- Myotonic dystrophies. These are more often type 2 than type 1. Myotonia may be subtle, cataracts are seen early in all patients, and cardiac arrhythmias are common.
- Limb girdle muscular dystrophy type 2 B (dysferlinopathy). In the legs, this often affects the gastrocnemius muscle, and this will be visible on MRI. In the arms, it most often affects the biceps, sparing the deltoids. CKs are typically very high.
- Facioscapulohumeral muscular dystrophy (FSHD). This involves facial weakness, especially obicularis oris, in 95% of cases, as well as scapular weakness and winging, inflammation on muscle biopsy in 75% of cases, and typically is endomysial or perivascular.
Metabolic myopathies
Among metabolic myopathies that can mimic myositis are disorders of carbohydrate metabolism such as McArdle’s disease, 6-phosphofructokinase deficiency, and Pompe’s disease (adult acid maltase deficiency); disorders of lipid metabolism such as carnitine deficiency and carnitine palmitoyltransferase 2 (CPT2) deficiency; and disorders of purine metabolism, such as myoadenylate deaminase deficiency.
A 27-year-old patient who complained of weakness with activity was referred for possible myositis and was found to have a CK of 3,650 IU/L that never normalized. Physical examination showed intact strength and no muscle atrophy or fasciculations, and an enzyme stain for myophosphorylase showed a normal staining pattern and complete absence of the enzyme on quadricep biopsy. A 22-year-old man with similar symptoms plus recent onset of brown/black urine after physical activity had CK of 110,000 IU/L when symptomatic, and also underwent biopsy after being referred for possible myopathy. Both patients were ultimately diagnosed with CPT2 deficiency, which is associated with risk of rhabdomyolysis triggered by prolonged exercise, diets low in carbohydrates and high in fat, or by fasting.
Myalgias are common, and CK levels are normal or only mildly elevated between episodes in CPT2 deficiency, Dr. Christopher-Stine noted.
Toxic myopathies
Drug-induced myopathies are among the most common etiologies of myopathy and can range from mild myalgia to massive rhabdomyolysis. They can cause mild to severe weakness and may be chronic. The mechanism of toxic injury is direct via myotoxins such as ethyl alcohol, glucocorticoids, lipid-lowering drugs, cocaine, antimalarial drugs, antipsychotic drugs, colchicine, and Ipecac syrup.
One case described by Dr. Christopher-Stine involved “statin myopathy.”
A 55-year-old man on atorvastatin complained of myalgias and brown urine, but had no definitive weakness. He had intact strength and diffuse myalgias that weren’t reproducible. His CK was 45,000 IU/L.
Statin myopathy, as seen in this patient, is usually self-limited and is not associated with autoimmunity or with anti-HMGCR autoantibody positivity.
The mechanism is unknown, but statin myopathy has an incidence of 1.2 per 10,000 patient-years. Myalgias, myositis, rhabdomyolysis, and asymptomatic hyperCKemia are commonly seen. This is in contrast to the immune-mediated necrotizing myelitis that can be secondary to statins and is responsive to immunosuppression, she noted.
Other myositis mimics
In addition to these common myositis mimics, certain other neurologic diseases (such as ALS and cervical myelopathy), endocrinopathies (such as hypothyroidism), and infections (like toxoplasmosis) can also be mistaken for myositis, Dr. Christopher-Stine said, noting that cases illustrating these mimics underscore the need for careful consideration of possible alternate diagnoses.
“While most noninflammatory myopathies are self-limited or have no therapies available, knowing the diagnosis can be helpful for genetic counseling of the patient and family, for mitigating risk factors, and for precluding the use of unwarranted immunosuppressive agents,” she said.
Dr. Christopher-Stine reported having intellectual property interest in a novel Inova Diagnostics autoantibody assay detection for anti-HMGCR. She was also the safety officer for the JBT-101 Trial sponsored by Corbus and funded by the National Institutes of Health.