The petrous apex is an area of the temporal bone of the skull. It is located about two or three inches in from the outer ear. This location makes the petrous apex very difficult to reach when medical intervention is required, which occurs if the area develops a lesion, such as a cyst, or a tumor.
The types of cysts that are most likely to occur in the petrous apex are usually fluid-filled. They may be petrous apex cholesterol granulomas or cholesteatomas, also called epidermoids. There are other types of cysts and lesions that occur less frequently.
Causes of Petrous Apex Lesions
No one is really sure what causes petrous apex lesions, although there are a few theories. The cholesterol granulomas may be due to air cells infiltrating the bone marrow. If these air pockets become filled with blood, it may stimulate the formation of a cyst. Cholesterol granulomas may also be related to a dysfunction of the eustachian tube in ways that are not entirely clear.
The cause of petrous apex cholesteatomas is more certain. They may either be present at birth or may develop during the patient’s life. If the lesion is congenital, it likely results from skin cells that are left in the area during embryonic development. Because the cells do not belong in the brain, they are encapsulated into a cholesteatoma, or skin cyst. This cyst may eventually grow and impinge on neighboring structures.
If a cholesteatoma is not congenital, it is called “acquired.” These lesions are the result of some type of malfunction of the eustachian tube. These cysts may be very large, reaching from the eardrum inward to the petrous apex.
Symptoms of Petrous Apex Lesions
Many people with petrous apex lesions will have no symptoms at all. Problems usually occur when the cysts begin to grow and press against other structures in the area. These structures may include the ear, eye and face.
A lesion may impact the inner ear itself or the nerves that communicate with it. Lesions can also affect the facial nerves, causing changes in the sensations that a patient feels in the face. It is also common for the abducens nerve to be affected. This is the nerve that is responsible for moving the eyeball within its socket.
A petrous apex lesion is usually diagnosed with an initial examination followed by an imaging tool. Imaging may be done with a CT or MRI scan of the temporal bone of the skull.
Treatment of Petrous Apex Lesions
Petrous apex cholesterol granulomas are fluid-filled, so the primary treatment is to drain the cyst. There are six different avenues that a physician may take to reach this area of the brain, but the most commonly used is called the infracochlear approach. This procedure drains the fluid at a point below the cochlea of the inner ear.
In contrast, petrous apex cholesteatomas must be fully removed. Some procedures approach the area through the inner ear, but sometimes it is also necessary to go through the middle fossa, which is below the brain.