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.
Not to be reproduced without permission ©
Clare Rusbridge, Stone Lion Veterinary Centre, 41 High St, Wimbledon, SW19 5AU
16/02/2016
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 (110years) 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
16/02/2016