8 Corneal Inflammation and Optical Coherence Tomography
8.1 Normal Corneal Texture
The normal cornea is seen as a homogeneous structure by optical coherence tomography (OCT). The usual corneal layers seen in conventional time-domain (TD) OCT are the epithelium, the stroma, and the endothelial layer (Fig. 8.1). The epithelial layer is seen highly reflective as a result of the difference in the refractive indices of air and the cornea with overlying tear film. Stroma is seen below the epithelium as a homogeneous structure. The Bowman layer is not routinely seen in conventional TD OCT, although it may be seen in patients with corneal pathologies like ectasia or keratoconus in spectral or Fourier-domain (FD) OCT. The Descemet membrane is seen as highly reflective layer with the underlying endothelium in the posterior cornea. The high reflection is again due to the difference in the refractive indices of aqueous and endothelium.
8.2 Pathology in Corneal Inflammation
Keratitis may be produced by infectious organisms or by noninfectious stimuli. Microbial keratitis is a common, potentially sight-threatening ocular infection that may be caused by bacteria, fungi, virus, or parasites. 1 It is often assessed by the status of epithelium, type of stromal inflammation, and site of stromal inflammation. The intrinsic virulence of an organism relates to its ability to invade tissue, resist host defense mechanisms, and produce tissue damage. Bacterial invasion is facilitated by proteinases that degrade the basement membrane and extracellular matrix and cause cell lysis. 2 The proteases in keratitis cause degradation of the basement membrane, laminin, extracellular matrix, proteoglycan, and collagen. The infective organism may enter the cornea after trauma or surgery (refractive surgery) easily owing to the presence of a break in the epithelium.
8.3 Role of OCT in Keratitis
Anterior-segment OCT provides a range of qualitative and quantitative information for the assessment of microbial keratitis; serial standardized examination allows objective assessment of microbial keratitis and monitoring of the disease course. 3 , 4 , 5 , 6 Depending on the clinical stage, the parameters in the OCT scan change during the course of the disease. Serial scans can be carried out through the same area of the cornea by adhering to a scanning protocol. The common features noted in OCT are stromal thickness, epithelial integrity, infiltration dimensions, endothelial edema, Descemet changes, scar, or fibrosis. Corneal high-resolution mode is routinely used for corneal infiltration quantification. Both TD and FD OCT have been used widely in the recent past for prognosis of corneal infection. 3 , 4 , 5 , 6
8.3.1 Early Stages of Inflammation
At the initial stages of microbial keratitis, even mild cases have a thickened cornea at the infiltrated area. 3 Inflammatory cells may appear to be hyperreflective in the corneal stroma in superficial and deep keratitis (Fig. 8.2). In infection involving the deeper cornea and anterior chamber, it may be seen as aggregates on the endothelial surface (Fig. 8.3). 7 Corneal ulcer prognosis can be observed by measuring the change in epithelial defect and infiltration thickness (IT) on serial OCT images. Stromal or corneal thickness is observed regularly during the course of treatment, and the decrease in corneal thickness shows response to treatment and resolution of inflammation. Epithelial healing can also be assessed by the absence of “heaped-up” layers and continuous regular reflection from the anterior surface. Anterior-surface OCT also can image and monitor the extent of stromal edema associated with the ulceration. Corneal stromal edema accompanying the ulcer can be visualized as diffuse or local thickening of stroma and increase in the convexity on the posterior surface (Fig. 8.4). Infiltration of microbes and underlying tissue damage is noted as hyperreflective regions in the clear corneal surface (Fig. 8.2). IT is an important parameter that aids in posttreatment monitoring. Descemet folds can be imaged as ruffles in the normally smooth endothelial surface (Fig. 8.5). 3 In early corneal edema, the anterior chamber can be visualized for cellular reaction clearly by OCT, unlike the late stages. 7