What causes mastoid bone opacification?

Causes of Mastoid Bone Opacification

Mastoid opacification is most commonly caused by middle ear effusion (MEE) associated with acute otitis media (AOM) or otitis media with effusion (OME), representing inflammatory fluid extending into the mastoid air cells rather than true mastoiditis in the majority of cases. 1, 2

Primary Infectious/Inflammatory Causes

Acute Otitis Media and Middle Ear Effusion

• AOM with MEE is the most frequent cause of mastoid opacification, occurring when inflammatory fluid from the middle ear extends into the mastoid air cell system 1, 2
• After an AOM episode, all children develop MEE which can persist and extend into mastoid air cells 1
• OME (glue ear) causes mastoid opacification through persistent MEE behind an intact tympanic membrane without acute infection 1
• Mastoid opacification is an extremely common incidental finding in asymptomatic children, with prevalence rates of 14-20% depending on age 3, 4
• The highest prevalence occurs in children aged 0-4 years (19-20%), inversely correlated with increasing age 3, 4

Acute Mastoiditis (True Infection)

• Acute mastoiditis represents suppurative infection of the mastoid air cells and is the most common complication of AOM 2, 3
• The key distinction from simple AOM with mastoid opacification is the presence of mastoid tenderness, retroauricular swelling, and protrusion of the auricle 2
• Mastoiditis can develop despite prior antibiotic treatment for AOM (33-81% of cases had received antibiotics previously) 2, 5
• The inflammatory process begins with hyperemia and osteoclastic erosion of bony walls in Haversian systems, progressing to active rarefaction and potential coalescence of air cells 6

Chronic Otitis Media

• COM causes persistent mastoid cavity opacification representing organized inflammatory tissue and granulation 7
• The mastoid opacification in COM represents a sequel to protective physiological responses rather than active infection in many cases 7
• Soft tissue density opacification on temporal bone CT is commonly seen in COM patients 7

Fungal Causes

Aspergillosis

• Aspergillosis of the mastoid sinus cells may occur as a result of chronic Aspergillus otitis media 1
• Mastoid aspergillosis can subsequently extend into the transverse sinus, resulting in venous thrombosis and severe neurological sequelae 1
• Classic CT findings include unilateral lesions, nodular mucoperiosteal thickening, focal bone destruction, and/or dense intrasinus concretions 1

Neoplastic Causes

Solitary Plasmacytoma and Other Tumors

• Tumors such as solitary plasmacytoma can produce air cell opacification and coalescence mimicking coalescent mastoiditis on CT 8
• MRI is superior to CT for differentiating tumor from infection, showing tissue signal characteristics and presence of soft-tissue masses 8
• Sinonasal tumors with mastoid extension can cause opacification 1

Important Clinical Distinctions

Imaging Interpretation Pitfalls

• Mastoid opacification on CT is nonspecific and does NOT equal clinical mastoiditis 3, 8
• The presence of sinus air-fluid levels or sinus opacification in immunocompromised hosts should prompt urgent evaluation 1
• CT findings require clinical correlation: only 3% of pediatric IMO cases required otolaryngology consultation, and none had clinical mastoiditis 4

When to Suspect True Mastoiditis vs. Incidental Finding

• Clinical examination trumps imaging: look for mastoid tenderness, retroauricular swelling, and auricle protrusion to diagnose true mastoiditis 2
• Bulging tympanic membrane, intense erythema, and limited TM mobility suggest AOM with secondary mastoid opacification rather than mastoiditis 2
• Failure to improve after 48 hours of appropriate antibiotics or clinical deterioration warrants CT temporal bone with IV contrast 2, 5

Risk Factors and Associations

• Male children have higher rates of IMO (16.6%) compared to females (11.0%) 4
• No significant seasonal variation in IMO rates 4
• Immunocompromised patients are at higher risk for fungal mastoiditis 1