Inflammatory Myopathy Health Article

Definition

Inflammatory myopathy is a term that defines a group of muscle diseases involving inflammation and degeneration of skeletal muscle tissues. They are thought to be autoimmune disorders. In inflammatory myopathies, inflammatory cells surround, invade, and destroy normal muscle fibers as though they were defective or foreign to the body. This eventually results in discernible muscle weakness. This muscle weakness is usually symmetrical and develops slowly over weeks to months or even years.

When using the term inflammatory myopathy, one is actually considering three separate disease entities, namely dermatomyositis (DM), polymyositis (PM), and inclusion body myositis (IBM). Although all of these diseases result in muscle weakness, each is unique in its development and treatment.

Description

Inflammatory myopathies include a diverse group of disorders ranging from localized varieties confined to a single muscle or group of muscles, to diffuse forms in which there is widespread involvement of the skeletal muscles.

Inclusion body myositis (IBM) mainly affects individuals over the age of 50. The onset is truly insidious with symptoms often having been present for more than five years before diagnosis. Clinically and histologically, IBM may appear identical to another inflammatory myositis called polymyositis, although differences are clear in more than half the patients.

Weakness in (IBM) may be localized in the extremities, or asymmetric, and it may be accompanied by diminished deep-tendon reflexes. Disease progression is usually slow and steady in some, while it seems to plateau in others, leaving them with fixed weakness and atrophy (muscle wasting) of the involved musculature. In the muscle tissue, a characteristic change in IBM is the presence of intracellular rimmed vacuoles (pockets). The muscle fibers with pockets are now recognized to contain abnormal deposits of amyloid proteins.

Polymyositis usually occurs after the second decade of life and is a subacute myopathy (one that occurs over time) that evolves over weeks or months, and presents with weakness of the arm and leg muscles. PM mimics many other myopathies. It should be viewed as a syndrome of diverse causes that occurs separately or in association with other autoimmune disorders. In PM, muscle fibers are found to be in varying stages of necrosis (tissue death) and regeneration.

Dermatomyositis (DM) is identified by a characteristic skin rash accompanying or, more commonly, preceding muscle weakness. DM affects children and adults and presents a varying degree of muscle weakness that develops slowly, over weeks to months.

Demographics

In the United States and Canada, IBM accounts for approximately 15–28% of all cases of inflammatory myopathies. IBM most frequently affects men with a male to female ratio of 3:1. No race predilection for IBM is known, but it is uncommon among African-Americans and has been reported in Europe and Asia. Assessing demographic data is difficult due to the fact that IBM patients often exhibit other medical problems.

Polymyositis (PM) is most common among black people and is most prevalent in women, with a male to female ratio of 1:2. In the United States, its incidence is one per 100,000 persons per year. Dermatomyositis (DM) affects mainly white people and is more prevalent in women, with a male to female ratio of 1:2. In the United States, the estimated incidence is 5.5 cases of DM per one million people.

Causes and symptoms

Inclusion body myositis (IBM) is thought to be a sporadic disease, meaning one that is not hereditary. The cause of IBM remains unknown, but is thought to be a form of autoimmune disease, where the immune system responds in a harmful manner to the rest of the body. Very rarely, IBM can be present within families, and it is not known whether this form is inherited or if family members have another susceptibility to whatever causes the sporadic form of the disease.

The trigger mechanism for all inflammatory myopathies remains unknown. Some scientists maintain that a viral illness causes an injury that activates a flawed immune response. Other scientists, noting that cancer sometimes occurs along side some types of inflammatory myopathy, are investigating the relationship between the two diseases. A genetic predisposition may exist for DM, and abnormal activities of certain white blood cells may be involved in the cause of both the skin and the muscle disease.

Weakness of muscle function in the area affected is usually the first symptom of inflammatory myopathy. The distribution of weakness is variable, and involvement of the knee extensor muscle and the wrist and finger flexor muscles are common. Fatigue is common, along with reduced tolerance to exertion, difficulty swallowing (dysphagia), and some forms of heart disease.

In polymyositis (PM), weakness and muscle pain on both sides of the body at rest or with use are the first signs of the disease. The weakness becomes chronic, lasting for weeks or months. If swallowing muscles are involved, dysphagia may occur. Joint pain and difficulty kneeling, climbing, or descending stairs, raising arms, and arising from a sitting or lying position are also noticeable.

People often present with skin disease as one of the initial manifestations of DM. A characteristic rash preceding or accompanying muscle weakness, or a confluent, purple-red rash with swelling in surrounding tissues appears. Other rashes seen with DM include swelling at the nail beds and a scaly purple eruption over the knuckles. Muscle involvement varies from mild to severe. The muscle wall of the heart or lung tissues may also become inflamed as a consequence of DM. Some cancers have been associated with DM, a finding much more common in adults over 60 years old.