Idiopathic Inflammatory Myopathies Overview

Idiopathic Inflammatory Myopathies (IIM)

• IIM is a group of diseases causing muscle inflammation with no known cause.
• Doctors use different methods to classify the various forms of IIM.
• Adult-onset dermatomyositis (DM) and polymyositis (PM) are common IIMs.
• A subtype of IIM is called overlap myositis which is associated with features of another connective tissue disease.
• Immune-mediated necrotizing myositis (IMNM) is a type of IIM characterized by specific muscle damage.
• Inclusion body myositis (IBM) is another type, differing in its muscle presentation from other types of inflammatory myopathies.
• Juvenile dermatomyositis affects adolescents.

Differential Diagnosis of Myopathies

• Diagnosing IIM involves excluding other conditions that may present similar symptoms, such as neuromuscular disorders, infectious myositis, muscular dystrophies, and various muscular disorders.
• Bacterial and viral infections can cause muscle inflammation.
• Metabolic myopathies such as glycogen storage disorders or nutritional deficiencies need to be considered.
• Some endocrine conditions like hypothyroidism and hyperthyroidism can have muscle involvement.
• Miscellaneous causes including sarcoidosis, organ failure, amyloidosis, or acute rhabdomyolysis need to be ruled out.
• Immune-mediated necrotizing myositis (IMNM) is a distinct type of myositis.

Classification of IIM

• Adult-onset DM
• Adult-onset PM
• Overlap myositis
• Immune-mediated necrotizing myositis (IMNM)
• Juvenile-onset DM (JDM)

Polymyositis and Dermatomyositis

• These conditions frequently present with muscle weakness.
• Muscle weakness, often starting in the proximal muscles, is a hallmark.
• Unusual ocular and facial weakness warrants another diagnosis investigation.
• Pain is often absent.

Dermatomyositis

• Presents similarly to polymyositis and comes with skin lesions, which may include distinctive Gottron's papules, a heliotrope rash, and a shawl sign.
• Skin lesions appear as scaly or purplish bumps on the extensor surfaces of the fingers and over the upper back, neck, and shoulders.

Clinical Features

• Muscle weakness, often symmetrical and affecting proximal muscles first, is common.
• Ocular and facial muscle weakness may be present but unusual and warrant further investigation.
• Pain is usually absent or mild.
• Pharyngeal muscle weakness leading to dysphonia and swallowing problems may indicate more severe disease.
• Interstitial lung disease occurs in around 30% of cases.
• Presence of antisynthetase antibodies is linked to interstitial lung disease.

Investigations

• Creatine kinase (CK) levels are elevated and useful in monitoring disease activity.
• Other enzymes like ALT, AST, and aldolase can also be elevated.
• Electromyography (EMG) helps differentiate myopathy from other nerve disorders.
• Magnetic resonance imaging (MRI) can facilitate muscle biopsy targeting.
• Muscle biopsies are key to definitively diagnosing inflammation, regeneration, and infiltration of muscle tissue.
• Screening for malignancy is a routine aspect of IIM management due to the threefold increased risk.

Management

• Oral corticosteroids, such as prednisolone, are common initial treatments.
• High-dose intravenous corticosteroids may be needed for severe cases.
• Immunosuppressants like azathioprine, methotrexate, and others are used to help manage symptoms and prevent relapse.
• Intravenous immunoglobulin may be used in refractory cases.

Differentiating Steroid Myopathy from IIM Exacerbation

• Weakness while on prednisone warrants further investigation, particularly if prednisolone dosage is above 20mg/day.
• Measuring CPK and aldolase can help distinguish, inflammation from muscle atrophy.
• Muscle MRI can confirm ongoing inflammation.

Amyopathic Dermatomyositis

• This subtype features skin manifestations without significant muscle involvement.
• Approximately half of these patients may eventually develop muscle disease over time.

Antisynthetase Antibody (ASA) Syndrome

• Proximal muscle weakness is commonly the initial manifestation.
• Mechanic's hand, a skin lesion characterized by cracking and fissures on finger pads, particularly on the radial side of the index finger, is often associated.
• Interstitial lung disease (ILD), arthritis, Raynaud's phenomenon, and fever may also occur.

Immune-mediated Necrotizing Myositis (IMNM)

• Muscle disease is primarily seen in this type.
• Symptoms range from acute to subacute progressive proximal muscle weakness accompanied by myalgias.
• Creatine kinase (CPK) levels are very high, exceeding the normal range by 10 to 50 times.
• Interstitial lung disease or skin involvement is generally absent.
• Muscle histology reveals scattered necrotic fibers along with macrophages, with a minimal inflammatory infiltrate.

Inclusion Body Myositis (IBM)

• Muscle disease is a prominent feature, usually presenting as proximal muscle weakness.
• There tends to be more distal muscle involvement, and it is often asymmetrical, meaning the weakness affects one side more than the other.
• Often, accompanying nerve damage is present.
• Muscle biopsy is crucial for diagnosis, revealing myopathic features with CD8+ T-cell infiltration, red-rimmed vacuoles, and amyloid deposits.

Juvenile Dermatomyositis

• It is the most frequent inflammatory myopathy in children and adolescents, not necessarily needing a search for malignancy.
• Symptoms often mimic adult-onset dermatomyositis, with periods of remission and relapse.
• Calcinosis, a calcium deposit in the skin, presents in about 30% of cases.

Case Scenarios

• Various case examples outline how physicians analyze muscle weakness cases to arrive at particular diagnoses. Relevant laboratory results provide medical insight.

Epidemiology

• Annual incidence of IIMs is relatively low.
• The peak onset age is bimodal for Dermatomyositis (DM), with one peak between 5-15 years old, and another between 45-65 years.
• Polymyositis (PM) typically peaks around 50-60 years old.
• Generally, females are affected twice to three times more often than males.