Rapid expansion in a previously indolent cholesterol cyst: a need for lifelong follow-up.

Cholesterol cysts of the petrous apex, also known as cholesterol granulomas, were first reported as a distinct clinical entity in the mid 1980s. Graham et al. (1) described the lesion as an expansile cyst containing cholesterol crystals within a fibrous capsule, typically arising within a well-pneumatized petrous apex. Most lesions are diagnosed when patients present with symptoms referable to the temporal bone or trigeminal nerve, and are subsequently treated with surgical drainage. Traditionally, these lesions were thought to arise as an inflammatory response to hemorrhage after obstruction of petrous apex air cells. However, Jackler and Cho (2) recently proposed an alternate hypothesis that might better explain the tendency of this lesion to occur in the well-pneumatized petrous apices of otherwise healthy temporal bones. According to their ‘‘exposed marrow hypothesis,’’ extensive pneumatization of the petrous apex transgresses areas of bone marrow in the petrous apex. Hemorrhage from the exposed marrow could then lead to blockage of outflow tracts and result in cyst formation. Sustained hemorrhage from this marrow and/or the resulting inflammatory reaction to anaerobic breakdown products of blood is thought to be the driving force for continued cyst expansion. Thedinger et al. (3) reported on a series of patients with cholesterol cysts of the petrous apex, which included two patients who were followed up radiographically for up to 4 years. These patients showed no cyst enlargement. Mosnier et al. (4) also reported two patients with follow-up imaging for up to 10 years who showed no sign of enlargement. Mosnier et al. (4) suggested differentiating these nonexpansile indolent lesions from the more aggressive expansile lesions that require surgical drainage. A series of magnetic resonance imaging (MRI) and serial computed tomography (CT) scans (Figs. 1–4) of a left-side petrous apex cholesterol cyst in a middle-aged woman demonstrated the stable appearance of the cyst over a 3-year period with rapid expansion of the cyst by the sixth year. The cyst was asymptomatic until it rapidly expanded. Figure 1 shows a cholesterol cyst that was found incidentally on an MRI scan obtained for an unrelated complaint. This mildly loculated, cystic-appearing, leftside petrous apex lesion demonstrates hyperintense signal on both T1(Fig. 1,A) and T2-weighted (Fig. 1,B) scans. A