INCIDENCE
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.
CLINICAL SIGNS
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.
CLINICAL COURSE
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.
DIAGNOSIS
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.
[snip]
TREATMENT
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
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.
[snip]
© Clare Rusbridge BVMS DipECVN MRCVS
