Introduction to Myopathy

00:00 In this lecture, we’re going to talk about myopathies. And this will be an introduction to how we approach patients who may have a myopathic disorder. As we recall when we are evaluating any patient with a nervous system condition or complaint, the first step is to figure out whether this is a central nervous system or a peripheral nervous system condition.

00:23 Central nervous system conditions are evaluated on a neurologic exam, and we think about problems with mental status and cranial nerves. In the peripheral nervous system, we’re really thinking about 3 localizations. Problems can occur in the muscle, the neuromuscular junction and the nerve. And we’re using the history and the physical exam to guide us to each of those locations. We further can subdivide problems in each of those areas to understand what’s going on with the patient. Muscle diseases are typically broken down into hereditary myopathies and acquired myopathies. In the neuromuscular junction, we think about pre- and post-synaptic problems and there’s a number of ways to further subdivide and characterize and classify nerve conditions to figure out what’s going on with the patient.

01:14 Whenever we’re evaluating a peripheral nervous system problem, there are 3 key features that we focus on. We want to do the full comprehensive neurologic examination and a good history. And on that examination, we’re going to focus on 3 areas: the distribution, the sensory findings, and the reflex exam. For muscle disorders, we tend to see that the distribution is proximal more than distal. The chief complaint is commonly weakness, patients present with weakness and that weakness is usually proximal, affecting the limb girdle and the shoulder girdle causing difficulty with getting up out of chairs or walking upstairs, reaching up to the top of cabinet and we’re seeing signs of proximal muscle weakness. For typical muscle disorders, we see that there are no sensory findings. Patients don’t complain of numbness and tingling and paresthesias. The sensory exam is normal. And this is important for differentiating peripheral nerve problems where sensory findings are prominent for muscle disorders. And then critical is the reflex examination. And for most muscle disorders, we see normal to slightly reduced reflexes. This is also an important differentiator for peripheral nerve disorders. Peripheral nerve pathology often causes reduced or even areflexia and with most muscle conditions we see normal to slightly reduced reflexes. So these 3 findings are critical when we’re evaluating cases or approaching a vignette to localize to the muscle. Once we’ve localized the presentation or problem to the muscle, we want to figure out what type of disorder we’re dealing with. And there are 5 broad categories of muscle disorders. The first is inflammatory, and inflammatory muscle disorders present with breakdown of muscle and increase in inflammatory enzymes. And that points us to be concerned about an inflammatory condition. Some of those types of conditions are polymyositis, dermatomyositis, and inclusion body myositis. We also see infections that affect the muscle. HIV-associated myopathy; trichinellosis is an interesting and very rare muscle disorder but infections are also important causes of myopathy or muscle disorder. Cancers don’t often develop in the muscle but paraneoplastic conditions are an important cancer or a neoplastic or paraneoplastic disorder to consider in the differential diagnosis of these patients. And there are a number of cancer associated myopathies, muscle disorders that are caused by a revving up of the immune system of paraneoplastic process that we may need to include in our differential diagnosis.

03:49 We must think of toxic and metabolic conditions that affect the muscle. The muscle is sensitive to many drugs, metabolites, nutritional disorders and so we think of thyroid myopathies, statin associated myopathy, steroid induced myopathy, a number of medications and certain metabolic myopathies that are in many differential diagnoses for these patients. And in the last consideration are the inherited muscle disorders. The first 4 categories were those acquired conditions and we evaluate for those first in many cases and then we need to think of inherited conditions. Muscular dystrophies are congenital myopathies. We’re going to talk about all of these conditions in subsequent lectures. For now, I would like you to understand the approach to the patient. First, we localize to the muscle and then we need to evaluate whether this is an immune mediated process, an infectious process, a paraneoplastic process, a toxic metabolic process or could this be running in families and be inherited.