MRI screening breeding animals

 

Why MRI screen?
To determine if the dog has syringomyelia. Early estimates of the heritability of syringomyelia suggest that it is high and consequently it should be possible to select against dogs with syringomyelia - i.e. removing dogs with syringomyelia from the breeding program will lessen the chance of syringomyelia in the offspring.

 

But I have heard that it is not that simple?
There are 3 main problems 1) the precise manner in which syringomyelia is inherited and whether that inheritance is modified or influenced by other factors has not been elucidated 2) syringomyelia can be a late onset disorder meaning that breeders can be not be truly confident that the dog is truly clear of the disease until the dog is middle aged. 3) Eliminating all dogs with syringomyelia from the breeding program may reduce the available gene pool and make other diseases more likely.
Work continues on the first problem and second and third issue will be addressed by the proposed (EBV) computer modelling system.

 

Does a MRI give conclusive results?
Yes a MRI is conclusive for syringomyelia (assuming that the images are correctly acquired).

 

I have been told that “syrinxes can develop a day after an MRI...so what good is the MRI except at the moment it is taken?”
Syrinxes cannot develop the day after MRI. It is not known how quickly they do develop in the Cavalier however it is likely to be over weeks. Most laboratory animal models of syringomyelia take at least 2 months for the pathology to develop.

 

I have heard that there is no point in MRI screening because neurologists cannot agree on the interpretation
A group of key UK neurologists and radiologists met, discussed and agreed on a protocol and interpretation of MRI for syringomyelia (for summary click here for PDF or click here for Word document). The consequence of this meeting was a report with guidelines which was then circulated to all remaining European and key North American neurologists with an invitation to comment or make suggestions. The final report is forming the basis of a proposed official scheme hopefully to be endorsed by the Kennel Club and British Veterinary Associations.

 

When is the proposed BVA / KC MRI screening scheme going to start?
Unfortunately the process of conception to an up-and- running scheme can be lengthy. The proposed scheme has been submitted to the BVA/KC Management Committee for their consideration.

 

So what are the breeding guidelines?
The current breeding guidelines can be downloaded here.

For a real life example of how a breeder used the guidelines over 3 generations click here

It is hoped that they can be replaced by the more accurate EBV system in the near future. The essence of the current guidelines was/is
1) removal of all dogs with early (less than 2.5 years of age) clinical or asymptomatic symptomatic syringomyelia from breeding programs.
2) young dogs (less than 2.5 years) MRI clear of syringomyelia should only be mated with older dogs MRI clear of syringomyelia (where older is defined as over 2.5 years). 
3) If asymptomatic syringomyelia affected older dogs must be used (e.g. to maintain genetic diversity), then they should be mated with older dogs MRI clear of syringomyelia (where older is defined as over 2.5 years).
4) If a dog had not been MRI scanned then ideally it should not be used particularly if less than 2.5 years.  If it must be used (e.g. to maintain genetic diversity) and is older than 2.5 years and not displaying signs of syringomyelia then it safer to assume that it is syringomyelia affected and mated to a older dogs MRI clear of syringomyelia.

 

What is the Estimated Breeding Value (EBV) computer model?
This system is designed to facilitate selection against disease while controlling inbreeding and loss of diversity.  The great advantage of the system is that is can simultaneously take account of several inherited diseases problematic for that breed (e.g. syringomyelia and mitral valve disease). Based on statistical likelihood the computer program generates an Estimated Breeding Values (EBV) for each dog.

 

An EBV is the best measure available for complex traits and the genetic potential of individuals and can be calculated for most Cavaliers even if they have not been MRI scanned, as long as they are related to dogs that have been scanned.  The predicted EBV of an individual is half the EBV of its sire plus half the EBV of its dam.  All dogs will have an EBV at birth but the EBV may be modified by the dog’s subsequent clinical record or MRI scan and by information coming from other relatives.

 

The system does depend on the input of accurate data.  The basis of the original database was information collected by Penny Knowler and Clare Rusbridge on clinical observations for SM and Chiari-like malformation (CM) on around 1,400 dogs of which ~ 700 had MRI scan results. This database was passed to the researchers, Dr Sarah Blott and Dr Tom Lewis who also have full access to the UK Kennel Club pedigree records for Cavaliers. Confidentiality of the information was maintained throughout.

 

New information added to the database is in the form of MRI reports from board certified neurologists and radiologists and over time the EBV will become more accurate as information on offspring becomes available, because insight is gained into which half of the sire and dam genes were actually inherited. For more information about the EBV system click here

 

Until the KC / BVA scheme is established are neurologists automatically forwarding results from MRI screening to the CKCS Health Breeding Programme at the Animal Health Trust?
Submission of MRI reports are currently at the owner’s discretion although some neurologists may forward on your behalf if requested. To be sure that information from your lines is being incorporated send copies of MRI reports and pedigrees to Dr Sarah Blott CKCS Health Breeding Programme Animal Health Trust, Lanwades Park, Kentford, Newmarket, Suffolk CB8 7UU This can also be done electronically to sarah.blott@aht.org.uk.

 

Why should I forward the MRI results from my dogs to the CKCS Health Breeding Programme at the Animal Health Trust?
Simply because the more information there is the more accurate it will be.  You are the (intended) end user of the scheme and you are the one most likely to want and need accurate information about your lines.

 

Why bother MRI screening - why not wait until the EBVs are available?
The EBVs are dependant on MRI information.

 

Why bother MRI screening - why not wait until a genetic test is available?
Action should be taken now – the condition could be more prevalent by the time a genetic test is available. For more information about the genome studies click here 

 

What age is it recommended to screen dogs? 
The minimum screening age is 12 months. It is also recommended that breeders determine the MRI status of their breeding dogs at 2-3 years and again when 6 years of age. This will provide further information about that individual dog’s estimated breeding value EBV (and therefore the EBV of that individual dog’s offspring).

 

I have heard that the dogs must have permanent identification for MRI screening?
Dogs presented for scanning must have permanent identification in the form of microchip / tattoo and Kennel Club Registration number
The microchip / tattoo number and the pedigree name, sex, breed and date of birth should be incorporated onto the MRI images.

What is a mini versus a full MRI scan?
A “mini” MRI is a limited MRI study of the back of the brain and upper neck provided at low cost by veterinary practices and intended as an economical screening test for breeders.

Example of a good quality mini MRI scan.  The minimum required images are
1) Sagittal T1W from intra thalamic adhesion to as far caudal as possible – The images must include a mid sagittal section of spinal cord visible in one section from the cisterna magna to the C4/C5 intervertebral disc space.  If this cannot be achieved because the dog has scoliosis secondary to syringomyelia then a dorsal image of the spinal cord must be included.
2) Sagittal T2W as above 3) T1W Transverse images though the maximum width of the syrinx if there is syringomyelia or as a block centred on C2/C3 and extending from at least mid point of the vertebral body of C2 and reaching the mid point of the vertebral body of C3

For details of MRI screening at Stone Lion Veterinary Centre click here
For details of other centres which offer MRI screening click here 

 

Is a mini MRI scan sufficient – I have been told that syringomyelia cannot be diagnosed without a full scale MRI?
A mini MRI scan has a high sensitivity (i.e. negligible false positive rate) but a lower specificity (i.e. higher false negative rate) – in other words there is a possibility that some dogs that have syringomyelia might be missed becasue the entire neuraxis is not imaged. Syringomyelia tends to develop first in the upper cervical spinal cord so screening of this area will detect early signs of disease in the majority of dogs. In ideal circumstances every dog would have a full MRI scan however breeders are limited by cost. A full MRI scan costs upwards of £1000 compared to £200 – 300 for a mini MRI scan and in reality the number of dogs that would be additionally identified by the full scan is not worth the economic outlay for most breeders. However for a dog with clinical signs of syringomyelia a full MRI is important to ascertain the true extent of the disease and rule out/in other spinal cord problems.

 

I have a dog that was determined to have syringomyelia on screening when young (less than 2 years old). He is now 5 years old and he still doesn’t seem to have signs of the disease. Surely he is OK to use at stud?
No at the current time it is not advisable to use dogs with early onset syringomyelia at stud – it is possible that in the future the EBV system might be able to help identity a safe mate.

What exactly is the problem using a dog with asymptomatic syringomyelia at stud? The dog appears happy and healthy!
Hopefully he will remain happy and healthy; the real significance of asymptomatic dogs is that their offspring appear to have a higher chance of being affected and more chance of being symptomatic
 

References (for full list click here)

 

Bynevelt M, Rusbridge C, Britton J. 2000 Dorsal dens angulation and a Chiari type malformation in a Cavalier King Charles Spaniel. Vet Radiol Ultrasound. Nov-Dec;41(6):521-4.

Cerda-Gonzalez S, Olby NJ, Pease TP: Morphology of the Caudal Fossa in Cavalier King Charles Spaniels. Veterinary Radiology & Ultrasound, Vol. 50, No. 1, 2009, pp 37–46.

Corfield GS, Burrows AK, Imani P, Bryden SL The method of application and short term results of tympanostomy tubes for the treatment of primary secretory otitis media in three Cavalier King Charles Spaniel dogs.. Aust Vet J. 2008 Mar;86(3):88-94

Dewey CW, Bailey KS, Marino DJ, et al. 2007 Foramen magnum decompression with cranioplasty for treatment of caudal occipital malformation syndrome in dogs. Veterinary Surgery. Jul;36(5):406-15

 

Granger N, Jeffery ND. 2009 MRI findings in severe chronic spinal cord injury of dogs BSAVA 52nd Annual Congress, Birmingham Scientific Proceedings: 415

Rusbridge C, Knowler SP 2006 Coexistence of occipital dysplasia and occipital hypoplasia/syringomyelia in the cavalier King Charles spaniel.. J Small Anim Pract. Oct;47(10):603-6.

Rusbridge C, Knowler SP, Pieterse L, McFadyen A. K 2009 Chiari-like Malformation in the Griffon Bruxellois Journal of Small Animal Practice Awaiting Publication

Rusbridge, C 2007 Chiari-like malformation with syringomyelia in the cavalier King Charles spaniel; long term follow up after surgical management  Veterinary Surgery 36:396–405, 2007

Rusbridge, C., Caruthers, H., Dubé, M-P., Holmes, M., Jeffery, N.D., 2007. Association between spinal cord dorsal involvement and pain in syringomyelia secondary to canine Chiari malformation. Journal of Small Animal Practice 48, 432–436

Stalin CE, Rusbridge C, Granger N, Jeffery ND. 2008 Radiographic morphology of the cranial portion of the cervical vertebral column in Cavalier King Charles Spaniels and its relationship to syringomyelia. Am J Vet Res. Jan;69(1):89-93