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,
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.,
Rusbridge, C
& Knowler S.P (2003) Inheritance of occipital bone hypoplasia (Chiari I
malformation) in Cavalier King Charles spaniels
Veterinary Record,153, 107-112.