Syringomyelia secondary to occipital bone hypoplasia (Chiari-type malformation)
Syringomyelia is an increasingly common diagnosis in Cavalier King Charles spaniels
The primary problem is an occipital bone hypoplasia i.e. the bone at the back of the skull is too small. This means that the caudal fossa (i.e. the back of the skull) is not large enough to comfortably accommodate the cerebellum and brainstem (hindbrain). As a consequence the brainstem can be kinked and the back of the cerebellum can be forced out the foramen magnum (the hole in the skull though with the spinal cord exits) into the vertebral canal.
The obstruction of the foramen magnum prevents the cerebrospinal fluid (fluid surrounding central nervous tissue) from circulating freely. The fluid is forced into the spinal cord creating a cavity termed syringomyelia, which literally translates as flute – spinal cord i.e. it implies the spinal cord is hollow like a flute. The term syringomyelia is often shortened to “syrinx”
The resulting damage to the spinal cord results in the typical signs of this condition of which the most common is shoulder scratching especially when excited or walking on a lead. The scratching is typically to one side only but may become bilateral. The scratching is presumed to be due to abnormal skin sensation (paraesthesia/dyskinesia). There is no skin or ear disease. Humans with this condition describe the sensation as varying from a feeling that insects are crawling on the skin to a severe burning pain.
Affected dogs are also sensitive around the head, neck and forelimbs and often cry/yelp/scream for apparently no reason. Pain may be related to head posture and some dogs prefer to sleep or eat with their heads up. It is common to have mild weakness/muscle atrophy of the forelimb on the same side as the scratching. More severely affected dogs may have weakness or a wobbling hindlimb gait. Some severely affected immature dogs develop a neck scoliosis i.e. their neck is twisted.
The only definite way to diagnose syringohydromyelia and the associated skull malformation is by a MRI scan. Unfortunately this expensive test is only available at specialist veterinary centres.
Treatment options are limited. Drugs can help but typically do not resolve the clinical signs. The aim is to reduce the discomfort i.e. the scratching and screaming. Some mild cases are helped by NSAID drugs e.g. daily dose of Metacam ® or Rimadyl ®. The best response is seen with corticosteroids. However these drugs are associated with side effects such as immunosuppression, weight gait and skin changes and long term mediation with these drugs is not advised. If there is no alternative then use the lowest possible dose to control signs and ideally give on alternate days. Gabapentin (Neurontin) has been successful in some dogs. This is not a licenced medication in dogs but is licenced as a neurogenic pain killer in humans. The dose is 10-20mg/kg 2-3x times daily which for a CKCS typically works out at a dose of 100mg 2-3x daily. Sedation may be seen at high doses. Neurontin can also be given with NSAIDs, steroids or opioid e.g. pethidine tablets at 2-10mg/kg TID/QID or methadone syrup at 0.1-0.5mg/kg TID/QID. The main disadvantage of Neurontin is that it is expensive. Acupuncture appears to help some dogs.
For dogs with significant pain or that are deteriorating, surgery is advised. As this is technically difficult, it is only available at specialist centres. The aim of surgery is to reduce pain and prevent further deterioration. There are two types of surgery performed for this disease 1) foramen magnum decompression where the hypoplastic occipital bone and sometimes the dorsal laminae of the atlas are removed to recreate a foramen magnum and 2) shunting the syrinx. Although surgery is successful in many dogs some may have a recurrence or still show signs of pain/scratching.
Many dogs with this condition can lead relatively normal lives, some dogs progressively deteriorate and are euthanatised when middle aged. The condition is also seen occasionally in other breeds most notably King Charles spaniels, Yorkshire terriers and Maltese terriers.
Genetics
Study of a family tree of 150 affected dogs and their ancestors has established that this is a common hereditary condition in this breed. All affected cases can be traced back though at least 3 out of 4 grandparents, through certain significant ancestors, to two bitches. As a consequence it is suggested that the inheritance is likely to involve 1 or more recessive genes. There does not appear to be sex predilection.
There is a tendency for more severe disease with an earlier onset with increased inbreeding especially when breeding from affected dogs. There appears to be 3 forms of the disease based on severity and age of onset 1) neonatal form (less than 6 months) presenting with clinical signs relating to hydrocephalous 2) juvenile form (6-15 months) initially presenting with scoliosis secondary to syringomyelia and 3) adult form (1-10years) typically presenting with shoulder scratching and pain secondary to syringomyelia. The disease is not linked to coat colour however selection for coat colour variation is believed to have influenced the development of the disease for example the disease is most common in Blenheim and Rubies which are recessive coat variations and must be bred from a more restricted gene pool. Avoidance of some lines which carry certain disease e.g. heart and cataract disorders is also affecting the incidence of syringomyelia by narrowing the gene pool.
Unfortunately syringomyelia is very widespread in CKCS lines, the number of potential carriers is huge and as such a breeding program based on avoiding certain dogs is not possible. The only way forward is likely to be developing a test for the gene so that affected dogs and carriers can identified and the latter safely bred. In the meantime breeders are advised the following.
· Affected case Identified by MRI or suspected on basis of clinical signs (scratching at shoulder area when walking on leash or when excited) Not to be used for breeding.
· Unaffected known carrier (sire, dam or offspring of an affected case). If mated with same can produce affected offspring. Only use very sparingly – i.e. retaining maximum possible variation in the gene pool but not saturating it. Mate only with unrelated dogs who have had no extended family history of the disorder
· Unaffected dog (it is likely that all modern CKCS will be carrier one or more of the genes). Don’t use closely related dogs, line breed only on one side. Keep track of offspring as time of onset of disease can vary.
References
Rusbridge, C. Macsweeny J.E., Davies, J.V., Chandler K., Fitzmaurice S.F., Dennis, R., Cappello, R. & Wheeler, S.J. (2000) Syringohydromyelia in Cavalier King Charles Spaniels Journal American Animal Hospital Associate 36 34-41.
Rusbridge, C & Knowler S.P (2003) Inheritance of occipital bone hypoplasia (Chiari I malformation) in Cavalier King Charles spaniels Veterinary Record,153, 107-112.
Not to be reproduced without permission ©
Clare Rusbridge, Stone Lion Veterinary Centre, 41 High St, Wimbledon, SW19 5AU 07/11/2003