Mastoiditis is defined as an infection and inflammation of the mastoid air cells of the temporal bone. Duration of the infection is used to categorize mastoiditis into acute and chronic (> 3 months). The mastoid is contiguous with the middle ear, and inflammatory processes and infection occurring in the ear are directly transmitted to the mastoid. Mastoiditis is most commonly seen as a complication of otitis media, but it can also be a result of cholesteatoma. There is some minor mastoiditis present with all otitis media, but it rarely becomes clinically important given routine antibiotic treatment for otitis media. However, while severe mastoiditis is now less often seen, it can be a serious complication given the proximity of the mastoid to important cranial features such as the posterior cranial fossa, the sigmoid sinus, the facial nerve, the semicircular canals, and the petrous tip of the temporal bone. The mastoid can serve as a conduit of extracranial infection to intracranial structures. Inadequately treated mastoiditis can result in deafness, sepsis, meningitis, sigmoid sinus thrombosis, abscess, and death.

If not treated properly with antibiotics, otitis media presentation can progress to mastoiditis in an average of 4.5 days, but it can occur in as little as 2 days. As the infection spreads through the mastoid air cells, the mucosal lining of these cells exhibit hyperemia and edema, followed by accumulation of fluid and pus within the air cells. The loss of vascularity and dissolution of calcium from the bony septa cause cell wall loss and coalescence of air cells into abscess cavities. The inflammation and infection can then spread to contiguous areas within the head and neck. The most common bacterial isolates are Streptococcus pneumoniae, Streptococcus pyogenes, Haemophilus influenzae, and Staphylococcus aureus. In cases of chronic mastoiditis, multiple organisms are often isolated, characterized by a predominance of gram-negative bacteria and anaerobes. In immunocompromised individuals, Mycobacterium tuberculosis, Aspergillus, and Rhodococcus equi are often found.

While mastoiditis can occur in a person at any age, most commonly it is seen in patients under 2 years old. Cases are more likely to be seen in the fall and winter, and incidence is higher in developing countries where access to antibiotics may be limited.

Mastoiditis presents with otalgia, with pain and tenderness extending to the postauricular region. In children, this may present as irritability. Pain is often worst at night. Otorrhea, vertigo, and nystagmus can also be present. Constitutional symptoms include high persistent fever and lethargy. Other presenting symptoms can include conductive hearing loss and facial palsy. The patient history most likely will include recent history of acute otitis media but may involve other bacterial or viral etiology. Immunocompromised patients are at an increased risk to develop mastoiditis. Recent antibiotic therapy increases the chance of antimicrobial resistance, which will make treatment more difficult.