Necrotizing otitis externa (NOE), also known as malignant otitis externa, is a severe invasive infection of the external auditory canal (EAC) which can spread rapidly to involve the surrounding soft tissue, adjacent neck spaces and skull base.
Predisposing conditions for NOE include diabetes and immunosuppression (i.e., diabetes or patients receiving chemo- and/or radiation therapy), and is usually seen in elderly patients. Pain can be out of proportion for typical otitis externa.
Pseudomonas aeruginosa is the pathogen in 98% of cases. The route by which the infection spreads is variable. It can spread anteroinferiorly to involve the suprahyoid neck spaces (parotid space and masticator space). Alternatively, the infection can also erode the cartilaginous-bone of the EAC resulting in a direct intracranial spread with resultant serious intracranial complications.
Serious complications include:
- skull base osteomyelitis
- subdural empyema
- cerebral abscess
- facial nerve palsy
- temporomandibular joint involvement
- dural venous sinus thrombophlebitis
On contrast-enhanced CT, there can be thickening and enhancing soft tissue and sometimes cortical bone erosion in the region of the external auditory canal with or without formation of phlegmon/abscess. In cases of abscess, cartilaginous bone ring enhancing collection with a necrotic low attenuation center can be observed.
Necrotizing otitis externa may be associated with inflammatory changes within adjacent structures such as the periauricular soft tissues, nasopharynx and parapharyngeal space. Additionally there may be opacification of the mastoid air cells and middle ear from direct extension 5.
Potential complications include skull base erosion with intracranial spread of infection.
Technetium-99m bone scanning is sensitive to osteoblastic activity and is highly sensitive for bony infection, with uptake in the temporal bone and skull base differentiating NOE from typical acute otitis externa 4.
Follow up imaging using gallium-67 citrate and indium-111 labeled leukocyte scans, in addition to serum ESR levels can gauge treatment response 4.
Imaging findings can be misinterpreted as a malignancy. Clinical symptoms usually help to differentiate.
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