A 62-year-old woman with a history of diabetes and hypertension presents to the emergency department complaining of a headache behind her left eye that came on gradually and has persisted for 5 days. She reports that the day before her daughter noticed slight redness to the left eyelid. The patient denies photophobia but reports nausea and an episode of emesis. She has no history of similar headaches and denies fever or other complaints.

The patient’s vital signs are normal except for a temperature of 37.3°C.  Physical examination reveals mild redness of the left upper eyelid and the left side of the scalp but is otherwise normal.

Differential diagnosis includes shingles, meningitis, and cellulitis. Laboratory testing reveals a normal complete blood count, an erythrocyte sedimentation rate of 30 mm/h, a normal basal metabolic panel except for a sodium level of 130 mEq/L, and a normal C-reactive protein level. Computed tomography of the brain is normal.

Lumbar puncture is performed. Cerebrospinal fluid (CSF) tube 1 revealed a white blood cell (WBC) count of 9×109/µL and a red blood cell count (RBC) of 131 ×106/μL µL; tube 4 revealed a WBC count of 2×109/µL and an RBC count of 300 µL×106/μL.

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