dry eye dx and tx

Staining the Dry Eye

BY WILLIAM TOWNSEND, OD

During the past decade our understanding of the pathophysiology of dry eye has grown exponentially. Despite these advances in basic science, we still use tests that are often decades old.

The same holds true for stains. In 1882, Pfluger first used sodium fluorescein to evaluate corneal abrasions in rabbit corneas. In 1933, Sjögren reported staining of the ocular surface in dry eyed patients with rose bengal, a derivative of fluorescein. Lissamine green has recently gained popularity in evaluating dry eye. These agents can contribute important diagnostic information about the status of the ocular surface, especially in individuals who suffer from dry eye.

It's important to understand the pathophysiology of staining and the information it provides about a patient's condition.

How Stains Work

Sodium fluorescein is a hydroxyxanthene dye that fluoresces in the presence of short wavelength light. Wilson et al (1995) reported that fluorescence depends on optimal concentration of dye in tissue; concentrations that are either too low or high will produce lower levels of fluorescence.

Fluorescein produces "staining" in one of three ways. It may fill gaps in the ocular surface such as a corneal abrasion; it may be absorbed by damaged tissue (traumatized epithelium); or it may penetrate intercellular spaces.

Punctate staining of the cornea is a common finding in dry eye disease. Feenstra and Tseng (1992) showed that fluorescein doesn't stain healthy cells, has no intrinsic toxicity and shows increased staining in dead cells. Tear components such as albumin or mucin do not block fluorescein staining. Staining with fluorescein is promoted by disruption of cell-to-cell junctions.

Rose bengal is also a hydroxyxanthene dye that's structurally similar to fluorescein, but has additional halides that give it its red color and also render it intrinsically toxic. Visible light enhances this toxicity.

Rose bengal stains healthy cells, showing preference for nucleus components. Clinicians once assumed that rose bengal stains dead and dying (devitalized) cells, but Feenstra and Tseng (1992) demonstrated that it stains all tissue with one important exception: mucins and other protective components on cell surfaces block the staining. The study also confirmed that rose bengal toxicity is directly related to increasing concentrations of the stain.

Lissamine green is a synthetic dye with staining characteristics essentially identical to those of rose bengal. Kim and Foulks (1998) reported that both are tolerated well at a concentration of 0.1%, but at 0.5% and 1% rose bengal is much more likely to produce toxicity and reduce cell viability. Lissamine green is less irritating than rose bengal, and Manning et al (1997) showed that patients prefer it.

One important difference between these two agents is that lissamine green doesn't stain healthy corneal cells, only those with membrane damage whereas rose bengal stains these cells and negatively affects their vitality.

Evaluating the Eye with Stains

In summary, fluorescein stains damaged tissue or tissue in which cell to cell junctions are compromised. Both lissamine green and rose bengal stain tissue that has lost its protective mucin coating. Tseng noted that there is often little correlation between fluorescein staining and rose bengal/lissamine green staining. Gilbard (1995) noted that ocular surface staining usually occurs first on the nasal conjunctiva, then as the condition worsens involves the temporal conjunctiva and finally, in the more progressive disease states, the cornea is stained.

Ocular surface staining with these dyes is an important facet of the dry eye evaluation. We need to understand the differences between them to better understand the underlying pathology that we observe in dry eye patients. CLS

For references, please visit www.clspectrum.com/references.asp and click on document #140.

Dr. Townsend practices in Canyon, Texas and is an adjunct faculty member at UHCO. E-mail him at drbilltownsend@gmail.com.