By Daphne Chiew Le Min, School of Medicine, Imperial College London
Dr Alex Black, Senior Lecturer at the School of Optometry and Vision Science, Queensland University of Technology, Australia, gave a lecture regarding “Low Luminance Vision Testing in Practice and Impact on Everyday Function” at the Singapore Primary Eye Care Symposium (SPECS) 2019 held from 23-24 July 2019 at One Farrer Hotel. He focused on the impact of poor vision in low light conditions, and the importance of low luminance testing in the diagnosis and management of patients with these conditions.
Under low lighting, visual function changes due to fundamental changes in pupil diameter as well as in the dynamics of rod and cone function. The different dynamics under varying light intensities allows us to divide vision into three large categories, namely scotopic (rod dominated, no colour vision, poor acuity), photopic (cone dominated, vibrant colour, high acuity) and mesopic (transitional zone within which both rods and cones are utilised).
Both luminance and illuminance can be used as means of measuring the amount of light reflecting into the subject’s eye, and allow the assessment of rod and cone function. Many studies have shown that low light vision (mesopic or scotopic) is associated with a reduction in visual acuity. However, most subjects are not typically affected by low-light conditions as the reduction in acuity is not significant if given sufficient time to adapt. However, for patients with certain visual defects, this is much more difficult, and as such it is important to determine how often those subjects use their low-light vision.
Under the current study, subjects spent a significant amount of time (29-36% of their day) spent in low light conditions. It was also shown that age is correlated to reduction in low-light contrast sensitivity – in those patients, low-light conditions were associated with severe reduction in night driving ability, which could contribute to social isolation and reduced independence. These negative impacts are exacerbated by the fact that elderly patients rely much more strongly on their visual system to maintain balance and stability, hence decreasing their overall confidence and mobility in low-light conditions.
Dr Black also shared results from a study depicting an overall reduction in best read value of the logMAR graph line when comparing photopic vs mesopic vision, and a significantly greater decrease (reduction of four lines as opposed to two) in the group of subjects with age-related macular degeneration (AMD) as compared to controls. Mesopic Pelli-Robson contrast sensitivity was also determined to have a stable decline as age increased, although it also declined more steeply in adults with AMD as opposed to those with normal vision. He suggested that mesopic visual acuity should ideally be tested at a level of 0.75cd/m2.
Finally, Dr Black mentioned the various challenges of low luminance testing, namely the difficulty in determining an appropriate level of light to reach mesopic range in clinical practice, the inability to reflect real-life situations and light fluxes accurately, as well as an absence of mesopic peripheral sensitivity.