Myositis

Myositis is a disease with many different outcomes, ranging from mild to debilitating. It is characterized by a chronic inflammation of the body's immune system that attacks one's own muscles.

The two major types of myositis are polymyositis and dermatomyositis. They are rare disorders affecting approximately 100,000 in the general population. There is a female predominance of 2:1 and a predilection for those in their fifth decade of life. While the two entities share a common immunologic defect, dermatomyositis is also associated with skin as well as muscle. It also carries a much higher malignancy rate. Two additional types of myositis, not as common as DM and PM, include Inclusion Body Myositis (IBM) and Juvenile Myositis. IBM is marked by progressive weakness of the muscles of the body typically occurring after the age of 50. Juvenile Myositis occurs in children with a triad of muscle weakness, skin rash and difficulty swallowing.

The diagnostic criteria for both dermatomyositis and polymyositis include five characteristics created by Bohan and Peter in 1973. They include: 1. symmetric proximal muscle weakness, 2. rash, 3. elevated serum muscle enzymes, 4. myopathic changes on an electromyography (EMG: nerve conduction study to determine the strength nerves), and 5. muscle biopsy abnormalities in the absence of histopathologic signs of other myopathies, such as those attributed to bacterial or viral infections.

Muscle weakness is the most common presenting symptom, shared by both DM and PM. The onset of the weakness may be acute or chronic progressing in several months. In the elderly population, the esophageal muscles are more likely to be affected, causing difficulty in swallowing (dysphagia) or regurgitation. Muscle tenderness occurs in 25 to 50 percent of cases.

The rash manifested in DM has a variety of presentations, but the most common rash, termed Got-tron's sign, is a symmetric, scaly, red/purple hued rash occurring over the knuckles of the fingers. Other distinct rashes seen include those that cover the neck and shoulders, or eyelids, or cracking of the nails and surrounding skin.

Muscle enzyme elevation is also a distinct characteristic of DM and PM. Creatine kinase (CK), lactate dehydrogenase (LDH), aldolase, aspartate aminotransferase [Page 1158](AST), and alanine aminotransferase (ALT) are the enzymes most commonly elevated. Most patients will experience an elevation of all of the enzymes. An Electromyography (EMG) is a test that measures the severity of any nerve disorders, measures the degree of nerve entrapment, and evaluates muscular and nerve disorders, such as myasthenia gravis. In the presence of DM or PM, an EMG will reveal high frequency discharges, increased activity and spontaneous fibrillations, and abnormal short-dura-tion, low amplitude motor potentials. These findings are not diagnostic, but rather support the diagnosis of DM or PM. It is a useful test to rule out muscle disease due to a neuropathic (nerve) etiology.

Muscle biopsies can accurately diagnosis DM or PM. It is recommended that it be obtained from a weak, but not completely atrophic, muscle specimen. An EMG, MRI can aid in the detection of weak muscles. While DM and PM may both exhibit structural changes in the tissue, there are distinct characteristics that identify the two. DM has a predilection for blood vessels within the muscle. There is an immunologic complex called the C5b–9 membrane attack complex.

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