A cholesterol granuloma is a slowly expanding cystic mass that develops as a chronic inflammatory response to cholesterol crystals within body tissue. This granuloma is essentially a foreign body reaction where the immune system attempts to wall off and contain an irritating substance. The mass consists of a fibrous capsule surrounding a thick, yellowish-brown fluid rich in lipids, cholesterol crystals, and blood breakdown products. While these cysts can occur in various parts of the body, they are most frequently encountered in the temporal bone of the skull, especially near the middle ear.
How Cholesterol Granulomas Develop
The formation of a cholesterol granuloma begins with localized bleeding into an area with poor drainage and aeration, such as the air cells of the temporal bone. This hemorrhage is often triggered by chronic inflammation, infection, or trauma. The trapped blood cannot be cleared effectively and begins to break down in the low-oxygen environment.
As red blood cells degrade, the hemoglobin releases cholesterol and other lipid byproducts. These molecules crystallize, forming sharp, irritating structures. The immune system sends specialized cells, such as macrophages, to engulf the crystals, but the cholesterol is resistant to absorption.
This chronic inflammation leads to the formation of granulation tissue, which encapsulates the fluid and crystals. Fragile new blood vessels often form within this capsule, which are prone to rupturing. This causes recurrent bleeding, fueling a cycle of expansion that creates an ever-growing mass and exerts pressure on surrounding structures.
Where They Occur and Associated Symptoms
Cholesterol granulomas most commonly occur in the petrous apex, a portion of the temporal bone deep within the skull base. They can also be found in the middle ear and mastoid air cells. When located in the petrous apex, they are concerning due to the proximity of cranial nerves and inner ear structures. Symptoms arise purely from the slow, expansile growth causing mass effect and bone erosion.
The specific symptoms depend on which nearby structures are compressed. A granuloma in the petrous apex may press on nerves responsible for balance and hearing, leading to hearing loss, ringing in the ears (tinnitus), dizziness, or vertigo. Pressure on other cranial nerves can cause facial pain, numbness, or muscle weakness.
If the granuloma is located in the middle ear space, the main symptom is typically a slowly progressive conductive hearing loss. Patients may also experience a dark, bluish discoloration of the eardrum, sometimes called a “blue eardrum,” caused by the cyst’s fluid showing through the membrane. Headaches or double vision (diplopia) can occur with larger lesions.
Identifying the Granuloma Through Imaging
Physicians use medical imaging to confirm the presence of a cholesterol granuloma and distinguish it from other skull base masses. Both Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) visualize the lesion and its effects on surrounding bone and soft tissue.
CT scans evaluate bony structures, typically showing an expansile lesion that has caused erosion or thinning of the adjacent bone. MRI is often the preferred modality for characterizing the mass due to its unique signal characteristics. A classic cholesterol granuloma appears with high signal intensity on both T1-weighted and T2-weighted MRI sequences.
This distinctive “bright on both T1 and T2” appearance is caused by high-protein fluid, methemoglobin from blood breakdown, and cholesterol crystals within the cyst. This consistent imaging signature helps differentiate the granuloma from other lesions, such as a cholesteatoma or a tumor. The lesion typically does not show significant enhancement after contrast injection.
Treating Cholesterol Granulomas
The approach to treatment is guided by the presence and severity of symptoms. For small lesions causing no symptoms, “watchful waiting” is adopted, involving periodic imaging to monitor the size and stability of the mass.
Surgical intervention becomes necessary when the patient experiences significant symptoms, such as progressive hearing loss, neurological deficits, or severe pain. The goal of surgery is not complete removal of the cyst wall, which is difficult and risky, but rather to drain the thick, chocolate-brown contents and establish a permanent pathway for drainage and aeration.
Creating a continuous drainage route prevents the fluid from re-accumulating and the mass from expanding. Various surgical approaches are used, often involving minimally invasive techniques like the endoscopic endonasal approach, which accesses the petrous apex through the nasal cavity. Maintaining the patency of the drainage pathway is key to long-term success, ensuring continuous aeration and preventing recurrence due to scar tissue blockage.