OVERVIEW

What is juvenile polymyositis?

Juvenile polymyositis (JPM) is an autoimmune inflammatory myopathy. Its main manifestations include proximal limb muscle weakness and immune-mediated inflammation. Some scholars have questioned whether polymyositis should be classified as an independent disease entity.

How many people have juvenile polymyositis?

The incidence of juvenile dermatomyositis (JDM) abroad is approximately 2-4 cases per million children annually (note: this refers to incidence, not prevalence). JPM is even rarer, estimated to account for less than 1/20 of JDM cases.

Unfortunately, there is no relevant epidemiological data in China, so the annual incidence and prevalence rates in the country remain unknown.

SYMPTOMS

What are the manifestations of juvenile polymyositis?

JPM differs from JDM in that it usually does not involve the skin, but like JDM, it often affects muscles, joints, and lung tissues. The specific manifestations are as follows:

• Muscle involvement: The clinical manifestations are essentially the same as those of "juvenile dermatomyositis."

• Difficulty walking, climbing stairs, getting out of bed, or standing up after squatting (positive Gower's sign).

• Inability to perform simple daily activities such as raising arms, combing hair, or dressing.

• Involvement of the neck flexors and back muscles may lead to difficulty lifting the head while lying flat or maintaining an upright neck (more prominent in infants and young children) or inability to maintain a normal sitting posture.

• Some patients may experience involvement of the throat and esophageal muscles, resulting in dysphonia, hoarseness, or dysphagia, with a risk of sudden aspiration. Nasal speech or nasal regurgitation of liquids should raise high suspicion.

• A few cases may develop dyspnea due to involvement of the chest wall and respiratory muscles.

• In the later stages of the disease or in cases of sudden onset, distal muscle involvement may occur, and muscle atrophy may develop in advanced stages.

• Rarely, facial and extraocular muscles may be affected.

• Joint involvement: JPM can also present with arthralgia and arthritis, most commonly symmetric arthritis of the small joints of the hands and feet. Arthritis is more common in patients with Jo-1 antibodies or other anti-synthetase antibodies. Sometimes, arthritis symptoms appear early and may mimic rheumatoid arthritis. This aspect is no different from juvenile dermatomyositis.

• Lung involvement: Compared with adults, pulmonary complications are less common in JPM patients. However, they remain one of the major causes of death in inflammatory myopathy patients. The specific manifestations are the same as those of juvenile dermatomyositis. For details, refer to the entry on "juvenile dermatomyositis."

CAUSES

What are the causes of juvenile polymyositis?

For the etiology, you can click to view the relevant content of the entry "Juvenile Dermatomyositis," as the two are quite similar. However, dermatomyositis is primarily caused by humoral immunity, leading to small vessel vasculitis and subsequent muscle fiber damage.

In contrast, polymyositis is mainly driven by cellular immunity, where CD8+ T cells and CD4+ T cells directly damage muscle fibers. Of course, other cells and proteins are also involved in the pathogenesis of polymyositis.

DIAGNOSIS

What tests are needed to diagnose juvenile polymyositis?

The diagnosis of juvenile polymyositis can be confirmed based on specific skin manifestations and laboratory tests. Common laboratory tests include:

• Elevated muscle enzymes: Troponin may be tested when differentiation from myocardial injury is needed. Muscle enzymes generally reflect myopathy activity but do not indicate muscle strength. Additionally, muscle enzymes may not be elevated when muscle atrophy is significant or during disease reactivation.

• Autoantibodies: These include antinuclear antibodies, found in up to 80% of JPM patients; "myositis-specific autoantibodies" are present in at least 30%–40% of patients; and myositis-associated autoantibodies, particularly in patients with overlap syndromes.

• Serum myoglobin and urinary myoglobin: Elevated levels of these proteins may be observed. Myoglobin is highly sensitive, but its levels fluctuate significantly in healthy individuals due to daily activity variations, making testing less common.

• Erythrocyte sedimentation rate (ESR): ESR is typically normal or only mildly elevated, even in patients with active myopathy.

Although "myositis-specific autoantibodies" are not present in all idiopathic inflammatory myositis cases, they are highly suggestive of the condition.

The aforementioned "myositis-specific autoantibodies" can be categorized into several types, including:

• Anti-aminoacyl-tRNA synthetase antibodies (anti-synthetase antibodies), such as anti-Jo-1 antibodies;
• Anti-signal recognition particle (SRP) antibodies;
• Anti-Mi-2 antibodies (a type of nuclear helicase).

In addition to these specific antibodies, newly discovered myositis antibodies have been identified, but their validation is limited and will not be discussed further here.

What muscle disorders should juvenile polymyositis be differentiated from?

Misdiagnosis is uncommon for JDM with "typical rash" + "muscle symptoms." However, misdiagnosis is more frequent for JPM presenting with isolated muscle symptoms.

It is important to note that JPM is extremely rare. For children with isolated muscle symptoms, JPM should not be the primary consideration. In fact, metabolic myopathies and congenital myopathies are more common. Rheumatologists must avoid being constrained by their specialty.

Specific differential diagnoses include:

• Drug- or toxin-induced myopathy: Alcohol, glucocorticoids, lipid-lowering drugs (e.g., statins), cocaine, antimalarials, colchicine, zidovudine, and penicillamine can induce myopathy. It may present as acute myopathy with painful muscle cramps, tenderness, and often swelling. Generalized muscle involvement is typical, with the calf muscles frequently affected. Creatine kinase and other muscle enzymes may be significantly elevated. Chronic myopathy may manifest as weakness and muscle atrophy. A clear drug history aids in differentiation.

• Rhabdomyolysis: Excessive activity, trauma, crush injuries, heat, seizures, bacterial or viral infections, and drugs can cause rhabdomyolysis. Symptoms include muscle pain, weakness, and dark-colored urine. Proximal muscle groups (e.g., thighs and shoulders), lower back, and calves are often affected, causing muscle pain. However, muscle symptoms are not always present. Notably, patients with certain metabolic disorders are more prone to rhabdomyolysis. Differentiation is straightforward with a clear history.

• Thyroid myopathy: Non-specific muscle stiffness or diffuse myalgia (often worsening after exercise) is common in hypothyroidism. Hypothyroidism is also frequently associated with elevated serum muscle enzymes. A distinctive sign of hypothyroid myopathy is myoedema, a small bulge on the muscle surface when tapped with a percussion hammer. However, this phenomenon is not specific to hypothyroid myopathy. Heart rate, cold intolerance, and thyroid function tests aid in differentiation.

TREATMENT

How should juvenile polymyositis be treated?

The treatment of JPM is relatively difficult. The current mainstream approach is high-dose corticosteroid therapy, but the side effects of steroids are quite concerning.

Particularly considering the impact of glucocorticoids on children's growth and development. Moreover, glucocorticoids themselves can also cause muscle atrophy, which is quite unfavorable for JPM. However, attempting treatment with fewer steroids is very challenging!

Clinical observations confirm that, compared to lupus erythematosus, JPM requires higher doses and longer courses of glucocorticoid therapy. To reduce steroid side effects, options such as immunoglobulin pulse therapy, combined methotrexate, cyclophosphamide, or azathioprine may also be considered.

DIET & LIFESTYLE

What should patients with juvenile polymyositis pay attention to in daily life?

JPM patients, like JDM patients, need vaccinations and appropriate exercise. For details, please refer to the relevant content in the "Juvenile Dermatomyositis" entry. However, it is worth noting that while dermatomyositis patients need to avoid sunlight, polymyositis patients may not have this requirement.

PREVENTION

Can Juvenile Polymyositis Be Prevented?

There are no reliable preventive measures. Reducing UV exposure and tobacco exposure to some extent may be effective.