Darkfield biomicrography of eye bank donor corneas

Evaluation of donated corneal tissue for transplantation relies on visual inspection by slit lamp biomicroscopy and specular micrography at the eye bank and then slit lamp biomicroscopy by the transplanting surgeon. An overall grade of tissue quality is assigned, but endpoints for biomicroscopic assessment are not universally accepted. We investigated the application of darkfield illumination to evaluate corneal transparency and provide a more standardized and reproducible method of tissue examination.

We built a fixture to securely hold a standard cornea storage container, and used two versions of photographic slit lamp biomicroscopes to examine donated corneal tissue with darkfield illumination.

In every cornea we examined, darkfield illumination portrayed increased subject detail as compared with conventional methods of biomicroscopy. The epithelium appeared as a richly textured "ground-glass" field. We noted variations in transparency that could not be detected with conventional slit lamp biomicroscopy. The actual surface area of lesions such as cornea verticillata and herpetic dendrites appeared larger in darkfield illumination because of its high sensitivity to subtle alterations of transparency.

Darkfield biomicrography of donated corneal tissue permits evaluation of the entire surface area, measurement of localized lesions of varying brightness, and assessment of epithelial cell coverage. As such, it extends our understanding of tissue clarity. Coupled with digital imaging technology, darkfield biomicrographs could be shared on the Internet and would allow transplanting surgeons to review tissue before selecting it for surgery. This technique could also be used in pharmaceutical research to measure changes in lesion size after therapy and could be incorporated into an eye bank quality assurance program. A new challenge for eye banking is to identify corneas in vitro that have undergone refractive surgeries such as laser in situ keratomileusis and photorefractive keratectomy-surgical alterations not easily seen at the slit lamp. Darkfield biomicrography may also be useful in detecting these lesions.