What is JM?

Juvenile Myositis (JM), including Juvenile Dermatomyositis (JDM) and Juvenile Polymyositis (JPM), is a group of rare and life-threatening autoimmune diseases, in which the body’s immune system attacks its own cells and tissues.

Weak muscles and skin rash are the primary symptoms of JDM, while muscle weakness without a rash is the primary symptom of JPM.

Even within these designations, JM affects every child differently. Some children experience a mild form of the disease, while others follow a more severe and potentially more debilitating course. Some of the more onerous secondary symptoms are calcinosis, digestive (GI) tract complications, vasculitic ulcers and contractures.

Incidence of JM

JM is a rare disease and its exact incidence is unknown. Approximately 2 to 4 children in a million in the United States are diagnosed with JM each year.

JM begins in childhood or the teen years. The average age of onset for JDM is between six to seven years old; 25% are age 4 or less. JPM usually develops several years later.

JM affects girls twice as much as boys. Once a child is diagnosed with JM, it is always considered to be the juvenile form, even if the patient continues with the disease into adulthood.

Genesis of the Disease

The immune system is a group of cells that normally protects the body from infection and other environmental factors. In an autoimmune disease like JM, however, the cells get active and they cannot stop. The immune system ends up attacking previously healthy tissues, harming the body instead of protecting it. In JM, the immune system mistakenly targets muscles, skin and other tissues.

The process that occurs when the immune system attacks healthy tissue is called inflammation. In JDM sufferers, the inflammation primarily occurs in the blood vessels that lie under the skin and in the muscles. With JPM, the attack is focused more directly against the muscle cells.

This inflammation causes the weak muscles and – in the case of JDM – skin rashes. If blood vessels or muscle cells in other parts of the body are inflamed, other systems of the body can be affected, such as the digestive tract, heart and lungs.

What causes JM?

Researchers believe the coming together of several factors – both environmental and genetic – cause JM. Children who develop this disease often have a family history of other autoimmune diseases, such as thyroid disorder, type I diabetes, rheumatoid arthritis, lupus, or Crohn's disease. If a child is genetically predisposed to JM, experts suspect a microbe, such as a virus or bacteria, might trigger a runaway immune response that will cause the body to attack itself. Other environmental factors, such as a heavy dose of sun exposure, may also play a role.

What are the symptoms of JM?

The primary symptoms of JM are weak or painful muscles, skin rash (with JDM), fatigue and fever. Some children experience joint pain as well.

What are other possible symptoms?

The following symptoms of JM are less common, but important to understand:

What tests are used to diagnose JM?

A doctor will first perform a complete physical exam, specifically looking for rashes and muscle weakness. If JM is suspected, blood tests are the next step in making a diagnosis. Muscle enzymes are measured, including creatine kinase (CK), aldolase, lactate dehydrogenase (LDH), aspartate aminotransferase (AST), and alanine aminotransferase (ALT). If these lab tests are elevated, enzymes are leaking from inflamed or damaged muscles into the bloodstream. Antinuclear antibodies (ANA) are also measured to see if the child's body is producing antibodies against its own cells. Several myositis-specific autoantibodies have also been identified, and these can be tested as well. Other blood tests are sometimes available to check out immune activation and/or blood vessel damage.

The next step in diagnosing JM is usually an MRI which can detect muscle inflammation and damage. If the MRI shows evidence of diseased muscles, a muscle biopsy may be performed to finalize the diagnosis. When the characteristic rashes are not present, it is important that a muscle biopsy be performed to exclude other causes of muscle weakness. A small amount of muscle is removed for examination under a microscope to determine if and how much the muscles and blood vessels have been affected by the disease.

If the patient has the rash and muscle weakness of JM, the lab tests are consistent with JM, and the MRI shows muscle inflammation, some doctors will not perform a muscle biopsy.

What is the treatment for JM?

Although medications can help alleviate the symptoms of JM, the disease has no known cure. The primary medications used to treat the symptoms of JM are corticosteroids, immunosuppressants and chemotherapy. These medications themselves can cause severe side effects, making JM challenging to treat. Many researchers believe that early and aggressive treatment is usually the best predictor of a better outcome of this disease.

High dose oral Prednisone or other corticosteroids often coupled with intravenous corticosteroids (Solumedrol) are usually the first line of treatment for JM. Since the side effects of corticosteroids can be very troublesome, Methotrexate (a chemotherapy drug administered at much lower doses than used for cancer patients) is usually introduced early to allow for tapering of the corticosteroids.

Second-line treatments include Cyclosporine and Intravenous Immunoglobulin (IVIG). Plaquenil is often used to control the rash.

For patients who do not respond to the first and second line of treatments, or in patients severely affected by the disease, Rituximab, Cytoxan and CellCept can be added. Protection from UVB is essential with sunscreens. Vitamin D with adequate calcium intake keeps bones strong.

These medications all have their own side effects, but the most common ones for Prednisone are:

In addition to medications, physical therapy is generally prescribed to increase strength and flexibility in muscles and joints.

What is the prognosis?

There is no cure for JM, but with advances in early diagnosis and aggressive treatment, the outcome has continued to improve. Some children experience a mild form of the disease and may go into remission. Others follow a more severe and potentially debilitating course that can be life-long. Some JM patients will have a loss of range of motion. Some will battle an array of serious complications, resulting in the inability to walk, ongoing pain, disfigurement and even death. Whether the course of the disease is mild or severe, JM is life-changing for all of these children and their families.