Chiari-like malformation (CM) and syringomyelia (SM)

What is ... ?: 

CM/SM is a very serious disorder that is common in the Cavalier King Charles spaniel and occurs occasionally in other small breeds. The effect of the Chiari-like bony malformation is that the back part of the skull is too small for the parts of the brain that are contained therein (the cerebellum and brainstem). These push against the foramen magnum (the opening from the skull to the spinal canal), causing obstruction of varying degrees, and thus abnormal movement and pressure of cerebrospinal fluid (CSF), which normally flows easily. The abnormal pressures cause the development of fluid-filled cavities (syrinxes) within the spinal cord.

Pain is the most important and consistent sign of CM/SM.
 

How is ... inherited?: 

Both CM and SM are known to be inherited in the Cavalier King Charles spaniel, but inheritance is complex. It is estimated that 95% of the CKCS population have CM and yet the severity of the malformation does not predict whether the dog will have SM. About 50% of CKCS have SM, but not all dogs with CM/SM have clinical signs (i.e., they are asymptomatic).

What does ... mean to your dog & you?: 

Almost all CKC spaniels have the bony Chiari-like malformation (CM), and about 50% have syringomyelia (SM) as well. Of those with CM/SM, about 35% will have clinical signs of the condition. The others may remain asymptomatic for their whole lives. Of those dogs who do show signs, about 45% will be affected in their first year of life, 40% between 1 and 4 years, and another 15% beyond 4 years.

The most important and consistent sign of CM/SM is pain, which your dog may show through crying out, withdrawn behavior, reluctance to exercise, intolerance of a neck collar or touch about the head and neck, and sleeping with the head raised. Affected dogs may have scoliosis (curvature of the spine), weakness and poor coordination and they may scratch on one side without touching the skin – called “phantom” scratching.

Whether a dog will develop clinical signs depends on the width of the syrinx cavity and its location in the spinal cord – those with wide and asymmetrical syrinxes are more likely to have signs. Some dogs will remain stable and only become worse very gradually over a number of years, while others will become severely disabled by pain and neurological deficits (poor coordination, etc) in a matter of months from the first signs. Dogs with narrow syringeal cavities may never have 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 ... diagnosed?: 

Magnetic resonance imaging (MRI) is required for diagnosis by a veterinary neurologist.

How is ... treated?: 

The main goal of treatment is to relieve the pain. Medical management includes drugs to decrease production of CSF and to relieve pain. Many CKCS with CM/SM are eventually euthanized for uncontrolled pain, despite medical treatment, especially those with a wide syrinx that show signs before 4 years of age.

For dogs in pain that are not responding to drug therapy, the most common surgery is cranial/ cervical decompression (removal of part of the bone at the back of the skull and part of the first vertebra) to relieve the pressure at the foramen magnum and obstruction to CSF flow. This is successful in reducing pain and neurological deficits in about 80% of dogs. There may be scarring and re-obstruction with as many as 50% of dogs eventually re-deteriorating. Medication may still be required after successful surgery due to spinal cord damage.

You can help your affected dog by avoiding touching or grooming sensitive areas (e.g., ear on one side), not using a neck collar, and raising the food bowl.
 

For the veterinarian: 

Pain may be intermittent and hard to localize. It is ascribed to obstruction of CSF flow and/or neuropathic pain syndrome probably due to damage to the spinal cord dorsal horn. Pain is often postural, so historical signs are important. There may be a variety of neurological deficits such as weakness and reduced proprioception, thoracic limb weakness and muscle atrophy (due to ventral horn cell damage), and pelvic limb ataxia (white matter damage or syringeal involvement of the lumbar cord). Dogs with a wider, asymmetrical syrinx are more likely to have pronounced pain and scoliosis, while dogs with a symmetric, narrow syrinx may remain asymptomatic. Scoliosis may appear before signs of neuropathic pain and may resolve despite persistence of the syrinx.

Rule-outs include other causes of pain and spinal cord dysfunction. MRI is essential for diagnosis of CM/SM. Radiographs and ultrasonagraphy are of limited use; myelography is contraindicated. With MRI, the size of the cerebellar herniation does not correlate with severity. Fluid-filled cavities within the spinal cord (typically upper cervical and thoracic) indicate SM. The maximum syrinx width is the strongest predictor of clinical signs.

See here for more information and a detailed treatment algorithm (Dr. Clare Rusbridge, veterinary neurologist).
 

Breeding advice: 

Because CM/SM is so common in the CKCS, and because affected dogs may not show any signs until past breeding age, and because the offspring of non-clinically affected parents with CM/SM appear more likely to be clinically affected with this devastating illness –for all these reasons, it is crucial that all CKCS dogs be tested by MRI screening before being used for breeding.

See current breeding guidelines here.

Resources: 

Dewey C. Caudal occipital malformation syndrome. In: Côté E, ed. Clinical Veterinary Advisor Dogs and Cats. Missouri: Mosby Elsevier, 2007:185-186.
Rusbridge C, Dewey C. Treatment of canine Chiari-like malformation  and syringomyelia. In: Bonagura JD, Twedt DC, eds. Kirk's Current Veterinary Therapy XIV. Missouri: Saunders Elsevier, 2009:1102-1107. 
www.cavaliercampaign.com/
www.veterinary-neurologist.co.uk/ 


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