Notes from the BVA /KC CMSM Scheme Seminar held on 7 October 2010
 
Ruth Dennis (Animal Health Trust)
Ruth gave a very clear description of MRI scanning and the types of MRI scanner. There are basically 2 types of scanner, a high field scanner (1.5 Tesla) such as used in hospitals, and a low field scanner (0.2 – 0.3 Tesla) which is generally used in veterinary practices.

Ruth then went on to explain and illustrate the different types of images that were required. A T1W image giving a more accurate detailed image, but a T2W giving better contrast. For the purposes of imaging for a Chiari-like malformation and a syrinx, both types of images are required. The cord oedema of a pre-syrinx is only visible on a T2W weighted image.

When getting a mini scan for breeding purposes a T1W image is the usual image. T2W images are harder to get with low field scanners because they take longer. Longer scans give better results.

Ruth also explained which images need to be taken, sagittal (lengthwise) and transverse (cross section), and how frequently (every 4mm) cross section slices need to be taken so that a small syrinx is not missed between.

Many dogs that have SM in the neck also have it all the way down. In some cases it can be worse further back.

This whole procedure takes some time to perform, and all the time the dog must remain completely still. This is usually done with anaesthesia (because the dog will have a tube into the throat and oxygen can be administered as required), but can be performed with sedation, the choice is up to the veterinary practice performing the MRI scan.

Microchips can interfere with an MRI scan, therefore dogs destined for MRI scanning must have the microchip implanted no further forward than the scapulae.

The BVA scheme will be a compromise between effectiveness and cost.

Mike Herrtage (University of Cambridge Dept. Of Veterinary Medicine)
Mike will be the Chief Scrutineer of the proposed BVA/KC scheme, and gave the details of the scheme.

The vet will contact the MRI centre.

The minimum age for a scan will be 1 year. There will be no upper limit.

Best performed at age 5/6 (but obviously this will be too late for breeding)

The incidence figures of SM on MRI scan at present are:
  • age group 1 (0.17 – 1.15y)   23.9%
  • age group 2 (1.16 – 2.28y)   31.5%
  • age group 3 (2.30 – 2.90)   41.5%
  • age group 4 (2.92 – 4.00y)   54.5%
  • age group 5 (4.00y – 10.9y)   54.9%
The dog must have permanent identification (microchip or tattoo) The following documents must be made available: KC registration document, any related transfer certificate, or change of name certificate. Owner must sign a declaration certifying details are correct, that the results can be published, and that the results can used for research.

The MRI scanning images are DICOM images (Digital Imaging & Communication in Medicine) and written on a CD (as used in hospitals). DICOM images are used as it is not possible to divorce patient details from patient.

Mike repeated the need for the dog to be still for several minutes.

Sagittal T1W images need to be taken to at least C4/C5, which on a low field scanner means starting at the pituitary gland. The positioning of the dog needs to be exact. Sagittal T2W images are taken to at least C4/C5. Then transverse T2W images need to be taken in the maximum of 4mm slices between C2 and C4.

The Veterinary surgeon doing the scan completes the form verifying the microchip number, dates and signs it.

The following grades will be given for Chiari-like Malformation:

Grade 0 — NO chiari malformation
Grade 1 — Cerebellum indented (not rounded)
Grade 2 — Cerebellum impacted into or herniated into foramen magnum

When viewing the images for syringomyelia it is sometimes possible to view a presyrinx, which is cord oedema. This may be transitional.

The following grades will be given for Syringomyelia:

Grade 0 — Normal (No central canal dilatation, no presyrinx, no syrinx)
Grade 1 — Central Canal Dilatation less than 2mm, dog aged 6 or more
Grade 2 — Central Canal Dilatation less than 2mm wide, dog aged under 6
Grade 3 — Syrinx or Presyrinx or Central Canal Dilatation 2mm or greater than 2mm

There will be a standardised assessment using 2 scrutineers from the panel. If necessary the chief scrutineer will make the final decision. There will be an appeals procedure. Anticipated 2 week turnaround.

The scheme cost is not finalised yet but is expected to be in the order of £80 – £100. This will be in addition to the cost of the scan.

Questions raised on the BVA scheme

Who will be on the panel? – not yet determined

How will breeders explain a Grade 2 on a puppy registration form to puppy buyers, as opposed to a blank space for a dog that has either been unscanned or is even symptomatic for Syringomyelia?

Is it appropriate to issue a certificate to a 1 year old dog which enables breeders to claim that their dog has been scanned clear, when it is effectively only an interim certificate at a very young age?

Clare Rusbridge (Stone Lion Veterinary Centre)
What causes Chiari malformation? Be very careful about deciding breeding programs on appearance of brain only, the condition is not necessarily associated with a larger cerebellar herniation.

Dogs with CM have a shorter skull base and supraoccital bones may have a compensatory increase in other bones so that the forebrain is accommodated however unlike the forebrain the Cerebellum grows into a predefined cavity and at a later stage in development. The skull bones encasing the cerebellum (basioccipital and supraoccipital bone) are those seemingly deficient in chiari malformation. Unlike the other skull bones there is no compensatory growth and the cerebellum has insufficient room. There is a hypothesis that it is down to a primary bone insufficiency or premature suture closure (craniosynosystosis) (this may be being studied at the RVC Foetal Tissue Project). There also appears to be smaller frontal sinuses which is also likely to be compensatory – to make more space.

Skull volume – When all ages are lumped together there is no difference between the skull caudal fossa volume between those CKCS that have SM and those that don’t have SM. BUT the volume is significantly smaller in early onset SM (under 2y) than in clear of SM over 5 years of age. Volume of brain within is significantly greater in SM cases, even greater in early SM cases. The larger the syrinx the larger the brain within the skull. The larger the syrinx, the larger the ventricles. Scanning will become less necessary as genomic values will be good enough.

Then Clare illustrated what the revised Breeding Guidelines associated with the BVA scheme are proposed to be. THESE ARE NOT COMPLETELY FINALISED YET:

Grade Age Breed To:
0a > 2.5 0a, 0b, 1, 2, 3a*
0b < 2.5 0a, 1
1 > 5 0a, 0b, 1, 2, 3a*
2 < 5 0a, 1
3a > 5 0a, 1
3b < 5 DO NOT BREED
     
CLINICAL SIGNS AT ANY AGE – DO NOT BREED
3a* = must have been clear of SM before 2.5 years

Clare has seen no CKCS without Chiari-like Malformation, only 2 with mild CM in the past 12 months.

Then Clare gave a talk on some research (still undergoing Peer Review prior to publication) on the Interim Breeding Guidelines for Syringomyelia – 4 year report.

This research is based on the Breeding Guidelines issued in 2006, using 465 dogs (CKCS and Griffon Bruxellois)

Full details cannot be reproduced here (as these are interim results that are undergoing Peer Review prior to publication) but a summary of the results:
  • Offspring without SM only occurred when there was at least 1 parent of Grade A status
  • There were higher numbers of SM clear offspring if both parents had Grade A status
  • There was no influence of gender on SM affectedness
  • All grade A* (where a grade of A* was assigned to Grade A dogs that had MRI aged 5 years or older) offspring had at least one Grade A* parent
  • Higher numbers of A* offspring resulted from matings where both parents were grade A
  • 100% offspring were SM affected if both parents were SM affected. The parents had often been bred before the SM developed
  • SM affected offspring may also result from SM unaffected parents
  • Using dogs of unknown status was risky for SM affectedness. 50% of older offspring were affected in A x U parental crosses and there were higher numbers of SM affected dogs with other parental combinations that included 1 Grade U (where Grade U is parent of unknown status)
The conclusions and recommendations of the INTERIM report:

To increase the number of SM unaffected offspring at least 1 parent should be ascertained to be free of SM by MRI at 2.5 years of age. In ideal circumstances both parents would be free of SM at 2.5 years of age and the true SM status of the grandparents at least 5 years old should be established. It is recommended that all breeding dogs from breeds susceptible to CMSM be MRI screened and results submitted to an officially recognised central database.

Clare hopes that there might eventually be a drug that will stop SM happening.

The researchers need more adult dogs for post-mortem.
 
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