Development of a Device to Measure Dark Adaptation

Age Related Macular Disease (AMD) is easily the leading cause of blindness in older people in developed countries. It affects between 30 and 50 million individuals worldwide, with around 30% of the over 65's showing early signs of the disease. Severe AMD has a devastating impact on the quality of life; it causes extensive visual impairment, making reading difficult and driving impossible. Patients lose their independence and become a major burden on public health systems.

Present treatment options are limited. Many new therapies are under development and all will need evaluation using a test with high specificity and sensitivity for early AMD. The present application will develop such an instrument. The prototype was funded by a previous i4i FS (feasibility study ll-FS-0110-14036). The new device measures sensitivity to a dim flickering light using the same principle as an established european conformity marked (CE marked) instrument. The original method involved lights of different wavelengths and higher intensities.

The instrument in this study assesses night vision, which is selectively damaged in early stage AMD. In low lighting, the investigators vision depends on specialized rod photoreceptors. Cone photoreceptors, which provide daytime vision, remain normal in the early stages of the disease. By the time patients complain of reduced (cone-based) visual acuity, they will have had the disease for many years and lost many thousands of photoreceptors.

In a clinical environment dark adaptation (DA) is measured by exposing the patient to a brief bright light and then measuring their sensitivity to a flickering light. This sensitivity recovery has a typical appearance; an early rapid recovery mediated by the colour vision system (cone photoreceptors) and a slower recovery of the low light system (rod photoreceptors).

If the measurements are made against a black background then a bi exponential model is suitable, however if a dim luminous background is used then the recovery is better modelled by a exponential bi linear model. We will use the exponential bilinear model, which has seven parameters. The parameters are as follows; the exponential cone phase is modelled by the cone threshold, cone coefficient, and cone time constant. The S2 rod phase is modelled by its commencement time. This is the time when the rod system becomes more sensitive than the cone system, sometimes called the cone-rod break point (alpha) and the slope of the subsequent thresholds. The late rod phase (S3) is similarly modelled by a transition time or rod-rod breakpoint (beta) and its slope.

If the initial bright light is sufficiently intense then the recovery of the rod phase (S2) is termed rate limited. There is evidence that this recovery rate is a useful measure of the health of the light sensing layer of the eye an adjoining tissues, the retina, retinal pigment epithelium and choroid. It is these tissues that are thought to be affected in age related macular disease.

In this study we will measure rate of rod recovery sometimes called the S2 phase. We want to see whether this measurement can be used to identify patients with and without early macular disease.