Background: Myotonic Dystrophy is an autosomal dominant neuromuscular disease with a global incidence of 1 in 8000 worldwide. This condition can be further categorized into myotonic dystrophy type 1 (DM1), having childhood onset, and myotonic dystrophy type 2 (DM2). The DM2 subtype is generally milder and often presents with proximal muscle weakness, especially in the hip girdle musculature, and disabling myalgic pain. Weakness in the neck flexion, finger flexion and tricep extension are also common symptoms. Genetic testing is the gold standard for DM2 diagnosis. Electromyography studies and muscular biopsies provide additional evidence in support of the diagnosis of DM compared to other inflammatory/metabolic myopathies. Despite these multiple diagnostic strategies, current practice lacks guidelines for efficient diagnosis and treatment initiation especially when clinical presentation is unclear.

Case Presentation: A 65-year-old woman of Korean descent with a past medical history of hypothyroidism presented to her primary care physician for upper and lower extremity swelling, upper extremity weakness, and a positive antinuclear antibody. After ruling out deep-vein thrombosis, furosemide improved her lower extremity edema. However, she continued to experience fatigue, poor appetite, nausea, intermittent dyspnea on exertion, and difficulty with ambulation. The patient went to the emergency department and was started on moderate-dose prednisone. Symptoms improved for two weeks. Then, the patient visited her rheumatologist, and was given an increased dose of oral prednisone with the suspicion of polymyositis, yet little benefit was gained. Muscle biopsy was not ordered due to prior use of corticosteroids. Instead, the patient underwent a nerve conduction study (NCS) and electromyography (EMG). Results from the EMG revealed signs of DM2, including myotonic discharges and positive sharp waves.

Discussion: During initial evaluation of extremity muscle weakness, current practice often begins with noninvasive measures including laboratory testing and imaging, followed by EMG if suspecting neurological involvement. Muscle biopsy can be done if the diagnosis remains unclear. However, biopsy may be complicated by corticosteroid treatment due to corticosteroid-induced myopathy of type II muscle fibers, delaying definitive diagnosis and pathology-specific management. Especially with this patient’s atypical presentation, deferring glucocorticoid treatment until after muscle biopsy may have improved access to diagnostic testing and earlier initiation of treatments appropriate for the pathology at play. Muscle weakness can encompass numerous etiologies, so early EMG and muscle biopsy may be useful tools in distinguishing the cause of symptoms and guiding management to bring greater and longer-lasting symptom relief to patients.