Inflammatory myopathies are rare diseases. Incidence range from 2.2–7.7 cases per million. Age at onset has a bimodal distribution with peaks observed between ages 10 and 15 years in children and between 45 and 60 years in adults.
Women are affected twice as commonly as men, with the exception of inclusion body myositis, in which men are affected more often.
Clinical Classification of the IdiopathicInflammatory MyopathiesPolymyositisDermatomyositisJuvenile dermatomyositisMyositis associated with neoplasiaMyositis associated with connective tissue diseaseInclusion body myositis
Clinical Features Proximal and symmetric muscle weakness is the cardinal clinical feature of the inflammatory myopathies. Weakness of the proximal muscles of the legs is usually noted first and results in difficulty arising from a chair or climbing stairs. Weakness of the proximal arm muscles may limit the ability to lift heavy items, to brush ones hair, or to reach up to shelves.
The detection of muscle weakness on physical examination typically relies on manual muscle strength testing and is usually rated on a scale of 0 to 5. Functional measurements of muscle strength are often helpful. These include determining how long it takes the patient to arise ten times from a chair without use of the arms or to walk 10 meters. The patients ability to rise from a squat or stand on his or her toes and heels can also be assessed
Pelvic and shoulder girdle musculature are affected most, but weakness of neck muscles, particularly the flexors, is also common. Ocular and facial muscles are virtually never involved. Dysphagia may develop secondary to esophageal dysfunction or cricopharyngeal obstruction.
Pharyngeal muscle weakness may cause dysphonia and difficulty swallowing. Pulmonary and cardiac manifestations may precede the onset of muscle weakness or develop at any time during the course of disease.
The clinical features of dermatomyositis include all those described for polymyositis plus a variety of cutaneous manifestations. Two cutaneous manifestations are considered pathognomonic.1. Gottron papules (symmetric lacy pink or violaceous raised lesions typically found on the dorsal and lateral aspects of the interphalangeal and metacarpophalangeal joints)2. Gottron sign (symmetric macular violaceous erythema overlying the dorsal aspects of the interphalangeal and metacarpophalangeal joints, olecranon processes, patellae, and medial malleoli)
Characteristic cutaneous findings :1. Heliotrope (violaceous) discoloration of the eyelids.2. Macular erythema of the posterior shoulders and neck (shawl sign).3. Anterior neck and upper chest (V sign).4. Dystrophic cuticles; and5. Periungual telangiectases and nailfold capillary changes.
Heliotrope rash of dermatomyositis demonstrating both erythemaand diffuse periorbital edema.
V-neck rash in a patient with dermatomyositis demonstratingphotosensitive nature over anterior chest.
“Shawl sign” in a patient with dermatomyositis featuring anerythematous rash across the upper back and extending onto the neckin thedistribution of where a shawl is worn.
Cuticular overgrowth with periungual erythema and capillarydilatation in a patient with dermatomyositis.
"Mechanics hands" refers to darkened or dirty- appearing horizontal lines and fissures that are seen across the lateral and palmar aspects of the fingers. This skin lesion can be seen in both dermatomyositis and the anti-synthetase syndrome subset of polymyositis.
Mechanic’s hands. Note the erythema and hyperkeratotic changesand cracking of the skin on the lateral aspects of the fingers in thispatient with the anti-Jo-1 autoantibody.
In juvenile dermatomyositis, the skin lesions and weakness are almost always coincidental, but the severity and progression of each varies greatly from patient to patient. Juvenile variant differs from the adult form because of the coexistence of vasculitis, ectopic calcification, and lipodystrophy.
Gastrointestinal ulcerations resulting from vasculitis can cause hemorrhage or perforation of a viscus. Ectopic calcification may occur in the subcutaneous tissues or in the muscles.
Amyopathic dermatomyositis : Biopsy-confirmed, classic cutaneous findings of dermatomyositis have normal muscle strength, muscle enzymes, electromyograms (EMGs). Dermatomyositis sine myositis: Above findings plus normal muscle histology
Muscle weakness associated with an underlying malignancy develops in a subset of patients with inflammatory myopathies. Malignancy may precede, or develop after, the onset of muscle weakness, usually the two are diagnosed within a 1- year period. The sites or types of malignancy that occur in association with myositis are those that are expected for the age and gender of the patient.
Inclusion body myositis mainly affects persons over the age of 50 years . It affects men twice as often as women. Symptoms are often present for 5–8 years before the diagnosis is made. Predominant weakness of the quadriceps, long finger flexors, and anterior calf muscles is characteristic.
Laboratory Findings An abnormal creatine kinase (CK) level is possibly the most sensitive indicator of skeletal muscle damage. Normal levels of CK may be found1. Very early in the course of polymyositis.2. Dermatomyositis.3. In advanced cases with significant muscle atrophy.4. In myositis associated with a malignancy.5. Inclusion body myositis.
Tests of acute phase reactants, the erythrocyte sedimentation rate and C-reactive protein levels, are abnormal in only some patients with myositis. The erythrocyte sedimentation rate is normal in about half of patients with polymyositis and is elevated above 50 mm/h (Westergren method) in only 20%.
Certain autoantibodies are found almost exclusively in patients with idiopathic inflammatory myopathies, and therefore are termed myositis-specific autoantibodies . Antinuclear antibodies (ANAs) may be found in the serum of over 50% of patients with inflammatory muscle disease
Most myositis-specific autoantibodies are directed against amino acyl-tRNA synthetase activities. The most common of these is anti-histidyl-tRNA synthetase (Jo-1), present in approximately 20% of patients with polymyositis
Patients with these autoantibodies typically manifest myositis (polymyositis more commonly than dermatomyositis) plus several extramuscular features including interstitial lung disease, arthritis, mechanics hands, and Raynaud phenomenon. The combination of these features and an inflammatory myopathy has been termed "antisynthetase syndrome."
EMG EMG is quite effective for :1. Differentiating between myopathic and neuropathic conditions.2. Localizing a neurologic lesion to the central nervous system, spinal cord anterior horn cell, peripheral nerves, or neuromuscular junction.3. In addition, knowledge of the distribution and severity of abnormalities can guide selection of the most appropriate site to biopsy if MRI is not available.
In polymyositis and dermatomyositis, EMG classically reveals the following triad:1. Increased insertional activity, fibrillations, and positive sharp waves.2. Spontaneous, bizarre high-frequency discharges.3. Polyphasic motor unit potentials of low amplitude and short duration.
Muscle histology In classic polymyositis, muscle biopsies reflect a T-cell mediated autoimmune process. The lymphocytic cell infiltrate is found predominantly in endomysial locations. T lymphocytes, especially CD8+ cytotoxic T cells, can be seen surrounding and invading non-necrotic fibers expressing class I major histocompatibility antigen.
The muscle biopsy in inclusion body myositis closely resembles that of polymyositis, with endomysial inflammatory infiltrates and CD8+ T-cell invasion of non- necrotic muscle fibers. However, a characteristic feature of inclusion body myositis is the presence of intracellular vacuoles. The vacuoles contain basophilic (red on Gomori trichrome stain) granules in their center or along their walls, leading to their "red-rimmed" appearance.
Treatment Glucocorticoids are the standard first-line medication for any idiopathic inflammatory myopathy. Initially, prednisone is usually given in a single dose of 1 mg/kg/d, but in severe cases, the daily dose can be divided or intravenous methylprednisolone can be used.
If a patient does not respond to glucocorticoid therapy, another agent is added, usually either azathioprine or methotrexate. Intravenous immune globulin is often beneficial in the treatment of dermatomyositis and polymyositis, but its effect is short-lived and repeat infusions are generally necessary every 6–8 weeks.
Other immunosuppressive agents or therapeutic modalities have been used in treatment-resistant patients, including1) Cyclophosphamide,2) Cyclosporine3) Tacrolimus4) Rituximab5) Etanercept6) Infliximab7) Mycophenolate mofetil8) Plasmapheresis9) Total-body (or total-nodal) irradiation.