Teaching Case

A patient without known co-morbidities complained of recurring morning headache for one month. The headache was described as beginning in the occipital region, irradiating into the right ophthalmic (V1) and maxillary (V2) sensory areas, and was accompanied by right eye epiphora and facial sweating. Clinical examination revealed no neurological deficit.

Initial MRI revealed bilateral cystic lesions of the petrous apex with an axial diameter of 2.1 and 2.4 cm on the right and left side, respectively. The lesions displayed high signal intensity on T1-, T2- and unenhanced fat-saturated T1w sequences. Diffusion weighted sequences showed no restricted diffusion. There was no enhancement after intravenous administration of gadolinium chelates. Anatomically, the lesions were in immediate vicinity of the intra-petrous carotid artery, the cisternal portion of the trigeminal (V), abducens (VI) and glossopharyngeal (IX) nerves. A thin slice CT with bone window settings showed a thin, regular bony cortex surrounding the expansile lesions. Due to the high signal on all sequences, the absent enhancement and the regular, thin margins, the differential diagnosis of bilateral cholesterol granulomas of the petrous apex was suggested. Follow-up MRI examinations obtained 3 and 16 months later showed no progression of findings.

Cholesterol granuloma (CG) is the most common lesion of the petrous apex, affecting 9-30% of individuals. It can also be found in the mastoid, middle ear or orbitofrontal region [2, 6, 7]. The pathogenesis of CG suggests air cell obstruction with subsequent negative pressure causing an inflammatory reaction. This inflammatory reaction leads to repeated bleeding, local tissue destruction and formation of cholesterol crystals, with subsequent foreign-body reaction and progressive bone remodelling [2, 3, 6]. Another theory explains the formation of cholesterol deposits from persistent bleeding caused by exposure of richly vascular marrow in exuberantly pneumatized petrous apices [8]. Proximity of the petrous apex to the cavernous sinus, Dorello’s canal (containing VI) and Meckel’s cave (containing trigeminal ganglion) may explain symptoms occurring when these structures are compressed. Frequent symptoms include hearing loss, vertigo and headache. Rarely, tinnitus, otalgia, diplopia, trigeminal neuralgia, and hemifacial spasm may occur [2, 3]. On CT, a sharply-defined, expansile lesion without internal matrix, calcifications or enhancement is typically seen. On MRI, a hyperintense signal is noted on all sequences, due to cholesterol crystals and methaemoglobin; a hypointense halo caused by haemosiderin deposits may also be present [1, 2, 3, 6, 7].
Differential diagnosis includes absent or incomplete pneumatization of the petrous apex, which is characterized by fatty signal due to bone marrow (characteristic hyperintensity on T1 and T2 suppressed on fat saturated sequences). Further differential diagnoses are cholesteatoma, mucocele, carotid artery aneurysm, apical petrositis and haemorrhagic metastasis. Although cholesteatomas are also expansile masses without contrast enhancement, they are hypointense on T1 and they have a restricted diffusion. Mucoceles are rare lesions resulting from air cell obstruction and resemble CG; however, they contain true mucus secreting epithelium. On T2, they display hyperintensity, whereas the signal intensity is variable on T1 as it depends on the protein content. Aneurysms of the petrous carotid artery may occasionally display similar MRI findings as CG. However, characteristic flow-voids and findings on MRI angiography are helpful for the differential diagnosis [1, 2, 3].
Clinical and radiological follow up is proposed for small CG. If symptomatic or progressive, surgical drainage can restore ventilation. After successful drainage, CG loses its T1 hyperintensity [2, 6]. Bilateral CG are rare and have been reported in isolated case reports. The absence of a history of trauma in the few cases reported so far suggests congenital obstruction of mastoid air cells [5].