From Tear Instability to Dry Eye: A Unifying Hypothesis

Abstract

Abstract: : Purpose:. Dry eye affects millions, but its etiology is unclear. Many studies have shown that the signs and symptoms of dry eye correlate poorly, although tear instability is common to almost all forms of dry eye. We have combined data from several of our studies to develop a hypothesis for the origin of dry eye symptoms. Methods: We developed the dry eye questionnaire (DEQ), and models using image analysis to quantify tear film breakup dynamics (TBUD). The forced staring model (FS–TBUT) instructed subjects to hold one eye open for as long as possible while tear break–up was videotaped. Subjects then filled out the McGill Pain questionnaire (MPQ). This was repeated 3x per eye. Before and after testing, subjects filled out questionnaires querying symptoms (CSQ). The tasks model (T–TBUD) directed subjects to play computer games while tear breakup and blinks were videotaped. Both used image analysis to generate maps quantifying the total area of breakup over the corneal surface by the intensity of tear film fluorescence in real time. Results: Over 280 subjects participated in these studies. In DEQ studies, many dry eye subjects' symptoms increased in intensity in the evening. FS–TBUD studies showed that (1) acute sensations of burning and stinging occurred during tear breakup, (2) dry eye subjects had more central breakup and a faster growth rate of areas of breakup, and (3) repeated tear breakup caused subsequent dry eye–like symptoms among both dry eye subjects and controls in the absence of surface staining. In T–TBUD studies, many dry eye subjects showed central tear breakup between blinks while engaged in visual tasks. Conclusions: DEQ studies suggest that the open condition causes dry eye symptoms to worsen over the day. Data from FS– and T–TBUD models suggest that dry eye subjects chronically suffer from poor wetting of the cornea due to tear instability, and that this causes symptoms even in the absence of surface staining. Because the sensation of burning often occurred during tear breakup, we hypothesize that local areas of tear breakup may undergo a transient hyperosmolar spike. This may chronically stress the ocular surface over the course of the day in dry eye patients, leading to worsening inflammation and symptoms.

Keywords: cornea: tears/tear film/dry eye • cornea: epithelium • inflammation