CM/SM – the fundamentals


What is syringomyelia (pronounced SIR-RIN-GO-MY-EE- LIA)?
Syringomyelia (SM) is a disease of the spinal cord characterised by fluid filled cavities (syrinxes) within the spinal cord substance.

MRI of the brain and neck from 3 year old female Cavalier with syringomyelia (asterisks) and Chiari-like malformation (CM).

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

Why is syringomyelia called syringomyelia?
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!).


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

What causes Chiari-like malformation (CM)?
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.

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).


What is central canal dilatation?
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). 

What is Chiari-like malformation (CM)?


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

What causes syringomyelia?
Syringomyelia occurs secondary to obstruction of cerebrospinal fluid (CSF). The central nervous system is suspended in a sack called the meninges which is filled with CSF. This fluid is important for protecting the nervous system. When the heart beats the brain expands with blood and pulses too, pushing the CSF into the vertebral canal through the hole in the skull (the foramen magnum or FM). If this hole is obstructed, then pressure increases in the nervous tissue and the CSF is forced through the FM like a jet – just like putting a thumb over the end of a hosepipe. One idea is that this jet decreases the pressure outside the spinal cord. A situation of high pressure in the spinal cord compared to low pressure outside causes the spinal cord to be sucked outwards much like a shower curtain is sucked inwards when the shower is switched on. These pressures on the spinal cord lead to fluid accumulation which ultimately coalesces into cavities. Other theories are being investigated.

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