Evaluating Severity of Microbial Keratitis Using Optical Coherence Tomography

Abstract

Microbial keratitis (MK) is an infection of the cornea caused a variety of pathogens that can lead to significant visual loss. The current diagnosis relies on clinical examination and microbiological testing. New in-vivo imaging techniques are now available that can assist the diagnosis and follow the clinical course of this disease. Using optical coherence tomography (OCT) it is possible to obtain high resolution imaging for the cornea. The purpose of this thesis is to demonstrate a link between the latest OCT technology and clinical outcomes by using a severity index scoring tool. A prospective study was undertaken including 68 eyes of suspected infective keratitis from 2016 to 2018. Corneal structures were evaluated by analysis of OCT images of patients with MK on the day of presentation and after 6 days. The effect of the infection on the corneal structure was characterized by observing key features. This included identifying and assessing corneal thickness, corneal infiltrates, corneal ulceration, and epithelial defects on the cornea. These characteristics were used to generate a severity score based on a point system. The hypothesis of this investigation was that a higher severity score would correlate with the requirement for surgical intervention. Features of microbial keratitis were identified on OCT analysis in 68 (100%) of cases on day zero, and 48 (71%) on day six. The most frequent findings were epithelial defect (100%) and stromal oedema noted as corneal thickening. There was corneal thickness changes in 57 (84%) cases on day zero, with 25 (37%) of these having a change between 5-10%, 18 (32%) having a change between 10-30%, 9 (16%) having a change 30-50%, and 5 (9%) having a change >50%. On day zero, there was corneal thickness changes in 89% of Pseudomonas aeruginosa, 26 % of Staphylococcus spp and 100% of Streptococcus pnumoniae (n=3) cases. There were 19 patients that required surgery. This included tarsorraphy or corneal gluing (12), deep anterior lamellar keratoplasty (DALK) (6), vitrectomy (1). The average score of the surgical patients was 19. The patients who required surgery had a significantly higher score than those that reached resolution without intervention (p= 0.042). There was no statistical association between a single feature on AS-OCT and a surgical outcome. There was a significant correlation between patients whose scores on day six were the same or higher than day zero and the requirement of surgery (p= 0.003). Patients with improvement in severity score from day zero to day six were more likely to not require surgery (p=0.027). This study demonstrated that characteristics evident on AS-OCT analysis correlated with clinical outcome and the need for surgical intervention. With a link established between the latest OCT technology and clinical course, further refinement of evaluation tools and image analysis severity characterization can occur.