Histological study of late phase fluorescence in the rabbit fundus.

MANY different explanations have been put forward to account for the phenomenon of late-phase fluorescence in fluorescein fundus photography. Until the exact mechanism is known, its value as a tool for the diagnosis of fundus pathology is limited. It seems quite clear that localization of fluorescein does occur in spite of the statement by Schiller, Schayer, and Hess (1953) that aqueous fluorescein sodium diffuses everywhere so quickly after intravenous injection that it can have no localizing value. It was felt that a new approach was needed to unravel some of the apparent contradictions in the interpretation of persistent fluorescence. We have already shown (Sollom and Brown, 1967) that a reliable experimental technique for producing tissue damage in rabbits is the application of trans-scleral diathermy. The subsequent events provide a range of pathological changes for study, in many ways similar to conditions in man, which have been investigated by the technique of fluorescein photography. In rabbits the diathermized area of fundus shows late-phase fluorescence throughout all stages of the healing process. In the experimental animal the lesion which fluoresces can be examined after enucleation to obtain a pathologist's report, whereas this is rarely possible in clinical practice. Moreover, routine processing in the pathology department can be performed in parallel with special examination of the second eye, when both have diathermy lesions at identical stages of healing. The present work is concerned with the special examination of rabbit eyes at intervals after trans-scleral diathermy. Our earlier work has shown that the picture of fluorescence in diathermy lesions does not materially alter between 5 and 15 minutes after the intravenous injection of 0 4 ml. of a 10 per cent. solution of aqueous fluorescein. In a series of 36 rabbits, diathermy lesions were produced and all showed a similar pattern oflate fluorescence. It was decided to inject the standard dose of fluorescein, wait 5 minutes, kill the animal with an overdose of intravenous barbiturate, and immediately enucleate the eyes. After identification the lesion would be prepared for frozen section. Immediate examination by fluorescence microscopy, using suitable filters, might show the characteristic green emission of fluorescein. Few guiding references could be found in the literature. Fluorescent tracing is a technique that is usually carried out in vitro and involves the use of fluorescein isothiocyanate conjugated with one of the plasma proteins. Flurorescence microscopy of the eye after intravenous fluorescein