Focal Inflammatory Myopathy in Cats
Myopathy is a general term that is used to denote any disorder of the muscles. Focal inflammatory myopathy is a localized form of the disease that affects specific muscle groups, in this case the masticatory (chewing) muscles and extraocular (eye) muscles.
The masticatory muscles are a group of four facial muscles that are used for chewing, and the extraocular muscles are a group of muscles, adjacent to eyeball, that control the movements of the eyes.
Focal inflammatory myopathy is suspected 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. This disease is relatively rare in cats.
Symptoms and Types
Masticatory muscles
- Problems with normal jaw movements
- Inability to get and keep food in the mouth
- Jaw pain
- Muscle swelling around jaw and face
- Progressive loss of muscle mass
Extraocular muscles
- Swelling around eye
- Protrusion of the eyeball from the eye socket
Causes
Immune-mediated
Diagnosis
You will need to give your veterinarian a thorough history of your cat’s health, including a background history of symptoms. After taking a detailed history, your veterinarian will conduct a complete physical examination on your cat.
Your veterinarian may attempt to manipulate your cat's jaw muscles to induce pain and swelling of the muscles so that the source of the problem is more evident. Your veterinarian will also try to open your cat's mouth, which often proves unsuccessful in these patients. 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 cat's immune system in order to limit 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 cat'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. In severe cases, stomach tubes may be required in order to feed your cat 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 cause 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 cat 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 cats with focal inflammatory myopathy.