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What is Craniomandibular Osteopathy (Lion Jaw) ?

Craniomandibular osteopathy (CMO), also known as “Lion Jaw” and “Westie Jaw”, is an inherited disease which commonly becomes noticeable within 3 to 8 months of age.  CMO is generally  “self-limiting” in that, once the puppy reaches one year of age, the disease usually will not worsen.  Any excessive bone growth which has occurred may not go away and, if it affects the function of the jaw either in mobility or pain, the excessive bone may have to be surgically removed or special diets and adjustments made to his lifestyle.

The simple definition of craniomandibular osteopathy is “enlarged jaw” affecting primarily the mandible, tympanic bullae and occasionally other bones of the head.

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Symptoms of Craniomandibular Osteopathy (Lion Jaw) in Dogs

This disease, as noted above, is inherited and usually presents in your puppy somewhere between 3 and 8 months of age, with excessive and abnormal bony growth continuing until about the age of 1 year before subsiding.  Here are some of the symptoms you would likely see in your puppy if this condition exists:

  • Pain with chewing, worsening over time
  • Loss or lack of appetite
  • Fever, usually intermittent
  • Difficulty picking up food and chewing, worsening over time
  • Excessive drooling
  • Jaw swelling
  • Bulging eyes due to swelling inside the skull


There are no specific types of this disease except as that which relates to the severity of the excessive abnormal bony growth.  The disease presents usually in the first six months of life and slows down and regresses at approximately one year of life.  Once this cycle concludes, the abnormal excessive bony growth not only regresses but sometimes has been noted to recede completely.

Causes of Craniomandibular Osteopathy (Lion Jaw) in Dogs

Craniomandibular Osteopathy (CMO) in dogs is a condition which is inherited, though the exact etiology is not known but an autosomal (chromosomal) mode of inheritance is a possibility in West Highland White Terriers.  The breeds most commonly affected by this condition are:

  • West Highland White Terriers
  • Scottish Terriers
  • Cairn Terriers
  • Boston Terriers

Less commonly found in these breeds:

  • Labrador Retrievers
  • Great Danes
  • Doberman Pinschers
  • Irish Setters
  • English Bulldogs
  • Boxers

Diagnosis of Craniomandibular Osteopathy (Lion Jaw) in Dogs

If you notice that your puppy seems to be experiencing pain when opening and closing the mouth or with chewing or if you notice that the jaw seems larger than it should in proportion to the size of his head, call your veterinary professional and schedule an examination as soon as possible.  Be sure to be prepared to give a complete history to your vet regarding which of the above symptoms you have noted, the severity and the duration of those symptoms and if you have noted anything else not specified above which seems abnormal or unusual for your puppy.  Your vet will need to do a physical examination and will likely need some additional testing to differentiate your puppy’s clinical signs from a number of other signs which are similar that signal other diseases and conditions.  The additional testing he will likely require would be radiographic (x-ray) imaging, complete blood count (CBC), chemical blood profile and biochemistry levels.  

The veterinarian may also require additional blood testing to rule out fungal and bacterial infections.  The radiographic imaging (x-ray) that will be needed will be of the head and skull as this will show any abnormal or excessive bone growth.  Your puppy may need some light anesthesia to accomplish this test.  There is a rare possibility that a bone specimen may be needed to rule out neoplasia, osteomyelitis (bone infection) and hypertrophic osteodystrophy (bone disease which restricts blood flow to the joint).

Treatment of Craniomandibular Osteopathy (Lion Jaw) in Dogs

There is currently no treatment for craniomandibular osteopathy (CMO) which will slow the progression of the abnormal and excessive bony growth of the jaw bones.  The excessive bony growth will stop on its own at about one year of age and the excessive bone generally will regress or even recede completely.  Since this condition is “self-limiting”, treatment of the symptoms (pain with movement, fever, appetite issues) will likely be the best course of action in the short term.  

If the excessive bone growth inhibits the function of the jaw, for example, if the bony growth prevents the jaw from opening and closing or continues to cause too much pain for the puppy to eat, then there are surgical options available to remove the abnormal, excessive bony tissue as well as dietary changes which could be incorporated to help the puppy gets his nutrition.  Your veterinary professional will consider all of these options as he develops his treatment plan for your puppy and will provide his best advice and recommendations for keeping your puppy comfortable while the condition runs its course.

Recovery of Craniomandibular Osteopathy (Lion Jaw) in Dogs

It is hoped that knowing this condition is temporary will provide some encouragement for you while you cope with adjustments that may be required to help your puppy through this period of his life.  Puppies who are suffering from severe excessive bony growth and are having problems eating may need a special diet of high calorie soups, broths or other liquids to get appropriate nourishment during this period of growth. If the dog is unable to tolerate receiving nourishment in this manner, a feeding tube inserted into the esophagus or stomach may be the only other option for nutrition during this period of growth.  

Also, knowing that the condition is hereditary should also serve to emphasize the negative consequences of any planned future breeding of both this puppy as well as any siblings in your household or kennel.  This trait is not one which is in the best interests of maintaining the purity of the breed.   You should also be aware that, in the end, the amount of extra bony growth around the jaw can have an effect on the quality of your dog’s life even after the disease has stopped.  Though this disease is not generally a fatal one, you also need to be aware that your dog may require a special diet or even the feeding tube for the rest of his life.