Patient assessment
The physical examination should involve localization and quantification of weakness, and assessment for fever, rash, atrophy/wasting/scooping of forearms, fasciculations, cranial nerve involvement, Raynaud’s phenomenon, nailfold capillary changes, arthritis, calcinosis, “mechanic’s hands,” signs of other autoimmune diseases, and lung crackles. Initial laboratory testing should include HIV and hepatitis B and C testing; measurement of CK, AST, ALT, aldolase, thyroid-stimulating hormone, and magnesium levels; a comprehensive metabolic panel and complete blood count; and measurement of erythrocyte sedimentation rate and C-reactive protein.
“Weakness may be secondary to a neuropathy, myopathy, or a problem at the neuropathic junction. Many causes of weakness can be readily identified by careful history taking, focused physical examination, and directed laboratory evaluation,” she said.
Features pointing toward a diagnosis of myositis include characteristic rashes, gradual symptom onset, proximal limb and truncal weakness, other connective tissue disease features such as Raynaud’s and arthritis, and the presence of lung disease, including interstitial lung disease or unexplained infiltrates, she said.
Features pointing away from a diagnosis of myositis include a family history of a similar illness, weakness that is associated with eating or fasting, neurologic signs, cranial nerve involvement, fasciculations, severe muscle cramping, early atrophy, and creatine phosphokinase levels that are either less than 2 times or more than 100 times the upper limit of normal.
Among the conditions to consider in the presence of the features that point away from a myositis diagnosis are muscular dystrophies, metabolic myopathies, and toxic (drug-induced) myopathies, to name a few, Dr. Christopher-Stine said.
She described a number of other cases to illustrate the need for – and to help develop – a differential diagnosis in patients presenting with apparent myositis.
Muscular dystrophies
A 38-year-old woman with limited scleroderma and anti-PM/Scl autoantibodies developed proximal weakness over 9 months and was eventually unable to walk up a flight of stairs. She had heliotrope rash and Gottron’s sign, her serum CK was 723 IU/L, and EMG showed an irritable myopathy.
Muscle biopsy showed inflammation, and she was treated with prednisone, but this led to worsening weakness. She complained of prominent fatigue and double vision at the end of the day, and these symptoms did not improve with steroids.
Anti-AChR and anti-MuSK antibodies were negative, but she had a decrement on repetitive nerve stimulation testing.
She was treated with pyridostigmine and experienced near-complete resolution of her proximal weakness and double vision. A chest CT scan showed thymic hyperplasia; thymectomy was recommended.
In another case, a 19-year-old woman who complained of leg pain after exercise was found to have intact strength but asymmetric calf hypertrophy. Her CK level was 5,000 IU/L, and she was referred to rule out acute myositis.
A quadriceps biopsy was performed and showed abnormal dystrophin immunostaining but no inflammation. A molecular genetic analysis showed deletions in Xp21 and she was diagnosed as a manifesting carrier of Duchenne muscular dystrophy. It was recommended that she be evaluated for cardiomyopathy and receive genetic counseling.
A number of other cases presented by Dr. Christopher-Stine highlighted other muscular dystrophies that can mimic myositis, such as: