The majority of the studies were carried out by Dr. S. Grozdanic, DVM, Ph.D., a veterinary ophthalmologist and a long time collaborator of Dr. Kuehn's.
Glaucoma is a disease that results in progressive degeneration of the optic nerve and the retinal ganglion cells. It is often triggered by elevated pressure inside the eye, but this is not always the case. Glaucoma is a common problem in many dog breeds, including the Basset Hound. In dogs glaucoma is often associated with very high intraocular pressure and while glaucoma drops may have a positive effect, it is often not enough to reduce the pressure to a safe level. In humans surgery may be used in individuals where medication alone does not lower the pressure enough, but these procedures do not work well in dogs.
Glaucoma can arise seemingly by itself. This case is referred to as primary glaucoma. The disease can also occur as a complication of other conditions such as trauma to the eye, inflammation, or pigment loss from the iris. These cases are referred to as secondary glaucoma. While the end effect may be the same, i.e. elevation of intraocular pressure, the mechanism how this happens is totally different. Please note that the following addresses only primary glaucoma in the Basset Hound.
Gonioscopy is a technique that uses a special lens to allow the ophthalmologist to view the iridocorneal angle. In our experience gonioscopy can predict glaucoma only in cases where the angle appears narrow or completely closed. While this may be the case in some animals we believe that more commonly the cleft, a narrow structure behind the angle, collapses. The cleft is very hard, if not impossible, to see using gonioscopy but can be visualized using High Resolution Ultrasound (see below).
A second complication during gonioscopy is the frequent presence of pectinate ligament dysplasia (PLD) in Basset Hounds. PLD are strands of connective tissue that can form in front of the angle. While the presence of some of these strands is normal, PLD refers to a situation where this material begins to look more like a sheet and not individual strands. The presence of PLD obscures the view of the angle in gonioscopy.
Based upon our current knowledge the presence of PLD is not a good indication whether a Basset will develop glaucoma or not. The vast majority of Bassets with PLD will not develop glaucoma, but some will. On the other hand, we have seen Bassets that develop aggressive glaucoma in the absence of any PLD.
High Resolution Ultrasound (HRUS):
In our experience the best prediction whether a Basset Hound will develop glaucoma can be made using High Resolution Ultrasound (HRUS). This technique allows visualization of the angle structures that are too far set back in the eye to be visualized by gonioscopy.
Most Basset Hounds will have an open angle and cleft as young dogs. However, in some individuals a general narrowing of the angle can be observed beginning around one year of age. In dogs destined to develop glaucoma the angle will continue to collapse until the cleft is completely closed.
Below are two scenarios that we frequently encounter in Basset Hounds:
HRUS imaging of the iridocorneal angle from a healthy dog (left) and an animal which eventually developed glaucoma (right). Click on image to enlarge.
The image on the left was taken from a dog which showed presence of severe PLD throughout all sectors of the eye upon gonioscopy while the width of the angle appeared normal. HRUS examination was performed and showed normal width of the cleft. This dog is now 7 years old, never produced affected puppies, has otherwise normal ocular examination and normal intraocular pressures before and after provocative testing. This dog is not likely to develop glaucoma despite complete PLD of the angle. Our recommendation was that this dog is OK to breed.
This finding is typical of 60-70% of healthy Basset Hounds that we have examined.
The image on the right was taken from a dog which showed normal width of the angle and complete PLD on gonioscopy examination. However, HRUS examination showed very narrow, near collapsed cleft. This dog was only 3 years old at the time of the examination. In this case the conformational changes did get worse with age, eventually resulting in a completely collapsed cleft and glaucoma. However, many dogs develop narrow or partially collapsed clefts and remain in this state for the rest of their lives. It is currently not clear if dogs with partially collapsed clefts are heterozygous carriers of the genetic defect. Our recommendation at this time is to avoid breeding dogs with narrow or collapsed clefts. Exams should be carried out on animals that are at least three years old. In our experience dogs that have open angles and clefts at that age remain healthy.
The tendency of some dog breeds, including the Basset Hound, to develop primary glaucoma has long been considered an indication that there is a hereditary basis to the disease. However, to date a disease causing mutation has not been identified and even the mode of transmission remains unclear. A mutation in the gene ADAMTS10 has been described as causing glaucoma in Beagles and other breeds. However, sequence analysis carried out in our lab demonstrates that this gene does not play a role in Basset glaucoma. To determine which gene is mutated in Bassets with this disease Iowa State University maintains a breeding colony of Basset Hounds. The colony is based upon two affected founder animals that developed primary glaucoma around three years of age in the absence of PLD.
In this colony glaucoma is transmitted as a simple recessive trait. Breeding of two affected dogs always results in affected offspring, yet the founders were derived from healthy individuals and matings to unaffected dogs have thus far not produced affected offspring.
However, analysis of other pedigrees has given indications that the inheritance pattern may be more complicated. It is possible that multiple genes act together to create glaucoma. It is also conceivable that more than one mutation exists in the Basset Hound population that can cause glaucoma.
One aim of our research is to understand the genetics of glaucoma in the Basset Hound. This would enable us to develop genetic tests that could be used to detect carrier and affected dogs at a very early age. Exclusion of these individuals from breeding programs would ultimately reduce the number of glaucoma cases in the Basset Hound.
The findings of such studies become more accurate the more dogs are involved. We urge Basset Hound breeders with large pedigrees including live affected individuals to participate in our study. Participation will involve clinical eye examination and a blood draw. The identity of the breeder or kennel will remain completely confidential. For more information please contact Dr. Kuehn
1. Why and how glaucoma develops in the Basset Hound is not entirely clear. The recommendations below are based upon the findings in the dogs we have worked with and may or may not be true for all Bassets or other breeds. Please understand that this is a work in progress.
2. Glaucoma in the Basset Hound is hereditary. Glaucoma appears to be inherited in a recessive fashion, and there are indications that more than one gene is involved. This means that a dog without glaucoma can produce litters with the disease. This also means that a Basset that has produced offspring that developed primary glaucoma could do so again if bred again.
3. Pectinate ligament dysplasia (PLD) does not seem to be a good indicator of whether or not a Basset will develop glaucoma. Rather, it appears that changes in the irdocorneal angle and cleft are better predictors.
4. The glaucoma-causing changes in the eye occur slowly as the dog ages. Puppies and very young dogs may have normal eyes but gradually develop narrow/closed angles and glaucoma later in life.
5. All breeding animals should have gonioscopy and HRUS exams at the age of 3 years (or older). Provocative intraocular pressure testing could also be considered, although there is some risk associated with this test.
6. There is currently no sure way to detect carrier animals i.e. healthy animals that may produce affected pups. However, dogs with narrow or partially collapsed clefts are suspicious.
A recently published scientific manuscript describing clinical findings from in Basset Hounds can be downloaded here.