model included a 6.0 or 7.0 mm diameter biconvex IOL (21.0 diopters [D]) with refractive index values of 1.46 and 1.55. For the low refractive index IOL (n = 1.46), the 6.0 mm diameter IOL was modeled with a 5.5 mm diameter optic surrounded by a 0.25 mm wide rim, and the 7.0 mm diameter IOL was modeled with a 6.0 mm diameter optic surrounded by a 0.5 mm wide rim. The thinner high refractive index IOLs (n = 1.55) were modeled with optical surfaces extending to the full optic diameter.
The IOLs were centered on the optical axis. The center of the haptic was used to position the IOLs in the axial direction, and then, IOL powers were adjusted to make the eye emmetropic.16 This resulted in the space between the posterior iris and the anterior IOL surface being 0.5 mm and 0.44 mm for the 6.0 mm and 7.0 mm high index IOLs and 0.44 mm and 0.3 mm for the low index IOLs, respectively. These designs have the general characteristics that are used for IOLs, whereas not representing any particular commercially available IOL.
Ray tracing and clinical reports demonstrate that the ND shadow is most prominent with smaller pupils and fades as the pupil dilates. As such, these calculations included a 2.5 mm diameter pupil (or a 3.0 mm apparent pupil) that was decentered nasally by 0.17 mm to create a nominal 2.5 degrees angle κ. Angles were evaluated relative to the optical axis, and then, 5 degrees was added to the input angles to account for the mean foveola location. Rays that hit the outer IOL edge were ignored, which effectively simulates how textured or round IOL edges widely scatter light over a large region.5,6 The IOL was modeled as free standing in aqueous humor.
This ray-tracing technique has been described previously.6 In brief, special optical surfaces were created to allow simultaneous analysis of rays passing through the IOL and rays missing the IOL. A Zemax macro was used to record the coordinates of ray intersections with the retina from object points at 6.0 m distance. A separate polarization routine that evaluated Fresnel reflections was used to calculate the fraction of refracted light rays that reached the retina, after reflections were removed. Calculations were adjusted for pupil obliquity. Simulated images were scaled to fill the intensity range and were adjusted using a gamma correction to enhance the visibility of lower intensity levels.
For evaluations, all angular locations were calculated relative to the second nodal point (NP2) of the eye, which is 7.0 mm from the anterior cornea.5–7,17 Recent work by Simpson confirms that angles subtended at the second nodal point are very similar to input visual angles over a very large angular range, with the retinal curvature adjusting ray intersections to compensate for increasing angles.17 There is modest nonlinearity in the angular scaling at very large angles, but the calculations in this study give values that can be directly compared because the same eye is used for each IOL.
Retinal ray intersections were imported into MATLAB for analysis and display. The boundary angles of the shadow region were determined from simulated intensity profiles as this allowed more accurate boundary determination in the presence of imaging aberrations.
These calculations showed that a low refractive index 7.0 mm optic (n = 1.46) increased the focused image field by approximately 3 degrees when compared with a 6.0 mm optic (Figure 2). The width of the shadow also increased from 7 to 10 degrees, and this moved the outer edge of the shadow more peripherally by more than 5 degrees. Consequently, the distinct narrow shadow (relative visual angle, 83.5 to 90.7 degrees) seen with a 6.0 mm optic is enlarged and shifted peripherally (relative visual angle, 86.3to 96.3 degrees) with a larger 7.0 mm optic (Figure 2). Calculations for the thinner, high refractive index optic (n = 1.55) show similar changes, but with some differences (Figure 3). The image size increased by about 1 degree, and the shadow width increased from 5 to 10 degrees, with the outer edge of the shadow moving peripherally approximately 6 degrees
Light at very large visual angles illuminates the far peripheral retina differently in phakia compared with that of in-the-bag pseudophakia.5–7,9 In the phakic eye, the peripheral retina is continuously illuminated to the extent that light is focused by the crystalline lens. By contrast, for in-the-bag pseudophakia, a narrow dark region can be present in the far peripheral nasal retina with a small pupil, and this is delineated posteriorly by the limit of the focused image and anteriorly by unfocused light that misses the optic (Figure 1). This nonphysiologic shadow within an illuminated region of peripheral retina may be responsible for the bothersome temporal shadow described in ND.
Recently, Bonsemeyer et al. found that using a 7.0 mm hydrophilic optic (refractive index 1.46) with a plate-haptic design effectively reduced clinical ND by 4-fold at 1 month after cataract surgery when compared with a 6.0 mm optic (refractive index 1.46) with a C haptic design (5.1% vs 20.6%, respectively).15 Between the 2 groups, there was no statistically significant difference in pupil size in scotopic and mesopic conditions. The authors hypothesized that the lower ND rates when using the larger optic diameter were due to an expanded image field, and our modeling showed that when using a larger 7.0 mm optic, the focused portion of the visual field was larger, by 2.8 degrees for this example. Expansion of the image field is limited because light rays that reach the peripheral 7.0 mm optic are traveling at a very large angle to the lens and are moving mostly across the optic, rather than into it.5
A larger effect is seen with the light that misses the IOL, which is shifted anteriorly to a more peripheral retinal region by 5.6 degrees (from a relative visual angle of 90.7 to 96.3 degrees; Figure 2). This makes the shadow no longer a distinct narrow band but converts it to a broader and more peripheral shadow. This broader and peripherally shifted shadow with a 7.0 mm optic may make ND less noticeable and may contribute to lower ND rates seen clinically with a larger optic. In addition, unfocused light that misses the 7.0 mm optic may fall outside the nasal retina, thus possibly eliminating this nonphysiologic light altogether. A similar but smaller change in peripheral retinal illumination is also seen when modeling an IOL with a high refractive index optic (refractive index 1.55). The routine use of a larger 7.0 mm optic, however, may create other surgical challenges such as the need for larger incisions and associated surgically induced astigmatism.
Consistent with this hypothesis are observed lower rates of ND when using sulcus-based IOLs, piggyback IOLs, or IOL optics repositioned anterior to the anterior capsule.11–13 Light rays are less likely to miss the IOL optic and illuminate the far peripheral retina when the space between the posterior iris and the anterior IOL is eliminated or reduced. The gap between the iris and IOL is eliminated when using a sulcus-based IOL or a piggyback IOL, and the space is reduced when using an IOL optic repositioned out of the bag and placed anterior to the anterior capsule. Ray modeling shows that the narrow shadow within the field of illuminated nasal retina is flooded with light and eliminated when using a sulcus-based or piggyback IOL, and the shadow is extended peripherally when the optic is anteriorized by repositioning it anterior to the anterior capsule.13,14
Recently, clinical studies suggest that surgeons can continue to use an in-the-bag 6.0 mm IOL and still reduce ND symptoms by up to 50% at 1 month postoperatively if the optic–haptic junction is oriented horizontally.1,10 Previous ray-tracing studies suggested that at the optic–haptic junction, light that would normally miss a 6.0 mm optic and illuminate far peripheral retina is, instead, internally reflected when it reaches the extended optic–haptic junction.9 Consequently, this light no longer reaches or illuminates peripheral retina. As a result, the nonphysiologic narrow shadow produced at the 6.0 mm optic edge is eliminated and replaced with a continuous nonilluminated region beyond a visual angle of 97 degrees and likely off the retina (Figure 3).
Why then would a 7.0 mm optic be more effective clinically in reducing ND symptoms than a horizontally oriented optic–haptic junction using a 6.0 mm optic when ray tracing suggests differently (Figures 2 and 3)? Most likely, the lesser clinical effect of a horizontal optic–haptic junction is because of the small size of the optic–haptic junction, which covers approximately 30 degrees or 1 o'clock hour of the optic. The small optic–haptic junction may not be large enough to internally reflect all input light from the temporal field. Otherwise, the junction may not be aligned at its most effective location, which is predicted by ray tracing to be 1 o'clock hour superior to horizontal when placed nasally.9 By contrast, the clinical benefit of using a larger 7.0 mm optic is always present, no matter where the optic–haptic junction is orientated.
Our study is limited in that this modeling only considers a specific average eye, and there are large variations in eye parameters across the population that could influence expected outcomes. Second, we did not model different optic–haptic junction designs that may also influence peripheral retinal illumination. Third, ray-tracing studies show that the ND shadow is most prominent in smaller pupils and gets fainter when the pupil enlarges as the peripheral retina is flooded with light that misses the IOL.5,6 For that reason, we chose to model an average photopic 2.5 mm pupil (3.0 mm apparent pupil), but we did not model other pupil sizes, nor did our modeling account for possible rapid changes in pupil diameter or motion of the eye, which could influence the awareness of the ND shadow. Finally, our modeling does not take into account the many other proposed potential causes of ND, such as light passing through the edge of the IOL, neuroadaptive changes, and effects from the capsulorhexis.11,18
In summary, ray modeling shows that a 7.0 mm optic does expand the extent of the focused image. Perhaps, more significant is that the distinct narrow shadow in the nasal retina seen with a 6.0 mm optic is changed to a broader, more peripheral dark region when the optic diameter is increased to 7.0 mm. This is because the larger diameter blocks or scatters some of the light that would otherwise miss the IOL, and this moves the shadow anteriorly onto more peripheral retina or possibly off the retina. The peripheral retinal illumination changes were more prominent with the low refractive index design modeled in this study. An expanded field of nonilluminated nasal retina combined with shifting nonphysiologic retinal illumination anteriorly may be less bothersome or may allow easier neuroadaptation, thus explaining why ND rates are lower when using a 7.0 mm diameter optic.
WHAT WAS KNOWN
- In a pseudophakic eye with a 6.0 mm optic, a narrow shadow-like region is formed in the far nasal periphery, bounded anteriorly by light missing the IOL and posteriorly by light refracted by the IOL.
- The narrow shadow in the nasal retina may correspond to the temporal shadow seen in negative dysphotopsia.
WHAT THIS PAPER ADDS
- Simulated images confirm that light focused by the IOL and nonphysiologic light that bypasses the IOL are shifted peripherally with a larger IOL diameter, and this illumination change is more pronounced with a low vs high refractive index optic.
- The narrow retinal shadow using a 6.0 mm optic is converted to a broader, more peripheral shadow when using a 7.0 mm optic and may explain lower negative dysphotopsia rates associated with a larger 7.0 mm optic.
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