Muscular Dystrophy Versus Polymyositis

02.01.2017
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Making the diagnosis of polymyositis or dermatomyositis and starting steriod treatment can save the patients' lives and keep them active.

In the many years I have seen children (and some adults) with neuromuscular disorders, I have had countless parents tell me that when they took their child to see a doctor because of increasing muscle weakness, the doctor said the child had muscular dystrophy. Sometimes this was even without doing a muscle biopsy or any lab tests. Now some doctors are doing a few genetic tests and nothing else and making the diagnosis of muscular dystrophy. Sadly, there are children and adults who have a treatable disorder, myositis, that if it is diagnosed and properly treated can lead to increased strength and many years of an almost normal life. 

Every child and adult with muscle weakness should at a minimum have a muscle biopsy performed with a local, not a general anesthetic. Then the lab test, CPK (CK in Europe) should be ordered. In Duchenne muscular dystrophy the CPK will be very high, whereas in poly or dermatomyositis, the CPK can be high or in about 25% of cases can be within normal limits. Very specific changes can be seen on the muscle biopsy with Duchenne muscular dystrophy, but in 25% of cases the biopsy in polymyositis can appear normal. Inflammatory cells can usually be seen in about 75%. Two-thirds of the mothers of boys with Duchenne dystrophy will have an elevated CPK and some will have heart abnormalities and some will have a moderate scoliosis. It is interesting that enlarged calves can be seen in some cases of polymyositis and they are enlarged in about 80% of boys with Duchenne dystrophy.

There are other symptoms that are different between the two disorders: the age of onset, the slow or rapid progression of muscle weakness, the neurological examination, particularly the deep tendon reflexes and the sex. Females are more affected with myositis than males. If a patient has dermatomyositis, skin changes will be seen.

Neck weakness can be a prominent presenting sign in polymyositis. Many years ago an adult patient came to see me. She was in a wheelchair and could not hold up her head and had been told she was dying of muscular dystrophy. She responded extremely well to prednisone and now many years later she is active, works as a volunteer and has multiple grandchildren. I get a yearly Christmas card. 

Image copyright: seacucumber/ pepino-do-mar ANNEKE/flickr/CC BY

Article last time updated on 28.02.2017.

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