CM is a quite common condition found in Cavalier King Charles Spaniels, Brussels Griffons, and a variety of other (typically small breed) dogs. CM is a boney malformation of the back part of the skull, which results in a skull that is too small for the brain. This abnormally-shaped bone causes crowding of the back part of the brain resulting in changes in the movement and pressure of cerebrospinal fluid (CSF) as it attempts to flow out of the brain. The abnormal movement of this fluid results in fluid-filled cavities within the spinal cord (SM). Pain is the most commonly reported clinical sign. This may present as quiet behavior, crying out, becoming less playful, low head carriage, and limping. Other possible symptoms include frequent scratching of the back of the head/ears/neck (sometimes without making contact with the skin–“phantom scratching”), weakness of the limbs, incoordination, and curvature of the spine.
How is the Condition Acquired?
CM and SM are inherited conditions in the Cavalier King Charles Spaniel and likely inherited in other high-risk breeds. Currently, essentially all Cavalier King Charles Spaniels have CM to some degree and approximately half of them have SM. Not all dogs with CM and/or SM have clinical signs.
When do Clinical Signs Develop?
Approximately 45% of Cavaliers who will become affected show signs by one year of age, 40% between one and four years of age and another 15% after four years of age. Frequently, a diagnosis is not made quickly since this condition is not commonly recognized by many veterinarians and the signs can be somewhat difficult to appreciate until the disease progresses. Nearly all cases will be progressive.
Dogs with narrow SM cavities may never show clinical signs. However, if they are bred, their offspring are likely to have CM/SM and the offspring appear to have a higher chance of developing clinical signs.
How is CM/SM Diagnosed?
The diagnosis of CM and SM is readily diagnosed with high-field magnetic resonance imaging (MRI) performed by a veterinary neurologist. Low field MRI and computed tomography (CT) scans are not recommended to achieve this diagnosis as they infrequently have sufficient resolution to fully characterize the condition. Myelograms are contraindicated in suspected CM/SM cases.
What is the Treatment?
Treatment is determined on a case-by-case basis, and requires evaluation and diagnosis by a veterinary neurologist. The two primary categories of treatment include 1) medical management and 2) surgical management. Medical management often includes specialized neuropathic pain medications and medications to decrease spinal fluid production. Many patients with CM/SM are eventually euthanized due to progression of their disease despite medical management. Surgical management should be considered for advanced cases, patients with large syrinx diameter and patients who do not fully improve with medical management. The surgical treatment consists of a routine surgical procedure to remove excess bone associated with the back of the skull.
Since CM/SM is inherited, what can be done to decrease the number of dogs that suffer from this condition?
Specific breeding guidelines have been put together to minimize the breeding of dogs who are likely to produce puppies who will become affected by this condition. The screening protocol requires an MRI, careful interpretation by a veterinary neurologist who is familiar with the grading of CM/SM and interpretation of breeding guidelines. Not all dogs with CM/SM will have overt clinical signs, so it is not safe to assume that a pet is appropriate for breeding on the basis of lack of symptoms. Two asymptomatic pets can result in a litter of puppies who develop CM/SM.