Blue Collar Scientist

One of the ways that dogs differ from their mammalian cousins, Homo sapiens, is that dogs have a complete nictitating membrane, often called colloquially the “third eyelid.”¹ This membrane has a number of functions, one of which is to act as additional protection for the eye. It also houses a gland which accounts for about 30% of the tear production for that eye. Tears keep the cornea hydrated and oxygenated, and they deliver nutrients - a necessary task, since the cornea does not have blood vessels to fulfill these functions.

Sometimes people notice that their dog has a small red mass in the inner corner of the eye. It was once thought that this was a tumor, and the standard treatment was to remove it. But in the 1980’s, this practice changed after it was discovered that the red mass was actually the gland that is normally held behind the third eyelid by connective tissue. When the gland moves out of place, perhaps because of connective tissue that becomes loose, blood circulation to the gland becomes impaired, and the gland swells and becomes red. This condition is commonly known as “cherry eye,” and is seen mostly in young dogs.

The condition can occur in any breed, but certain breeds are especially susceptible, including english bulldogs, pugs, cocker spaniels, boston terriers, shihtzus, beagles, pekingese, bassett hounds, and neapolitan mastiffs.

The condition looks like this:

In the past, the standard treatment for this problem was to remove the gland. But since the gland produces 30% of the tears for that eye, removing it can lead to a condition called keratoconjunctivitis sicca - which we will mercifully shorten to KCS. It is also commonly known as “dry eye.”

KCS is a serious disease with no cure, and if untreated it is very painful. There are two different kinds of KCS. In the first type, the glands do not produce enough tears; in the second, the glands are not making the right type of tears. Tears are made up of three components. There is a lipid layer to prevent tears from evaporating, a liquid portion that helps to distribute the tears across the cornea as well as providing nutrients, and a mucous layer to help the cornea get the nutrients from the liquid layer into the cells of the cornea. Without the liquid portion of the tears, the mucous portion will stick to the cornea, and it will look as if there is a very bad infection. It also feels as if you have sandpaper on your eyelids as you blink. If the condition is bad enough, the eyelids will be unable to blink across the cornea. The cornea will start to dry out and die, and will become pigmented or darkly colored. At this stage, the animal will be blind.

KCS looks something like this:

So all this can be caused by the removal of a prolapsed gland normally held behind the nictitating membrane, which used to be the standard treatment. What about today? Well, science marches on, and we have much better methods of treating the condition now. The current standard of care is to replace the gland behind the third eyelid and suture it into place. There are a number of different ways to do this, and they have varying rates of success and difficulty. Over the course of the several days following surgery, assuming there are no complications, the swelling of the gland will go down, and the gland will return to normal, as though nothing had ever happened. A 100% recovery is the usual, but the most common complication is for the gland to pop out again, requiring additional attention.

A few breeds, notably bulldogs and mastiffs, have anatomical peculiarities that make it more difficult to get the gland to stay where it is put. Typically this is because the cartilage in the third eyelid is bent, or not as stiff as it should be. In these circumstances the surgical procedure is slightly altered for the patient in order to minimize the chance of a second prolapse - for example, the cartilage can be reconstructed and the gland sutured more securely as a result of the alteration.

If these surgical techniques are used, there’s a 17% chance that the animal will develop KCS, or dry eye. This may sound like poor odds, but not once you compare them to the alternative. If the gland is removed, there’s a 50% chance of developing the condition - just over three times the risk. In fact, it is better to leave the gland alone, in its prolapsed state, than to remove it - but best of all is to put it back into position where it can work normally again.

Naturally, such a medical success story is the subject of bad advice, mythology, and woo on the interwebs. You need not look far at all to find websites that discuss certain breeds of dog, and how horrible it is to repair a prolapsed gland. The flavor of the woo-meisters’ arguments is that veterinarians are ignorant and awful for doing replacement surgeries (with high success rate - yes, even on Neapolitian Mastiffs), rather than just doing the obvious thing and hacking out something that looks ugly and infected (leading to a 50% chance of disabling complications). KCS, and resulting blindness, is apparently an acceptable alternative.

At least these people are telling dog owners to go see their vets - but usually just to get prescriptions for medications that the writers seem to think will help. One of them talks about the use and dosing of atropine eyedrops to reduce the pain of the condition. Atropine is used as a pain reliever, but only in cases of uveitis (inflammation in the front chamber of the eye). Its use is so limited because it has serious side effects. It dilates the iris, usually for 2-3 days, and sometimes up to a week or more. Overdoses can cause ventricular fibrillation, tachycardia, dizziness, nausea, loss of balance, loss of balance, hallucinations, excitement, and death. A common mnenomic used to remember some of the signs of atropine overdose is “hot as a hare, blind as a bat, dry as a bone, red as a beet, and mad as a hatter” - referring to the fever, blindness, dehydration, red skin, and hallucinations that overdose victims experience. As a result of all these side effects and dangers, we don’t typically use atropine for cherry eye, either before or after surgery.

If cherry eye is untreated, or if the gland is removed, and KCS (dry eye) develops, there are some treatments that will help if it is diagnosed early enough. These range from prescription eye ointments that need to be administered 2-3 times daily, to artificial tears which can be given as many times in a day as you can give them. In both cases, this must be done for the rest of the animal’s life. Obviously, this results in a lot of annoyance and discomfort for the patient, and places heavy demands of time and effort on the part of the pet owner, so it is best to just get the correct surgical treatment in the first place. If these medications aren’t administered properly, or don’t work, and the condition progresses, the complications from KCS can be severe enough that it is best to remove the eye entirely.

This doesn’t sound like an option that is worth the risk.

1. Homo sapiens has a vestigial version. [↩]

Tags: cherry eye, cornea, dry eye, eyeball, keratoconjunctivitis sicca, nictitating membrane, ocular, surgery, tear

This entry was posted on Monday, April 14th, 2008 at 4:08 am and is filed under medicine. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.