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Assessment of Visual Performance: Comparison of Normal Subjects and Post-Refractive Surgery Patients

Chisholm, C. (2003). Assessment of Visual Performance: Comparison of Normal Subjects and Post-Refractive Surgery Patients. (Unpublished Doctoral thesis, City, University of London)


This work forms part of a study to assess the implications of excimer laser surgery for commercial aviation in light of reports of reduced visual performance and glare problems in some individuals following surgery. The literature indicates that comeal refractive surgery can increase both forward light scatter and optical aberrations, leading to a reduction in retinal image contrast. The project comprised a retrospective study of subjects that had undergone either photorefractive keratectomy (PRK) or laser assisted in situ keratomileusis (LASIK). Control subjects were also examined for comparison purposes since insufficient laser patients were available for a longitudinal study. The experimental work involved the measurement of forward light scatter using the City University Light Scatter Program and the development and utilisation of four novel tests of visual performance. An additional aim of the project was to gather information from the tests and subsequently produce a suitable technique for assessing visual performance following comeal refractive surgery that was sensitive to retinal image degradation.

Take-off, approach and landing are the most visually demanding tasks undertaken by the pilot during a flight. All three tasks require rapid scanning of a range of instrumentation panels to assimilate the relevant visual information, along with a degree of awareness of the external environment.

The novel tests of visual performance included a visual search task, selected as the principal parameter for assessment since visual search is used extensively on the flight deck for assembling information from the display systems and the outside world. The mean glimpse duration was also measured during the search task. The variability of the visual search data led to the development of a model of visual search to provide an insight into the principal factors that determine the outcome of visual search, and two further tests were developed to provide input data for the search model. These tests involved the measurement of contrast detection and orientation discrimination (i.e. a spatial acuity task) thresholds respectively. These data provided useful and relevant information on the subject’s visual performance in addition to providing the data needed for the visual search model.

Fifty-two LASIK, 32 PRK and 53 control subjects were assessed during the study. Despite the long mean follow-up time of the PRK group (approximately three years), a statistically significant increase in both contrast acuity thresholds and visual search times was revealed, suggesting that the reduction in performance is permanent. This was associated with a statistically significant increase in the angular spread of straylight on the retina. The increase in total scatter as indicated by the integrated straylight parameter (k’), did not however reach statistical significance. Since surgically induced aberrations are thought to be of a similar magnitude following PRK and LASIK, these findings indicate the important role the distribution of straylight may have in determining retinal image contrast and hence visual performance.

The LASIK group showed a small but not statistically significant increase in mean contrast acuity thresholds and mean visual search times compared to the control group. This is consistent with the findings for forward light scatter - a small, but not statistically significant increase in the scatter index (n) and the integrated straylight parameter (k’). These findings suggest that changes in scattered light as a result of LASIK do not significantly affect visual performance.

The large stimulus employed by the test meant that absolute contrast detection thresholds were relatively insensitive to retinal image degradation. Both the visual search task and contrast acuity thresholds revealed a reduction in visual performance post-PRK, but the contrast acuity threshold test appeared to be the most sensitive, identifying all symptomatic subjects as outliers and detecting the improvement in performance with follow-up time. Based on these findings we developed a Contrast Acuity Assessment (CAA) test. The parameters selected for the test were based on the results of a detailed analysis of visual tasks in the cockpit. The ‘standard observer’ and normal ±2a range were established, based on the assessment of 100 control subjects. The usefulness of the test is to identify those subjects with thresholds above the upper 2a limit, as needing further investigation before the granting of a medical certificate. Preliminary data were obtained from a small group of refractive surgery subjects, many of whom produced data that clustered around the ‘standard observer’. Others produced thresholds that fell outside the normal 2j range, often showing a characteristic A-pattern’ implicating increased forward scattered light and/or aberrations. All symptomatic subjects showed contrast acuity thresholds outside the 2a range.

The Contrast Acuity Assessment (CAA) test provides a new measure of functional visual performance both for daytime and low levels of ambient illumination. Individuals can be assessed and compared to the normal range, using parameters relevant to the instrumentation found on the flight deck of a commercial aircraft. The test is sensitive to changes in retinal image contrast associated with increases in forward light scatter and/or aberrations as a result of excimer laser surgery and therefore can be used to identify those whose visual performance is inadequate for commercial aviation.

Publication Type: Thesis (Doctoral)
Subjects: R Medicine
Departments: School of Health & Psychological Sciences > Optometry & Visual Sciences
School of Health & Psychological Sciences > School of Health & Psychological Sciences Doctoral Theses
Doctoral Theses
[thumbnail of Chisholm thesis 2003 PDF-A.pdf]
Text - Accepted Version
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