Masked Mastoiditis Presenting As Pneumococcal Meningoencephalitis in an Elderly Patient: A Diagnostic Pitfall in the Emergency Department.

Otogenic infections remain a potentially under-recognized cause of pneumococcal meningoencephalitis in adults. In elderly patients, acute mastoiditis may present without overt otological findings ("masked mastoiditis"), delaying diagnosis and increasing the risk of intracranial complications. An 86-year-old unvaccinated male patient with multiple comorbidities initially presented to the emergency department with non-specific upper respiratory symptoms and normal otoscopy and was discharged with presumed viral infection. Four days later, he returned with acute neurological deterioration and hemodynamic instability. Cranial computed tomography demonstrated left mastoid and middle ear opacification with petrous bone erosion and suspected intracranial extension. Magnetic resonance imaging confirmed a left temporobasal abscess. Blood cultures and urinary antigen testing were positive for Streptococcus pneumoniae. The patient was diagnosed with pneumococcal meningoencephalitis secondary to acute mastoiditis and was treated with intravenous antibiotics, corticosteroids, intensive care support, and surgical mastoid drainage. The hospital course was prolonged, with persistent cognitive impairment at discharge. Masked mastoiditis may lead to life-threatening intracranial infection despite normal otoscopic findings. In elderly patients presenting with fever and altered mental status, clinicians should consider occult otogenic sources. Early neuroimaging is crucial when lumbar puncture is contraindicated or delayed, particularly in clinically unstable patients.