This article presents the latest information on the presentation, diagnosis, imaging characteristics, management, and outcomes for petrous apex cholesterol granulomas. An in-depth review of the pathophysiology and surgical approaches is presented along with a summary of other petrous apex lesions and their imaging characteristics.
Key points
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Petrous apex cholesterol granuloma has a unique appearance on MRI that distinguishes it from other lesions. These lesions are hyperintense on both T1 and T2 weighted images.
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Asymptomatic petrous apex cholesterol granulomas can be managed with observation using serial imaging.
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A thorough headache history should be obtained to determine if this symptom is a result of the more common entities of migraine or muscle tension as opposed to a petrous apex cholesterol granuloma.
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Symptomatic lesions can be managed with marsupialization, with either an endonasal approach or a lateral transtemporal approach, depending on the individual’s anatomy.
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Symptom resolution or improvement is achieved in the vast majority of patients regardless of the surgical approach, and one should consider recurrent symptoms or enlargement of the cholesterol granuloma as opposed to lack of aeration in determining disease progression.
Introduction
Diagnosis and management of petrous apex lesions present unique challenges secondary to their centralized location and critical adjacent and in situ structures. The petrous apex forms the most medial aspect of the temporal bone and is defined laterally by the otic capsule, petrous carotid artery, and the semicanal of the tensor tympani muscle. The superior surface of the petrous apex is the floor of the middle fossa, and it extends from the arcuate eminence to Meckel cave. The posterior surface of the petrous bone 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 superior petrosal sinus defines the border between the middle and posterior fossa surface of the petrous apex. The internal auditory canal arbitrarily divides the petrous apex into anterior and posterior segments when viewed from above, which is important with respect to selecting surgical approaches. Petrous apex cholesterol granuloma is a non-neoplastic inflammatory lesion of the petrous apex that is often found incidentally on cranial imaging.
Introduction
Diagnosis and management of petrous apex lesions present unique challenges secondary to their centralized location and critical adjacent and in situ structures. The petrous apex forms the most medial aspect of the temporal bone and is defined laterally by the otic capsule, petrous carotid artery, and the semicanal of the tensor tympani muscle. The superior surface of the petrous apex is the floor of the middle fossa, and it extends from the arcuate eminence to Meckel cave. The posterior surface of the petrous bone 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 superior petrosal sinus defines the border between the middle and posterior fossa surface of the petrous apex. The internal auditory canal arbitrarily divides the petrous apex into anterior and posterior segments when viewed from above, which is important with respect to selecting surgical approaches. Petrous apex cholesterol granuloma is a non-neoplastic inflammatory lesion of the petrous apex that is often found incidentally on cranial imaging.
Epidemiology and pathophysiology
Asymmetric pneumatization and effusion are the most common entities identified on medical imaging of the petrous apex. It has been estimated that petrous apex effusions outnumber cholesterol granulomas 500 to 1. The incidence of petrous apex cholesterol granuloma has been estimated at 0.6 cases per 1 million population. Cholesterol granulomas are 10 times more common than petrous apex cholesteatomas.
Two hypotheses exist for the pathogenesis of cholesterol granuloma. Uniform to both hypotheses is that blood enters a mucosalized space, and the anaerobic breakdown products of blood, including cholesterol crystals, incite a foreign body giant cell reaction, thus resulting in cyst formation. The original theory proposes that negative pressure resulting from eustachian tube function is responsible for bleeding into a mucosalized space. Jackler and Cho proposed an alternative hypothesis, as middle ear hemorrhage and a hyperpneumatized temporal bone are rarely seen in patients with chronic eustachian tube dysfunction. Another argument by these same authors against the classic hypothesis is that once the air cell is filled with blood, pressure should be equalized, thus no further bleeding would occur, which is necessary for further expansion and enlargement of a cholesterol granuloma. Jackler and Cho offered an alternative exposed marrow hypothesis that cholesterol granulomas form when there is an osseous dehiscence between bone marrow and a pneumatized air cell. They supported this theory by demonstrating that 6 out of 13 patients with cholesterol granuloma indeed had an osseous dehiscence between bone marrow and an air cell in the contralateral petrous apex, whereas control patients with pneumatized petrous apices had no evidence of dehiscence. The exposed marrow theory has been supported by additional temporal bone histologic studies.
Presentation
Petrous apex pathology can present with a variety of symptoms, but is often incidentally discovered on imaging for unrelated symptoms. Headaches, hearing loss, ear pressure, and dizziness are common chief complaints from otolaryngology patients, and these complaints can also be seen in patients with petrous apex lesions. Most headache and petrous apex pathologies are not related to the lesion but are more commonly secondary muscle tension or migraines. Retro orbital pain and generalized temporoparietal headaches are sometimes seen in patients with a petrous apex lesion. Pain in the distribution of the ipsilateral trigeminal nerve in the presence of a petrous apex mass can be explained by compression or irritation of the cisternal segment or the Gausserian ganglion and is seen in approximately 20% of patients with cholesterol granuloma. Double vision is occasionally identified and results from compression of the sixth nerve in Dorello canal. Facial spasm/weakness results from irritation or compression of the seventh nerve in the internal auditory canal, cerebellopontine angle, or the fallopian canal and occurs in up to 20% of patients with petrous apex cholesterol granuloma. Sensorineural hearing loss, ear fullness, tinnitus (65%), and dizziness (50%) from compression of the eighth nerve or erosion into the otic capsule are seen with involvement of the internal auditory canal or lateral extension of petrous apex cholesterol granuloma. Dysphagia, hoarseness, and shoulder pain/weakness are rarely seen with aggressive malignancies originating in the petrous apex such as chordoma, chondrosarcoma, or metastatic disease. Unilateral serous otitis media is sometimes identified with petrous apex lesions, which compress or erode into the eustachian tube.
Evaluation and diagnosis
A complete history and physical examination are standard parts of the evaluation of any patient with complex skull base pathology. A thorough examination of the cranial nerves, with attention to ocular motility, facial function, hearing, balance, and facial sensation, is of paramount importance. Otoscopy can sometimes reveal a unilateral effusion or retrotympanic mass depending on the extent of the lesion. Tuning fork examination provides basic information on the presence of sensorineural or conductive hearing loss and confirms the results of audiometry. A head impulse test is a simple and rapid way to assess for a vestibular deficit involving the lateral semicircular canal.
Pure tone and speech audiometry provides objective information on the extent of middle ear and eighth nerve involvement. The audiogram is critically important when selecting a surgical approach in the event that management of the petrous apex lesion is required.
Radiographic studies are the single most important diagnostic modality. The differential diagnosis and radiographic features for petrous apex lesions include a variety of inflammatory and neoplastic conditions ( Table 1 ). Imaging also assists with determining a potential surgical approach in the event operative management is required. Computed tomography (CT) provides a detailed view of the osseous anatomy of the skull base. Structures readily defined by CT include the otic capsule, carotid canal, jugular foramen, fallopian canal, semicanal of the tensor tympani muscle, eustachian tube, and all of the foramina within the skull base. CT allows for delineation of a petrous apex lesion relative to adjacent structures within the temporal bone. Intralesional calcifications are also easily visualized on CT and can help narrow the differential diagnosis. Surgical access to a petrous apex lesion is often determined using CT and can also be used for image guidance at the time of the procedure. Petrous apex cholesterol granulomas demonstrate smooth erosion of the petrous apex. Erosion into the internal auditory canal, otic capsule, fallopian canal, petrous carotid canal, jugular foramen, or outside the confines of the petrous apex is readily demonstrated on CT ( Fig. 1 ). CT angiography/venography is sometimes necessary in cases in which the cholesterol granuloma has eroded into the carotid canal or jugular foramen with resultant vascular compression ( Fig. 2 ). CT angiography/venography provides critical information on surgical access relative to the distorted vascular anatomy. CT is also sometimes utilized to follow patients who are asymptomatic or who have undergone surgical management to monitor for recurrence or progression. Aeration of the petrous apex after surgical management has not been shown to correlate with recurrence.
| Lesion | MRI | CT | Other | |||
|---|---|---|---|---|---|---|
| T1 Without Gadolinium | T1 With Gadolinium | T1 Fat Saturated Gadolinium | T2 | |||
| Cholesterol granuloma | Hyperintense | No enhancement | No enhancement | Hyperintense | Smooth erosion | |
| Petrous apicitis | Hypointense | Rim enhancement | Rim enhancement | Hyperintense | Destroyed septae and possible cortex | |
| CSF cyst/cephalocele | Hypointense | No enhancement | No enhancement | Hyperintense | Smooth erosion | May connect to Meckel cave No restricted diffusion on diffusion weighted imaging |
| Epidermoid | Hypointense | No enhancement | No enhancement | Hyperintense | Smooth erosion | Restricted diffusion on diffusion weighted imaging |
| Effusion | Iso- to hypointense | Slight enhancement | Slight enhancement | Hyperintense | Intact septation | Hyperintense on FLAIR |
| Mucocele | Isointense | No enhancement | No enhancement | Hyperintense | Destroyed septae | |
| Asymmetric pneumatization | Hyperintense | No enhancement | Hypointense, no enhancement | Hypointense | Marrow on lesion side, air cells contralateral side | |
| Carotid aneurysm | New thrombus–hypointense, Old thrombus–hyperintense | Hyperintense | Smooth expansion of carotid canal, heterogeneous contrast enhancement | MRI–central flow void, onion skin appearance | ||
| Chordoma | Hypo- to isointense | Enhancement less intense than chondrosarcoma | Enhancement | Hyperintense | Lobulated, bone destruction with residual fragments | Centrally located in clivus with lateral spread to petrous apex |
| Chondrosarcoma | Hypo- to isointense | Enhancement | Enhancement | Hyperintense, heterogeneous | Infiltrative, remnants of eroded bone | Centered in petrous apex in region of foramen lacerum, calcified areas may show as signal voids |
| Metastasis | Depends on primary | Enhancement | Enhancement | Depends on primary | Bone erosion | |
| Paraganglioma | Isointense | Enhancement, flow voids | Enhancement, flow voids | Hyperintense | Opacified and destroyed air cells | Vascular blush on angio |
| Meningioma | Iso- to hypointense | Enhancement | Enhancement | Iso- to hyperintense | Hyperostosis, Iso- or hyperdense calcifications | Dural tails, sessile–broad interface with bone or tentorium |
| Schwannoma | Isointense | Enhancement | Enhancement | Hyper- or Hypointense | May show dilation of internal auditory canal | Centered over porus acousticus |
| Plasmacytoma | Isointense | Enhancement | Enhancement | Hypointense | Irregular to smooth bone erosion | Fluorodeoxygluocos PET enhancement |
| Vascular malformation | Intermediate | Enhancement | Enhancement | Heterogeneous hyperintense | Destruction of osseous septae | |
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