Focal Inflammatory Myopathy in Dogs
The term myopathy is a general clinical term for a disorder of the muscles. Focal inflammatory myopathy affects specific muscle groups, in this case the masticatory muscles, which are the facial muscles involved in chewing, and the extraocular muscles, the group of muscles that are adjacent to the eyeball and that control the movements of the eye.
Focal inflammatory myopathy is suspeced to be due to autoantibodies, or antibodies that are known to react against the body's own tissues. Antibodies are proteins found in the blood and which are used by the immune system to identify and destroy foreign invaders, such as bacteria and viruses. In effect, the antibody has crossed signals, mistakenly attacking the body as though reacting to a pathogen. Focal inflammatory myopathy describes a condition in which these autoantibodies begin targeting the muscles of the affected animal.
A genetically based familial form has been found to occur in cavalier King Charles spaniels, rottweilers, Dobermans, and samoyeds, in which the masticatory muscles are affected. A similar form, affecting the extraocular muscles, has been seen in golden retrievers.
Symptoms and Types
Masticatory Muscles
- Problems with normal jaw movements
- Inability to pick up a ball
- Inability to get and keep food into the mouth
- Jaw pain
- Muscle swelling
- Progressive loss of muscle mass
Extraocular muscles
- Swelling around the eye
- Protrusion of eyeball from the socket
Causes
Immune-mediated
Diagnosis
You will need to give your veterinarian a thorough history of your dog's health, including a background history of symptoms. After taking a detailed history, your veterinarian will conduct a complete physical examination on your dog.
Your veterinarian will try to open your dog's mouth, which often proves unsuccessful in these patients. In an attempt to induce pain and swelling of the muscles so that the source of the problem is more evident, your veterinarian may attempt to manipulate your dog's jaw muscles. Laboratory tests will include a complete blood count (CBC), biochemistry profile, and urinalysis.
The biochemistry profile may indicate higher levels of serum creatine kinase, indicating muscle injury. More specific testing includes taking a muscle tissue sample, especially important in masticatory diseases. This test can help in reaching a confirmatory diagnosis. More advanced testing may include demonstrating the autoantibodies against the muscle fibers. Diagnostic imaging will include X-ray of the jaw bones and ultrasound of the eye orbit to examine the swollen extraocular muscles. Magnetic resonance imaging may also be used to examine the muscle inflammation.
Treatment
As focal inflammatory myopathy is an immune-mediated disease, immune-suppressive drugs will be used to suppress the dog's immune system in order to suppress the abnormal immune response. The dose is adjusted and maintained at lower doses to prevent restricted jaw mobility. In most patients long-term treatment will cover a minimum of six months before there is a resolution of symptoms.
Living and Management
Abnormal jaw movements remain a major problem because it limits the dog's ability to take food into its mouth. If the disease becomes chronic, the muscle bulk of the jaws and face may reduce considerably, further complicating the jaw's movements and the ability of the dog to use them. In severe cases, stomach tubes may be required in order to feed your dog a liquid or gruel diet to maintain health. Your veterinarian will brief you about the proper care and use of the stomach tube, including how to clean before and after use. This is essential, as improperly cleaned, contaminated medical aids can result in severe infection.
Long-term use of immune-suppressive drugs are harmful for the patient's overall health. It is important to strictly follow the dosage and frequency of the medication in order to avoid complications related to their use. Never modify the dosage of the immune-suppressive drugs or stop the treatment without prior consulting with the veterinarian. If you have any concerns you should consult with your veterinarian first. You will also need to isolate your dog to some degree while it is under treatment to protect it from outside illnesses, and from transmissible illnesses from other animals or pets.
Most patients respond well to the immune-suppressive drugs and jaw mobility will return to normal. However, in chronic cases, the prognosis is often not good due to the loss of muscle bulk. Timely treatment is the single most important factor in the treatment of dogs with focal inflammatory myopathy.