Imaging of temporal bone brain hernias: atypical appearances on magnetic resonance imaging

Brain tissue with or without its dural covering may protrude into the tempo ral bone through a defect in the tegmen tympani. Infection or granulation t issue, with or without cholesteatoma may make diagnosis difficult. While co mputed tomography (CT) may demonstrate a bony defect, it is difficult to di stinguish between herniated brain, cholesteatoma or granulation tissue. Mag netic resonance imaging (MRI) clearly demonstrates healthy herniated brain tissue, but ischaemic or necrotic elements may mimic other lesions. We pres ent a series of five patients with brain hernia, highlighting the spectrum of preoperative radiological appearances,and some of the difficulties encou ntered in interpreting these images. MRI demonstrated herniation of healthy brain in two patients but in three, Showed irregular soft tissue with sign al characteristics consistent with cholesteatoma. Surgery in these three pa tients demonstrated necrotic brain with coexisting cholesteatoma in two. Br ain hernias are often necrotic and may have morphological and signal charac teristics similar to cholesteatoma with which they may coexist.