How long after an ear infection does mastoiditis start?

7 days: Acute infection of the mastoid air cells is a complication of acute otitis media. It was more common in the pre-antibiotic ear than now. Symptoms usually appear 7 days after acute otitis media. Patients with mastoiditis are acutely ill, have fever, extreme ear pain, and occasional ear drainage. The area behind the affected ear will be red, tender and edematous.

Depends: Mastoiditis is an inflammation and infection of the bone next to the ear. It can be from a localized spread of infection from an ear infection and can happen rapidly (if a severe infection) or slowly depending on the infection and immune response. If they develop, it is usually over a week or two from the beginning of an ear infection.