Cholesterol granuloma of the petrous apex. (Imaging Clinic).

A cholesterol granuloma of the tip of the petrous bone is a smoothly expanding mass that contains cholesterol crystals, hemosiderin-laden macrophages, fibrous tissue, chronic inflammatory cells, and blood vessels. On computed tomography (CT), this lesion appears as a well-marginated expansile mass (figure 1). Magnetic resonance imaging (MRI) of a cholesterol granuloma shows a hyperintense mass in conventional pulse sequences (figure 2), which on magnetic resonance angiography (MRA) can resemble an aneurysm (figure 3). The clinical implementation of MRA has proved to be very useful in vascular lesions, but the fact that a cholesterol granuloma can be mistaken for an aneurysm is a potential diagnostic pitfall for the unwary. (1)

It is presumed that the formation of a cholesterol granuloma is caused by an obstruction of the air cells in the petrous apex. The obstruction induces mucosal engorgement, which leads to a rupture of blood vessels and hemorrhage. Red blood cell degradation into cholesterol crystals produces a foreign giant-cell reaction, which is characterized by inflammation and small-vessel proliferation, followed by rupture of the blood vessel. Granulation tissue forms secondary to repeated hemorrhage and leads to an expansile mass of the petrous apex. (2)

Clinically, this lesion can produce sensorineural hearing loss, tinnitus, hemifacial spasm, facial numbness, and trigeminal neuralgia. It usually occurs in young or middle-age adults. The traditional treatment is drainage and stent placement via a transtemporal approach. An extended middle cranial fossa approach with extradural removal of the lesion and obliteration of the cavity decreases the recurrence rate. (3)

References

(1.) Pen J, Turski PA, Masaryk TJ. MR angiography techniques and clinical applications. In: Atlas SW, ed. Magnetic Resonance Imaging of the Brain and Spine. 2nd ed. New York: Raven Press, 1996:1547-618.

(2.) Chaljub G, Vrabec J, Hollingsworth C, et al. Magnetic resonance imaging of petrous tip lesions. Am J Otolaryngol 1999;20:304-13.

(3.) Greenberg JJ, Oot RF, Wismer GL, et al. Cholesterol granuloma of the petrous apex: MR and CT evaluation. AJNR Am J Neuroradiol 1988;9:1205-14.

From the Department of Otolaryngology (Dr. Hunt and Dr. Rigby) and the Department of Radiology (Dr. Palacios), Louisiana State University, New Orleans.