Dr. Schmidt broke down his talk into several categories of mimics of inflammatory myopathies. For clinical mimics, this refers to patients presenting with myalgias (muscle pain). The long list of potential causes of myalgias includes toxins like alcohol, such drugs as statins, thyroid disease, myopathies (i.e., metabolic, mitochondrial or muscular dystrophies), polymyalgia rheumatica, fibromyalgia and neuropathic processes. Dr. Schmidt noted that Becker muscular dystrophy can present with a proximal paresis and onset can range anywhere from 10 to 60 years of age. These patients can demonstrate a creatine kinase (CK) elevation of up to 10 times the upper limit of normal. Clinicians should look for hypertrophy of the calf muscles in evaluating for this condition. Chronic inflammatory demyelinating polyneuropathy is a condition with a subacute onset and typically affects patients ages 40–60 years (more men than women), and forms of the condition can present with a pure motor deficit without a concurrent sensory deficit. CK can be elevated to two to five times the upper limit of normal, and nerves may appear thickened when imaged with ultrasound. Myasthenia gravis is a disease that presents with fatigable weakness, often involving the arms and legs, with a bimodal distribution of disease onset (one peak is between ages 20–40, the other at 70 years and older). Clinicians should be on the lookout for patients describing double vision, drooping eyelids, impaired chewing and swallowing, or slurred speech. Dr. Schmidt also warned clinicians not to be fooled into thinking that amyotrophic lateral sclerosis (ALS), a progressive neurodegenerative disease that affects the motor neurons in the brain and spinal cord, is actually inclusion body myositis (IBM); he noted that one can be fooled by the pattern of weakness in the hands, which can be strikingly similar-appearing in both conditions. Detection of myopathic motor unit potentials (MUPs) in flexor digitorum profundus muscles may argue more strongly for ALS, though neurogenic MUPs can be seen in IBM as well.
For laboratory mimics of myositis, Dr. Schmidt noted two important concepts: 1) it is possible to find both false positive and false negative myositis-specific antibodies in the general population and 2) a normal CK does not rule out myositis. With an acute elevation in CK, clinicians should consider rhabdomyolysis, particularly with a history of toxin exposure, intense physical activity, trauma or seizures. With a more chronic elevation in CK, it is important to consider the possibility of muscular dystrophies and metabolic myopathies, and to ask about family history of muscle disease that may imply the need for genetic testing. Dr. Schmidt informed the audience that, in evaluating patients with elevated CK, muscle ultrasound is a relatively newer, emerging modality that is being used, especially in cases of suspected IBM.
