Dry Eyes
Dry Eyes
What is it?
The term “dry eyes” has created tremendous confusion. It describes a group of conditions with different symptoms, some of which include watering of the eyes, which doesn’t seem to make sense. To add to the confusion, the watering eyes are frequently treated with lubricating eye drops. This apparent contradiction creates frustration, erodes confidence and patients frequently give up on their treatment. However, reduced tear production can indeed produce watering and lubricating eye drops are frequently useful in treatment. More information on how this works can be found here.To understand what is happening it is useful to understand tear production.
Tears are produced in 3 ways.
- Basal tear production. This happens all the time. Tears are produced in a slow but constant flow to provide nourishment and protection to the cornea and wash debris from the ocular surface.
- Reflex tear production. Tears are released in large quantities, usually in response to pain or irritation on the surface of the eye. Think of an eyelash in the eye. A large gush of tears is required to flush the lash out. These tears are not somehow instantly produced and released. The lacrimal gland stores a supply of tears which can be instantly released when required.
- Emotional tearing. Crying – when you are sad, happy or amused. It is emotionally driven and is not relevant in this discussion.
Tears have 3 components, as outlined below. Most of the tear film is made of aqueous which is produced by the lacrimal gland. The gland is located in the eye socket more or less behind the outer edge of the eyebrow. After the aqueoushas been released under the top lid, it flows over the eye to form the tear film and collects on the margin of the lower lid. From here it flows towards the nose and drains down a small hole in the inner corner of the lid and then into the nose. This explains why your nose runs when you have been crying.
As we get older the lacrimal gland struggles to keep up the basal tear production.
However, reflex tearing is still possible.
If basal tear production is reduced to the point where the cornea starts to dry out it will cause irritation. Irritation causes reflex tearing.
Tear quality is also an important factor in dry eyes. The lacrimal gland produces aqueous which tends to dry quickly and does not spread out evenly over the eye surface without some assistance. The spreading is facilitated by a very thin layer of mucous that lies on the surface of the cornea. However, the tears would still dry out very quickly if it weren’t for a thin layer of oil that floats on the surface. This oil comes from glands in the eyelid and these glands are important. They are called Meibomian glands and are involved in many ocular surface conditions. The glands are located in the tarsal plate at the back of each eyelid. So, we have 3 components to a healthy tear film.
If there is a problem with this relationship the tears will become unstable and dry out.
This will cause irritation and, … reflex watering.
To summarise:
Reduced basal tear production causes - dry eye and irritation
Reflex tearing causes – Watering
What causes it?
We have seen here that a healthy tear film has 3 components. A mucous layer, aqueous layer and oil layer.A problem in the formation of any of these components may result in tear film instability and dry eye symptoms. Problems with the production of mucous is uncommon and limited to a few specific inflammatory conditions. Problems with aqueous production is common and largely age related although there are also specific conditions that may be responsible. Problems with the production and release of oil are also common and frequently associated with a condition called blepharitis.Do I have it?
Dry eyes cause a wide range of symptoms, including grittiness, irritation, soreness, intermittent blurring and inability to read comfortably for extended periods. These symptoms are not specific to dry eyes but are suggestive and warrant a review.How is it treated?
The principle of treatment is to support the tear film with lubricating eye drops while targeting the underlying cause with specific measures.
Not all underlying causes can be effectively managed and cases involving abnormal production of mucous and aqueous are likely to require long term use of lubricants.
Blepharitis is the most common cause for abnormal oil production and may be reasonably controlled in most cases. Blepharitis is covered in greater detail elsewhere.