We have also observed many dogs with CLM/SM who cried out when picked up and while defecating. Some dogs with this disorder will also become somewhat inactive and/or overtly exhibit pain with abrupt weather changes or stressful situations. We suspect that sudden pressure changes, regardless of the cause, are transmitted to the region of the spine around the syrinx, leading to transient worsening of clinical signs.
Abnormal sensations are, for us in the veterinary world, a guess. Dogs can’t tell us that they feel strange itching sensations, but they certainly act as if this is the case in many instances. Our assumption is that the persistent scratching activity commonly seen with CLM/SM (especially prevalent in the CKCS) is a form of abnormal sensation, most likely due to the syrinx cavity interfering with sensory pathways that must traverse the spinal cord to reach the brain for interpretation (Fig 7). Support for this theory is that most dogs with such scratching activity do not actually make contact with the skin while scratching (“air guitar”), suggesting that the skin is not the source of the irritation. Pain and scratching are common signs associated with CLM/SM. Other clinical signs include loss of balance and difficulty walking on all four legs-sometimes notably worse in the front limbs.
Some dogs with this disorder will display an abnormal curvature of the spine, called scoliosis. It is best appreciated by looking down on the dog while it is standing stationary; the abnormal “bowing” of the spine is contributed to by the syrinx cavity damaging neurons responsible for keeping the back straight.
In humans with Chiari malformation, there is approximately a 10% incidence of seizure activity. We have observed a similar phenomenon in CKCS with CLM/SM. This breed however, is also prone to epilepsy, so it is often impossible to ascertain whether such patients have one or two active neurologic disorders. Although 97.3% of dogs were found to have neck pain on physical examination, according to a large scale study being conducted at the The Canine Chiari Institute at LIVS, other clinical signs should be recognized and are summarized in table 1.
How commonly does CLM/SM cause pain and clinical signs?
Clinically affected dogs are probably a minority of the total CLM/SM affected population. The exact incidence is unknown however even if the number of clinically affected dogs were only 4% as a recent study suggested (ACKCSC Health Survey), then thousands of CKCSs across the world would be affected - 447 new cases a year in the UK alone. According to recent data developed at the The Canine Chiari Institute at LIVS, of 110 dogs with SM, significantly more dogs, (61%) exhibited clinical signs compared to dogs without clinical signs (39%). Additionally, only 59% of the 227 dogs evaluated had clinical signs according to their owners, however, 97.3% of dogs were found to have neck pain on physical examination. Others have found greater than 25% of CKCSs subject to MRI screening for asymptomatic dogs were found to have SM. The real significance of these numbers is that the offspring of asymptomatic dogs appear to have a higher chance of being affected and more chance of being symptomatic