A 57-year-old woman presents to her primary care physician with weakness for the past 6 months with symptoms that have worsened recently. She feels fatigued, depressed, and has gained 10 pounds which she attributes to feeling too weak to go to the gym. She was recently treated for an ear infection with an antibiotic which she completed 3 weeks ago, and experienced a rash which she believes was an allergic reaction to her treatment. The patient has a medical history of diabetes mellitus that is well-controlled with insulin and metformin. She has been admitted multiple times for hypoglycemia secondary to missing meals. Her temperature is 99.5°F (37.5°C), blood pressure is 127/68 mmHg, pulse is 80/min, respirations are 17/min, and oxygen saturation is 98% on room air. Physical exam is notable for 2/5 strength in the patient's upper and lower extremities, sparse fine hair on her body, and a rash on her face and arms. Laboratory values are ordered as seen below. Serum: Na+: 139 mEq/L K+: 3.3 mEq/L Cl-: 100 mEq/L HCO3-: 25 mEq/L BUN: 20 mg/dL Glucose: 70 mg/dL Creatinine: 1.1 mg/dL Ca2+: 10.2 mg/dL AST: 12 U/L ALT: 10 U/L A muscle biopsy is obtained as seen in Figure A. Which of the following is the most likely explanation for this patient's presentation?
Dermatomyositis
Hypothyroidism
Inclusion body myositis
Polymyositis
Inclusion body myositis
This patient who presents with weakness and a rash has a biopsy suggestive of inclusion body myositis.

Inclusion body myositis presents with an insidious onset of weakness, a mildly elevated CRP and CK, and a rash. A muscle biopsy in this disease will demonstrate endomysial inflammation with basophilic-rimmed vacuoles within the muscle fiber sarcoplasm. Electromyography is typically abnormal, and the most accurate/confirmatory test is a muscle biopsy. There is no cure for this disease and no standard course of treatment. Corticosteroids are not usually effective but intravenous immunoglobulins can be effective.

Naddaf et al. review the evidence regarding the diagnosis and treatment of patients with inclusion body myositis. They discuss how this disease is still refractory to treatment. They recommend a muscle biopsy to establish a definitive diagnosis.

Figure/Illustration A is a muscle biopsy demonstrating endomysial inflammation with basophilic-rimmed vacuoles (red circle) within the muscle fiber sarcoplasm. These findings are classically seen in patients with inclusion body myositis.

Incorrect Answers:
Answers 1 & 5: Dermatomyositis and polymyositis present similarly with muscle weakness and an elevated CK, CRP, and aldolase. However, dermatomyositis presents with dermatologic findings. A biopsy would demonstrate inflammation surrounding the muscle fascicle (dermatomyositis) or within the muscle fascicle (polymyositis). Treatment may include azathioprine and methotrexate.

Answer B: Hypokalemia could present with weakness and cardiac abnormalities. Though this patient is hypokalemic, her biopsy demonstrates inclusion body myositis. Treatment of hypokalemia is with potassium repletion.

Answer C: Hypothyroidism presents with depression, fatigue, scarce lanugo, myopathy, and weight gain (as this patient has). Though myopathy from hypothyroidism is plausible, her symptoms of weakness have a confirmatory biopsy. Treatment of hypothyroidism is with thyroid hormone replacement.

Bullet Summary:
Inclusion body myositis presents with weakness and an elevated CRP/CK with a muscle biopsy demonstrating endomysial inflammation with basophilic-rimmed vacuoles within the muscle fiber sarcoplasm.