Primary open angle glaucoma in Norwegian Elkhounds (POAG)

Glaucoma is a heterogeneous group of disorders that lead to blindness due to the death of retinal ganglion cells and damage to the optic nerve. It is the most common cause of the blindness and visual impairment in humans and dogs worldwide. In general, the glaucoma is classified as primary (in most cases inherited) and secondary (acquired due to eye damage - e.g. injury, tumour and others).

The primary glaucoma is sub-divided in three groups by the iridocorneal angle (the angle between the iris, the cornea and the white):

• Primary open angle glaucoma (POAG)
• Primary angle closure glaucoma (PACG)
• Primary congenital glaucoma (PCG)

The POAG glaucoma is characterized by elevated intraocular pressure (IOP), loss of retinal ganglion cells and atrophy of the optic nerve.

The clinical symptoms occur between 9 and 18 months of age. They are very different and depend on the duration and rate of disease development and the age of the dog. The main symptoms are:

• Painful eyes
• Blepharospasm - lid spasm, squinting
• Behavioural changes - apathy, loss of appetite,  looking for dark places, depression, aggressive behaviour, eye rubbing
• Mydriasis - dilatation of pupil
• Cornea oedema that causes greyish-blue up to cloudy colour of the cornea
• A red (bloodshot) eye - typical for glaucoma
• Elevated pressure of aqueous fluid
• Visual changes, orientation problems
• Exophtalmos - protrusion of eyeball from the socket
• Lens luxation - dislocation of the lens from the normal position
• Shallow anterior chamber  - narrowing of chamber angle

The success of any glaucoma treatment depends upon how early the condition is diagnosed. The preventive measures include genetic testing.

In the Norwegian Grey Elkhound, a missense mutation c.1159G>A in the ADAMTS10 candidate gene has been described as the cause of POAG. The protein encoded by ADAMTS10 plays an important role in skin and lens formation and has also an effect on heart development.

Mutation that causes POAG is inherited as an autosomal recessive trait. That means the disease affects dogs with P/P (positive / positive) genotype only. The dogs with P/N (positive /negative) genotype are clinically without any symptom. They are genetically considered carriers of the disease (heterozygotes). In offspring of two heterozygous animals following genotype distribution can be expected: 25 % N/N (healthy non-carriers), 25 % P/P (affected), and 50 % N/P (healthy carriers). Because of high risk of producing affected offspring, mating of two N/P animals (carriers) can not be recommended.

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Reference:

Ahonen, S.J., Kaukonen, M., Nussdorfer, F.D., Harman, C.D., Komáromy, A.M., Lohi, H. : A novel missense mutation in ADAMTS10 in Norwegian Elkhound primary glaucoma. PLoS One 9:e111941, 2014. Pubmed reference: 25372548.