PETROUS APEX BENIGN NEOPLASMS AND SLOW-FLOW VASCULAR MALFORMATIONS
- Magnetic resonance imaging and computed tomography can almost always make a reasonably definitive diagnosis of a benign tumor of the petrous apex.
- Imaging is essential in initial treatment planning and following surveillance strategies depending on the extent of disease and approach to treatment.
Benign neoplasms of the petrous apex are uncommon. Slow-flow vascular malformations are even more uncommon but are considered in this chapter because their growth pattern most commonly resembles that of a benign or at least very low grade tumor. Vascular malformations and tumors essentially arise from the tissues of the region and must be differentiated from the relatively short list of mucosal, congenital, other vascular and malignant diseases that affect this part of the postero lateral portion of the central skull base. There is naturally considerable overlap with the same or similar conditions affecting the cavernous sinus that are discussed in Chapters 72 through 75. The most common of these discussed in this chapter are meningiomas and nerve sheath tumors. Chordomas (Chapter 34), chondrosarcomas (Chapter 39), and paragangliomas (Chapter 33) are discussed separately. Other benign tumors are rare and include lesions of mesenchymal and osseous origin such as fibromyxomas and osteochondromas and even pituitary tumors discussed in Chapters 32 and 35 through 40.
Prevalence and Epidemiology
These are mainly sporadic tumors, usually with no known predisposing conditions. Patients with neurofibromatosis types I and II and mosaics of those conditions are far more likely than the general population to have a nerve sheath tumor and are at increased risk for meningioma.
Facial, periorbital, and periocular pain or other mainly trigeminal- and abducens nerve–related dysfunction occurs when the mass involves the mesial aspect of the petrous apex at its interface with entrance of the trigeminal nerves to its cistern (Figs. 124.1 and 124.2). Dural involvement can produce localized headaches that most typically produce otalgia and/or are retro-orbital or referred to the skull vertex similar to the pattern of sphenoid sinus referral patterns. A mass situated more laterally in the internal auditory canal or at the first genu/geniculate ganglion may result in facial paralysis (Figs. 124.3 and 124.4). Hearing loss may be conductive due to eustachian tube obstruction or impingement on the ossicles (Figs. 124.2D and 124.4). Sensorineural hearing loss, vertigo, and/or tinnitus may be due to cochleovestibular nerve or direct inner ear involvement. Cranial nerve deficits tend be grouped as VII through XII and II through VI depending on the site of origin and extent of the mass, although the deficits are typically of only one or two nerves at the time of presentation—most commonly, cranial nerves V and VI (Figs. 124.1 and 124.4). Carotid artery occlusion can lead to signs and symptoms of ischemia but is rarely, if ever, associated with these lesions at presentation.
The anatomy of interest is that of the petrous apex (Figs. 104.15–104.17) and mastoid air cell tract development as those air cells populate the petrous apex, which is described in detail in Chapter 104 (Fig. 104.48). It is also useful to understand the relationship of cranial nerves V and VI to the petroclival fissure region and the petrous segment of the carotid canal (Fig. 112.2). Collateral knowledge about the entire temporal bone and related cranial nerves is also a general requisite for interpreting studies of patients with petrous apex masses.
Pathology and Patterns of Disease
The appearance, patterns of growth, and pathophysiology of benign tumors in general are discussed in Chapter 21; those of nerve sheath tumors and meningiomas are discussed more specifically in Chapters 29 and 31, respectively. Slow-flow vascular lesions (Fig. 124.5), commonly referred to as hemangiomas when they affect the temporal bone, are discussed in Chapter 9.
These solid and often homogenously well-vascularized benign tumors may compress cranial nerves V, VI, VII, and VIII and become adherent to major vessels including the dural sinuses, carotid artery, and jugular vein (Figs. 124.1 and 124.2). Involvement of these transiting structures may be associated with obvious remodeling of the internal auditory canal, carotid canal, and jugular fossa. The otic capsule bony and membranous labyrinth may eventually be involved (Figs. 124.3 and 124.4). Petroclival meningiomas arise from the arachnoid villi of the clivus or the mesial petrous apex. Meningiomas may be largely intraosseous; however, most grow in an exophytic manner along the dural surfaces (Figs. 124.1 and 124.3).
Schwannomas of the petrous apex typically arise from cranial nerves V, VII, and VIII (Figs. 124.2 and 124.4). Sometimes, large schwannomas from the lower cranial nerves may grow into the apex.
Slow-flow vascular malformations will generally have a narrow zone of transition in bone with the telling sign of a stippled or lacy calcification pattern within the interstices. The magnetic resonance imaging (MRI) pattern is predictable but nonspecific (Fig. 124.5).
Pathologically Altered Function
Typically, there are no deficits created early in the pathogenesis of these chronically progressive tumors. At some point, the bony eustachian tube may be compressed with resulting middle ear disease, most commonly otitis media with effusion. Extensive lesions can cause compressive cranial neuropathies and inner ear dysfunction.
Techniques and Relevant Aspects
Specific protocols for computed tomography (CT) and magnetic resonance (MR) studies for investigating petrous apex masses appear in Appendixes A and B. These are typically studied with dedicated contrast-enhanced imaging of the temporal bone. Computed tomographic angiography and MR angiography are included as necessary. Diagnostic catheter angiography is commonly done as a prelude to endovascular interventions or occasionally to confirm the diagnosis.
Pros and Cons
MRI provides a definitive diagnosis when typical morphologic features and growth patterns of the mass are expressed on various pulse sequences. Differentiation from all other petrous apex pathology is relatively simple based on the MRI findings (Figs. 124.1–124.5). MRI is typically preferred for posttreatment surveillance.