What is the diagnostic approach for mastoiditis?

How to Check for Mastoiditis

The diagnosis of mastoiditis is primarily clinical, based on the presence of mastoid tenderness, retroauricular swelling, and protrusion of the auricle, which distinguish it from uncomplicated acute otitis media, followed by confirmation with high-resolution CT of the temporal bone with IV contrast. 1

Clinical Examination Approach

Key Physical Findings That Distinguish Mastoiditis from AOM

The critical diagnostic features on physical examination include:

• Mastoid tenderness on palpation of the postauricular area 1
• Retroauricular swelling and erythema (present in 94.7% of cases) 2
• Protrusion or displacement of the auricle (present in 91.2% of cases) 2
• Fever (present in 75.4% of cases) 2

These findings are not present in uncomplicated AOM and are the hallmark clinical signs that should prompt suspicion for mastoiditis 1. However, a critical pitfall is that classical postauricular signs may be present in only 10% of patients in some series, making the diagnosis challenging 3.

Otoscopic Examination

Perform otoscopy to evaluate for:

• Bulging tympanic membrane (most reliable sign of AOM) 1
• Intense erythema of the tympanic membrane 1
• Limited or absent tympanic membrane mobility indicating middle ear effusion 1
• Otorrhea if tympanic membrane rupture has occurred 1

Note that otoscopic findings in mastoiditis are similar to AOM, so relying solely on otoscopic findings may miss mastoiditis—always examine the mastoid area in cases of severe or persistent AOM 1.

Assessment for Complications

Evaluate for signs suggesting intracranial or serious complications:

• Headache, vertigo, or meningismus 1
• Neck rigidity 1
• Seizures or neurological deficits 1
• Cranial nerve involvement 4

These findings suggest complications such as meningitis, intracranial abscess, subdural empyema, labyrinthitis, or dural venous sinus thrombosis 1.

Imaging Studies

CT Temporal Bone with IV Contrast (First-Line Imaging)

High-resolution CT of the temporal bone with IV contrast is the optimal initial imaging modality for confirming mastoiditis and evaluating for complications 1, 5. CT provides:

• High spatial resolution to assess bone destruction, coalescence of air cells, and erosion of bony structures 1
• Detection of subperiosteal abscess 2
• Visualization of other intratemporal complications 5
• Superior sensitivity compared to plain radiographs, which are often not diagnostic early in the disease course 6, 3

The addition of IV contrast improves detection of inflammatory tissue and abscesses 1.

When to Obtain CT Imaging

Obtain CT temporal bone when:

• Clinical suspicion for mastoiditis exists based on physical examination findings 1
• Failure to improve after 48 hours of antibiotic treatment 1, 4
• Clinical deterioration occurs 1
• Complications are suspected based on neurological signs 1

MRI Head and Internal Auditory Canal Without and With IV Contrast

MRI is superior to CT for detecting intracranial complications and should be obtained when intracranial involvement is clinically suspected 5, 1. MRI provides:

• Higher sensitivity and specificity for intracranial complications compared to CT 5
• Superior detection of dural venous sinus thrombosis, meningitis, brain abscess, or cerebritis 1
• Better differentiation between normal and involved soft tissues 5
• Superior visualization of labyrinth involvement 5

CT Venography Head with IV Contrast

CTV should be considered when dural venous sinus thrombosis is suspected, as it adequately demonstrates intraluminal filling defects or lack of contrast filling of the dural venous sinuses 5.

Laboratory Studies

While not diagnostic, laboratory tests can support the clinical assessment:

• Complete blood count with differential to assess systemic inflammation 1
• C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) are typically elevated, with CRP being initially higher in acute mastoiditis and declining faster than ESR 3
• Blood cultures if systemic infection is suspected 1

Common Pitfalls and Caveats

• Mastoiditis can develop despite prior antibiotic treatment for AOM (33-81% of cases had received antibiotics previously), so don't be falsely reassured by recent antibiotic use 1, 7
• Plain radiographs are insensitive, particularly early in the disease course, and should not be relied upon 6, 3
• Cultures may be negative in 33-53% of cases, so negative cultures do not rule out mastoiditis 1, 7
• Classical postauricular signs may be absent in up to 90% of patients, requiring high clinical suspicion 3
• Delaying imaging when complications are suspected can increase morbidity and mortality 1
• Failing to obtain imaging in patients who don't respond to antibiotics within 48 hours can delay diagnosis 1