Discussion
In this study, the vision and flight simulator performance of a group of older adults with MIOLs was compared with that of a group of age-matched adults with monofocal IOLs. This comparative study was undertaken to explore concerns that MIOLs have negative effects on functional vision, and specifically, claims by CASA that multifocal contact lenses and IOLs ‘cause unacceptable impairment of vision’, which precludes individuals with these visual corrections from holding a pilot’s license.6 Performance was assessed both under daytime and night-time flying conditions, given that night-time flying may be more problematic in those with multifocal lenses given the larger pupils associated with low light levels.
The findings demonstrated that the visual function of the two groups did not differ significantly in terms of their visual acuity or contrast sensitivity, either under photopic or mesopic light conditions, which is in accordance with previous studies.3 However, mesopic contrast sensitivity when measured with the Mesotest, both with and without glare, was significantly worse in the MIOL relative to the monoIOL group, which has not been previously reported. This may be due to the very low luminance levels used in the Mesotest test (0.032 cd/m2) in our study. The MIOL group also had significantly larger halo areas than the monofocal group, which is in accordance with previous reports of self-reported glare sensitivity,11 as well as increased halo size12; these differences may reflect the problems reported by MIOL patients with night-time driving.2
The literature review done in preparation for this study failed to find any agreed on or scientifically agreed on definition of the terms halo or glare. The tests used, halometry and Mesotest, are considered irregular tests used to try and model the visual experiences of patients after cataract surgery. However, there is no scientifically validated measure or instrument available to measure these phenomena.
What has been observed when testing the MIOL participants is that they describe rings around the test LED.
It is speculated that these rings may in fact be Fraunhoffer diffractive rings associated with a circular aperture of the pupil and the diffractive rings of the IOL. This observation may open up a more productive area of future research to explore as a more scientifically valid explanation of the visual experiences of patient with these IOLs, compared with the more qualitative terms halo and glare.
The defocus curves demonstrated that MIOL subjects had significantly better vision than monoIOL subjects for myopic blur, which is in accordance with previous studies.3 13–17
Significant differences were found—as expected—in the need for near correction with 100% spectacle freedom for near vision in the MIOL group and two-thirds of the monoIOL group needing spectacles for near-vision tasks.
In terms of vision-related flight simulator tasks, there was no significant difference in performance between MIOL and monoIOL subjects for correct recognition of any of the console-based values, including heading, altitude, speed and fuel flow values, accurately reporting from the airport approach chart or reading off the radio frequency values. Importantly, both IOL groups were able to perform these tasks at relatively high levels of accuracy (many subjects achieving 100% accuracy), regardless of whether the flight was made in clear daytime or night-time conditions. These findings are in accordance with those reported for the defocus curves, demonstrating relatively high levels of visual acuity at near and intermediate distances for the MIOL group, with worse results for the monoIOL group. Importantly, the monofocal subjects were able to use near corrections (half-eye reading, bifocals or multifocal spectacles) as required for the flight simulator tasks as indicated in the CASA guidelines.10 Even for the lower light levels of the simulated night-time flights, performance was not significantly impaired for either group and no performance differences between groups were evident. It should, however, be noted that habitual rather than best-corrected spectacles were used for the vision and flight simulator assessment, which may not reflect the situation for a pilot undergoing medical license assessment.
There was no significant difference in accuracy of colour recognition of runway lights between both groups regardless of light conditions.
This study demonstrated that the ability to complete vision-related flight simulator tasks was not significantly different for the MIOL subjects compared with monofocal IOL subjects. Clearly, comparison of flight simulator performance in a group of pilots with MIOLs to that of pilots with monofocal IOLs would permit inclusion of a wider range of flying tasks, rather than those limited to non-pilots who acted as co-pilots in the simulated flight. However, given that licensed pilots with MIOLs are not permitted to fly, this research was not possible. Nevertheless, we believe that the tasks selected were visually demanding and representative of normal flying conditions as represented in a simulator.
Whereas Australian pilots are not permitted to fly after implantation of MIOLs, the US Federal Aviation Agency (FAA) allows MIOLs in pilots.18 The IOL model must be approved for use in the USA and the implantation has to be done at least 3 months prior to the FAA physical examination. In the UK, according to the Civil Aviation Authority, multifocal and bifocal implants are not compatible with pilot certification. Accommodating lenses may be acceptable following a review with a consultant aviation ophthalmology adviser.19
Performance-based testing has been helpful in changing aviation policy over concerns about the quality of night vision for other forms of eye surgery, in particular LASIK.20 Previously, LASIK was not authorised for aviators in the US Navy and US Air Force because of concern about the postoperative quality of vision. The results of the study were pivotal for the decision to permit LASIK in aviators and astronauts. The US military has meanwhile accepted laser vision correction as a way to improve performance.20
In conclusion, this is the first study to explore the flight simulator performance of individuals with MIOLs. The data support previous studies in terms of visual performance with MIOLs. Our findings suggest that the performance of vision-related flight simulator tasks was not significantly impaired in subjects with MIOLs relative to that of age-matched monofocal IOL subjects, who under current licensing arrangements are legally eligible to hold a pilot’s license.