Imaging description

Malignant otitis externa (MOE) is an uncommon infection of the external auditory canal (EAC) that has a propensity to spread outside the temporal bone. MOE is seen in elderly diabetic patients, and Pseudomonas aeruginosa is the most common organism responsible. Infection from the EAC spreads through the small perforations along the floor of the cartilaginous portion of the EAC and extends medially to the skull base including the jugular canal and stylomastoid foramen, nasopharynx, parapharyngeal space, parotid space, and infratemporal fossa. More medial and superior extension results in involvement of the petrous apex, dural sinuses, meningies, and brain [1]. Involvement of the stylomastoid foramen leads to facial nerve paralysis, and this is the most commonly involved cranial nerve in MOE. Involvement of the jugular foramen causes deficits of cranial nerves IX, X, and XI, and petrous apex involvement may result in trigeminal and abducens nerve palsies.

The diagnosis of MOE is difficult and requires the presence of clinical, radiologic, and microbiologic findings. Demonstration of bone involvement in the mastoid and soft tissue involvement outside the EAC is diagnostic of MOE in the appropriate clinical setting (Figs. 66.1, 66.2). CT is usually the first imaging study obtained, and it is very accurate in demonstrating bone erosion, which is present in about 70% of cases [2]. Often, cortical bone erosion is very subtle and requires careful scrutiny of all cortical margins of the mastoid bone. Tc-99m MDP bone scan is much more sensitive than CT and is positive in essentially all cases of MOE, although its specificity is quite poor. Soft tissue involvement outside the EAC can be seen on CT, although findings are often subtle. MRI is much more accurate in the assessment of the extent of disease because of its superior contrast resolution and multiplanar capability (Fig. 66.1).