Skull Base, Petrous Ridge

A 24-year-old man with a history of severe right-sided tinnitus and decreased hearing underwent MR imaging. The scan demonstrated a multilobulated, T1- and T2-hyperintense lesion centered within the right petrous apex (Image A). The mass demonstrated a T1- and T2-hypointense rim compatible with chronic blood products. A CT scan demonstrated smooth bony erosion surrounding the mass (Image B). The specimen was resected and is shown in Image C, Image D, and Image E.

Tissue Site
Skull base, petrous ridge

Image A: T1-weighted and T2-weighted MRI.

Image A: T1-weighted and T2-weighted MRI.

Image B: CT scan.

Image B: CT scan.

Image C: H&E stain.

Image C: H&E stain.

Image D: H&E stain.

Image D: H&E stain.

Image E: H&E stain.

Image E: H&E stain.

The whole slide image provided is an H&E-stained slide of the sphenoid sinus from a craniotomy.

  1. The clear, needle-shaped crystals represent what type of material?

    1. Calcium pyrophosphate

    2. Cholesterol

    3. Hematoidin

    4. Hemosiderin

    5. Urate

  2. What is the most likely etiology of the mass?

    1. Chronic fungal sinus infection

    2. Epidermoid cyst

    3. Neoplasm of bony origin

    4. Obstruction of a nasal sinus with blood and inflammation

    5. Tophaceous gout

  3. What is a cholesteatoma of the middle ear?

    1. An abnormal skin growth in the middle ear behind the ear drum

    2. A benign proliferation of Schwann cells involving the middle ear

    3. An abnormality resulting in a squamous-lined cyst of the middle ear

    4. A deposit of uric acid crystals in the form of monosodium urate

    5. Uncommon benign hemorrhagic cyst arising in the petrous apex

View Answer Key

The lesion shown is a cholesterol granuloma arising in the petrous apex. The most common lesions of the skull base and associated sinuses are dermoid and epidermoid cysts, which are sometimes congenital malformations composed of squamous epithelial-lined cysts. However, the cholesterol granuloma must also be included in this differential diagnosis. Cholesterol granulomas are among the nonneoplastic, inflammatory lesions of the cranial sinuses that commonly produce space-occupying, destructive lesions. Limited aeration, obstructed drainage, and bleeding into a nasal sinus predispose to the formation of a cyst. Air absorption leads to blood accumulation and inflammation. The expanding fluid accumulation results in bony erosion into marrow spaces and more hemorrhage. Some lesions may expand slowly remaining asymptomatic for many years while other lesions may expand more rapidly resulting in hearing loss, vertigo and lower cranial nerve palsies. Characteristically, on MRI, cholesterol granuloma exhibits high signal intensity on both T1- and T2-weighted images (Image A). A typical appearance on CT is a sharply marginated expansile lesion with bony remodeling (Image B). It is nonenhancing with contrast and isodense to brain tissue.

Image A: T1-weighted and T2-weighted MRI.

Image A: T1-weighted and T2-weighted MRI.

Image B: CT scan.

Image B: CT scan.

The cholesterol granuloma is a reaction to crystallized cholesterol, typically localized within the petrous apex. These lesions have been reported to be most frequently associated with polyps of the maxillary antrum. Not to be confused with cholesteatoma of the middle ear, which is the result of an ingrowth of squamous epithelium, the cholesterol granuloma represents the product of an attempted phagocytic clean-up of an inflammatory or hemorrhagic process. Although a source of potential confusion, granulomas, including cholesterol granulomas, are also found in epidermoid and dermoid cysts, as well as craniopharyngiomas, most commonly with hemorrhage and rupture.

Although occasionally considered in the radiographic differential diagnosis of epidermoid and dermoid cysts, cholesterol granulomas of the cranial sinuses have an entirely different histologic appearance. The cholesterol granuloma is a poorly-circumscribed lesion composed of hemorrhage and leukocytes variably attributed to erosion into bone marrow or ongoing inflammation and hemorrhage associated with multinucleated histiocytes (Image C, Image D, and Image E). As a result, the macrophages become engorged with nondegradable cell membrane products, the predominant component being cholesterol. The accumulated cholesterol crystalizes into needle-shaped structures and becomes embedded within the macrophages, resulting in a persistent foreign body response.

Image C: H&E stain.

Image C: H&E stain.

Image D: H&E stain.

Image D: H&E stain.

Image E: H&E stain.

Image E: H&E stain.

Diagnosis
Cholesterol granuloma


Take Home Points

  • Cholesterol granuloma often arises in the petrous apex and is associated with cranial nerve deficits.
  • Cholesterol granulomas are inflammatory lesions associated with chronically obstructed nasal sinuses, commonly related to the presence of a nasal polyp.
  • The cholesterol is thought to arise from degenerated inflammatory cells, hemorrhage, and bone marrow elements.

References

  1. Gore MR, Zanation AM, Ebert CS, et al. Cholesterol granuloma of the petrous apex. Otolaryngol Clin North Am. 2011 Oct;44(5):1043-58.
  2. Hoa M, House JW, Linthicum FH, et al. Petrous apex cholesterol granuloma: pictorial review of radiological considerations in diagnosis and surgical histopathology. J Laryngol Otol. 2013 Apr;127(4):339-48
  3. Royer MC, Pensak ML. Cholesterol granulomas. Curr Opin Otolaryngol Head Neck Surg. 2007 Oct;15(5):319-22.
  4. Shrirao N, Mukherjee B, Krishnakumar S, et al. Cholesterol granuloma: a case series & review of literature. Graefes Arch Clin Exp Ophthalmol. 2016 Jan;254(1):185-8.

Answer Key

  1. The clear, needle-shaped crystals represent what type of material?
    A. Calcium pyrophosphate
    B. Cholesterol
    C. Hematoidin
    D. Hemosiderin
    E. Urate
  2. What is the most likely etiology of the mass?
    A. Chronic fungal sinus infection
    B. Epidermoid cyst
    C. Neoplasm of bony origin
    D. Obstruction of a nasal sinus with blood and inflammation
    E. Tophaceous gout
  3. What is a cholesteatoma of the middle ear?
    A. An abnormal skin growth in the middle ear behind the ear drum
    B. A benign proliferation of Schwann cells involving the middle ear
    C. An abnormality resulting in a squamous-lined cyst of the middle ear
    D. A deposit of uric acid crystals in the form of monosodium urate
    E. Uncommon benign hemorrhagic cyst arising in the petrous apex

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