Fluorescein is a synthetic organic dye used widely in medicine and science as a diagnostic agent. Its utility stems from its capacity to glow brightly under specific light conditions, a property known as fluorescence. Created in 1871 by chemist Adolf von Baeyer, the dye quickly became a valuable tool in organic chemistry and medical diagnostics.
The Unique Property of Fluorescence
Fluorescein functions by utilizing the principle of fluorescence, which involves the absorption of light at one wavelength and the subsequent emission of light at a longer wavelength. When a molecule absorbs a photon, its electrons jump to a higher, excited energy state. The electron then loses energy before dropping back down to its original state, releasing the remaining energy as a photon of light. This emitted light has a lower energy level and a longer wavelength than the light originally absorbed, a difference known as the Stokes shift.
For fluorescein sodium, the dye is most efficiently excited by blue light (approximately 494 nanometers [nm]). Once excited, the molecule emits light in the yellowish-green spectrum, peaking around 521 nm. Specialized cameras or filtered viewing instruments shine the blue excitation light while filtering out reflected blue light, allowing only the green fluorescent signal to be observed. This mechanism provides a high-contrast image that reveals details not visible under standard illumination.
Essential Diagnostic Uses
The dye is indispensable, particularly in ophthalmology, where it is used in two different ways. The first is topical staining, where the dye is applied directly to the surface of the eye via a drop or saturated paper strip. The dye does not penetrate intact epithelial cells. Instead, it pools in areas where the protective outer layer of the cornea or conjunctiva has been damaged or lost. Using a cobalt blue filter, a clinician observes the damaged area glowing bright green, demarcating conditions like corneal abrasions, foreign bodies, or epithelial defects.
The second application is fluorescein angiography, which requires the dye to be injected intravenously. Once in the bloodstream, the fluorescein travels rapidly to the eye, circulating through the blood vessels of the retina and the underlying choroid. A specialized camera photographs the dye as it passes through the ocular vessels, providing a dynamic map of blood flow and revealing leaks or blockages. This technique is routinely used to diagnose and monitor conditions such as diabetic retinopathy, age-related macular degeneration, and retinal vein occlusions.
Methods of Delivery and Safety Considerations
The method of delivering fluorescein dictates the safety profile. For topical staining of the eye’s surface, the dye is applied as a dilute solution or from a sterile paper strip. This localized application results in minimal systemic absorption, and side effects are generally limited to mild, temporary stinging or irritation at the application site.
For angiography, the dye is administered intravenously to circulate systemically. Following injection, patients may experience a temporary yellowish discoloration of their skin and bright yellow urine, as the dye is naturally excreted by the kidneys over the next 24 to 36 hours. Nausea and vomiting are also common, though transient, side effects that can occur shortly after the injection. While rare, severe hypersensitivity reactions, including anaphylaxis, can occur after intravenous administration, requiring medical personnel to be prepared for immediate intervention.