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Sunday / June 26.
HomeminewsCorneal Staining May Not Be What It Seems

Corneal Staining May Not Be What It Seems

New research on corneal staining and multi-purpose solution-associated hyperfluorescence has revealed that many previous assumptions may be incorrect -and that the phenomenon requires further investigation.

Preeminent research experts – Dr. Paul Karpecki, Prof. Frank Bright, Prof. Nathan Efron and Dr. Philip Morgan – met in the U.S recently to discuss the science of fluorescein, corneal staining, and preservative-associated transient hyperfluorescence (PATH). They concluded that despite common assumptions in optometry, the characteristics of PATH point to it being a harmless entity that is distinct from corneal staining.

Fluorescein

First synthesised in 1871, fluorescein is a molecule not found in nature. Many factors can impact the fluorescent intensity and diagnostic utility of fluorescein. First, the charge of fluorescein is directly related to the pH of the environment and its fluorescence is highly pH dependent (fluorescence intensity increases as pH rises until a pH of 8 is reached). Also, at high concentrations (>0.001 per cent) it is self-quenching (energy emitted by fluorescein molecules is absorbed by nearby fluorescein molecules rather than being emitted as light), so diminished fluorescence occurs with increasing concentration above this threshold.

Preservative Uptake and Release

All multi-purpose solutions (MPS) contain one or more preservative agents such as polyhexamethylene biguanide (PHMB), polyquaternium-1 (PQ-1), myristamidopropyl dimethylamine (MAPD; also known as Aldox), and alexidine. The total amount of preservative contained in MPS formulations is very small, ranging from one to 15 parts per million. Although MPS preservatives
are taken up by soft contact lenses while soaking, the amount absorbed may be too low to quantify.

A number of factors can influence the uptake of preservative into a soft contact lens including: the soft contact lens material…

A number of factors can influence the uptake of preservative into a soft contact lens including: the soft contact lens material, type(s) of preservative contained in the MPS, and overall MPS formulation including components such as buffers, wetting agents, or other molecules that carry an electro-static charge. The amount and rate of uptake for each preservative can vary between hydrogel and silicone hydrogel lenses, as well as among the different silicone hydrogel lens materials.

When placed on the eye, the soft contact lens begins to release the absorbed preservative into the tear film. The rate at which this occurs is dependent on the preservative and type of contact lens material. The preservative is ultimately dissipated through the normal turnover of tear film.

Fluorescein and Preservative Interactions

The MPS preservatives released from soft contact lenses interact with fluorescein dye when it is applied to the eye. Fluorescein is negatively charged and MPS preservatives are positively charged, attracting them to one another. The level of attraction depends on the MPS preservative. Prof. Bright’s studies have found that the attraction between fluorescein and the preservative polyhexamethylene biguanide (PHMB) is up to 50 times greater than the attraction between fluorescein and the preservative polyquaternium-1 (PQ-1). The substantially higher affinity of fluorescein for PHMB may account for the higher levels of hyperfluorescence at a two-hour time point with PHMB-based solutions versus MPS with other preservatives at all time points.

Corneal Staining vs MPS-Associated Hyperfluorescence

Although a number of eye care professionals (ECPs) have labeled this phenomenon as ‘corneal staining’, many characteristics of PATH point to it being a distinct entity from corneal staining. One such difference is the etiology of PATH versus corneal staining. While the etiology of PATH is due to the benign preservative interaction with fluorescein, pathological corneal staining is due to damage to the epithelium. Additionally, PATH is generally asymptomatic, has a superficial punctate pattern, and is transient, lasting only several hours following lens insertion, and is not associated with future complications. These characteristics, especially together, are not observed in cases of corneal staining during pathological situations.

Conclusion

While a significant effort has been made to connect PATH with adverse outcomes over the past decade, to date, no negative sequelae have been shown to be associated with PATH. A large body of literature shows the hyperfluorescence at the time of peak PATH is not pathological corneal staining, a measure of biocompatibility, a cause of infiltrative keratitis or infection, an indicator of cellular damage, or associated with future adverse events. It is essential that clinicians understand what they are observing in patients and why. It is only then the best clinical decisions for patient’s health and satisfaction can be made.

Complete proceedings from the forum are available online at: www.revoptom.com/supplements.

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