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Acute mastoiditis

Acute mastoiditis refers to a suppurative infection of the mastoid air cells, typically presenting as a complication of acute otitis media in childhood. Clinical presentation includes postauricular tenderness, erythema, swelling with auricle protrusion, otalgia, and fever.

Coalescent mastoiditis

Simply the term given to acute otomastoiditis when mucoperiosteal disease extends to involve the bone. The septae which normally separate one mastoid air cell from another are resorbed. This change is best appreciated on thin section bone-algorithm through the temporal bones. Comparison to the contralateral mastoid may be useful in detecting early changes, although normal asymmetry should be taken into account.

Coalescent mastoiditis complications

Subperiosteal abscess, sigmoid plate erosion and venous sinus thrombosis, and intracranial extension such as epidural or subdural empyema or brain abscess. MRI can complement CT when intracranial complications are suspected
Look For First
  • Erosion of mastoid air cell bony septa (establishes coalescent mastoiditis diagnosis)
  • Partial-to-complete opacification of mastoid air cells with mucosal enhancement on post-contrast imaging
  • Erosion of lateral mastoid wall (subperiosteal abscess) or sigmoid plate (epidural abscess)
  • Rim-enhancing fluid collections or masses with fat stranding deep to sternocleidomastoid (Bezold abscess)
Key Image Findings
  • CT shows partial-to-complete opacification of the mastoid air cells, which is non-specific for incipient mastoiditis but becomes diagnostic when accompanied by clinical evidence of mastoid inflammation.
  • Erosion of mastoid air cell bony septa is the radiological hallmark of coalescent mastoiditis, representing progression from periostitis to actual bone destruction and osteomyelitis.
  • Erosion of the lateral wall of the mastoid indicates subperiosteal abscess formation, while erosion of the sigmoid plate suggests epidural abscess formation adjacent to the sigmoid sinus.
  • On MRI, T1-weighted imaging shows low signal intensity in affected mastoid, T2-weighted shows high signal intensity, and post-gadolinium imaging demonstrates mucosal contrast enhancement which is present in the majority of cases.
  • Diffusion restriction on DWI/ADC sequences may be present and supports acute infection; however, fluid signal intensity alone should not be interpreted as mastoiditis without mucosal enhancement and/or diffusion restriction.
  • Bezold abscess appears as a rim-enhancing collection or mass with surrounding fat stranding located deep to the sternocleidomastoid muscle, representing lateral extension of infection.
  • Citelli abscess presents as erosion and infection extending into the occipital bone or appears as a collection within the digastric triangle.
  • Post-contrast imaging is essential for evaluating associated soft tissue complications, intracranial extension, epidural abscess formation, dural venous sinus thrombosis, and cerebral abscess.
Differential Diagnosis
  • Acute otitis media without mastoiditis: fluid in middle ear cleft alone without mastoid air cell involvement or bony septation erosion; clinical context (lack of postauricular signs) is key distinction.
  • Non-infectious mastoid opacification: serous mastoiditis or post-radiation changes show fluid/hypodensity without erosion, mucosal enhancement, or diffusion restriction.
  • Mastoid cholesteatoma: demonstrates erosive changes similar to coalescent mastoiditis but shows characteristic MRI signal patterns and different clinical presentation (chronic discharge).
  • Neoplastic lesion involving mastoid: typically shows asymmetric enhancement and should have clinical imaging for primary tumor; history and gradual onset distinguish from acute infection.
  • Aspergillosis: more aggressive appearance with extensive erosion and frequent facial nerve dysfunction; typically in older or immunocompromised patients.
  • Tuberculous otomastoiditis: demonstrates slow progression with caseous necrosis and granulomatous inflammation; clinical and microbiological context is essential for distinction.
Discussion

Acute mastoiditis with periostitis represents the initial clinical stage where infection spreads through mastoid emissary veins into the periosteum, whereas coalescent mastoiditis involves infiltration and destruction of bone representing frank osteomyelitis.

Streptococcus pneumoniae is the most common causative organism (65-80% of cases), but Haemophilus influenzae, though less frequent, is more aggressive and more likely to cause complications including meningitis.

CT is the imaging modality of choice for initial investigation, with post-contrast imaging critical for detecting soft tissue complications, intracranial extension, and abscess formations.

The mere presence of mastoid fluid on imaging does not change management of uncomplicated acute otitis media; the diagnosis of mastoiditis requires clinical signs or symptoms of mastoid inflammation.

Complications including subperiosteal abscess, Bezold abscess, Citelli abscess, petrous apicitis, epidural abscess, meningitis, and dural sinus thrombosis may occur and require imaging surveillance and escalated treatment.

Approximately 30% of the population has a pneumatized petrous apex, which increases the risk for petrous apicitis and intracranial complications if infection extends superiorly.

Reporting Pearls

When reporting mastoiditis, clearly describe whether imaging findings represent incipient mastoiditis (opacification with enhancement only) or coalescent mastoiditis (with bony septal erosion), as this distinction guides treatment escalation and complication risk; always specify the presence or absence of associated complications such as subperiosteal/epidural/intracranial abscess, facial nerve involvement, or dural sinus disease.

Pitfalls
  • Interpreting simple mastoid fluid opacification as mastoiditis without clinical context or additional imaging findings (mucosal enhancement, diffusion restriction): mild fluid is present in many uncomplicated acute otitis media cases.
  • Overlooking bony septal erosion on CT, which is essential for establishing coalescent mastoiditis diagnosis and indicates need for more aggressive treatment consideration.
  • Failing to assess for complications on post-contrast imaging, particularly lateral wall erosion (subperiosteal abscess), sigmoid plate erosion (epidural abscess), and intracranial extension, which significantly impact management.
  • Not recognizing Bezold and Citelli abscesses which represent lateral and inferior extensions respectively and may be the primary presenting finding rather than obvious mastoid involvement.
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