Syringomyelia is a disease of the spinal cord characterized by fluid filled cavities (syrinxes) within the spinal cord substance (Fig 2a). Fluid is normally found in small quantities within the center of the spinal cord in a space called the central canal. This amount is so small, it is barely detectable on MRI evaluations (Fig 2b). When the fluid volume in the central canal increases a small amount, it is called hydromyelia and is believed by some to be a precursor to syrinx formation (Fig 2c).
|Fig 2A||Fig 2B||Fig 2C|
Syrinx size can vary greatly and in some cases, occupy the majority of the spinal cord substance. Syrinxes exert pressure on the spinal cord, compressing and stretching its fibers and in some cases resulting in permanent nerve damage, pain, craniocervical scratching and paresis(difficulty walking). Syringomyelia can also occur as a result of any condition that changes CSF flow, such as spinal fractures and tumors. Syrinx formation in the cervical region is well documented in both human and veterinary patients since the cervical region is easily incorporated in the cervicomedullary MRI study used clinically, however little is known about the incidence in other spinal regions. Because of this observation, the entire central nervous system is now imaged when patients are evaluated as part of the CLM/SM screening program at The Canine Chiari Institute at LIVS. The majority of dogs in studies at Stone Lion Veterinary Hospital and the Canine Chiari Institute at LIVS with cervical syrinxes were found to have syrinxes in other spinal cord regions with whole spine MR imaging.
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, a greek god with the head and torso of a man and the hind quarters of a goat. He chased her to a river and as Pan threw his arms around what he thought was the nymph Syrinx, 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. It is from Syrinx we derive the term syringomyelia.
What causes syringomyelia?
Hydromyelia describes a dilated central canal, actually central canal dilation is a more precise term and is favored over hydromyelia. The mechanism of the fluid accumulation and the nature of the fluid remains controversial with several theories being reported.
In dogs with CLM, the cerebellum often protrudes through the FM. There is typically both constriction at the junction of the cervical spine and brain as well as compression of the cerebellum by an abnormally shaped supraoccipital bone. The constriction where the brain meets the spinal cord sets up a pressure differential between the brain and spinal compartments. This pressure differential is believed to be the cause of spinal cord fluid accumulation (SM), which is a common phenomenon in CLM.
Several theories have been proposed to explain the development of SM in dogs with CLM. All theories have in common the principle that the abnormal pressure differential between intracranial (brain) and spinal compartments created by the CLM is responsible for spinal fluid accumulation (Fig4). 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. Likely there is more than one mechanism at play. It has been theorized that the formation of a syrinx may occur by one mechanism, while its propagation may be from another.
Why do not all dogs with CLM develop SM?
|Fig 5||Fig 6|
One of the enigmas of CLM/SM is the difficulty of predicting / explaining which dogs with CLM will subsequently develop SM. This suggests that there are as of yet other unidentified anatomical or environmental factors influencing the development of SM. CLM is almost ubiquitous in the CKCS. Cerda-Gonzalez et al (2009) found that 92% had at least one craniocervical morphologic abnormality detected in MR images.
What other causes of SM are there?
Syringomyelia can occur as a consequence of any alteration to the normal flow pattern of CSF such as arachnoid cysts / adhesions (Fig 5), malformations such as spina bifida and spinal dysraphism, tumors (Fig 6), inflammatory disease such as (in the cat) feline infectious peritonitis, and secondary to trauma.