A Patient With Rapidly Progressive Muscle Weakness and Rather Elevated Creatine Kinase

A 44-year-old man presented with a progressive proximal weakness for 2 months and an elevated serum creatine kinase (CK) of 17,209 U/L (normal, <22–269 U/L). Physical examination revealed symmetrical Medical Research Council 2/5 proximal strength of the upper and lower limb girdle. The remainder of the examination including motor reflex testing, cranial nerve and sensory testing, and dermatological examinations were within normal limits.

Computed tomography scan of the chest, abdomen, and pelvis showed no evidence for malignancy. Lab workup includes HIV, thyroid-stimulating hormone, T4, Jo-I antibody, anti-Smith antibody, and anti-RNP antibody. Hepatitis A, B, and C panels were all negative. Chest radiograph did not show abnormalities.

What is the Differential Diagnosis?

This patient appeared to have a rapidly progressive myopathy with rather elevated CK. Polymyositis is a possibility, although this is very unusual in patients who do not have connective tissue disease. Dermatomyositis is even less likely, because there is no skin rash. Adult-onset Pompe disease is a possibility, although the rapidly progressive weakness is somewhat against this. Another possibility is limb-girdle muscular dystrophy, for which the rapid progression is unlikely. He is too young to have inclusion body myositis.

Other conditions include carnitine deficiency or another metabolic myopathy. A necrotizing myopathy is seen in those who have been on statins and have the HMG-CR antibody. Some patients with signal recognition particle (SRP) antibodies can also have a necrotizing myopathy.

What to do next?

An EMG showed evidence of many fibrillation potentials and brief polyphasic motor units, suggestive of a necrotizing or an inflammatory myopathy, although this can also occur in other conditions such as some limb-girdle muscular dystrophies, toxic myopathies, and Pompe disease and other metabolic myopathies.

What Should be Done Next?

The patient had an elevation of signal recognition antibodies.

A muscle biopsy revealed an active necrotizing myopathy with fiber atrophy, necrosis, phagocytosis, and some regenerating fibers with minimal inflammation ( Fig. 102-1A and B ) with only rare scattered CD45-positive lymphocytes in the interstitium or vessels. There were CD4 lymphocytes in muscle fibers ( Fig. 102-1C and D ).