Lipid Deficiency

Most of the lipids at the surface of the tear film come from the meibomian glands, which are located inside the eyelids. These glands produce a complex lipid-rich secretion called meibum. Meibum reaches the eyelid margin through tiny gland openings, and blinking helps distribute lipid material across the tear film.

The resulting lipid-rich surface is extremely thin, but it contributes to the physical behavior and stability of the tear film. When this system is working well, the lipid component spreads and reorganizes with blinking and interacts with the rest of the tear film. When it is disrupted, the tear film may become less stable.

Meibomian Glands & Lipids: how the system normally works

This distinction is important. A common explanation of dry eye goes something like this:

“The meibomian glands produce oil → the oil forms a protective layer → if there is not enough oil, the watery tears evaporate too quickly.”

There is some truth behind this model, but modern tear-film research shows that the reality is more complicated.

The lipid component is not simply a fixed waterproof coating sitting on top of the tears. It is a dynamic, highly organized mixture of different lipids that changes during the blink cycle and interacts with the rest of the tear film underneath it. Its composition, spreading, organization and stability can all matter.

This means that measuring “how much lipid is there?” does not necessarily tell us everything about how well that lipid is functioning.

There are several ways in which the lipid component of the tear film can become abnormal:

• •there may be less meibum reaching the tear film
• •the meibum reaching the surface may have altered properties or composition
• •the lipid material may not spread normally across the ocular surface
• •the lipid layer may become irregular or unstable
• •blinking may fail to distribute it effectively
• •or several of these problems may occur together

The result can be a tear film that becomes unstable more quickly between blinks.

This is why lipid deficiency should not be understood simply as a measurement of lipid quantity. It describes a problem with one functional component of the tear-film system.

The tear-film lipid layer has traditionally been described mainly as an anti-evaporative barrier — a surface layer that helps reduce the loss of water from the tears.

Lipids do play an important role in tear evaporation. But the relationship is not as simple as “thinner lipid layer = faster evaporation” or “thicker lipid layer = better protection from evaporation.” Research has not established a straightforward direct relationship between measured lipid-layer thickness and tear evaporation rate.

This does not mean that lipids are unimportant. It means that thickness alone does not tell the whole story. The composition and organization of the lipid layer, the way it spreads, its interaction with the rest of the tear film, blinking, the exposed surface area of the eye, humidity, airflow and other factors can all influence what happens to tears between blinks.

So rather than imagining the lipid layer as a simple lid placed over water, it is better to think of it as part of a dynamic system that helps determine how the tear film behaves and how stable it remains.

After every blink, the tear film needs to spread across the ocular surface and remain sufficiently stable until the next blink. The lipid component participates in this process.

If it is abnormal, the tear film may become less able to maintain a smooth and stable surface between blinks. Tear-film breakup may occur sooner. When areas of the tear film break up, parts of the ocular surface underneath can experience greater local stress.

Repeated tear-film instability can then interact with other processes involved in dry eye, including increased tear concentration, ocular-surface damage, inflammation and changes in sensory signaling.

This is one reason dry eye can become more complicated over time: a problem that begins in one part of the system can eventually affect others.

Lipid deficiency and meibomian gland dysfunction (MGD) are very closely connected. In fact, MGD is the major clinical cause of problems with the lipid component of the tear film.

The connection is quite straightforward. The meibomian glands produce meibum, the secretion that provides most of the lipids found at the surface of the tear film. If these glands become obstructed, their secretion changes, or less functional meibum reaches the eyelid margin, the lipid component of the tear film can also become abnormal.

We can think of the process like this:

MGD→abnormal meibum production or delivery→abnormal tear-film lipids→tear-film instability

Because they describe the problem at two different levels. MGD describes what is happening in the meibomian glands. Lipid deficiency describes what is happening in the tear film.

This distinction may sound technical, but it becomes useful when we start trying to understand an individual case of dry eye. For example, a person can have signs of MGD without necessarily having clinically significant dry eye disease. And problems with the way lipids are distributed across the tear film can also be influenced by other parts of the system, including blinking.

So lipid deficiency and MGD are not two unrelated conditions. Quite the opposite: they are often different parts of the same chain of events.

Meibomian Gland Dysfunction (MGD): what actually happens inside the glands — coming soon

Not exactly.

The term evaporative dry eye has been used for many years to describe dry eye in which excessive evaporation plays an important role. But modern dry-eye classification is becoming more specific. TFOS DEWS III describes potential dry-eye drivers according to the part of the system that is disrupted. Tear-film deficiencies include problems involving the lipid, aqueous and mucin/glycocalyx components. Other drivers can involve blinking and eyelid closure, the lid margin, the ocular surface, inflammation, anatomy or neural function.

This distinction matters because evaporation describes a process, while lipid deficiency describes a particular abnormality within the tear-film system that can contribute to that process. They are closely related, but they are not exactly the same thing.

There is no single test that tells us everything about the lipid component of the tear film.

Interferometry can be used to observe the lipid layer and, with some instruments, estimate its thickness. But lipid-layer thickness varies between individuals, and there can be considerable overlap between measurements found in people with and without dry eye. A number alone therefore cannot tell us whether the entire lipid system is functioning normally.

Clinicians can also examine the meibomian glands, assess whether meibum can be expressed and what it looks like, evaluate the eyelid margins, observe blinking and measure tear-film stability. Each of these tests provides a different piece of information.

This is an important principle that will appear repeatedly throughout this guide: no single dry-eye test tells the whole story.

Lipid-Layer Interferometry: what are we actually looking at?

Dry eye is often more complicated than one abnormal tear-film component. Someone with a lipid problem may also have reduced aqueous tear availability, incomplete blinking, ocular-surface damage, inflammation, eyelid abnormalities or altered sensory function.

And these problems can influence one another. An unstable tear film can increase stress on the ocular surface. Surface stress can contribute to inflammatory and neural changes. Blink behavior can change in response to discomfort, while abnormal blinking can further affect tear-film distribution.

Dry eye can therefore become a network of interacting problems rather than one isolated defect. This is one reason identifying the important drivers in an individual patient can matter so much.