Skeletal muscle are those voluntary muscles that attaches to bones and is responsible for movement of the different body parts. The muscles are active to varying degrees in each individual, which affects its size and structure. The muscles are prone to the same physiological changes and pathological alterations as any other organ of the body. Inflammation is the protective response of tissue to injury. When this occurs in muscles, it is known as myositis. This should be differentiated from other related terms like myalgia which is muscle pain, myopathy which is any disease of the muscles and rhabdomyolysis where there is degeneration of skeletal muscle tissue.
What is myositis?
Myositis is the medical term for inflammation of the skeletal muscle. The inflammation may vary in severity and may be isolated to one muscle, muscle group or several muscles throughout the body. The key features of muscle inflammation are swelling, pain and disruption of normal function (muscle weakness). Myositis may be due to various causes, both acute and chronic mechanisms that can cause mild discomfort of the area and muscle weakness to severe debilitating pain and complete loss of muscle function.
Muscle inflammation itself is not an uncommon condition. The muscles are constantly working on a daily basis and sometimes undergo strain when it is forced to act in a manner that it is not accustomed to. As a part of the protective structure of the body, the muscles may also experience injury from an external force. There is usually no other clinical features or disturbances of other organs and systems in these cases. This acute inflammation is often referred to as sore muscles but recovers quickly within a few hours or days. There is minimal need for treatment apart from rest and there is no long term complication or permanent damage.
However, some causes of muscle inflammation may be more complex and can have a long term effect which can permanently compromise muscle function.This may include autoimmune disease, infections, toxicity of certain drugs and substances. Most chronic forms of myositis appear to be associated more often with autoimmune causes.
Types and Causes of Myositis
There are several types of myositis which can be broadly divided into the infectious and non-infectious causes.
- Infectious Myositis
- Parasitic worms
- Non-infectious Myositis
- Inclusion body myositis
Myositis associated with injury and strain is rarely considered as a separate type because it is so common and almost always recovers completely in a short period of time. Similarly drug-induced myositis is a temporary inflammation of muscles associated with the use of certain drugs. This may include drugs used for cholesterol, gout, malaria and cancer. It may also be seen with cocaine use and excessive alcohol consumption or alcoholism. However, both physical and chemical injury can cause a dangerous condition known as rhabdomyolysis where the muscle is rapidly broken down.
Infectious myositis is inflammation of the muscles due to an acute, subacute or chronic infection. Bacterial infections (pyomyositis) is a severe often acute infection that tends to arise when bacteria spread to the muscles from another site of infection, either superficial or deep. At times, bacterial myositis may be associated with an insect bite like Lyme disease which arises with tick bites. The ticks in these cases are the carriers and reservoirs of the Borrelia species of bacteria that cause the actual infection.
Infectious myositis may also be caused by other pathogens including :
- Viruses, like HIV-1, influenza, coxsackieviruses, and echoviruses.
- Protozoa and parasites including trypanosomes and helminths (worms) like the tapeworm.
Non Infectious Myositis
This type of myositis is largely due to autoimmune mechanisms – the body’s immune system attacks muscle cells. The exact cause of this autoimmune mechanism, however, is largely unknown although it may be associated with familial history, environmental factors or be related to systemic autoimmune disorders like systemic lupus erythematosus (SLE).
In this type of myositis, both the skin and skeletal muscles are inflamed. The distinctive rash is often seen more frequently on certain parts of the body particularly the knuckles, elbows and knees. It typically causes a scaly rash with dryness and roughness of the skin. Muscle weakness affects the proximal muscles, progresses slowly and affects both sides in the same pattern of distribution.
The muscle inflammation, however, is not isolated only to the skeletal muscles of the limbs and can affect even smooth muscles and cardiac muscle. There are several other clinical manifestations that may affect many other parts of the body other than the muscles or skin. This may include the lung, blood vessels, heart and throat which contributes to the overall clinical presentation. If the skin rash occurs with no muscle weakness then it is known as amyopathic dermatomyositis.
Juvenile dermatomyositis is seen in children under the age of 18 years and is similar to dermatomyositis seen in adults. It is marked by extensive gastrointestinal symptoms and calcinosis which is hard lumps or sheets under the skin (contractures).
This type of polymyositis is very similar to dermatomyositis with the exception of the skin rash that is not seen in polymyositis. The progression of muscle weakness and distribution, as well as the involvement of areas other than the skeletal muscle, is the same as with dermatomyositis.
Inclusion Body Myositis
This type of myositis starts with the distal muscles (away from the midline of the body) and has a predilection for the flexor muscles of the wrist and fingers and the extensor muscles of the knee. Muscle weakness is not symmetrical as it is with dermatomyositis and polymyositis.
Signs and Symptoms
The signs and symptoms may vary to some degree depending on the type of myositis. The clinical features have been grouped together for the different types of non-infectious chronic myositis – dermatomyositis (DM), juvenile dermatomyositis (JDM), polymyositis (PM) and inclusion body myositis (IBM).
- Red to purple skin rash usually itchy and painful. (DM/JDM)
- Dry, rough scaling skin. (DM/JDM)
- Thickening of the skin. (JDM/PM)
- Muscle weakness :
- Sudden (DM/PM)
- Gradual (JDM/PM)
- Neck, hip, back and shoulders (DM)
- Neck, arms and legs (JDM)
- Forearms, shoulders, back, hips and thighs. (PM)
- Hands, fingers, elbows, thighs, knee, legs and feet. (IBM)
- Discomfort or pain as the muscle weakness increases. (IBM)
- Difficulty getting up after a fall or when seated. (All)
- Hoarse voice or whispering tone (dysphonia). (DM/JDM)
- Difficulty swallowing (dysphagia). (All)
- Tiredness. (DM/JDM)
- Gastrointestinal features. (JDM)
- Irritability. (JDM)
Article reviewed by Dr. Greg. Last updated on December 3, 2011