In this part of my dry eye series, I want to talk a little about what tears are made of and why the individual components are very important to the development of dry eye symptoms. There are three basic components to our tear film. When I say tear film, I mean the very thin mixture of fluid that covers the surface of the eye after a blink of the eyelids occur. The three basic components of the tear film are meibum, aqueous, and mucous, and they are all produced by different sources.
The first layer, or meibum, is an oily substance that is produced by glands named Meibomian glands. The glands run from the eyelid margin (the part of the eyelid near the eyelashes) in a linear fashion through the eyelids to which they belong. During a blink, the eyelid muscle puts pressure on the glands, which moves the oil out of tiny gland openings at the eyelid margin. The oil is then dragged up onto the ocular surface when the lid goes back into the open position. This oily layer sits on top of the tear film because its purpose is to reduce friction from the eyelid blink movement, and more importantly, to keep water in the layer below it from evaporating.
The middle, and thickest layer, is the aqueous layer. It's made of saline (salt and water), and is produced by Accessory Lacrimal Glands that are located on the back side and in the superior-most portion of the upper lids. These glands are very similar to the salivary glands in our mouths, but their output is specifically made for the ocular surface. The aqueous portion of tears help move foreign material and physiological byproducts off of the eye and is constantly being replaced by new aqueous gland output.
The accessory glands make tears at a normal rate. We call this our our basal tear production rate. Let's not confuse basal production with reflex or emotional tears. These are tears that are produced by the main Lacrimal glands that rest above each eye. When the eye senses pain from injury or feels something foreign in the eye, the brain tells the Lacrimal gland to dump some of its contents out to rinse the eye of foreign materials or to protect the injury. Main Lacrimal gland output contains a lot of aqueous, but it also contains other substances that protect the eye, as well as those that cause inflammation. When we cry from emotions, the tears that get dumped onto the ocular surface are different than reflex tears in that they contain more protein based hormones. I have always found it to be so cool that the type of tears that are pushed onto the surface of the eye are dependent on which nerves tell the gland to function (physical pain or emotion). The eye is an amazingly designed organ.
The mucous layer sits at the bottom-most part of the tear film. It is produced by the corneal and conjunctival epithelial cells that line their respective surfaces. The mucous layer acts as a medium between the epithelial layer and the aqueous layer, providing a barrier of entry to micro-organisms and acting as a lubricant to reduce friction from blinking eyelids. Mucous is also produced by goblet cells that line the inner parts of the eyelids. Goblet cell mucous is pushed out onto the tear film and gets mixed in to the tear film during blinking.
Why eyes get dry when tears go awry?
When one of the layers of the tear film is dysfunctional, changes occur on the surface of the eye. If the meibum is irregular, a higher evaporative rate of the aqueous will occur, which causes an increase in osmolarity of the aqueous layer. This causes the epithelium to release inflammatory mediators that call in immune cells. The immune cells then infiltrate the Accessory Lacrimal glands causing them to dysfunction, which leads to a decrease in aqueous. This becomes a viscous cycle.
So when when a patient comes in with dry eye complaints, we have to find out which component of the tear is dysfunctional. In Part 3 of this series, I will go into more depth on inflammatory dry eye, some of the in-office testing that can be done, and what we look for when considering what needs to be fixed.
Hope all is well!
Mike Morabito, O.D.
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