|Artist rendition of normal inner ear. The petrous apex is the mos middle part of the temporal bone.|
Cholesterol granulomas of the petrous bone are tumor like growths, made of inflammatory cells. The term is actually a pathological one, but generally it is applied to MRI images (where there is no pathology). So basically it occurs when radiologists guess about the pathology of something they see on MRI. When tissue is obtained, there may be a foreign body, a giant cell reaction to cholesterol, with fibrosis and round cells. They are very similar to congenital cholesteatomas in both their location as well as management, but they are more benign than cholesteatomas as well as much more common.
The petrous apex is the most medial (middle) part of the temporal bone and is defined laterally by the otic capsule, the petrous carotid artery, and the canal of the tensor tympani muscle. THe superior surface of the petrous apex is the floor of the middle fossa. The posterior surface extends from the endolymphatic duct operculum to the clivus. The jugular fossa, vertical petrous carotid canal and inferior petrosal sinus make up the inferior border of the petrous bone.
The incidence of petrous apex cholesterol granuloma has been estimated at 0.6 cases per 1 million population. Thus these lesions are exceedingly rare. Cholesterol granulomas are 10 times more common than petrous apex cholesteatomas (Isaacson, 2015). On the other hand, it has been estimate that petrous apex effusions outnumber cholesterol granulomas 500 to 1.
The terminology has changed through the years. In the past, these were called "congenital epidermoid cysts".
The theory regarding their formation suggests that there is poor ventilation of a normally pneumatized space in the temporal bone. The cholesterol is postulated to be related to local tissue breakdown. This is difficult to follow. Jackler and Cho offered an alternative-- the exposed marrow hypothesis. They suggested that cholesterol granulomas form when there is an osseous dehiscence between bone marrow and a pneumatized air cell. (Jackler and Cho, 2003)
These lesions are not common in children (Radhakrishnan et al, 2014), who comments that "Differential considerations of petrous apex lesions in children include "leave me alone" lesions, infectious or inflammatory lesions, fibro-osseous lesions, neoplasms and neoplasm-like lesions, as well as a few rare miscellaneous conditions".
There are a large number of alternative causes of petrous apex lesions, which are discussed below under the diagnosis section.
Presenting symptoms commonly include headache, diplopia, ear pressure, vertigo or dizziness, and occasionally hearing loss and/or facial weakness. About half of these patients have a history of previous trauma to the ear (e.g. surgery).
Double vision has been reported (Roemer et al, 2017), but is not very common. Gradinego's syndrome (petrous apicitis) has also been reported (Lattanzi et la, 2015)
Pain in the distribution of the ipsilateral trigeminal nerve is seen in about 20% of patients with cholesterol granuloma, and is attributed to compression or irritation of the cisternal segment of the Gasserian (Trigeminal) ganglion. (Isaacson, 2015)
Damage, if it occurs, is generally due to erosion and expansion. This generally occurs gradually over years.
Dizziness and/or hearing loss may occur if the cholesterol granuloma erodes into the inner ear.
Lesion just below the inner ear attributed by radiology to cholesterol granuloma of the petrous apex. Symptoms include headache. This lesion is high signal on T1 and T2.
Axial MRI showing right petrous apex lesion, anterior and mesial (towards the middle) to the inner ear. The arrow shows the high signal on T2. On the left is T1, which is low signal. This is not a typical cholesterol granuloma which is high signal on both T1 and T2.
Diagnosis is usually from the MRI and CT scan of the temporal bone (Dinh et al, 2016). Usually it is discovered on the MRI, and a temporal bone CT scan follows.
The classic signs of cholesterol granulomas on CT scan are a smoothly marginated expansile mass that is nonenhancing and isodense with brain. It is not wise to do too many CT scans of the temporal bone, as they involve high radiation.
On MRI they are hyperintense on both T1 and T2, and should not exhibit a change on gadolinium enhancement.
Differentiating cholesterol granulomas from congenital cholesteatoma's can be tricky -- perhaps sometimes impossible. On T1 the granuloma should "light up" from cholesterol, but be lower signal intensity for the congenital cholesteatoma. However, there is overlap in imaging characteristics (Dinh et al, 2016), and ultimately one may find out at the time of surgery.
Issacson (2015) provided a differential diagnosis of petrous apex lesions, based on imaging characteristics. The table below is abstracted from his table:
Differential diagnosis of Petrous Apex Lesions, abstracted from Issacson (2015) Lesion T1 wo gad T2 CT Other Comment Cholesterol Granuloma Hyperintense Hyperintense Smooth erosion Petrous apicitis Hypointense Hyperintense Destroyed septae CSF cyst Hypointense Hyperintense Smooth Erosion No restricted diffusion on DWI Epidermoid Hypointense Hyperintense Smooth Erosion Restricted Diffusion Effusion Iso to Hypointense Hyperintense Intact septation Hyperintense on Flair Effusions outnumber Cholesterol granulomas 500:1 Mucocele Iso Hyperintense Destroyed septae Asymmetric pneum. Hyperintense Hypointense Marrow on lesion side Carotid aneurysm Either hypo or hyper Hyperintense Smooth expansion of carotid canal MRI central flow void, onion skin appearance. Chordoma Hypo to iso Hyperintense Lobulated, bone destruction Generally starts in clivus Chondrosarcoma Hypo to iso Hyperintense Infiltrative Centered in petrous apex Metastasis Depends on tumor Depends on primary Bone erosion Unlikely place for a metastasis Paraganglioma Iso Hyper Opacified Vascular blush on angio Meningioma Iso to hypo Iso to hyper Hyperostosis Dural tail Schwannoma Iso Hyper or hypo May show dilation of IAC Centered over IAC Also see intralabyrinthine schwannoma and Neurofibromatosis Plasmacytoma Iso Hypo Irregular to smooth bone FDP-PET enhancement Vascular malformation Intermediate Heterogeneous hyper Destruction of osseous septae.
Radiology is not the same as pathology -- the "diagnosis" from radiology is rarely certain. In order to come to a relatively firm conclusion, one needs MRI, CT, and some sort of contrast procedure. Frequently there are follow-up MRI scans.
The main two management options are surgery and watchful waiting (Sweeney et al, 2015) . The goal of surgery is removal of as much as possible and providing it with permanent aeration. Watchful waiting is currently advised when there is no growth. Surgery usually requires wide exposure and often necessiates a craniotomy with associated brain retraction. This can result in encephalomalacia, seizures, intracranial hemorrhage, and CSF leakage. (Isaacson, 2015)
Considerable literature recently has reported endoscopic approaches to these masses. As often they are not malignant, this seems somewhat reasonable.