CANINE CHIARI-LIKE MALFORMATION AND SYRINGOMYELIA
Clare Rusbridge BVMS DECVN PhD MRCVS, European and RCVS Specialist in Neurology
Chiari-like malformation (CM) is a condition characterised by a mismatch between
the caudal fossa (skull) volume and its contents, the cerebellum and brainstem. The neural structures are displaced into the
foramen magnum obstructing cerebrospinal fluid (CSF) movement. A consequence of this is syringomyelia (SM) where fluid filled
cavities develop within the spinal cord (Fig 1). The primary clinical sign of CM/SM is pain, either due to obstruction of
the CSF pulse pressure and/or a neuropathic pain syndrome due to damage to the spinal cord dorsal horn. This disease has also
been referred to as occipital hypoplasia (Rusbridge et al 2006) and caudal occipital malformation syndrome (COMS) (Dewey et
al 2005). CM/SM is sometimes erroneously confused with Arnold Chiari malformation (cerebellar and medulla herniation associated
with myelomeningocoele) and occipital dysplasia (incomplete ossification of the supraoccipital bone).
FIGURE 1: Midsagittal T2 weighted MRI of the brain and upper cervical spinal
cord from 3 year old female CKCS with syringomyelia (asterisks) that first developed signs of pain at 1.7years old. Clinical
signs included shoulder scratching at exercise and when excited. She would not tolerate her right ear to shoulder area to
be touched or groomed. She frequently screamed and her owners were not able to exercise without her becoming distressed. She
also had mild pelvic limb weakness. She was managed with a foramen magnum decompression and despite persistence of the syrinx,
made a satisfactory post operative that was maintained for 1.8years. Following this deterioration she was managed medically
for a further 3.8 years and is currently 7 years old.
The pathogenesis of canine CM/SM is not fully understood. An important contributory
factor is thought to be that the brain is too big for the skull and early studies suggest that there is an inappropriately
short skull base (basioccipital bone (Fig 1). Cavalier King Charles spaniels (CKCS) with clinical signs related to syringomyelia
are more likely to have a smaller ratio of cauda fossa (i.e. back of skull) volume to total brain volume compared to unaffected
CKCS (Cerda-Gonzalez et al 2006). However it is likely there are other unidentified anatomical or environmental factors. Studies
comparing skull dimensions did not demonstrate a significant difference between the size of the back of the skull in CKCS
with and without syringomyelia (Curruthers et al 2006, Cerda-Gonzalez et al, 2006).
The precise pathogenetic mechanism of development of syringomyelia is much
debated (reviewed by Rusbridge et al, 2006; Greiz, 2006). The most popular theory is that obstruction of CSF flow results
in relative increase in spinal cord pressure and decrease in pressure in the CSF space around the spinal cord, the consequence
of which is repeated mechanical distention of the spinal cord. This in turn results in dilatation of the central canal and
accumulation of tissue fluid which eventually coalesces into cavities.
The CKCS is overwhelmingly overrepresented for cases of CM/SM. There is no
colour or sex predisposition. As shortened skull is a risk factor, any breed with a degree of brachycephalism and/or miniaturization
could potentially be predisposed to CM/SM. To date the condition has been also reported in King Charles spaniels, Brussels
griffons, Yorkshire terriers, Maltese terriers, Chihuahuas, Miniature dachshunds, Miniature/toy poodles, Bichon Frise, Pugs,
Shih Tzus, Pomeranians, Staffordshire bull terriers, a Boston terrier, French bulldogs a Pekingese, a miniature Pinscher and
a couple of cats. Recent studies suggest 35% of SM-affected dogs have clinical signs of the condition. The youngest reported
dogs with SM have been 12 weeks old. Dogs may be presented at any age although the majority of dogs (approximately 45%) will
develop first signs of the disease within the first year of life and approximately 40 % of cases have first signs between
1 and 4 years old. As many as 15% develop signs as mature dogs with the oldest reported case first developing signs of disease
aged 6.8 years. Due to the vague nature of signs in some cases and lack of awareness about the disease there is often a considerable
time period (mean 1.6 years) between the onset of signs and confirmation of a diagnosis.
The most important and consistent clinical sign of CM/SM is pain however this
may be difficult to localise. Owners may describe postural pain; for example, affected dogs may suddenly scream and/or lie
with the head on the ground between the paws after jumping up or during excitement. It is also common to sleep with the head
in unusual positions, for example elevated. Discomfort often appears worse in the evening and early morning or when excited
and can be associated with defecation or may vary with weather conditions. Pain is positively correlated with syrinx width
and symmetry (Fig 2); i.e. dogs with a wider asymmetrical syrinx are more likely to experience discomfort, and dogs with a
narrow syrinx may be asymptomatic, especially if the syrinx is symmetrical. Dogs with a wide syrinx may also scratch, typically
on one side only, while the dog is walking and often without making skin contact, such behaviour is often referred to as an
“air guitar” or “phantom” scratching. Dogs with a wide syrinx are also more likely to have scoliosis.
In many cases the scoliosis slowly resolves despite persistence of the syrinx.
SM may result in other neurological deficits such as thoracic limb weakness
and muscle atrophy (due to ventral horn cell damage) and pelvic limb ataxia and weakness (due to white matter damage or involvement
of the lumbar spinal cord by the syrinx). Seizures, facial nerve paralysis and deafness may also be seen; however, no direct
relationship has been proven and this association may be circumstantial.
CM alone appears to cause facial pain in some dogs with owners describing ear
and facial rubbing/scratching. It has been proposed that CM and compression of the brain stem can result a pain syndrome (Thimineur
et al, 2002). In this circumstance it can be difficult to be certain that the CM, as apposed to ear, oral or skin disease,
is the cause of the distress especially as CM is a common incidental finding in the CKCS breed.
FIGURE 2: T2 weighted transverse image through a wide syrinx (asterisks) demonstrating
the asymmetrical involvement of the right spinal cord dorsal horn.
Progression of disease is variable. Some dogs remain stable or deteriorate
minimally over years. Other affected dogs can be severely disabled by pain and neurological deficits within 6 months of the
first observed signs.
Magnetic resonance imaging (MRI) is essential for diagnosis and determining
the cause of SM (Fig 1). In the instance of CM/SM the cerebellum and medulla extend into or through the foramen magnum which
is occluded with little or no CSF around the neural structures. The size of the cerebellar herniation is not correlated with
severity. There is typically ventricular dilatation. SM is indicated by fluid-containing cavities within the spinal cord.
The upper cervical and upper thoracic segments are typically most severely affected. Maximum syrinx width is the strongest
predictor of pain, scratching behaviour and scoliosis; 95% of CKCS with a maximum syrinx width of 0.64cm or more will have
associated clinical signs.
CT and radiographs have limited value. In severe cases cervical images may
suggest widening of the vertebral canal especially in the C2 region and/or scoliosis. Flexed and extended radiographs of neck
can be used to rule out vertebral abnormalities such as atlantoaxial subluxation and for an indication of the likelihood of
intervertebral disc disease.
Ultrasonography through the cisterna magnum may confirm cerebellar vermis herniation
however as CM is so common in the CKCS this information has limited value. Likewise a syrinx may be identified if within the
cranial/cervical segment; however, failure to detect a syrinx does not eliminate the possibility of one more caudally. CM/SM
does not appear to increase risk of anaesthesia.
The most important differential diagnoses are other causes of pain and spinal
cord dysfunction such as intervertebral disc disease; CNS inflammatory diseases such as granulomatous meningoencephalomyelitis;
vertebral abnormities such as atlantoaxial subluxation; neoplasia; and discospondylitis. When scratching or facial/ear rubbing
is thepredominant clinical sign, ear and skin disease should be ruled out. The scratching behaviour for SM is classically
to one distinct area. It is a common incidental finding for CKCS to have a mucoid material in one or both tympanic bullae
and in the majority of cases this is not associated with clinical signs. Some cases with scoliosis appear to have a head tilt
which could be confused with vestibular dysfunction. If in doubt cervical radiographs can confirm scoliosis.
The main treatment objective is pain relief. The most common surgical management
is cranial/cervical decompression (also described as foramen magnum or suboccipital decompression) establishing a CSF pathway
via the removal of part of the supraoccipital bone and dorsal arch of C1. This may be combined with a durotomy (incision of
the dura with/without incision of subarachnoid meninges) with or without patching with a suitable graft material. Cranial/cervical
decompression surgery is successful in reducing pain and improving neurological deficits in approximately 80% of cases and
approximately 45% of cases may still have a satisfactory quality of life 2 years postoperatively (Rusbridge 2007). However
surgery may not adequately address the factors leading to SM and the syrinx appears persistent in many cases (Rusbridge 2007).
The clinical improvement is probably attributable to improvement in CSF flow through the foramen magnum. In some cases scaring
and fibrous tissue adhesions over the foramen magnum seem to result in re-obstruction and 25% to as many as 50% of cases can
eventually deteriorate (Dewey et al 2005, Rusbridge 2007). This can be as early as 2 months postoperatively. Recently, a cranioplasty
procedure used in human cranial/cervical decompression surgery has been adapted for use in dogs. The procedure entails placement
of a plate constructed of titanium mesh and polymethylmethacrylate (PMMA) on pre-placed titanium screws bordering the occipital
bone defect (Dewey et al 2006). An alternative method of managing SM is direct shunting of the cavity. In humans this is not
a preferred technique as long term outcome is poor due to shunt obstruction and/or spinal cord tethering. There has been a
single report of syringo-subarachnoid shunting in a dog using an equine ocular lavage tube. However post-operative MRI revealed
that SM was still prominent although there was a clinical improvement in the dog (Skerritt and Hughes 1998).
Due to the persistence of SM and/or spinal cord dorsal horn damage it is likely
that the post-operative patient will also require continuing medical management for pain relief and in some patients medical
management alone is chosen because of financial reasons or owner preference. There are three main drugs used for treatment
of CM/SM: drugs that reduce CSF production; analgesics; and corticosteroids (Fig 3). If the dog’s history suggests postural
pain or discomfort relating to obstruction of CSF flow then a trial of a drug which reducing CSF pressure, e.g. furosemide,
cimetidine or omeprazole, is appropriate. This can also be very useful if it is difficult to determine if the cause of discomfort
is CM versus, for example, ear disease. CSF pressure reducing drugs may be sufficient to control signs in some dogs, but additional
analgesics are likely to be necessary for an individual with a wide syrinx. In this circumstance we suggest that non steroidal
anti-inflammatory drugs are the medication of first choice partly because there are several licensed products. However, for
dogs with signs of neuropathic pain, i.e. allodynia and scratching behaviour (suspected dysesthesia); a drug which is active
in the spinal cord dorsal horn is more likely to be effective. Because gabapentin has established use in veterinary medicine
we suggest that this is the drug of first choice but amitriptyline or pregabalin may also be suitable. Corticosteroids are
an option if pain persists or where available finances prohibit the use of other drugs. Because the mechanisms of development
of neuropathic pain are multifactorial, appropriate polypharmacy is likely to be more effective than treatment with single
agents. Anecdotally, acupuncture and ultrasonic treatments have been reported to be useful adjunctive therapy in some cases.
The dog’sactivity need not to be restricted but owner should understand that dog may avoid some activities and grooming
may not be tolerated. Simple actions, for example raising the food bowl and removing neck collars, can also help.
Prognosis for CM/SM managed medically is guarded especially for dogs with a
wide syrinx and/or with first clinical signs before 4 years of age. Study of a small case series (14 CKCS) managed conservatively
for neuropathic pain suggested that 36% were eventually euthanatized as a consequence of uncontrolled pain. However 43% of
the group survived to be greater than 9 years of age (average life expectancy for a CKCS is 10.7 years). Most dogs retain
the ability to walk although some may be significantly tetraparetic and ataxic.
Current breeding recommendations for CKCS concentrate on removal of dogs with
early onset SM (i.e. within the first 2.5 years of life) from the breeding pool (for precise recommendations and grading system
REFERENCES AND FURTHER READING
Carruthers H, Rusbridge C, Dubé, M-P, et al Association between cervical and
intracranial dimensions and syringomyelia in the cavalier KingCharles spaniel In: Rusbridge C, Chiari-like malformation and
Syringomyelia in the Cavalier King Charles spaniel. ISBN 90-393-4456-6; 978-90-393-4456-9. 82, 2006
Cerda-Gonzalez S, Olby
NJ, Pease TP: Morphology of the Caudal Fossa in Cavalier King Charles Spaniels.
J Vet Internal Med 20: 736, 2006
Dewey CW, Bailey KS, Marino DJ, et al. Foramen magnum decompression with cranioplasty
for treatment of caudal occipital malformationsyndrome is dogs.
Dewey CW, Berg JM, Barone G et al: Foramen magnum decompression for treatment
of caudal occipital malformation syndrome in dogs. J Am Vet Med Assoc 227: 1270, 2005
Greitz D: Unravelling the riddle of syringomyelia. Neurosurg Rev 29:251, 2006
Levine DN: The pathogenesis of syringomyelia associated with lesions at the
foramen magnum: a critical review of existing theories and proposal of a new hypothesis. J Neurol Sci; 220:3, 2004.
Lillington, K, http://www.sm.cavaliertalk.com - a resource for current information
on treatment and breeding recommendations with links to Chiari-like malformation and syringomyelia discussion and support
Rusbridge C, Greitz D, Iskandar BJ: Syringomyelia: Current concepts in pathogenesis,
diagnosis and treatment. J Vet Internal Med 20: 469, 2006
Rusbridge C, Jeffery, NJ: Pain mechanisms and treatment in Chiari malformation
and syringomyelia in the dog. In press The Veterinary Journal (2007), doi:10.1016/j.tvjl.2006.12.007.
Rusbridge C, Knowler SP: Inheritance of occipital bone hypoplasia (Chiari type
I malformation) in cavalier King Charles spaniels. J Vet Internal Med; 18:673, 2004.
Rusbridge C: Chiari-like malformation with syringomyelia in the cavalier King
Charles spaniel; long term follow up after surgical management. In press Veterinary Surgery.
Skerritt GC, Hughes D: A syndrome of syringomyelia in the cavalier King Charles
spaniel, and its treatment by syringo-subarachnoid shunting. In Proceedings from the 12th Annual Symposium of the European
Society of Veterinary Neurology, Vienna, 23: 1998.
Smith SR. For the Love of Ollie; a story of compassion and courage Trimatrix
Management Consulting Inc, 2047 Pen Street, Oakville, Ontario, Canada, available though http://www.fortheloveofollie.com,
2006 This book is a valuable resource for owners and their veterinarians and was written by the owner of 2 CM/SM affected
dogs. It describes her journey from first realising that her pet was in pain to the post-operative period. There are contributions
from other CM/SM affected dog owners and a simple explanation of the disease and treatment.
Thimineur M, Kitaj M, Kravitz E et al: Functional abnormalities of the cervical
cord and lower medulla and their effect on pain: observations in chronic pain patients with incidental mild Chiari I malformation
and moderate to severe cervical cord compression. Clin J. Pain 18:171, 2002
Researchers no longer need cavalier blood for research, as they have sufficient
amounts for the DNA/genome scan to begin.
However, please note that blood IS needed from SM-affected dogs of breeds OTHER
than CKCS and also from NORMAL dogs WITHOUT SM that have been MRId (this includes dogs with
the Chiari-like malformation [CM], but NOT SM). Full information and downloadable forms are below.
How to donate:
If you have a dog of a breed other than CKCS diagnosed with syringomyelia,
or a dog of another breed that has been MRId and is NORMAL
or has the Chiari-like malformation (CM) but NO SM, please contact:
Stone Lion Veterinary Centre
41 High Street,
SW19 5AU, UK.
Confidential fax line 00 44 208 7860525
Centre for the Study of Brain Diseases
P0: 824 329
CHUM - Notre
Simard Pavilion, Room Z-8911
2065 Alexandre de Sève St.
H2L 2W5 CANADA
Fax: (514) 412-7602
Phone: (514) 890-8000, ext. 24857