Benign Extracranial Skull Base Tumors

Juvenile Angiofibroma

Highly vascular and locally invasive, juvenile angiofibromas occur almost exclusively in adolescent males. Although morphologically benign, these tumors can exhibit aggressive local growth, extending along planes of least resistance or pre-formed pathways and, when large, can invade directly by bone erosion.

Juvenile angiofibromas represent less than 0.05% of all head and neck tumors, with a median age at presentation of around 13 years. It is thought the hormonal changes at puberty are the primary influence on the growth of this tumor. The site of origin is unclear but is likely to be from the sphenopalatine foramen. The tumor spreads laterally into the pterygopalatine fossa, thereby gaining access into the infratemporal fossa as well as the nasal cavity/paranasal sinuses and nasopharynx posteriorly, and the orbit superiorly. Intracranial spread into the anterior cranial fossa may occur, but more likely is invasion into the middle cranial fossa (although it usually remains extradural). Early symptoms of nasal obstruction and intermittent epistaxis are found in over 80% of patients. Nasendoscopy invariably confirms the presence of a mass obstructing the posterior nares. At diagnosis, approximately two thirds of patients have localized disease, and 20% have intracranial involvement [15]. Erosion of the medial pterygoid plate associated with enlargement of the sphenopalatine foramen is a constant feature on CT scan. Involvement of the sphenoid sinus, infratemporal fossa, orbit or middle cranial fossa may be demonstrated but is more easily demonstrated on MRI. The distinction between the mass and fluid in an obstructed sinus and the vascularity of the lesion (signal voids from the vessels within the tumor) can also be made on MRI.

There is little factual evidence to support a common belief that, with increasing age, angiofibromas will show spontaneous involution. It must be assumed that all untreated angiofibromas possess potential for aggressive growth.

Management usually involves surgery (rarely radiotherapy may be indicated in extremely extensive tumors where surgery may carry a high risk of operative mortality). In view of its vascularity, the majority of surgeons would seek the help of the interventional neurora-diologist, with pre-operative angiography and embolization. About 10% of cases recur after surgical resection (probably a reflection upon the difficulty of gaining good surgical access to this area).

Glomus Tumors (Paraganglioma) of the Temporal Bone

Paragangliomas are a group of histologically similar benign neoplasms that arise from neuroectodermally derived paraganglionic cells associated with autonomic ganglia. Two sites of these extra-adrenal paraganglia include:

• Intravagal - these are at the level of the jugular or nodose ganglion. Tumors arising here give rise to glomus vagale tumors.

• Jugulotympanic - these occur along Jacobson's nerve (tympanic branch of IX) and Arnold's nerve (tympanic branch of X), but the majority are found in the adventitia of the jugular bulb in the jugular fossa. Tumors arising from these sites give rise to glomus tympan-icum (arising within the tympanic, or middle ear, cavity) and glomus jugulare tumors (arising from the jugular bulb).

Glomus vagale tumors usually present as a parapharyngeal mass, with no obvious neurologic problems. Their involvement of the jugular foramen and possible spread along the internal carotid artery towards the cavernous sinus can make their management difficult.

Jugulotympanic paragangliomas usually present with a unilateral hearing loss and pulsatile tinnitus. More rarely, they present with neuropathy involving the lower cranial (IX,X, XI,XII) or facial nerves. On examination, a vascular mass is seen behind the tympanic membrane or, in larger tumors, the mass may have eroded though the drum and floor of the bony ear canal to present within the ear canal itself.

Multiple or synchronous tumors occur in 3-10% of patients [16], and there is a familial incidence with an autosomal dominant mode of transmission (these patients have a higher incidence of synchronous paragangliomas).

These tumors demonstrate a slow and insidious pattern of growth. They tend to migrate through the temporal bone by vascular channels, naturally occurring fissures and foramina and, most importantly, along air cell tracts. Intracranial extension into the posterior and middle fossae does occur and increases potential morbidity and mortality. Rarely, these tumors may metastasize, usually to cervical lymph nodes.

The clinical picture of jugulotympanic para-gangliomas is characteristic, but the clinical findings non-diagnostic. The mainstay of tumor diagnosis is radiological and the main diagnostic objectives are:

• Determine the site and extent of the tumor.

• Determine the presence of synchronous lesions.

• Determine the degree of major vascular involvement.

• Identify any intracranial extension.

• Determine central nervous system collateral circulation.

High-resolution CT scanning or MRI may assess the site and extent of tumor. MRI is superior in assessing intracranial extension and is useful for identifying synchronous tumors. Bilateral carotid angiography is used to determine involvement of the internal carotid artery (ICA) in larger tumors and the degree of collateral blood flow. It will also identify an aberrant ICA/intrapetrous carotid artery aneurysm (both are included in the differential diagnosis of a vascular lesion behind the tympanic membrane), as well as the rare case of anomalous venous drainage, where all intracranial venous return occurs via a single sigmoid sinus/internal jugular vein on the involved side (a contraindication to surgery). If performed pre-operatively, it also allows embolization of the lesion.

The management options for these tumors once again include:

• Conservative management.

• Radiotherapy (or possibly stereotactic radiosurgery in some cases). This may have a place as a primary form of treat

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