Cholesterol granuloma (CG), also sometimes called a chocolate cyst of the ear or blue domed cyst, is a special type of middle ear granulation tissue which is particularly prone to bleeding and is a frequent cause of a hemotympanum. Cholesterol granulomas represent the most common cystic lesion of the petrous apex.
Cholesterol granulomas typically affect young to middle-aged patients often with a history of chronic otitis media. There is no recognized gender predilection 3.
The presentation will depend on location 5:
- if in the middle ear
- conductive hearing loss
- cranial nerve dysfunction, e.g. CN VII
- if at the petrous apex
- if in the mastoid
Any aerated portion of the temporal bone may develop a cholesterol granuloma. The mastoid air cells are the most common location.
The pathogenesis is controversial with two major schools of thought 3,5:
- obstruction-vacuum theory: where eustachian tube dysfunction is thought to be the underlying abnormality 1,5 and causes mucosal edema with repeated episodes of bleeding
- exposed marrow theory: where hyperplastic mucosa invades the underlying bone and exposes bone marrow, which in turn bleeds 3
In either scenario, trapped blood undergoes degeneration and is surrounded by a chronic inflammatory response. There may also be a superimposed infection.
Histologically, cholesterol granulomas are composed of yellowish-brownish fluid which contains 4,5:
- cholesterol crystals: accounting for the high T1 and T2 signal
- multinucleated giant cells
- red blood cells and blood breakdown products
This is surrounded by a fibrous connective tissue capsule with fragile blood vessels which are prone to rupture, thus preventing resolution 3.
Typically, there is an expansile well-marginated lesion with thinned overlying bone. This may be dehiscent when the lesion is large. Faint peripheral enhancement post contrast may be seen.
Appearance is related to location. When a cholesterol granuloma is located in the petrous apex, it may be more aggressive in appearance, e.g. bony erosions and extension to the carotid canal or cerebellopontine angle. However, when they are located in the middle ear, associated erosion is rare 1.
- T1: overall high signal due to cholesterol component and methemoglobin +/- low signal rim due to hemosiderin rim, and thinned adjacent bone
- central high signal +/- peripheral low signal due to hemosiderin rim
- thinned adjacent bone
- does not attenuate on FLAIR
- fat suppression: remain high signal 7
- T1 C + (Gd): no central enhancement although faint peripheral enhancement may be difficult to see due to intrinsic high T1 signal of lesion which is not saturated (not an adipose tissue)
- DWI/ADC: no restricted diffusion 6
Treatment and prognosis
If symptomatic, surgical excision is required, and this must include the cyst wall. A number of surgical approaches exist and are chosen based on both the location of the cholesterol granuloma and the degree of hearing loss 5. In some cases, a mastoidectomy may be necessary 3. However high recurrence rates have been reported.
Given its signal characteristics, a cholesterol granuloma should be easily distinguished from other petrous apex/middle ear lesions, such as 2:
- normal asymmetry of fatty marrow/pneumatization
- middle ear effusions
- effusion of pneumatized petrous apex
- base of skull tumors
A few lesions can have similar appearances, including:
hydrated mucocoele: rare, but may have identical signal on MRI
- high protein = high T1
- high water = high T2
- no enhancement
- thrombosed ICA aneurysm 2
- signal is usually more complex because of layered blood products
- flow void centrally if not completely thrombosed
- 1. Imaging of the temporal bone. Joel D. Swartz, H. Ric Harnsberger. New York : Thieme, 1998. ISBN:0865777004 (find it at amazon.com)
- 2. Connor SE, Leung R, Natas S. Imaging of the petrous apex: a pictorial review. Br J Radiol. 2008;81 (965): 427-35. doi:10.1259/bjr/54160649 [pubmed citation]
- 3. Temporal Bone Imaging. Ellen Hoeffner (Editor), Suresh Mukherji (Editor), Dheeraj Gandhi (Editor), Diana Gomez-Hassan (Editor). Thieme ISBN:1588904016 (find it at amazon.com)
- 4. Imaging of the Head and Neck. Mahmood F. Mafee, Galdino E. Valbasson, Minerva Becker, J. S. Lewin, S. G. Nour (Contributor), A. L. Weber (Contributor), M. Becker (Contributor). Thieme Medical Publishers ISBN:1588900096 (find it at amazon.com)
- 5. Wiet R. Ear and Temporal Bone Surgery. Thieme. (2006) ISBN:1588901211. Read it at Google Books - Find it at Amazon
- 6. Baráth K, Huber AM, Stämpfli P et-al. Neuroradiology of cholesteatomas. AJNR Am J Neuroradiol. 2011;32 (2): 221-9. doi:10.3174/ajnr.A2052 - Pubmed citation
- 7. Razek AA, Huang BY. Lesions of the petrous apex: classification and findings at CT and MR imaging. Radiographics. 2012;32 (1): 151-73. doi:10.1148/rg.321105758 - Pubmed citation