Ocular-Surface Staining

Two important tissues make up much of the exposed ocular surface:

• •the cornea — the clear window at the front of the eye
• •the conjunctiva — the thin tissue covering the white part of the eye and lining the inside of the eyelids

These are living tissues covered by epithelial cells. The tear film sits on top of them and constantly interacts with them.

Normally, the ocular surface provides a healthy barrier and a smooth interface for the tear film. When that surface becomes stressed or disrupted, special dyes can make some of those changes easier to see.

Many changes on the ocular surface are microscopic. A clinician may not be able to see them simply by looking at the eye. Diagnostic dyes work a little like highlighters. They make certain abnormalities easier to see under magnification and special illumination.

Two dyes are particularly useful in dry-eye assessment: fluorescein and lissamine green. They do not behave in exactly the same way, which is why they can provide somewhat different information.

Fluorescein is a yellow-orange dye that appears bright green when viewed under blue light. It is commonly used during eye examinations. You may already have encountered it during a tear-breakup-time test.

Fluorescein can highlight areas where the normal barrier of the ocular surface has been disrupted. It is particularly useful for examining the cornea. When many tiny fluorescent dots appear, a clinician may describe this as punctate corneal staining. “Punctate” simply means that the staining appears as small dots.

Not in the way that phrase might suggest.

The surface of the cornea is made of layers of epithelial cells with complex connections and protective surface structures. When the integrity of this superficial barrier is disturbed, fluorescein can enter areas where it normally would not.

So staining is better understood as a sign that the normal surface barrier has been disrupted, rather than as tiny holes appearing in the cornea. The amount of staining matters, but so do its location and pattern.

Lissamine green is another dye used to examine the ocular surface. It is particularly useful for looking at the conjunctiva and the area around the eyelid margin. It can highlight superficial cells whose membranes or protective surface environment have been altered.

Because fluorescein and lissamine green reveal somewhat different aspects of the ocular surface, clinicians may use both.

If the tear film repeatedly becomes unstable, parts of the ocular surface may experience increased stress. The tear film may break up too quickly. Some areas may become poorly lubricated. Local tear concentration may increase. Friction between the eyelid and ocular surface may increase. Inflammatory processes may become involved.

Over time, these stresses can interfere with the normal health and integrity of surface epithelial cells. Staining can be one visible sign of that disruption. This creates an important relationship:

tear-film instability→ocular-surface stress→surface disruption→poorer tear–surface interaction→further tear-film instability

So ocular-surface damage can begin as a consequence of tear-film problems and later become part of the cycle maintaining them.

Not by itself. Ocular-surface staining is an important objective sign used in modern dry-eye diagnosis. But staining is not unique to dry eye. The ocular surface can also become disrupted because of:

• •injury
• •infection
• •contact-lens-related problems
• •exposure
• •other ocular-surface conditions

That is why staining always needs to be interpreted together with your symptoms, history and the rest of the eye examination.

Not all staining patterns mean the same thing. A clinician may look at questions such as:

• •Is the staining mainly on the cornea or conjunctiva?
• •Is it concentrated in one particular area?
• •Is it mostly on the lower part of the eye?
• •Is it central?
• •Is it spread across the exposed surface?
• •Is one eye very different from the other?

The pattern can sometimes provide clues about what is stressing the ocular surface. For example, a small localized area of staining may suggest a different problem from widespread staining across the exposed eye. This is why simply knowing a staining “score” does not tell the entire story.

Clinicians and researchers need a standardized way to describe what they see. Different scoring systems divide the ocular surface into regions and grade how much staining is present. This makes it easier to compare findings and follow changes over time.

As a patient, however, you usually do not need to memorize these scoring systems. The more useful questions are: is staining present, where is it, how extensive is it, what does the pattern look like — and how does it fit with the rest of my examination?

Yes. Not everyone with dry eye has obvious ocular-surface staining. For example, someone may have significant tear-film instability without much visible surface disruption. Dry eye can also fluctuate, so staining may change over time. A normal staining result therefore does not tell us that every other part of the tear-film system is functioning normally.

Yes. This is another example of the mismatch that can occur between symptoms and signs.

Some people have significant ocular-surface abnormalities but relatively little discomfort. One possible reason is altered corneal sensation. If sensitivity is reduced, the surface may be affected without producing the amount of discomfort we might expect.

So few symptoms do not always mean that the ocular surface is healthy.

Not necessarily in a simple one-to-one way. More extensive staining can indicate greater ocular-surface involvement. But staining measures only one part of the dry-eye picture.

Someone may have severe symptoms with relatively little staining. Another person may have substantial staining with fewer symptoms. Tear-film stability, tear volume, eyelid function, inflammation, meibomian gland function and sensory nerve function may all contribute to the overall picture.

So staining should not be treated as a universal “dry-eye severity score.”

Because putting dye into the eye changes the natural tear film. Once fluorescein or another dye has been added, later tear-film measurements may be affected.

That is why clinicians generally try to perform measurements of the undisturbed tear film first and staining later. This is another example of why the order of dry-eye testing matters.

It can help show:

• •whether the ocular surface is disrupted
• •where that disruption is occurring
• •how extensive it appears to be
• •whether it changes over time

It can also provide important objective evidence when diagnosing dry eye.

By itself, staining cannot tell you:

• •why the surface became abnormal
• •whether MGD caused it
• •whether aqueous tear production is low
• •whether inflammation was the original problem
• •how much discomfort you should feel

Those questions require other pieces of the diagnostic puzzle.