History: A 74-year-old female with a history of hypertension and GERD presents to the ED with dizziness that came on suddenly about 6 hours ago. She also describes imbalance, generalized weakness, and double vision. She has never had these symptoms before. She takes only metoprolol and ranitidine, neither of which is new. She denies use of any alcohol, pain medications, or sedatives.
Exam: Her vital signs are all within normal limits except for her blood pressure, which is 171/99 mmHg. The head and neck exam is notable for bilaterally small pupils, which are both ~1.5 mm in both dim and full light (non-reactive). She also has a mild disconjugate gaze, with the right eye deviated slightly to the right (see facial image in Figure 1, above right; please click to enlarge). Results of the chest and abdominal exam are both normal. The neurologic exam is notable for mild ataxia.
1. What is the differential diagnosis of bilateral miosis?
2. What is the most likely cause in this case?
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