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.