Chapter 42: Myopathies and Neuromuscular Junction Disease

PART 1—GENERAL APPROACH TO MYOPATHIES

Myopathies are diseases of skeletal muscle. Chronic or subacute weakness from myopathy is usually evaluated in the clinic. In the hospital setting, myopathy is most likely to present as a new condition for evaluation in the critically ill patient or the patient with an acute myopathy with rapidly progressive weakness. These situations will be discussed separately.

What are the causes of acute myopathy in the hospital population?

There are many disorders that may present as a subacute or acute undiagnosed problem in the hospital setting. These include inflammatory myopathy, metabolic myopathy, toxic myopathy, and myopathy due to a systemic disorder (Table 42-1).

When considering the weak patient in the hospital, other disorders besides myopathy must be considered. Disorders of the neuromuscular junction, motor nerves, anterior horn cells, and motor tracts of the brain or spinal cord may mimic a myopathy.

The approach to diagnosis of myopathy begins with a history. This includes medications and other supplements and symptoms and signs of involvement of the heart, pulmonary system, skin, and joints.²³

What symptoms may occur with a myopathy?

Symptoms are usually considered to be either positive or negative symptoms. The most common symptom, weakness, is a negative symptom. Exercise intolerance, fatigue, and muscle atrophy are other negative symptoms. Positive symptoms include cramps, contractures, muscle hypertrophy, muscle stiffness, myalgia, and myoglobulinuria.

What symptoms is the patient reporting?

Most patients with myopathy report weakness. Proximal weakness is the most common pattern seen in myopathy. Proximal weakness of the lower extremities is seen in rising from a chair and in climbing and descending stairs. Proximal weakness in the upper extremities is seen particularly while raising one’s arms above the head to reach for an object or for grooming. If the patient complains of foot drop or hand weakness, distal involvement is present. This can occur in myopathy, but may be a clue of an alternative diagnosis. Cranial muscle involvement may occur with dysphagia, dysarthria, double vision, and ptosis, but this pattern of involvement is much less common.

What myopathies may cause pain?

A feature of metabolic and mitochondrial myopathies is especially fatigue and sometimes pain after exercise. Myalgias may occur in myopathy, but most patients with myopathy ...