Pupil optics with multifocal intraocular lenses

In their study of the pupil diameter of Japanese individuals as a function of age,1 Nakamura et al. measured 140 eyes of 70 subjects 19 to 80 years of age using an infrared open-view pupillometer (ie, a device that does not obstruct the eye not being measured) at distance and near fixation under 5 lux and 500 lux ambient illumination. They found the mean pupil diameter as a function of age to be consistently smaller than values reported in the literature, and they speculate that this may be due to their unique subject cohort. There are simpler explanations. First, the low-light pupil diameter measurement was at 5 lux, whereas most investigators attempting to simulate night-driving conditions use 1 lux or less.2,3 At 5 lux, there is enough foveal cone activity that small high-contrast print such as that found in a medical journal can be read fairly easily (Jay Bradley, MD, personal communication, May 2009). Second, the measurement at 500 lux was performed first and the authors do not specify a standardized dark adaptation time before the 5 lux test. This is a critical experimental detail that must be carefully controlled for all subjects.4 Insufficient photoreceptor dark adaptation causes an erroneously small pupil diameter measurement. The investigators also make the confusing statement that “the pupil size measured by most instruments decreased 14% due to the refraction through the cornea.” Infrared pupillometers “see” the entrance pupil (EP), which is a magnified virtual image of the physical pupil (PP); magnification is about 14% across a wide range of central corneal curvature5 [PP = EP(1 – AK/1.3375), where A is anterior chamber depth in meters and K is central corneal power in diopters]. The EP is the anterior optical aperture stop of the eye, although it is not a physical aperture; all light directed by the cornea through the EP will traverse the PP. The optical circumstance of multifocal intraocular lenses (IOLs) is curious. If the EP is 4.0 mm, the PP is about 3.5 mm. For multifocal IOLs that supply distance vision through the IOL center and near vision through an annulus around the center, the PP under reading conditions must be larger than the annulus to provide good near focus. Surgeons should take into account the effect of corneal magnification when making a decision about multifocal IOL optics; that is, they must calculate the PP from the EP so they are comparing “physical to physical,” not “virtual to physical.” To address the issue, Nakamura et al. divided the EP data by 1.14 for analysis and found that above 59 years of age, less than 50% of their subjects had a PP of more than 2.1 mm under reading conditions (bright light, near fixation). Although the experimental data are likely inaccurate for the reasons I specify, this point is well taken and deserves clarification.