Corneal staining: do we really understand what we are seeing?

Sodium fluorescein ('fluorescein') is widely utilised for the evaluation of ocular surface integrity. It is particularly well suited to this task due to its fluorescent properties and its high visibility at low concentrations. Ocular surface fluorescence observed clinically is affected by a number of factors including concentration of fluorescein, thickness of the fluorescein layer, the wavelength of the exciting light source and whether or not a barrier filter is used as part of the viewing system. In this review we have attempted to provide a critical evaluation of the published literature relating to three potential cellular mechanisms involved in corneal surface fluorescence: surface pooling, uptake by cells and ingress around cells. Despite the widespread adoption of use of fluorescein for the assessment of the ocular surface, we contend that the clinical understanding and interpretation of corneal surface fluorescence is based upon assumption, extrapolation and clinical intuition rather than solid evidence-based science underpinning the basic causative mechanisms of this phenomenon. Further research in order to understand the basic interactions between the ocular surface cells and fluorescein should be made a priority in the fields of optometry and ophthalmology.