A cholesterol granuloma is a benign mass that develops from a long-term inflammatory reaction. This cyst-like formation is a collection of fluid, lipids, and cholesterol crystals surrounded by a fibrous wall. While they can occur anywhere, they are clinically significant when they arise in the confined spaces of the skull base. Their slow expansion in this area can affect surrounding nerves and bone.
How the Granuloma Forms
The process begins with blood products or exposed bone marrow in an enclosed space, often triggered by trauma, infection, or poor ventilation. A common theory suggests that an obstruction, such as mucosal swelling, blocks the air cells. This causes the remaining gas to be resorbed, creating a vacuum that ruptures local blood vessels and draws blood into the air cell.
The trapped blood then breaks down, releasing its contents, including hemoglobin. As red blood cells degrade, the cholesterol within the hemoglobin is released, forming sharp, irritating cholesterol crystals. The immune system recognizes these crystals as foreign material and mounts a chronic inflammatory response.
This response involves immune cells like macrophages and foreign body giant cells attempting to engulf the cholesterol crystals. Cholesterol is difficult for these cells to absorb, which perpetuates the inflammation. The ongoing inflammation causes nearby fragile blood vessels to rupture, leading to recurrent hemorrhaging and further expansion of the mass. This cycle of bleeding, breakdown, and inflammatory reaction results in the formation of the cholesterol granuloma, encased in a fibrous capsule.
Primary Locations of Development
Cholesterol granulomas are most frequently found in the temporal bone. The most clinically relevant site is the petrous apex, the pyramid-shaped tip of the temporal bone located near the middle ear. This area is predisposed to granuloma formation because it contains air cells that can easily become obstructed and poorly ventilated.
Another common location is the middle ear cavity, sometimes referred to as “chocolate ear” due to the dark, thick fluid it contains. Less frequently, they can occur in the mastoid air cells, orbit, or sinuses. The location is important because the surrounding anatomy dictates the type and severity of symptoms that will eventually appear.
Identifying the Symptoms
The symptoms are directly related to the granuloma’s size and pressure on neighboring structures. Since they grow slowly, the resulting mass effect can erode bone and compress delicate nerves. If the granuloma is located in the petrous apex, it can affect the nearby cranial nerves.
This compression often causes facial numbness or pain due to pressure on the trigeminal nerve. Pressure on the nerves controlling eye movement can lead to double vision. Patients may also experience vague or localized headaches, often felt behind the eye. When the mass is centered in the middle ear, the primary complaint is typically a slowly progressive conductive hearing loss.
Management and Treatment
Diagnosis begins with specialized imaging. Magnetic Resonance Imaging (MRI) is often used because the cholesterol and protein content of the cyst gives it a unique, bright signal on T1- and T2-weighted scans, helping distinguish it from other lesions. A Computed Tomography (CT) scan is also used to assess the extent of bone erosion and plan the surgical approach.
Small lesions that are not causing any symptoms are typically managed with a strategy of watchful waiting. This involves monitoring the granuloma’s size with serial MRI or CT scans to ensure it is not expanding rapidly. For lesions that are symptomatic or show signs of aggressive growth, surgical intervention becomes necessary.
The main goal of surgery is to drain the fluid and establish a permanent drainage pathway to prevent recurrence. Because the petrous apex is a challenging area to access deep within the skull base, the surgical approach is tailored to the granuloma’s exact location and the patient’s existing hearing status. Surgeons may use minimally invasive techniques, such as an endoscopic approach through the nose, to access and decompress the cyst.