Before discussing abnormal, we need to briefly review normal eyes and development in the dog. Forgive me if this seems too basic for some of you. The globe, or eyeball, sits within the orbit, the boney socket of the skull that surrounds and protects the eye. It is attached by a variety of small muscles that control eye movements, as well as miscellaneous soft tissue structures. The conjunctiva is the tissue that lines the eyelids and the sclera, or white, of the eye. A few scleral vessels will be visible in the normal eye. The eyelid margins are lined by small glands, called meibomian glands. The nictitating membrane, or third eyelid, is located in the inner corner. A gland that helps produce tears lies just behind the third eyelid. Tear ducts are located in the upper and lower lids, also at the inner corner, and drain into the nasal cavity through the nasolacrimal duct. The cornea is the clear surface of the eye. The next visible structure is the iris, the colored portion of the eye, and the pupil, the hole in the middle, which controls the amount of light entering the eye by dilating and constricting. The lens lies behind the pupil, and cannot be visualized in a normal eye. The eye is filled with fluid, aqueous and vitrious humur, that circulates and maintains normal intraocular pressure within the globe. Fluid drains at the angle formed by the attachment of the iris to the ciliary body, the muscles that control pupil size. The back of the eye is made up of the retina. The retina is composed of cells that convert light energy to electrical energy, that is carried by the optic nerve to the brain, where vision is interpreted. Animal retinas differ from humans by the presence of the tapetum, a highly reflective portion of the retina. This is the reason that animal eyes reflect light back at you in the dark. The optic disc is the area that the optic nerve connects to the retina. The fundic exam is used to refer to examination of the retina and associated structures in the back of the eye. This is often referred to as the "window to the brain", as many diseases affecting the brain may show changes on fundic exam.
The embryonic eye has a membrane covering the region of the pupil, that is part of the blood supply of the developing eye. This membrane gradually dissintegrates and is usually gone by the time the eyes open. The eye open at approximately 10-14 days after birth. The cornea may have an initial cloudiness, appearing blue, caused by increased fluid content. This disappears in 2-4 weeks, as the fluid balance approaches that of the adult cornea. The normal bluish coloration of the iris will change to adult coloration within a few weeks as well. The tapetal membrane also starts as grayish blue and gradually develops adult coloration, which is dependent on coat coloration.
Corgis get many eye disorders, some acquired, others are congenital. Not all congenital defects are inherited, although many are genetic. I will discuss those disorders that are commonly screened for during the CERF examination first, which include many which have a suspected, if not proven, genetic component, working from the front of the eye to the back.
Some puppies will develop an infection behind the eyelids prior to the lids opening. The eyes will appear swollen, and a small amount of pus may be visible at the inner corner. The source of the infection may enter through the umbilicus or from tail docks. It is very important that the pus not sit in contact with the cornea for any length of time. The toxins developed by the white blood cells in the pus are very damaging to the cornea. If the inner corner has begun to open, the eyes can be flushed with sterile saline, then an antibiotic oinment placed under the lids. If there is no opening, heat packing may help or your veterinarian will need to very very carefully make a slit to allow drainage. The entire lid should not be opened unless absolutely necessary. If not treated promptly, the corneas may become permanently scarred, the lids may form adhesions to the cornea, causing permanent blindness in the affected eye.
Epiphora - This is nothing more than excessive tearing. The causes many be due to increased tear production, such as irritation to the eye, allergic conjunctivitis, infection, etc. It may be due to normal tear production, with abnormal eye conformation. In other words, a more globoid, protruding eye or poorly fitting eyelids, allowing tears to spill over the rim of the lids instead of going into the nasolacrimal duct. Some bloodlines in the corgi commonly have these conformational traits leading to epiphora. The last cause of epiphora is blocked tear ducts, preventing normal drainage of tears. One means of diagnosing blocked tear ducts is to apply stain to the eye, and watch for its presence at the nostril, because of the normal drainage through the nasolacrimal duct. Trauma, infection, or chronic allergies may cause damage to the duct, preventing normal drainage, as can the presence of thick mucoid discharge.
Entropion - This is the rolling in of the eyelids, causing the lashes to rub on the surface of the cornea. Entropion is rarely seen in the corgi, but is common in many other breeds, such as Shar Peis and chows, as well as lambs. The irritation to the cornea causes pain, tearing, and even corneal edema, or opacity, and ulcerations. Surgical correction is usually performed, although this may be delayed in a puppy until he has reached approximately adult size, as many entropions may correct themselves as the puppy grows. Antiinflammatory and antibiotic eye medications may be used in the interim in mild cases.
Distichiasis - This condition consists of eyelashes that are in an abnormal position, pointed towards the surface of the eye. This causes irritation to the cornea, and may lead to chronic conjuctivitis and tearing. A careful exam of the eyelids with magnification usually detects these stray hairs. Some vet ophthomologists will remove them through the equivalent of electrolysis or freezing.
Dermoids - Again, not a condition seen very frequently in the corgi, although someone on corgi-l recently inquired about a dermoid. These are masses of tissue contain contents of normal skin, including hair, sebacious (oil) glands, and fat, and usually are located on the outer margin of the eye, where the cornea and sclera join. Dermoids are readily visible as soon as the puppy's eyes open, and can be surgically removed.
Persistent Pupillary Membrane - Aka PPM. Remember that membrane covering the fetal pupil? PPM's are strands of that membrane that are still visible in the puppy and adult dog. PPM's are felt to be inherited in the Basenji, but the jury is still out in the corgi, although the incidence appears to be increasing. Most corgis have only 1-2 strands per eye, unlike the Basenji, who may have hundreds. They can occur iris to iris, meaning they cross the pupil itself and both ends attach to the iris. These do not cause vision deficits unless they appear as a spider web over the pupil. The other, less common, presentations are iris to cornea, attaching at one end to the back surface of the cornea, often causing a localized area of opacity, and iris to lens, attaching to the front surface of the lens, causing a small focal cataract. Our ophthomologists have no hesitations with dogs showing iris to iris PPMs being included in a breeding program. Spider web appearances, or iris to cornea, iris to lens, should be approached with more caution. Some PPM's may be visible in very young puppies, yet disappear as the dog ages, indicating a delay in the deterioration of a normal structure. The best method of checking for PPMs is to exam the eyes both before and after the pupils are dilated. Some PPMs at the margins of the pupil may not be visible once the pupils are dilated.
Heterochromia - Here is something you can impress your friends with, especially when you see blue merle cardigans, or other merles. This fancy term descibes variation in iris color, such as the merles who have merled eyes, both brown and blue coloration in the same eye. Blue eyes, which are occassionally seen in normal coat colors in Pembrokes, have been associated with deafness in white cats and some breeds of dogs. Curiousity, has anyone ever noted any deafness in blue-eyed corgis?? As many of the cats with one blue eye are deaf in the opposite ear, this may be very difficult to detect without special tests. Just wondering.
Cataracts - Cataracts are an opacity of the lens, affecting vision in the dog. Please note...the cornea is clear and the iris can be seen with a cataract, but the normal black hole of the pupil will appear with a whitish coloration. People often confuse diseases of the cornea, causing opacity, with cataracts. When the cornea is opaque, the iris and pupil will not be readily visible. Cataracts can occur in any aged dog, although those seen in young and middle aged dogs are much more likely to be inherited than "senile" cataracts seen in the older, geriatric patient. Inherited cataracts usually have a typical age of onset, a typical location within the lens itself, and a typical appearance, within individual breeds. Cataracts can occur from a variety of other causes, such as trauma to the lens, irritation to the lens, such as seen with PPM, diabetes, and aging changes. The severity of cataract formation determines the amount of vision loss. Many cataracts can be successfully removed with surgery, using similar techniques as that used in humans. There are some differences between dog and human lenses that makes the risk of complications slightly higher in the post-op period, plus it is difficult to tell the dog to keep quiet after surgery! Cataracts may also be confused with nuclear sclerosis, a normal aging change in the lens. The lens appears more blue & cloudy, vs white. The key is that a tapetal reflex can still be seen, and your vet can still see through nuclear sclerosis to the back of the eye. Your dog can also see out of nuclear sclerosis. A true cataract blocks light from getting to the back of the eye, thus vision deficits, prevents visualization of the back of the eye, and prevents the tapetal reflex from being obtained. Cataracts are one of the reasons that the CERF exam is only good for one year, as cataracts, even inherited, can develop at varying times in a dogs life. Many dogs have already produced offspring by the time they develop inherited cataracts, making this a breeder's worst nightmare in those breeds where it is a severe problem.
Retinal dysplasia - This has been diagnosed in several corgis, and a subtype, retinal folds, can be seen in dogs who have been CERFed. Instead of the retina lying smooth and flat on the back surface of the eye, there are areas that form ridges, folds, or puckers. No one knows whether these areas are more likely to lead to detachment, with resulting vision loss, or not. Retinal dysplasia is catagorized on the CERF form as geographic, folds, or detached. At this time, dogs with retinal folds can be CERFed, but geographic, involving larger areas of the retina, cannot. Some "folds" seen in young puppies may smooth out as the dog ages. It is almost as though the retina is too big for the size of the eye.
Progressive retinal atrophy - Aka PRA Anyone coming from collies and shelties is extremely familiar with this disorder, but it has been identified in a large number of breeds. The name says it all...it is a slowly progressive destruction or atrophy of the retina, leading to eventual blindness. The changes are irreversible and usually affects both eyes. The age of onset and speed of development varies with different breeds. Corgis tend to have a medium age of onset, another reason for repeated CERF examinations. PRA is considered to be inherited as a recessive trait in most breeds. Carriers can really only be detected through test breedings or production of affected offspring. Some affected dogs can be detected in the very early stages on a fundic exam, before any vision loss is apparent. Unfortunately, many affected dogs have already been introduced into a breeding program before changes are detected. The incidence is corgis is felt to be rather low at this time, as vision loss due to retinal disease in older corgis is relatively uncommon. However, recessive genes can be carried without being expressed for several generations, particularly when there is a lot of outcrossing in the breeding program. A good article on inherited eye diseases in the corgi can be found in the 1992 PWCCA yearbook or September '92 PWCCA Newsletter.
We are fortunate that the incidence of significant inherited or congenital eye disorders in the corgi is quite low. As breeders, the best means we have of keeping the status at this level is to examine the eyes of all breeding dogs annually and each puppy in every litter before they go into pet or show homes. Cataracts and PRA will require keeping in contact with our pet owners, hoping to be informed if any affected dogs from our breedings show up. Recognizing that some defects, such as retinal folds, can be present in animals who have been CERFed means that breeders must keep the lines of communication open, and be willing to share the actual results of eye examinations, not just CERF numbers, with each other. We are fortunate that corgi breeders, as a whole, have always shown this willingness to discuss the problems in the breed, whereas other breeds have not.
Persistent Pupillary Membranes seems to be the biggest area of concern in our breed right now, particularly as these dogs can no longer be CERF'ed. If we are going to determine the heritability of PPMs, we will need to start keeping accurate records of affected dogs, particularly affected puppies from two affected parents, etc.
Personal theory about PPMs - as this is a remnant of a normal embryonic structure, anything that many cause slow maturation of the puppy may result in PPMs. Talking to breeders across the country, it seems as if PPMs are more common in puppies who were stressed at delivery, such as dystocias, c-sections, weak puppies requiring tube feeding,etc. In my own case, one litter of affected puppies developed infections before the eyes opened. Coincidentally, the only eyes affected with PPM were also the eyes with the infections. Makes you wonder, doesn't it. The only other PPM affected puppy came from a bad c-section, where the puppies came out blue and cold, which is highly unusual for my sections. How about the rest of you, any feelings about this issue???
Lyn Johnson DVM and the Tartan Corgi Crew
Texas A&M College of Veterinary Medicine
College Station, TX