Alternative Implantation Sites
The standard implantation site for microchips in dogs, cats, rabbits and ferrets within the UK is subcutaneously (under the skin) midline between the shoulder blades. However, there are occasions where different implantation sites may be used. One noteable example is that in Mainland Europe many animals are implanted in the left side of the neck as standard.
More relevant to the UK is a situation relating to breeds of dog such as the Cavalier King Charles Spaniel, which suffer from Chiari malformation and syringomyelia. This is a widespread condition in certain breeds, often requiring MRI of the cervical and possibly thoracic spine for diagnostic and breed health testing purposes. Problems may arise as the metal components present in the microchip can cause distortion or blank zones to appear around the microchip on the image itself. This leads to problems with accurate interpretation if this distortion affects an area that needs to be examined.
With standard MRI machines operating at 0.5 -1.5 Tesla, the zone of distortion around the microchip isn't too large, but can cause a problem if it happens to be in the region of a syrinx lesion. Zones of distortion may also be larger in machine operating at a higher Tesla.
Therefore there has been some debate about where to place the microchip in these dogs. Correct placement of the microchip in the midline and between the shoulder blades should mean that the microchip does not interfere with any scans of the cervial spine. However, one expert, Dr. Clare Rusbridge PhD. MRCVS of Fitzpatrick Referrals has recommended that they be placed further back than usual. Speaking at a syringomyelia symposium organised by the Griffon Bruxellois Club of the UK, Dr. Rusbridge recommended that microchips be placed as low as possible over the thoracic vertebrae. They should be placed along the middle of the back/shoulders to ensure that they don't overlie the area of concern.
We recommend taking veterinary advice for microchipping affected breeds, especially in dogs that are going to be scanned as part of a breed health testing scheme. Some other toy breeds have the condition and breeders should discuss this with their veterinary surgeon.
If an alternative site is chosen, we would recommend this is noted on the microchip paperwork for future reference.
The images below are two MRI scans of a Cavalier King Charles Spaniel's cervical spine. This dog has Chiari malformation and syringomyelia. The latter is the cigar-shaped structure in the spinal cord at C2. The left hand image is a sagittal T1 scan so that CSF is dark; the one on the right is a sagittal T2 scan in which the CSF is white. Both show the distortion caused by a microchip located in the caudal dorsal neck.
The images were kindly supplied by Dr.Geoff Skerritt FRCVS of ChesterGates, a world leader in veterinary MRI and one of the leading referral centres in the UK.