1.3 Confocal Microscopy

Applanated

Air-jet

cornea

higher hysteresis values (on average) than do keratoconic corneas (Fig. 1.19a, b).

While the technology looks promising, there is a question as to how extinct the method can distinguish normal corneas from mildly keratoconic and early keratoconic tissue. Data plots of normal and KC corneas show that there is significant overlap in hysteresis values in the two groups (Fig. 1.20). Most recently, new parameters are being developed in the new software of the machine to diagnose corneas that look normal but may develop post-lasik ectasia.

The Ocular Response Analyzer (Reichert) is till now the only available machine that measures the biomechanical properties of the cornea. It utilizes a rapid pulse of air to move the cornea inward and outward under the force through a “dynamic bi-directional applanation process.” An electro-optical system monitors changes in the shape of the cornea. Applanation pressure is recorded at two points: when the cornea is bending inward and when the cornea is returning to its normal state (Fig. 1.17). Due to bending resistance of the tissue, there are delays in the expected times when the cornea bends and returns to a normal state and the difference in the applanation pressure at these two points is termed corneal hysteresis (Fig. 1.18). Depending on the properties of the cornea for different conditions, one would expect to see different hysteresis values. For example, normal corneas have significantly

The list of confocal microscopy signs associated with KC includes: thinning of the stroma; elongated, exfoliating superficial epithelial cells; enlarged wing and basal epithelial cells; bright reflective material deposited within basal epithelial cells; prominent, thickened sub basal nerves with additional structural changes seen along nerve fibers; increased stromal haze; pronounced reflectivity and an irregular arrangement of the stromal keratocytes; structurally abnormal anterior stromal keratocyte nuclei; lower densities of anterior and posterior stromal keratocytes; Z-shaped folds in the anterior, mid, and posterior stroma; folds in Descemet’s membrane; pleomorphism and enlargement of endothelial cells; increased endothelial cell density.

While these signs may occur with KC, most are not specific to KC, and may be also seen with other corneal disorders.

Fig. 1.18 Principle of ocular response analyzer. Due to bending resistance of the tissue, there are delays in the expected times when the cornea bends and returns to a normal state and the difference in the applanation pressure at these two points is termed corneal hysteresis. Corneal hysteresis with other parameters measured by this machine reflects corneal properties which differ according to corneal conditions and pathologies

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