Development of a Device to Measure Dark Adaptation
Condition: Macular Degeneration; Age-Related Macular Degeneration
MeSH terms: Macular Degeneration
Study type: Observational

Summary: 
      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.
    

Detailed description: 
      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.
    

Observational model: Case-Control
Time perspective: Cross-Sectional
Sampling method: Non-Probability Sample
Study population: 
        Participants will be recruited from a the greater manchester area by a call for volunteers,
        through the local newspaper, a university email list and through the Hale Barns Macular
        Disease Association
      
Keywords: Retinal Rod Photoreceptor Cells; Dark Adaptation