Craniomandibular osteopathy (CMO) is a non-cancerous dog bone disorder that exclusively affects the bones of the head in dogs, usually in on the lower jaw bone (ramus of the mandible) or over the angle of the mandible and tympanic bulla. CMO is a developmental disease in dogs causing extensive bony changes in the mandible and skull. It is also called mandibular periostitis, temporomandibular osteodystrophy or "lion jaw."

The cause of Craniomandibular osteopathy is believed to be hereditary and Terriers are prone to the disorder. It is not cancerous or caused by inflammation. The disease is inherited as a simple autosomal recessive trait. This means that both parents must have at least one gene for CMO (i.e. they are defined carriers). In this disease, the production of an affected puppy provides the only method of identifying carriers. The most commonly affected breeds are West Highland White Terriers, Scottish Terriers and Cairn Terriers. It has been recognized in other terrier breeds and in Boxers, Labs, Great Danes and Dobermans. There is no sex predilection, with males and females affected equally. Neutering and spaying seems associated with reduced risk of the disorder. It usually occurs between the ages of 3 and 8 months, but it can occur as early as 3 - 4 weeks and rarely as late as 9 - 10 months. Experienced breeders and veterinarians usually recognize it earlier than 4 months of age by clinical signs or by palpation. The disorder is usually self-limiting, but may require medication to make the dog comfortable.

Symptoms include firm swelling of the jaw, drooling, pain, and difficulty eating and pain on opening the mouth; sometimes there is actually an inability to open the mouth. Dogs may drool and be depressed. Often the body temperature will fluctuate over time, with fever occurring in phases every 10-14 days. In severely affected dogs, the masticatory muscles (those involved in chewing) may atrophy and there may be lymphadenopathy (swollen glands). Canine distemper has also been indicated as a possible cause, as has E. coli infection, which could be why it is seen occasionally in large breed dogs. The disease is most often diagnosed by clinical signs and palpation with definitive confirmation by lateral and/or ventral/dorsal radiographs of the skull, depending on the location of the specific lesion. All board-certified radiologists can diagnose the disease, as can many other experienced veterinarians.

Craniomandibular osteopathy is treatable in almost every case, except the most severe. The amount of medication and length of treatment varies greatly depending on the severity of the disease, and needs proper petsafe. Many puppies with CMO will need to be on some dose of cortisone until they are 10 months old or longer. Therapy is usually targeted at making the dog more comfortable through the use of pain relievers and anti-inflammatory drugs such as prednisone. Proper nutrition must be provided, and in severe cases, it may be necessary to place a gastrostomy (stomach) tube. X-rays are the main method of confirming the diagnosis. Both sides of the jaw are usually affected, although some dogs are affected only on one side. A biopsy may be necessary to confirm the diagnosis in breeds for which this disorder is uncommon, especially if only one side of the jaw is affected. The disorder usually resolves on its own, although anti-inflammatory drugs may help reduce some of the clinical signs. Occasionally, a dog is euthanized because of inability to relieve the extreme discomfort.

The disease is usually self-limiting, with the progression of the disease slowing down at around 11 to 13 months of age. Sometimes, it is followed by a slow regression of the disorder, although radiographic abnormalities or impaired function may remain. Several drugs have been tried, however, with good response. There are no specific preventive care measures. People seeking purebred terriers, especially West Highland white terriers, should question breeders carefully about the occurrence of the disorder in any lines, as CMO is inherited in Westies, and is believed to be inherited in Scottish terriers as well. Optimal treatment for your pet requires a combination of home and professional veterinary care, with good rest on comfortable dog crates. Follow-up can be critical, especially if your pet does not rapidly improve. Administer all prescribed medication as directed. The disease often stops progressing around 11 to 13 months of age, and then may regress partially or completely.