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Inflammatory MyopathiesInflammatory Damage to Skeletal Muscle Causes Weakness
When inflammatory cells invade skeletal muscle or its immediate environs, the resulting insult to the muscle can cause a loss of strength.
There are three major inflammatory myopathies in humans: polymyositis, dermatomyositis and inclusion body myositis. In each case, inflammatory cells invade the area around skeletal muscle fibers, their blood vessels or the muscle fibers themselves. While the exact cause of these myopathies is unknown, for some of them it is believed that there is likely to be an autoimmune component. How Are These Myopathies DifferentWhile all of these myopathies can present with muscle weakness, the pattern of weakness and the presence of other signs and symptoms can differentiate between them. Damage to muscle fibers as a result of the inflammation can lead to the release of a muscle enzyme, creatine kinase (CK), into the blood stream. While polymyositis and dermatomyositis are routinely associated with significant increases in CK, inclusion body myositis does not necessarily associate with any change in CK levels in the blood. Characteristics of DermatomyositisAs the name suggests, dermatomyositis (DM) also involves the skin (from ‘derm’). In this disorder a patchy skin rash, often reddish or purple in color, appears prior to the occurrence of muscle weakness. The rash occurs because of inflammation in the blood vessels that are just under the skin. When muscle weakness arises it too is due to inflammation, but now in the skeletal muscle. Examining a biopsy of muscle tissue in the microscope shows inflammatory cells in between the muscle fibers and typically in dermatomyositis many inflammatory cells surrounding the blood vessels that are in the muscle. Characteristics of PolymyositisIn polymyositis (PM), muscle fibers are also subject to infiltration with inflammatory cells, but there is not significant involvement of the blood vessels as there is in dermatomyositis. Weakness can progress slowly or rapidly but usually begins with what are known as the proximal muscles, those that are closest to the trunk. Oftentimes, weakness can present as difficulty rising from a seated position, due to problems with the quadriceps muscles in the thigh. More women than men are typically afflicted with polymyositis. Characteristics of Inclusion Body MyositisInclusion body myositis (IBM) can show varying degrees of inflammatory cell infiltration of the skeletal muscle, from substantial to minor. The characteristic finding upon examination of a muscle biopsy however, is the presence within the muscle cells themselves of self-contained vacuoles, or membrane bound inclusion bodies. These inclusions are often filled with the sticky peptide known as beta-amyloid, the same beta-amyloid peptide that is found in the brain cells in patients suffering from Alzheimer’s disease. Yet, patients with IBM do not have Alzheimer’s disease. What Are the Treatments for Inflammatory MyopathiesMost of the inflammatory myopathies will respond to treatment with agents that modify the immune response. For both DM and PM, treatment with agents such as prednisone and other corticosteroids can provide significant relief of symptoms. For some people with more difficult to treat forms of PM, stronger immune modulating drugs can be used. IBM, on the other hand, is not generally controlled using anti-inflammatory medications, even though for most cases there is a clear inflammatory component. Other therapies are being tried and research continues to provide new clues as to possible future treatments. For additional information visit The Myositis Association
The copyright of the article Inflammatory Myopathies in General Medicine is owned by Kenneth Rosen. Permission to republish Inflammatory Myopathies in print or online must be granted by the author in writing.
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