Skeletal muscles are prone to wear and tear and undergo physiological and pathological changes like other organs of the body. Sometimes these muscles become inflamed which is the typical protective response of muscles to injury. Inflammation of this muscle tissue is known as myositis. This is quite different from other muscle ailments like myopathy (disease of the muscles), rhabdomyolysis (degenerative disease of the muscles) and myalgia (muscle pain). Myositis can be caused due to various factors which can include both acute as well as chronic mechanisms. These may cause just mild discomfort in the affected area or can cause acute and debilitating pain leading to complete loss of muscle function.
Types and causes of myositis
There are various types of myositis, which can be broadly classified as infectious myositis and non-infectious myositis.
1. Infectious myositis
It is caused by bacteria, viruses, protozoa and parasitic worms whereas non-infectious myositis is caused due to dermatomyositis, polymyositis and inclusion body myositis.
Infectious myositis is caused due to acute or chronic infections. Generally, bacterial infections (pyomyositis) spread from one site to another and tend to be quite acute. It could be caused because of insect bites, tick bites, etc. The ticks carry and act as reservoirs of the Borrelia species of bacteria which is the root cause of the actual infection.
Viruses like HIV-1, Echo viruses, Coxsackie viruses and influenza viruses are also the cause of infectious myositis. Protozoa and parasites like helminths (worms) like tape worm and trypanosomes also can cause infectious myositis.
2. Non-infectious myositis
It is caused mainly due to autoimmune mechanisms. Though the exact reason for this is not known, it is attributed to familial history, environmental factors or systemic autoimmune disorders like systemic lupus erythematosus (SLE).
Non-infectious myositis can be of different types – dermatomyositis (DM), juvenile dermatomyositis (JDM), polymyositis (PM) and inclusion body myositis (IBM).
When a person is affected by dermatomyositis, the skin and skeletal muscles both are inflamed. It is generally seen in certain body parts like elbows, knuckles, and knees. The proximal muscles are affected and they affect both sides. This is not restricted to skeletal muscles and can affect lungs, blood vessels, heart and throat. Sometimes, only the skin is affected and not the muscles, in such cases it is called amyopathic dermatomyositis.
When dermatomyositis occurs in children under 18 years of age then it is known as juvenile dermatomyositis. Extensive gastrointestinal symptoms and calcinosis which are hard lumps or sheets under the skin are observed.
In polymyositis, skin rash is not seen. However, the progression of muscle weakness and the way it affects areas other than skeletal muscle is similar to that of dermatomyositis.
Inclusion body myositis starts with distal muscles, that are muscles which are away from the mid-line of the body. It is generally seen in flexor muscles of the wrist, fingers, and knees.I t does not affect in a symmetrical way as it does in dermatomyositis and polymyositis.
The symptoms of myositis differ depending on various factors like degree and type of myositis affecting the person. Some of the more common symptoms which are seen as clinical features are detailed as below:
Dry, and rough scaling of skin-seen in dermatomyositis and juvenile dermatomyositis.
Red to purple colored skin rash which can be itchy and painful – seen in dermatomyositis and juvenile dermatomyositis.
Thickening of skin-seen in juvenile dermatomyositis and polymyositis.
Voice change like getting hoarse voice and whispering tone – seen in juvenile dermatomyositis and dermatomyositis.
Having difficulty in swallowing – seen in all types of myositis.
Having difficulty in getting up after sitting or after falling – seen again in all types of myositis.
Feeling tired – seen in dermatomyositis and juvenile dermatomyositis.
Feeling irritable and having gastrointestinal features – both symptoms are seen in juvenile dermatomyositis.
Muscle weakness can be of varying degree and can affect different body parts. It can be classified as below:
a. Muscle weakness occurs suddenly in dermatomyositis and polymyositis.
b. Muscle weakness occurs gradually in juvenile dermatomyositis and polymyositis.
c. Muscle weakness occurs in neck, arms and legs in juvenile dermatomyositis.
d. It occurs in forearms, back, shoulders, hips and thighs in polymyositis.
e. It occurs in thighs, knees, legs, feet, fingers and hands in inclusion body myositis.
f. When muscle weakness increases it causes acute discomfort or pain in inclusion body myositis.