Aqueous Deficiency

In simple terms, aqueous deficiency means that the aqueous component of the tear film is insufficient. There is less tear fluid available on the ocular surface than the system needs to function normally.

You may also come across the term aqueous-deficient dry eye, or ADDE.

Traditionally, dry eye has often been divided into two broad categories:

• •aqueous-deficient dry eye — associated primarily with insufficient aqueous tears
• •evaporative dry eye — associated primarily with excessive tear evaporation

This distinction is still useful, but dry eye is now understood as a much more interconnected disease. TFOS DEWS III therefore looks more specifically at the different parts of the ocular-surface system that may be disrupted. Within the tear film itself, these include problems involving the aqueous, lipid and mucin/glycocalyx components.

Aqueous deficiency is one of these possible tear-film problems.

The aqueous component provides most of the fluid volume of our tears. It helps create the environment in which the ocular surface functions. It contributes to lubrication, allows the eyelids to move smoothly over the surface of the eye, helps flush away small particles and cellular debris, and carries many substances important for ocular-surface health.

Together with lipids, mucins, the glycocalyx, blinking and the ocular surface itself, it also contributes to maintaining a stable tear film between blinks.

So when aqueous tear volume becomes insufficient, we are not simply losing “water.” We are changing the environment in which the entire ocular surface has to function.

Imagine the tear film as a very small fluid environment covering the exposed surface of the eye. If the amount of available fluid decreases, that environment becomes more difficult to maintain.

The tear film may become thinner and less stable. There is less fluid available to lubricate and protect the ocular surface. Normal clearance of material from the surface may become less effective. And the balance of dissolved substances within the tears can change.

One particularly important consequence is tear hyperosmolarity.

Tears contain salts and many other dissolved substances. If water is lost from a smaller volume of tears, those substances can become more concentrated. This increased concentration is known as hyperosmolarity.

Hyperosmolarity is an important part of dry-eye disease because it can place stress on cells of the ocular surface and contribute to processes involving inflammation and surface damage.

This helps explain why aqueous deficiency is not simply a matter of the eye feeling “less wet.” Reducing the amount of available tear fluid can change the biological environment of the ocular surface. And once the ocular surface becomes stressed, other parts of the dry-eye system may become involved as well.

Having enough aqueous tear fluid is also important for maintaining the tear film between blinks. When tear volume is substantially reduced, the tear film can become more vulnerable to breakup. Once breakup occurs, areas of the ocular surface become exposed to greater local stress before the next blink restores the tear film.

This creates an important connection between aqueous deficiency and evaporation. The two are often presented as opposites: not enough tears versus tears evaporating too quickly. In reality, they can interact. If there is already less tear fluid on the eye, evaporation is occurring from a smaller available volume. And if tear-film breakup develops, local evaporation and ocular-surface stress can increase further.

So a problem that begins primarily as aqueous deficiency can eventually acquire an evaporative component as well. This is one reason dry eye often becomes a mixed problem rather than remaining in one neat category.

Not necessarily.

The lacrimal glands are the major source of the aqueous component of tears, so reduced lacrimal secretion is an important pathway to aqueous deficiency. But these are still two different levels of the system. Lacrimal gland function describes one of the systems responsible for producing aqueous tears. Aqueous deficiency describes the problem we see at the level of the tear film — insufficient aqueous tear fluid available on the ocular surface.

That distinction is similar to the one we made when discussing MGD and lipid deficiency. In this part of the guide, our question is simply: what part of the tear-film system is not functioning normally? The separate question — why has aqueous tear availability become reduced? — comes later.

Yes. And this can understandably seem contradictory. If someone has “not enough tears,” how can their eyes sometimes water?

The answer is that tear production is dynamic. The ocular surface continuously sends sensory information through the nervous system, which helps regulate tear secretion. When the eye is strongly irritated, this system can trigger reflex tearing. That is why wind, smoke, cutting an onion or getting something in your eye can suddenly produce a large amount of tears. Dry-eye-related irritation can also stimulate reflex tearing in some situations.

So an episode of watery eyes does not automatically tell us that normal tear volume and tear regulation are adequate throughout the day. The eye can experience insufficient tear availability under ordinary conditions and still produce episodes of reflex tearing.

This is one reason symptoms alone cannot tell us exactly which part of the dry-eye system is affected.

Clinicians have several ways of assessing the aqueous component and tear volume.

One useful feature is the tear meniscus — the small reservoir of tears that can be seen along the lower eyelid. Its height can be assessed non-invasively, and a low tear meniscus can provide evidence that tear volume is reduced.

The Schirmer test is another familiar test in which a strip of filter paper is placed at the lower eyelid to measure how much of the strip becomes wet over a certain period.

But neither of these should be thought of as a perfect standalone answer to the question “Do my eyes produce enough tears?” Tear production and tear volume are dynamic, and tests are influenced by their measurement methods and conditions. As with other aspects of dry eye, no single test tells the whole story.

Tear Meniscus Height: understanding tear volume

The Schirmer Test: what it measures — and what it doesn't

A person with aqueous deficiency may also have problems involving other parts of the tear-film and ocular-surface system. For example, aqueous deficiency can coexist with:

• •lipid abnormalities
• •tear-film instability
• •eyelid or blinking problems
• •ocular-surface damage
• •inflammation
• •changes in sensory function

These problems can influence one another. This is why describing someone simply as having “aqueous dry eye” may not capture everything that is happening. The more useful question is often: which parts of the system are disrupted in this particular person?

Dry Eye Is Often a Mixed Problem

There are many possible reasons why aqueous tear availability can become reduced. These can include conditions affecting the lacrimal glands, systemic and autoimmune diseases, certain medications, age-related changes, problems involving neural regulation and other factors.

But those are questions about why aqueous deficiency developed — not about what aqueous deficiency itself is. We explore them separately later in this guide.