To identify the clinical factors and cognitive errors that lead to the misdiagnosis of myasthenia gravis (MG) as stroke.

MG patients (2013-2017) with at least 1 year follow up were analyzed. Demographic and clinical characteristics were collected. The type of cognitive error, clinical factors contributing to the misdiagnosis as stroke were recorded. Diagnostic errors were categorized according to Patient Safety Network’s categories of cognitive bias. Statistical analysis was done using Fischer’s Exact test.

21 patients -13 women, 19 Caucasians (90.5%), mean age of 66.7+/-4.07 were included. 10 were MGFA class 3a, 7 were MGFA class 2b, 3 were MGFA3b. 18 patients were acetylcholine receptor antibody positive, 1 was MuSK positive. Slurred speech (8 patients, 38.1%) was the most common symptom that resulted in misdiagnosis, followed by hemibody weakness (7 patients, 33.3%), and dysphagia (3, 14.3%). Smoking (12, 57.1%) and hypertension (11, 52.4%) were common risk factors for stroke. Small vessel disease was suspected as the most common etiology (15, 71.4%).  10 patients received thrombolytic therapy and 16 patients were on antiplatelets. 4 patients who presented with recurrent symptoms were placed on anticoagulants. 18 patients presented with their symptoms acutely to the emergency department and 3 of them presented to the clinic. Availability bias due to lower threshold to diagnose stroke was the most common cognitive error (9 patients, p<0.05) followed by anchoring bias (7 patients, due to hemibody weakness) and premature closure (appropriate imaging was not ordered, 3 patients, p<0.05).

Acute presentation of bulbar symptoms and focal weakness to the emergency department results in misdiagnosis of MG as stroke.