What is Polymyositis?
Polymyositis is a rare inflammatory myopathy that causes muscle inflammation and weakness. It is an autoimmune disorder that attacks the muscles, causing them to become weak and easily fatigued.
Polymyositis is classified as an idiopathic inflammatory myopathy, meaning it is an inflammatory disease that affects the muscles, but does not have a known cause. It mainly affects the proximal muscles closest to the trunk of the body, like those in the hips and shoulders. This can make it difficult to perform everyday tasks like climbing stairs, getting out of a chair, or reaching overhead.
In polymyositis, the body’s immune system mistakenly identifies muscle proteins as foreign invaders and launches an attack against them. This causes chronic muscle inflammation that damages muscle fibers over time. As the inflammation progresses, the muscles steadily weaken and waste away.https://www.mayoclinic.org/diseases-conditions/polymyositis/symptoms-causes/syc-20353208
Causes
The exact cause of polymyositis in dogs is unknown, but it is believed to be an autoimmune disease where the body’s immune system mistakenly attacks its own muscle cells. While the triggers that initiate this abnormal immune response are unclear, genetics may play a role in susceptibility.
Some research suggests polymyositis may be caused by a combination of genetic and environmental factors. Certain dog breeds like German Shepherds, Collies, and Shetland Sheepdogs appear predisposed, indicating a possible genetic component. Exposure to viruses, bacteria, drugs, or toxins may act as environmental triggers provoking the immune system to malfunction and attack muscle tissue.
However, the precise sequence of events remains elusive. More research is needed to uncover the complex interplay between genetic risks and environmental exposures leading to polymyositis in canines.
Symptoms
The main symptom of polymyositis in dogs is progressive muscle weakness, especially in the legs, neck, jaw, and throat muscles.1 This leads to difficulty standing up, walking, climbing stairs, or jumping. Dogs may show a stiff, stilted gait and have trouble getting up after lying down.
Dogs with polymyositis often have trouble swallowing, known as dysphagia.2 This can lead to regurgitation of food, drooling, gagging, or coughing while eating and drinking. The throat muscles and esophagus are weakened, making it hard to swallow properly.
As the disease progresses, dogs experience muscle atrophy and loss of muscle mass, causing a sunken or wasted appearance.3 The muscles feel soft and flabby rather than firm. This muscle wasting is most noticeable on the head, legs, and shoulders.
Dogs with polymyositis often show signs of pain or soreness when the affected muscles are touched or manipulated. They may cry out or nip when muscles are pressed on during a veterinary exam.
Diagnosis
Diagnosing polymyositis in dogs involves a combination of a physical exam, blood tests, electromyography, and a muscle biopsy:
The physical exam allows vets to check for symptoms like weakness, muscle atrophy, difficulty standing or swallowing, and enlarged muscles. They will manipulate and palpate muscles to check for pain and inflammation.
Common blood tests look for elevated muscle enzymes like AST, ALT, and CK, which can indicate muscle damage. Increased white blood cell counts may also point to inflammation.
Electromyography tests the electrical activity in muscles. It can detect abnormal muscle weakness and spontaneous electrical activity associated with muscle inflammation.
A muscle biopsy involves removing a small sample of muscle tissue and examining it under a microscope. This allows vets to directly see muscle damage and inflammation consistent with polymyositis.
With a combination of these diagnostic tests, vets can confirm a diagnosis of polymyositis in dogs.
Treatment
There are several main treatments used for polymyositis in dogs:[1]
Immunosuppressive drugs are commonly used, including corticosteroids like prednisone or prednisolone. High doses of steroids like prednisolone (1-2 mg/kg, PO, every 24 hours for 3-4 weeks) are typically the first line treatment. Other immunosuppressive medications may also be used, such as azathioprine or cyclosporine.[2]
Physical therapy can help dogs regain muscle strength and mobility that has been lost due to the muscle inflammation. Massage, range of motion exercises, stretching, and other techniques may be beneficial.
Assistive devices like harnesses, slings, or carts may be needed to help dogs with severe muscle weakness to remain mobile and participate in physical therapy.
Prognosis
The prognosis for dogs with polymyositis can be quite variable depending on the severity of the condition. In mild cases caught early, dogs generally respond well to treatment and can make a full recovery. However, in more advanced or severe cases, the prognosis is guarded to poor.
With aggressive treatment, many dogs with polymyositis can stabilize and have a good quality of life, even if some muscle weakness persists. However, treatment is often lifelong and relapses may occur if immunotherapy is discontinued. Severely affected dogs may eventually become unable to walk or even stand.
Overall, the earlier polymyositis is diagnosed and treatment begins, the better the prognosis. Mild cases treated promptly have an excellent prognosis and can regain normal muscle function. But in chronic, advanced or poorly responsive cases, muscle damage may be irreversible leading to a more guarded outlook. Close monitoring and follow-up care is essential, even after initial improvement.[1]
Complications
Some of the potential complications of polymyositis in dogs include:
Aspiration pneumonia – Weakness in the muscles involved in swallowing can lead to aspiration of food, saliva, or liquids into the lungs. This can cause aspiration pneumonia, a type of pneumonia caused by foreign matter entering the lungs. Dogs with polymyositis are at increased risk for this potentially serious complication (1).
Permanent muscle damage – If the inflammatory process of polymyositis is not controlled, it can lead to permanent damage and scarring of the muscles. This can result in ongoing weakness and disability even after treatment (2).
Breathing difficulties – The muscles involved in breathing can be affected in polymyositis. Weakness of the chest muscles and diaphragm can make it difficult for dogs to breathe properly. In severe cases, mechanical ventilation may be needed to support breathing (3).
Careful monitoring and supportive care are important for promptly identifying and managing complications like these in dogs with polymyositis.
(1) https://www.merckvetmanual.com/musculoskeletal-system/myopathies-in-small-animals/polymyositis-in-dogs
(2) https://pubmed.ncbi.nlm.nih.gov/6987202/
(3) https://www.kingsdale.com/polymyositis-in-dogs-causes-symptoms-and-treatment
Prevention
There are no known ways to prevent polymyositis in dogs. Since the underlying cause is unknown, there are no specific preventative measures.
However, managing any underlying or triggering conditions may help reduce the risk. Providing excellent nutrition, maintaining vaccinations, controlling parasites, and avoiding trauma or overexertion can support your dog’s overall health.
Practicing good hygiene, providing a clean living environment, and avoiding feeding raw or undercooked foods can help prevent infections that may trigger polymyositis. Limiting contact with other dogs with infectious diseases is also wise.
Routine veterinary care to monitor your dog’s health is advisable. If polymyositis develops, early detection and treatment can improve outcomes. Discuss any concerns about prevention with your veterinarian.
Living with Polymyositis
Caring for a dog with polymyositis requires some lifestyle adjustments. Exercise is an important part of a dog’s health, but needs to be modified for a dog with muscle inflammation. Low-impact exercise like short walks and swimming are recommended over high-impact activities like running and jumping. Physical therapy can also help improve muscle strength and function. Sessions with a canine physical therapist typically involve range of motion exercises, stretching, and massage.
Dogs with polymyositis are also at risk of aspiration, which is when food or liquids accidentally enter the lungs instead of the stomach. This can occur when muscles involved in swallowing are weakened. Owners should monitor their dog during mealtimes and avoid exercising for at least 1 hour after eating. Food and water bowls can be elevated to make eating and drinking easier. In some cases, a veterinarian may recommend switching to canned food or gruel to make swallowing easier.
With treatment and supportive care, many dogs with polymyositis can manage the condition and continue to have a good quality of life. However, owners do need to make adjustments to their dog’s routine and monitor them closely. Working together with a veterinarian can help create the best treatment plan.
Summary
Polymyositis in dogs is an inflammatory disease that affects muscles and causes weakness. While the exact cause is unknown, it is likely an autoimmune disorder or reaction to an infection. Symptoms include difficulty standing up, muscle loss, trouble swallowing, and more. The disease is diagnosed through bloodwork, imaging, electromyography, and sometimes muscle or nerve biopsies.
Treatment focuses on suppressing the immune system inflammation with corticosteroids and other immunosuppressants. Physical therapy can also help maintain muscle strength and function. With aggressive treatment, the prognosis for polymyositis can be good, though relapses are possible. It’s important to start treatment as soon as possible to halt muscle damage. With dedicated care and monitoring, dogs with polymyositis can have a good quality of life.
