A novel device for assessing dark adaptation in field settings

Abstract

Background: Aberrant dark adaptation is common to many ocular diseases and pathophysiological conditions, including vitamin A deficiency, cardiopulmonary diseases, and hypoxia. Scotopic vision and pupillary responsiveness have typically been measured using subjective, time-consuming methods. Existing techniques are particularly challenging for use in developing country settings, where vitamin A deficiency remains a major public health problem. Our aim was design a compact, low cost, and easily operated device to assess dark adaptation in the field.

Methods: The Portable Field Dark Adaptometer (PFDA) incorporates a digital camera, a retinal bleaching flash, and a Ganzfeld light source inside a pair of light-obscuring goggles. After a ~10 min period of dark adaption, the infrared camera digitally records afferent pupillary responses to graded light stimuli (-2.9 to 0.1 log cd/m(2)). We tested this device in a variety of field settings to assess: a) ease of use and b) whether test data could clearly and accurately depict the well-known dose-response relationship between light intensity and pupil contraction. A total of 822 videos were collected. We used an open source video analysis software to measure pupil size in pixel units. Pupillary responsiveness was expressed as the percent change in pupil size from pre- to post-light exposure. Box plots, t test, and multi-level mixed effects linear regression modeling were used to characterize the relationship between light intensity and pupillary response.

Results: The PFDA was employed with only minor technical challenges in Bangladesh, Kenya, Zambia, and Peru. Our data show a clear linear increase in pupillary constriction with increasing log light intensity. Light intensity was a strong predictor of pupillary response, regardless of baseline pupil size.

Conclusions: The consistent physiological response demonstrated here supports the use of the PFDA as a reliable tool to measure dark adaptation. As a next step, PFDA measurements will be validated against biochemical indicators of vitamin A status and hypoxemia. Ultimately, this new technology may provide a novel approach for nutritional assessment, with potential clinical applications.

Fig. 1

Schematic diagram of the Portable Field Dark Adaptometer (PFDA) developed to assess impaired pupillary responses to a graded series of Ganzfeld light stimuli applied within a pair of “dark-room” goggles (a,b) with an embedded microcircuit design (c) and regulated by a laptop-powered controller box (d)

Fig. 2

Infrared imaging allows the visualization of an eye in complete “dark-room” conditions while also enhancing pre (a) and post-stimulus (b) pupillary measurement due to the clear differentiation of the pupil-iris boundary, irrespective of iris color

Fig. 3

Comparison of the angular profile in light intensity between the VA-20 Ganzfeld source in the commercial SST-1 device, the mock-up of the dual diffuser stack in the Portable Field Dark Adaptometer, and the ideal Lambertian reflectance across an angular illumination range of -70 to +70°

Fig. 4

Trend in pupillary response across nine log-incremental steps of light intensity (left to right) in Bangladesh (a), Kenya (b), Zambia (c), and Peru (d). Pupillary response is defined as the percent change in pupil diameter from pre- to post-stimulus