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CM/SM – the fundamentals
Syringomyelia (SM) is a disease of the spinal cord characterised by fluid filled cavities (syrinxes) within the spinal cord substance.
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MRI of the brain and neck from 3 year old female Cavalier with syringomyelia (asterisks) and Chiari-like malformation (CM).
Syrinx implies hollow tube or reed. Myelia or myelo refers to marrow - the old medical term used to describe the spinal cord. Therefore syringomyelia implies a hollow spinal cord. In Greek mythology syrinx was a nymph who was the recipient of unwelcome attention from Pan. He chased her to a river and she called upon the water nymphs to help her. They consented and as Pan threw his arms around what he thought was the nymph he found himself embracing a clump of reeds. He heaved a sigh, the air sounded through the reeds, and produced a melancholic sound. The god, amused by this and the sweetness of the melody, said, "Thus, then, at least, you shall be mine." And he took some of the reeds, and placing them together, of unequal lengths, side by side, made an instrument he called Syrinx, in honor of the nymph. From syrinx we have the term syringomyelia, syrinx and syringes (and of course pan pipes!).
According to the Munro-Kellie doctrine the central nervous system and its accompanying fluids are enclosed in a rigid container whose total volume remains constant. Therefore when the heart beats and there is increase in volume of intracranial blood, cerebrospinal fluid (CSF) is displaced from the cranial to the spinal subarachnoid space through the foramen magnum thus avoiding a deleterious increase in intracranial pressure. The spinal dural sac is distensible, further increasing the compliance of the system and minimising rises in central nervous system pressure. Disturbance of the normal free flow of CSF through the foramen magnum appears to be a major factor responsible for the formation of a syrinx in the cervical spinal cord. However there may be other possible factors influencing the pathogenesis of a syringomyelia such as failure of absorption or drainage of extracellular fluid, intracranial hypertension, imbalance in the production and absorption of CSF, disruptions of the blood-spinal cord barrier or alterations of aquaporin expression. The currently most accepted theory of pathogenesis of syringomyelia is that obstruction to CSF flow in the subarachnoid space results in a mismatch in timing between the arterial pulse peak pressure and CSF pulse peak pressure. Earlier arrival of peak CSF pressure compared to peak spinal arterial pressure encourages flow of CSF into the perivascular space. The perivascular space changes in size during the cardiac cycle and is widest when spinal arteriole pressure is low. If at that time peak CSF pressure is high then the perivascular space could act as a ‘leaky’ one-way valve. From the perivascular space, fluid flows into the central canal ultimately resulting in a syrinx (28-30). However this theory also leaves many unanswered questions and further study is required. In the dog syringomyelia is associated with a number of different pathologies with a common theme of CSF flow obstruction. The most common cause is Chiari-like malformation, which is a complex abnormality characterised by overcrowding of the craniocervical junction and obstruction of CSF flow through the foramen magnum. It is unclear why some dogs with Chiari-like malformation develop syringomyelia and some do not.
Chiari-like malformation (CM) is the most common cause of foramen magnum obstruction and syringomyelia in the dog. CM is a condition characterised by mismatch in size between the brain (too big) and the skull (too small). There is not enough room for the brain and the back part (cerebellum and medulla) is pushed out the FM.
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Comparison between the back of the brain in a Staffordshire bull terrier (top) and a Cavalier (bottom). The MRI images on the left are transverse “slices” though the foramen magnum. In the Staffordshire bull terrier an unobstructed space around the spinal cord can be appreciated (white ring with asterixis). This is absent in the Cavalier. The MRI images on the right show the normal cerebellum anatomy in the Staffordshire bull terrier and a cerebellar herniation (arrow) in the Cavalier
CM is not yet fully understood. Somehow the miniaturisation process in the Cavalier went awry and unlike many other toy breeds the brain did not decrease in size in proportion with the skull. The Cavalier appears to have a brain more appropriate for a bigger dog. Studies in the Griffon Bruxellois (Rusbridge et al 2009) have suggested that CM in this breed is characterised by a short skull base. This shortening results in an compensatory increase in the size of some of the other skull bones meaning that the forebrain is adequately accommodated however there is no compensatory increase in size of the back of the skull meaning that there is not enough room for the cerebellum and brain stem.
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MRI scans and head profiles from 10kg and 16kg Cavalier. Both dogs were registered pedigree dogs although the 16kg animal is considerably bigger than the breed standard. The volume of the brain tissue is similar in both dogs however the volume of the skull, is considerably smaller in the 10kg dog. Consequently there is insufficient capacity for the brain and the cerebellum is herniated into the vertebral canal (arrow) and there is also syringomyelia (asterisks).
The central canal is the narrow cerebrospinal fluid-filled space that runs longitudinally through the length of the entire spinal cord. It is contiguous with the ventricular system of the brain and is the adult remnant of the neural tube. Under normal circumstances it is only just appreciable on a MRI. If it is abnormally dilated then this will be recorded in MRI reports however small central canal dilation (less than 2 millimeters) does not appear to cause a problem to the individual dog. Nevertheless it is part of the spectrum of CM/SM and some young dogs with central canal dilatation have subsequently progressed to syringomyelia and some dogs with central canal dilation have produced offspring with syringomyelia (Rusbridge et all 2009). Central canal dilation may be difficult to appreciate especially on low tesla MRI machine therefore MRI screening should include images in at least 2 orientations (sagittal and transverse). 

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Transverse sections though the cervical spinal cord demonstrating normal central canal and central canal dilatation (arrows)

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.
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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.
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.
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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).
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.
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.
Brachycephalicism and miniaturisation are risk factors for Chiari-like malformation (33). The condition is most commonly reported in toy breed dogs, in particular CKCS, King Charles spaniels, Griffon Bruxellois, Affenpinschers, Yorkshire terriers, Maltese, Chihuahuas, Pomeranians, Boston terriers and Papillons (34). Chiari-like malformation has also been recognised in cross-breed dogs particularly CKCS crosses. Partly because of its popularity as a pet, the Cavalier King Charles spaniel is overrepresented and Chiari malformation is considered ubiquitous in this breed (1, 31, 35). Up to 65% of the Griffon Bruxellois breed has Chiari-like malformation (21, 36); data for other breeds is not available. Chiari-like malformation may also be seen in cats and is again more common in brachycephalic varieties such as the Persian. The incidence of symptomatic Chiari-like malformation is not known and is difficult to determine because the most common clinical sign is pain. Pain is a complex amalgamation of sensation, emotions and (in humans) thoughts and manifests itself as pain behaviour which in a dog may not be recognised by owners or their veterinarians.In addition pain associated with Chiari-like malformation is rarely constant or focal. In humans the key features of Chiari-related headaches are their relationship to any Valsalva-like manoeuvre, their brief duration - often lasting only seconds – and their posterior, suboccipital location. In a dog this might manifest as a yelp on a rapid change of position, for example being picked up. It is difficult to attribute non-specific and brief signs to a specific aetiology especially when a condition is common in a breed and can be asymptomatic. The reported number of human patients with asymptomatic Chiari malformation type 1 varies between a third and a half of those diagnosed with the condition by MRI .
Due to the relationship with Chiari-like malformation, prevalence of syringomyelia is also high in brachycephalic toy-breeds (34). Again not all animals with syringomyelia are symptomatic and like Chiari-like malformation it is difficult to obtain reliable incidence data. In humans the reported frequency of syringomyelia in people who have Chiari malformation type 1 malformation ranges from 65 to 80% (44) and the frequency of asymptomatic syringomyelia has been reported as being 23%. Syringomyelia has a varying age of onset, there is 46% prevalence in (allegedly) asymptomatic breeding CKCS but prevalence (symptomatic and asymptomatic) increases with age and may be as high as 70% in dogs over six years of age (1). In the Griffon Bruxellois 42- 52% of dogs have syringomyelia and this is not always in association with a classical Chiari-like malformation
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.
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.

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