Malignant External Otitis

Malignant external otitis, also known as necrotizing external otitis, refers to osteomyelitis of the skull base (characteristically Pseudomonas aeruginosa), usually caused by persistent external otitis in diabetic or immunocompromised patients.

Early in the disease, CT may show thickened mucosa of the floor of the external auditory canal and auricle. As the disease progresses, there is is erosion of bone and maybe even extension into the middle cranial fossa or clivus.

Differential considerations include:

• Squamous cell carcinoma of the external auditory canal: The two conditions may coexist. Look for history of cutaneous squamous cell carcinoma. But beware that ity may be impossible to differentiate squamous cell carcinoma of the external auditory canal from malignant external otitis radiographically.
• Cholesteatoma of the external auditory canal: Look for a soft tissue mass with underlying bony destruction and intramural bony flakes.
• Post-inflammatory medial canal fibrosis: There is fibrous tissue in medial aspect of the external auditory canal without bony erosion. It is usually caused by chronic otitis externa or surgery
• Keratosis Obturans: Rare condition classically seen in patients with chronic sinusitis and bronchiectasis. There is obstruction of the external auditory canal due to abnormal accumulation of desquamated keratin. Look for homogeneous soft tissue within the external auditory canal, which may be mildly enlarged. No bony erosion is seen.

References

• Narozny W et al. Infectious skull base osteomyelitis-still a life-threatening disease. Otol Neurotol. 2006;27(7):1047-8.
• StatDx.