Congo red vital staining of cornea and conjunctiva

Vital staining with an aqueous solution of 1% Congo red has been studied in the slit lamp. In 98 cases the dye was mixed with 1% lissamine green, in 120 eyes subsequent staining was performed with 0.125% fluorescein, and in 80 cases the mucous thread from the inferior conjunctival fornix was microscoped. Congo red stains dead cells, degenerate cells, and mucus. The dye discloses keratitis, corneal erosion, contact lens damages, corrosions, etc. It stains like lissamine green and rose bengal, though less frequently and less intensely than these. Congo red is a pH indicator. Acid reaction beyond its pH-range (3.0-5.2) has not been demonstrated. Amyloid-specific colour reaction (red-green dichromatic polarisation) has been noticed in mucous fibrils, most often in relation to infectious conjunctivitis and corrosion, never in normal eyes. The phenomenon is believed to indicate degeneration of the mucous fibrils (on the analogy of toluidine-blue-stained mucus), whereas not presence of genuine amyloid. It is, in other words, an important phenomenon in the differential diagnosis. Congo red is hardly indicated in ordinary clinical practice for vital staining of cornea and conjunctiva. Fluorescein, combined with rose bengal or lissamin green should be preferred.