Incidental petrous apex findings on magnetic resonance imaging.

Abstract

We performed a retrospective chart review to categorize a group of petrous apex findings that were noted incidentally on magnetic resonance imaging (MRI) in 88 patients. These patients were among those who had been seen at a tertiary care center between July 1988 and July 1998. These incidental findings, which were unrelated to the presenting clinical manifestations, included asymmetric fatty bone marrow (n = 41), inflammation (19), cholesterol granulomas (14), cholesteatomas (9), and neoplasms (5). Followup imaging and clinical surveillance of these patients has not demonstrated any significant change in the incidentally detected lesions. In all cases, the incidental MRI findings represented benign pathology.

Introduction

The petrous apex can be the site of a variety of pathologic conditions. The nature of these conditions ranges from inflammatory to neoplastic, and lesions can arise from the petrous bone itself or from adjacent neurovascular structures. [1] An accurate diagnosis is dependent on the patient's history, physical examination, and findings on magnetic resonance imaging (MRI) and computed tomography (CT). The imaging features are key, and the need for surgical intervention should be based on the correlation of imaging and clinical findings.

The purpose of our study was to categorize a group of petrous apex radiographic findings that were incidentally noted on MRI in patients who were undergoing diagnostic testing for a variety of head and neck symptoms.

Materials and methods

We identified 88 patients--52 females and 36 males, aged 16 to 71 years (mean: 39.8)--with petrous apex lesions that had been incidentally noted on MRI at our institution between July 1988 and July 1998. From a retrospective review of patient records and other reports, we obtained information on the indications for each MRI, the incidental finding, and the long-term outcome. Histologic data were also obtained on the few patients who required surgical intervention (n = 8) or biopsy (1).

Indications for MRI (signs and symptoms). Seventy-five of the 88 patients had no presenting signs (table). Among those who did, the most common were trismus and decreased facial sensitivity, which altogether were observed in only 10 patients. The most common presenting symptoms were headache and facial pain (table). The distribution of these signs and symptoms was in no way related to the distribution of the incidental findings on MRI.

incidental findings. Forty-one patients were noted to have an asymmetric enhancement of bone marrow in the petrous apex, and 19 patients demonstrated asymmetric inflammation. Fourteen had small cholesterol granulomas, 9 had asymptomatic cholesteatomas, and 5 had neoplasms (2 meningiomas, 2 trigeminal nerve neuromas, and 1 osteoblastoma).

Outcomes. Eight of the 88 patients had their lesions surgically excised (4 cholesteatomas, 2 cholesterol granulomas, and 2 neoplasms). The surgical approaches used to achieve skull base access to the petrous apex in the 8 operated patients were an orbitocraniozygomatic approach in 4 patients, a middle fossa approach in 2, and a transmastoid approach in 2.

The MRI surveillance protocol for the 80 patients who had not undergone surgery (1 of these patients did undergo a CT-guided biopsy) included repeat studies at 1-, 3-, and 5-year intervals.

All 41 patients who had asymmetric bone marrow in the petrous apex showed no change on imaging overtime. Of the 19 patients with inflammatory changes, 2 exhibited an increase in signal intensity overtime, 6 had a decrease in inflammation, and 11 were unchanged. Of the 12 patients with cholesterol granulomas, 4 showed an increase in the size of their lesions, none had a decrease in size, and 8 were unchanged. Of the 5 patients with cholesteatomas, 2 showed an increase and 3 were unchanged. Finally, all 3 of the neoplasms followed with MRI (which included selected coronal cuts with gadolinium enhancement) showed a slight increase in size.

In all of these 80 patients, followup MRI found increases in 11, decreases in 6, and no change in 63. None of the 11 increases was significant enough to warrant surgical intervention.

Discussion

The petrous portion of the temporal bone is a pyramid-shaped structure that extends medially and anteriorly toward the central skull base. The core of the petrous bone houses the labyrinth, cranial nerves VII and VIII, the internal carotid artery, and the jugular bulb. The petrous apex varies in its degree of pneumatization, but it includes the peritubal and apical air cell tracks. [2] The peritubal region surrounds the eustachian tube and is anterolateral to the carotid canal. The apical area is posteromedial to the carotid canal. The degree of petrous apex pneumatization is often asymmetric, and the presence of air cells in this region can predispose the petrous apex to inflammatory diseases.

The clinical manifestations of petrous apex disease can be subtle or nonexistent. Advanced lesions in this area can cause a mass effect, bone erosion, or inflammation with a resultant dysfunction of the adjacent cranial nerves III through VIII. Patients might report headache, eye pain, otalgia, diplopia, facial twitching, unsteadiness, hearing loss, or otorrhea. [3] Small lesions of the petrous apex are usually asymptomatic.

Diagnostic imaging. Specific petrous apex pathology can be diagnosed only with appropriate diagnostic imaging. CT is ideal for evaluating bony anatomy and bone destruction, whereas MRI is more specific for evaluating soft tissue and neurovascular anatomy. [4] MRI with intravenous gadolinium enhancement is also important to identify the presence and extent of neoplasms in this region (figure 1). MRI can differentiate lesions based on the various signal intensities of the different weighted images. Fast spin-echo techniques can add resolution to MRIs, and fat-suppression techniques distinguish adipose tissue from pathology. [5]

Pathology. A variety of pathologic processes can involve the petrous apex. They can be classified as an intrinsic lesion or as a secondary invasion from an adjacent neurovascular structure. Inflammation of the petrous apex can be caused by apical air cell effusion or by the more advanced skull base osteomyelitis. CT findings in the case of inflammation generally include opacification of air cells, while MRI signals are hyperintense on T1-weighted (T1W) and T2-weighted (T2W) images (figure 2). Simple fluid effusions can become infected and lead to bacterial petrous apicitis, osteomyelitis, or chronic obstruction, which in turn can lead to the formation of petrous apex mucoceles.

Petrous apex cholesteatoma, which is believed to arise from congenital epithelial rests, can lead to the accumulation of epithelial cells. [6] A cholesterol granuloma, on the other hand, is caused by petrous apex mucosal hemorrhage, obstruction, and inflammation, which can lead to the formation of a thick, fluid-filled, hemorrhagic cyst with a surrounding fibrous capsule. [7] Both cholesteatomas and cholesterol granulomas can remain clinically silent for several years. While CT shows bone erosion in both lesions, the MRI features are more specific. On MRI, cholesteatomas are hypointense with respect to brain tissue on T1W images and hyperintense on T2W images (figure 3). Cholesterol granulomas are typically hyperintense on both T1W and T2W images (figure 4). Surgical extirpation is the recommended treatment for petrous apex cholesteatomas, while surgical excision or surgical drainage with internal stenting is often recommended for the management of cholesterol granulomas.

The most common vascular lesions of the petrous apex include aneurysms, arteriovenous malformations, and carotid cavernous fistulae. [8] All of these lesions can be identified on contrast-enhanced CT, MRI, or magnetic resonance angiography. However, the most accurate diagnosis and possible endovascular treatment might involve interventional angiography.

Primary neoplasms of the petrous apex include meningiomas, trigeminal neuromas, chordomas, and chondrosarcomas. [9] Arachnoidal cysts, which are not true neoplasms, can also involve the petrous apex and cause symptoms that mimic neoplasms in this region. Metastatic petrous apex neoplasms are extremely rare, but some have been reported to have spread from distant sites such as the breast, lung, prostate, thyroid, and kidney. [10]

Fibrous dysplasia can involve the petrous apex as well as other portions of the temporal bone. [11] On CT, a finding compatible with fibrous dysplasia has the characteristic "ground glass" appearance. Perhaps the most common confounding element in diagnosing pathology in the petrous apex by CT or MRI is asymmetry of the bone marrow because it can mimic several of the lesions discussed above (figure 5).

Acknowledgment

The authors thank Amy Donohue for the critique and preparation of this manuscript.

References

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(6.) Smith PG, Leonetti JP, Kletzker GR. Differential clinical and radiographic features of cholesterol granulomas and cholesteatomas of the petrous apex. Ann Otol Rhinol Laryngol 1988;97:599-604.

(7.) Gherini SG, Brackmann DE, Lo WW, Solti-Bohman LF. Cholesterol granuloma of the petrous apex. Laryngoscope 1985;95:659-64.

(8.) Pisaneschi MJ, Mafee MF, Samii M. Applications of MR angiography in head and neck pathology. Otolaryngol Clin North Am 1995;28:543-61.

(9.) Weber AL, Brown EW, Hug EB, Liebsch Cartilaginous tumors and chordomas of the cranial base. Otolaryngol Clin North Am 1995;28:453-71.

(10.) Nelson EG, Hinojosa R. Histopathology of metastatic temporal bone tumors. Arch Otolaryngol Head Neck Surg 199l;1l7:189-93.

(11.) Megerian CA, Sofferman RA, McKenna MJ, et al, Fibrous dysplasia of the temporal bone: Ten new cases demonstrating the spectrum of otologic sequelae. Am J Otol 1995;16:408-19.

                        Original indications for
                         MRI in the 88 patients
Signs                 n  Symptoms         n [*]
None                 75  Headache            61
Trismus               5  Facial pain         15
Decreased facial      5  Orbital pain         7
 sensitivity             Tinnitus             4
Serous effusion       1  Facial numbness      1
Abducens palsy        1  Hearing loss         1
Nasopharyngeal mass   1  Doplopia             1
(*.) One patient had both facial pain
and headache, and another had both
facial and orbital pain.