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Necrotizing otitis externa

Severe invasive infection of the external auditory canal, typically in elderly or immunocompromised patients (diabetes, chemotherapy, radiation) with severe pain out of proportion to clinical findings. Imaging is indicated when necrotizing otitis externa is suspected to assess for skull base involvement and intracranial complications.

Key Images

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Look For First
  • Thickening and enhancement of soft tissues around the external auditory canal on contrast-enhanced CT
  • Cortical bone erosion at the temporomandibular junction or skull base region
  • Abnormal uptake on technetium-99m bone scan in the temporal bone and skull base (differentiates from typical otitis externa)
Key Image Findings
  • Contrast-enhanced CT shows thickening and enhancing soft tissue in the external auditory canal region, often with surrounding edema in periauricular tissues and nasopharynx.
  • Cortical bone erosion of the external auditory canal cartilaginous-bony junction, which may appear as loss of the normal cortical margin on CT.
  • Phlegmon or abscess formation with enhancing rim and necrotic low-attenuation center within the cartilaginous bone ring of the external auditory canal.
  • Opacification of adjacent mastoid air cells and middle ear from direct extension of infection seen on axial and coronal CT.
  • Extension into suprahyoid neck spaces including parotid space and masticator space on CT demonstrates the aggressive anterior-inferior spread pattern.
  • Technetium-99m bone scan demonstrates osteoblastic activity with abnormal uptake in temporal bone and skull base, which is highly sensitive and differentiates necrotizing otitis externa from typical acute otitis externa.
  • Potential skull base erosion on CT with intracranial spread of infection, including involvement of adjacent structures and risk of serious intracranial complications.
  • Gallium-67 citrate and indium-111 labeled leukocyte imaging may show persistent uptake during treatment, used in conjunction with serum ESR levels to gauge treatment response.
Differential Diagnosis
  • Mastoiditis or other skull base osteomyelitis from alternate etiology: may have similar imaging appearance but clinical context and culture results differentiate; mastoiditis typically follows acute otitis media.
  • Squamous cell carcinoma of the external auditory canal: presents with mass and bone erosion but typically in older patients with smoking history; can coexist with infection.
  • Erosive otitis externa: occurs in immunocompetent patients with exposed bone and small sequestrum; lacks aggressive soft tissue enhancement and spread seen in necrotizing otitis externa.
  • Iatrogenic skull base osteomyelitis from surgery: temporal relationship to prior surgery and different location based on surgical approach helps differentiate.
Discussion

Pseudomonas aeruginosa is the causative organism in 98% of cases, reflecting the water-loving nature of this pathogen in the moist external auditory canal environment.

Immunocompromised states (diabetes, chemotherapy, radiation therapy) are key predisposing factors that allow rapid progression from simple external otitis to life-threatening skull base infection.

The infection can spread along two main routes: anterior-inferiorly into suprahyoid neck spaces or direct intracranial spread through erosion of the cartilaginous-bony external auditory canal.

Serious complications including meningitis, subdural empyema, carotid artery pseudoaneurysm, and dural venous sinus thrombophlebitis require aggressive early imaging and treatment to prevent morbidity and mortality.

Pain severity is a clinical clue: pain out of proportion to otoscopic findings should raise suspicion for necrotizing otitis externa rather than typical external otitis.

Serial imaging with nuclear medicine studies (gallium-67 and indium-111) combined with ESR monitoring provides objective assessment of treatment response and guides duration of therapy.

Reporting Pearls

Clearly document the presence and extent of cortical bone erosion at the external auditory canal, specify which neck spaces or intracranial structures are involved, and describe the pattern of spread (anterior-inferior vs. intracranial) to guide surgical planning and assess for life-threatening complications.

Pitfalls
  • Confusing necrotizing otitis externa with uncomplicated acute otitis externa based on canal symptoms alone; the key is imaging demonstration of bone erosion and soft tissue involvement beyond the canal.
  • Failing to obtain contrast-enhanced CT and missing subtle bone erosion at the cartilaginous-bony junction, which may be the earliest imaging sign of progression.
  • Misinterpreting mastoid air cell opacification as isolated mastoiditis without recognizing the external auditory canal pathology as the primary source of infection.
  • Underestimating the aggressiveness of the infection by not systematically evaluating the skull base, intracranial structures, and neck spaces for extension and complications.
Source: Reference source

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