Muscle Weakness
Dr. Aggarwal provided important clinical pearls with respect to the historical aspects of IIM. Although fatigue and myalgias can certainly be seen in patients with IIM, muscle weakness is a key element consistent across nearly all forms of these diseases. The onset of muscle weakness in dermatomyositis, polymyositis and juvenile dermatomyositis can be quite acute and the progression of weakness typically occurs over weeks to months—although with IBM a very slow progression over the course of years is the norm.
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In contrast to metabolic myopathies, which often cause intermittent weakness, IIM demonstrates progressive, chronic weakness that tends to increase over time. When assessing for these diseases, it is important to evaluate for extra-muscular manifestations of myositis, such as dysphagia; signs and symptoms of cardiomyopathy; lung disease, which can manifest as interstitial lung disease or respiratory muscle weakness; arthritis; Raynaud’s phenomenon; rash; calcinosis; and/or such systemic symptoms as fevers and weight loss.
In any patient being evaluated for IIM, the clinician must also consider the possibility of metabolic myopathy, genetic myopathy, muscular dystrophy, thyroid disease and toxic myopathies that can be associated with alcohol or drug use, as well as with certain medications, such as statins and glucocorticoids.
Dr. Aggarwal stated the laboratory findings in IIM must be interpreted carefully and within the right clinical framework. Not only will creatine kinase and aldolase be elevated in many forms of IIM, so too will aspartate and alanine transaminases and lactate dehydrogenase. About 20% of patients with dermatomyositis, unlike polymyositis, will demonstrate normal muscle enzyme levels.
Autoantibodies
Specific autoantibodies correlate well with certain phenotypes within the category of IIM. For example, antibodies against signal recognition particle and HMG-CoA-reductase are commonly found in patients with immune-mediated necrotizing myopathy, whereas antibodies against melanoma differentiation-associated protein-5 (anti-MDA5) occur in patients with very specific skin findings (i.e., skin ulcerations, tender palmar papules), a propensity for severe and often rapidly progressive interstitial lung disease and frequent occurrence of clinically myopathy disease.
Additional studies that can aid in diagnosis include electromyogram and nerve conduction studies, magnetic resonance imaging of the muscles using specific myositis protocols and muscle biopsy.
IIM Treatments
Dr. Aggarwal concluded his lecture discussing treatments for IIM. First-line agents remain methotrexate and azathioprine, with second-line agents consisting of mycophenolate mofetil, tacrolimus and cyclosporine; a combination of first- and second-line agents can also be used in patients with disease that is refractory to methotrexate or azathioprine alone. Third-line agents include rituximab and cyclophosphamide, and such medications may be particularly appropriate for patients with interstitial lung disease. Experimental treatments at this time include ACTH gel, abatacept, and tofacitinib.