What is pre syrinx?
The first stage of syrinx development appears to be (interstitial) oedema i.e. fluid within the spinal cord typically around a dilated central canal– so called pre-syrinx. The fluid is within the spinal cord substance rather than coalesced into distinct cavities.

Developing syringomyelia in an 18 month Cavalier. There is a small syrinx (arrow) with surrounding oedema or pre-syrinx (asterixis). There is also a pre-syrinx in the thoracic spinal cord.

What is ventricular dilatation / ventriculomegaly?
The ventricles are four communicating cavities within the brain that are continuous with the central canal of the spinal cord.  The four ventricles consist of the two lateral ventricles, the third ventricle and the fourth ventricle: They are filled with CSF, which is formed by structures called choroid plexuses located in the walls and roofs of the ventricles. When there is an obstruction at the FM the CSF can “back up” in the ventricular system, dilating the cavities. When ventricular dilatation is considered an incidental finding it is often described in MRI reports as ventriculomegaly. Severe ventricular dilatation resulting in brain compromise is described as hydrocephalous. Hydrocephalous as a consequence of CM is rare and most cases with ventricular dilatation have no associated clinical signs. Ventriculomegaly is also seen in other situations unrelated to CM and is particularly common in brachycephalic (dome headed) breeds.

What other causes of syringomyelia are there?
Syringomyelia can occur as a consequence of any obstruction to the cerebrospinal fluid such as arachnoid cysts / adhesions, malformations such as spina bifida and spinal dysraphism, tumours, inflammatory disease such as (in the cat) feline infectious peritonitis, and secondary to trauma.


How common is trauma a cause of canine syringomyelia?
In humans post-traumatic syringomyelia occurs up to 30% of patients following traumatic paraplegia and tetraplegia. This typically occurs within 5 years of the accident (range 2 months to 32 years). In humans post traumatic syringomyelia represents approximately 11% of the cases of syringomyelia, compared to 50% due to Chiari malformation. Post traumatic syringomyelia is rare in the dog presumably because most animals with devastating spinal injures tend to be euthanatized. Granger and Jeffery (2009) investigated nine chronically paraplegic dogs; MRI  detected small (~ 3.5cm long) syrinxes in two dogs. Neither dog had clinical signs relating to the syrinx.


Can syringomyelia occur after a birthing injury?
In humans one study of idiopathic syringomyelia (i.e. where the cause of syringomyelia is not apparent on MRI for example no evidence of CM or structural anomaly) found a high incidence of difficult labour in the mothers. It was suggested that birth trauma and in particular forceps delivery might result in arachnoiditis in the foramen magnum area. This has not been reported in dogs but it is theoretically possible.


What is arachnoiditis?
Arachnoiditis is inflammation of the middle layer of membranes covering the brain and spinal cord and can occur as a complication of procedures such as myelograms, spinal operations, epidural injections, and injury to the spine. Arachnoiditis has not been reported in dogs but it is theoretically possible and some studies are investigating the possibility.


CM/SM – the nitty gritty 


Is syringomyelia in Cavaliers an inherited condition?
Both CM and syringomyelia have been shown to be inherited in Cavaliers however the inheritance is not simple and may be influenced by other unknown factors (Lewis et al 2009; personal communication)


Why do not all dogs with CM develop syringomyelia?
One of the enigmas of CM/SM is the difficulty of predicting / explaining which dogs with CM will subsequently develop syringomyelia. This suggests that there are as yet other unidentified anatomical or environmental factors influencing the development of syringomyelia.  CM is almost ubiquitous in Cavalier King Charles spaniels (CKCS); Cerda-Gonzalez et al (2009) found that 92% had at least one craniocervical morphologic abnormality detected in MR images. However severity of the CM does not predict the presence of syringomyelia although Cavaliers with clinical signs from syringomyelia are more likely to have smaller cauda fossas (i.e. back of skull) volume compared to unaffected Cavaliers (Cerda-Gonzalez et al 2009).


What breeds are affected with CM/SM?
The Cavalier 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, Griffon Bruxellois, Chihuahuas, Yorkshire terriers, Maltese terriers, Miniature dachshunds, Miniature/toy poodles, Bichon Frisé, Pugs, Shih Tzus, Pomeranians, Staffordshire bull terriers, Boston terriers, French bulldogs a Pekingese, a miniature Pinscher and a couple of cats.


How common is CM/SM in Cavaliers?
CM is very common in Cavaliers, Cerda-Gonzalez et al (2009) found that 92% had at least one craniocervical morphologic abnormality detected in MR images.
All scientific papers looking at groups of 16 or more asymptomatic Cavaliers have found a high incidence of syringomyelia ranging from 26.5% (Cerda-Gonzalez et al 2009 – 49 dogs) to 65.4% (Rusbridge et al 2007 – 55 dogs). These figures increased to 42% and 74.5% respectively when symptomatic dogs were added to the population. 
However the population in these papers was not random or age matched, the asymptomatic dogs were variously composed of those attending budget screening services; voluntary participants in funded studies; and dogs having investigation for other neurological disease. To complicate the difficulty of estimating incidence, syringomyelia is a late onset disease i.e. dogs free of disease when young adults will not necessarily remain free of the disease.
Recently a study was proposed to determine the incidence in a random sample of 200 middle aged Cavaliers. Unfortunately so far this study has failed to secure the necessary funding.


But surely dogs with clinical syringomyelia are a tiny minority of the total Cavalier population?!
They are the tip of the ice-berg however even if the number of clinically affected dogs was only 4% as a recent study suggested then thousands of Cavaliers across the world would be affected  In the UK, 44731 Cavalier puppies were registered over a 4 year period. If a 4% incidence almost 1789 will have clinical syringomyelia i.e. 447 new cases a year in the UK alone. The number of dogs with asymptomatic syringomyelia is much higher (see previous question) and the real significance of asymptomatic dogs is that their offspring appear to have a higher chance of being affected and more chance of being symptomatic

How common is CM/SM in the other breeds?
The incidence in other breeds is unknown – only isolated cases have been reported. CM/SM is increasingly recognised in the Griffon Bruxellois however the incidence is not known as the only study to date examined predominantly a single extended family (Rusbridge et al 2009).  In this breed syringomyelia may be seen without CM. There are other studies looking at the incidence and characteristics of the disease in this breed.


When was canine CM/SM first discovered?
The first reports of syringomyelia in Cavaliers were in 1997 almost simultaneously from the UK, South Africa and Australia. This coincided with the availability of spinal MRI for animals. However the disease was around before this time; however, because of the lack of appropriate diagnostic tests, it was not recognised. The earliest known case with case history and radiographs suggestive of syringomyelia was presented to the Royal Veterinary College in 1987.


CM/SM – clinical signs


What are the clinical signs of syringomyelia?
The most important and consistent clinical sign of CM/SM is pain. Signs are very variable and can include vocalisation; withdrawn behaviour, unwillingness to exercise, difficulty or vocalisations on jumping, difficulty lowering the head to eat, and sleeping with the head in elevated positions. Sign of pain may be more apparent in the evening and early morning or when excited and can be associated with defaecation or may vary with weather conditions.


Pain is positively correlated with syrinx width and symmetry 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 (curvature of the spine).yringomyelia may also result in other neurological deficits such as weakness and poor coordination and slow correction of abnormal limb position. Seizures, balance problems, facial nerve paralysis and deafness may also be seen; however, no direct relationship has been proven, this association may be circumstantial and it is important to rule out other causes.

Can CM alone cause clinical signs?
CM alone appears to cause facial pain in some dogs with owners describing ear and facial rubbing/scratching. This may be because of an increase in intracranial pressure or because compression of the brain stem can result a pain syndrome.  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 Cavalier breed and most dogs with CM alone do not appear to have signs.

Why do some dogs with syringomyelia seem completely normal?
Not all dogs with syringomyelia have clinical signs. The presence of signs is correlated to the cross-sectional width of the syrinx and the area of the spinal cord damaged. Signs of pain, scratching and / or scoliosis are more likely with wide and asymmetrical syrinxes.


So if a syrinx is detected either by screening or because of investigation of another disease do I need to be concerned?
Syrinxes can progressively expand and a dog which is asymptomatic in early life may eventually become painful. However a narrow symmetrical (less than 3 millimetres wide) syrinx or central canal dilation is unlikely to develop a pain syndrome. Some owners opt for intermittent neurological examination or even repeat MRI to assess for progression. The real significance of asymptomatic dogs is that their offspring appear to have a higher chance of being affected and more chance of being symptomatic. For this reason breeders are advised to MRI screen their breeding animals.

What age of dog is affected by syringomyelia?
The youngest reported dogs with syringomyelia 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.


Transverse T2 weighted MRI brain sections from Cavaliers demonstrating (from left to right) asymmetrical ventricular dilatation (arrow – normal; asterixis – dilated); Moderate ventricular dilatation (arrow); Marked ventricular dilatation (arrow).

T2 weighted transverse image through a wide syrinx (asterisks) demonstrating the asymmetrical involvement of the right spinal cord dorsal horn.

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