Overview

Polymyositis (PM) is a type of autoimmune disease with an unknown cause and often patients also develop interstitial lung disease (ILDs). In this article, we will briefly look at what polymyositis and interstitial lung disease is, and then we will discuss the symptoms, diagnostics and management strategies currently used for PM. Throughout this article, you will be able to see how ILDs play a key role in PM and how it is diagnosed and managed.

What is interstitial lung disease?

Lungs are important organs in the body which carry out the vital function of oxygenating blood cells so that the rest of the body can get the oxygen it requires. In ILD, inflammation and fibrosis (scarring) are found in the lungs. There are a large group of disorders that fall under the category of ILDs which all have a wide range of causes and result in a decline in the function of the lungs.¹

What is polymyositis?

Myositis are autoimmune disorder where an individual’s immune cells mistake their own cells as foreign cells and start to attack them resulting in muscle inflammation. It is a connective tissue disorder (CTD) where the exact cause is unknown.²

PM is a subtype of myositis involving the cellular immune system and is characterised by the absence of skin inflammation symptoms that are usually observed in individuals with other types of myositis.^2,3 Lung issues are however a common complication.²

Common symptoms of polymyositis

As it involves muscle inflammation, weakness in muscles such as the legs and arms are common symptoms. For example, patients may feel weakness and pain whilst walking and climbing stairs or difficulty lifting objects above their heads/lifting their arms. This can be very distressing for patients and result in poorer quality of life as they are unable to carry out basic day-to-day activities.³

Muscle weakness can either start out slow (subacute) or progress fast (acute), although in most cases development is slow, typically over weeks. Some patients may also develop arthritis.³

As mentioned in the previous section, ILDs are also commonly noted to occur in PM patients as muscle weakness results in issues such as respiratory insufficiency or aspiration pneumonia.² Signs of this would include having a non-productive cough (dry cough with no mucus) and progressive dyspnoea (shortness of breath). Some patients may also start coughing up blood and experience chest pain.²

Patients in the later stages of PM can develop dysphagia (swallowing difficulties), aspiration pneumonia, nasal speech (when air escapes down the nose whilst talking), and respiratory failure as a result of the involvement of the thoracic muscles and diaphragm.²

There is also an increased risk of cancer for those with PM and other idiopathic inflammatory myopathies (IIM).²

Factors that increase the risk of cancer for these patients include being older, having dysphagia, and interstitial lung disease. Therefore, ILM increases cancer risk in polymyositis patients².

Diagnosing polymyositis

Patients usually go to see the physician due to muscle weakness and pain. At the consultation, some possible questions that the doctor might ask include whether they can reach something on the top shelf of the kitchen without feeling muscle weakness, and if they feel weakness when doing basic activities such as climbing the stairs or walking. Doctors will check for any rashes on the skin to rule out some types of myositis.⁴ They must also rule out other conditions such as muscular dystrophy before they can diagnose the patient with PM.³

The doctor will also use their stethoscope to listen to the sounds of the lungs whilst they tell you to breathe out (forced expiration). Airway disease can be ruled out if there are no wheezes or abnormal loud sounds heard during forced expiration.⁴

The gold standard for early diagnosis is high-resolution computed tomography (HRCT) to assess pulmonary involvement.²

Blood tests are also an important diagnostic tool as they can also aid in ruling out other disorders with similar symptoms. In hospitals, respiratory function tests particularly lung capacity (vital capacity) would also be carried out.⁴ 

Fluid from the lungs would also be analysed through flow cytometry and lab tests, such as culturing to test for different bacterial or viral infections as possible causes and in some cases, a lung biopsy may also be required.⁴

Another diagnostic tool which also gives insight into how the disease can be managed is antibody profiling. Patients who have myositis-related ILD that are anti-ARS (antibodies) positive are more likely to respond well to treatment with anti-inflammatory drugs.⁴

Once diagnosed, treatment can then begin, and disease progression can then be monitored for effective disease management².

Managing polymyositis

Challenges

Managing PM can be quite challenging, especially since the exact cause is unknown and diagnosis is also complex. For the best possible outcome for the patient, many specialists are required to work together including rheumatologists, dermatologists (concerning skin conditions), cardiologists, neurologists, physiotherapists, and pulmonologists.³

Treatment

In mild to moderate cases of ILD where there is minimal interstitial disease, doctors may be able to avoid giving steroids as immunosuppressive drugs alone usually suffice for treatment/management. In severe cases, corticosteroids and induction therapy are used for treatment.²

Complications involving therapeutic drugs

Since it is a muscle inflammation disorder, it is possible in most cases to improve or prevent PM altogether using anti-inflammatory drugs like corticosteroids.⁴ However, like all medicines, they can have side effects and may be unsuitable for some patients as they are very strong medications.

Immunosuppressive drugs are often used for treatment, however, they are known to cause lung damage making it more challenging to identify the cause of the ILD in PM patients as it could have either developed as a result of the PM or developed during treatment. This is known as drug-induced pneumonitis.²

Summary

Polymyositis is an autoimmune disease with no known cause where interstitial lung diseases are commonly reported. Therefore, identifying and monitoring ILDs are useful for both diagnosing and treating patients with polymyositis. It can also however make it difficult to sometimes assess the effects of therapeutic drugs as lung damage can be a side effect of the drugs or caused by PM itself. Still, it is important to closely monitor ILD in PM patients for better outcomes.