Clinical Reasoning: A 36-Year-Old Man With Asymmetric Muscle Weakness

A 36-year-old White man with no relevant medical history was referred to our clinic for evaluation of progressive muscle weakness of both lower limbs. He reported difficulty climbing stairs and standing up from a chair progressing over the prior 2 years. He preferred to lead with his left leg when climbing stairs because of more significant weakness of the right leg. He denied upper extremity weakness, abnormal sensation, muscle cramping, contractures, myalgias, change of urine color, skin rash, dysphagia, dysarthria, dyspnea, or exposure to new medications or toxins. He denied a family history of similar conditions. Neurologic examination was remarkable for asymmetric lower extremity muscle weakness. Muscle atrophy was noted in right quadriceps femoris, hamstrings, and posterior calf (Figure 1). The Medical Research Council grade was 4/5 for bilateral hip flexion, abduction, and adduction. There was asymmetric weakness of knee extension (4-/5 right and 4/5 left), knee flexion (4-/5 right and 4+/5 left), and ankle plantar flexion (4/5 right and 5/5 left). Foot dorsiflexion was normal bilaterally. He had a mild Trendelenburg gait. He did not have scapular winging, scoliosis, or percussion or grip myotonia. The remainder of neurologic examination was normal, including deep tendon reflexes. Previous evaluation at an outside facility revealed elevated serum creatine kinase (CK) 4,283 unit/L. Prior electrodiagnostic testing including nerve conduction studies (NCS) and needle electromyography (EMG)–reported findings of myopathy. Muscle biopsy was subsequently performed in left quadriceps femoris and reportedly revealed rare inflammatory cells; additional details were not available for review. He had been taking prednisone (40 mg/d) for 8 months and oral methotrexate (15 mg/wk) for 2 months followed by azathioprine (50 mg/d) for 6 months, which did not improve his symptoms. An electrocardiogram (ECG) was normal.