meningioma to both optic canals or of a unilateral secondary ONSM across the planum to the contralateral side. Similar to other meningiomas, all types of ONSMs are associated with neurofibromatosis type 2. The presenting feature is a slowly progressive optic neuropathy and variable visual loss, disturbances of color vision, visual field defects, pain, double vision, and transient visual obscuration, which is almost always associated with optic disc swelling [38]. The triad of visual loss, optic atrophy, and optociliary shunts is almost pathognomonic for ONSM, though this may occur late in the course [39].

Both CT and MRI with contrast are useful for imaging of meningioma. Precontrast CT is superior in detecting calcification within the tumor, which is said to indicate slow growth and adjacent hyperostotic bone changes [40, 41]. MRI is better for detecting intracanalicular ONSM because of the absence of bone signal [41]. MRI is superior in determining if cavernous sinus meningiomas are entering the orbit through the superior orbital fissue/orbital apex. MRI is superior than CAT scan in determining if a optic nerve meningioma is primary or secondary because of the finer detail that can be seen with MRI in the region of the planum, anterior clinioid, optic canal, orbital apex region.

30.4.5 Sinonasal and Nasopharyngeal Tumors

Tumors that involve the paranasal sinuses, nasal cavity, and nasopharynx are often clinically silent at early stages and, thus, at diagnosis may involve a wide area of the adjacent skull base vital structures. Paranasal sinus tumors are most frequently located in the maxillary sinuses, whereas nasal cavity tumors most often arise from the nasal septum [42]. Maxillary sinus spreads through the roof into the inferior orbit, through the posterior wall into the pterygopalatine fossa, and upward through the inferior orbital fissure to the orbital apex or posterolaterally into the infratemporal fossa. Tumors in the ethmoid sinuses can spread laterally into the medial orbit through the thin lamina papyracea. Frontal sinuses antero-superiorly and the sphenoid sinus postero-superiorly have only a bony wall separating them from the orbit, and this bony wall can be breached by tumor spread. Sphenoid body tumors can compress the optic nerve in the canal during expansion in a medial direction.

Nasopharyngeal carcinoma (NPC) is usually a squamous cell carcinoma that arises from the epithelium and, in some populations, is related to Epstein–Barr virus. NPC frequently arises behind the medial crura of the eustachian tube opening (Rosenmueller fossa) in the nasopharynx and may extend laterally into the parapharyngeal space through the sinus of Morgagni, a natural defect in the lateral wall. The nasopharynx is surrounded posteriorly and laterally by a tough pharyngobasilar fascia that can occasionally be invaded by NPC to involve the clivus. The skull base can be destroyed by superior extension of NPC into the sphenoid sinus or cavernous sinus or lateral extension into the foramen ovale and/or foramen lacerum [42].

Sinonasal tumors and NPC often present with vague symptoms, and affected patients are frequently treated for sinusitis or other maladies before a suspicion of

Fig. 30.2 Antegrade perineural spread tracking anteriorly from the Meckel cave through the cavernous sinus en route to the orbit

CT and MRI are complimentary for the diagnosis of sinonasal and NPC. Skull base erosion is more easily appreciated on CT scan, while perineural spread is better delineated on MRI, which is also more effective in monitoring recurrence. Positron emission tomography is used to detect distant metastasis [42].

30.4.6 Schwannoma

Schwannomas are benign tumors of the cranial nerves and account for 8% of primary brain tumors. Schwannomas can arise from any of the cranial nerves except the optic nerves, which lack Schwann cells, and rarely in the olfactory nerve (see below). Schwannomas arise most commonly from the vestibular division of the eighth nerve, followed by the trigeminal nerve sheath, facial nerves, and lower cranial nerves [43]. Schwannomas are more common in females in most anatomic locations. Vestibular tumors are usually isolated, but approximately 5% are associated with neurofibromatosis, mainly type 2. Patients with neurofibromatosis type 2 have bilateral tumors in more than 90% of cases [43]. Trigeminal schwannomas are rare, accounting for only 2–3% of intracranial schwannomas [44]. Trigeminal schwannomas can arise from the trigeminal nerve root, gasserian ganglion, or one of the three peripheral branches and may have a complicated pattern in the middle cranial fossae. They have also been found in the infratemporal fossa and orbit [45]. Though rare, a few cases of olfactory groove schwannomas have been reported in

young males. The origin of these tumors is not known, but theories have pointed to embryonic terminal nerves, peripheral nerves in close proximity, such as the anterior ethmoidal nerve, or ectopic foci of Schwann cells [44]. Vidian nerve schwannoma is very rare and located in the retromaxillary space. It can grow silently and become extremely large by the time symptoms present and the diagnosis is made [46].

Jugular foramen schwannomas can arise from any of the lower cranial nerves and pass through the jugular foramen and identification of the involved nerve is often difficult. The tumor may be predominantly intracranial, predominantly extracranial, both intracranial and extracranial, or located mainly in the canal [43]. In a study of 53 patients with jugular foramen schwannomas, the most common deficits observed were deficits of the vagus nerve, followed by deficits of the vestibular/cochlear nerve and glossopharyngeal nerve. In vestibular schwannomas as well as other prepontine schwannomas, the lesions grow toward the cerebello-pontine angle, and the facial–acoustic complex is affected early, manifesting as hearing loss that may be followed by hoarseness and absence of the gag reflex [47]. On examination, absent corneal reflex, nystagmus, sensorineural deafness, and cerebellar signs, such as ataxia and pyramidal tract involvement in the form of hemiparesis, may be seen due to distortion of the brain stem. Patients rarely display facial weakness even when a schwannoma is closely associated with the seventh nerve [43].

Motor nerve schwannomas of the third, fourth, sixth, and twelfth cranial nerves are rare and have a long duration of symptoms [48]. Patients may present with headache, nausea, and vomiting due to raised intracranial pressure and diplopia. Neuro-ophthalmic examination may reveal anisocoria, ptosis, and papilledema. Other complaints may include facial numbness or burning pain, dysphagia and dysarthria, tinnitus, numbness of the tongue, and taste disturbance due to involvement of lower cranial nerves. These symptoms may be accompanied by findings of hemiparesis, hemianesthesia, dysmetria, and ataxia, indicating long tract as well as cerebellar compression [49]. Occasionally, pathological laughter and aggressive behavior have been described with fourth cranial nerve palsy [48, 50, 51]. In the case of lesions traversing the base of skull foramina, CT may reveal enlargement of foramina, and T1-weighted MRI may show isointense lesions with dense enhancement after contrast administration [52].

30.4.7 Pituitary Tumors

Pituitary tumors are usually benign neoplasms located in the sella turcica and are the third most common primary intracranial neoplasm, accounting for 9.5% of primary intracranial neoplasms (Central Brain Tumor Registry of the United States, 2007). Nonfunctional pituitary adenomas occur more frequently than hormone-secreting adenomas, namely, prolactin-secreting and growth hormone-secreting adenomas. Compared with their hormone-secreting counterparts, the nonfunctional pituitary adenomas go undetected for a longer time since there are no alerting symptoms and grow to larger sizes before becoming symptomatic due to compression of normal

pituicytes or visual disturbances. Patients with pituitary tumors may present with hormonal disturbances (either underor over-secretion), headache, or gradual visual disturbances. However, the extent of visual loss may not correlate with the duration of symptoms as patients may not notice a gradual loss of peripheral vision. Visual field defects were found in half of the patients with a histologically proven pituitary tumor. Bitemporal hemianopia was the most common visual defect (seen in 17.2% of patients) [53]. Visual loss in patients with pituitary tumors occurs as a result of long-standing compression of the adjacent visual apparatus due to suprasellar extension of the tumor [53]. Compression of the optic chiasm gives rise to bitemporal hemianopia. If the chiasm is prefixed, the patient may manifest with signs of optic tract compression; if the chiasm is postfixed, the patient may present with signs of optic nerve compression. Optic tract compression causes homonymous field defects, and optic nerve compression causes optic nerve visual field defects. Patients may have a compressive optic neuropathy, causing visual acuity to be affected. Additionally, if the patient has significant chiasmal compression, affecting not only the nasal crossing fibers but also the temporal fibers that do not cross, vision can be compromised in both eyes [53]. Diplopia occurs by involvement of one or both cavernous sinuses when the tumor expands laterally through the dura. Third to sixth cranial nerve dysfunction may be a sign of tumor growth, tumor recurrence, or pituitary apoplexy.

Pituitary apoplexy refers to a sudden expansion in intrasellar contents that results in a sudden expansion of the mass. This is usually due to hemorrhage or infarction of a preexisting adenoma but has also been reported, rarely, in the normal pituitary gland in pregnancy [54]. In 60–80% of cases, pituitary apoplexy is the first presentation of a pituitary tumor, which can be misdiagnosed as meningitis or a subarachnoid hemorrhage because of its abrupt onset and similar signs. Precipitating factors for apoplexy have been reported in 25–30% of cases, such as closed-head trauma, hypotension, hypertension, anticoagulant therapy, and use of dopamine agonists [54–57]. Though more common in nonfunctional pituitary adenomas all types of pituitary adenomas are at risk of apoplexy.

Headache is the most consistent symptom of pituitary apoplexy; headache in such cases is sudden and severe; can be retro-orbital, frontal, or occipital; and is often accompanied by nausea or vomiting [54]. Patients can also manifest with sudden onset or worsening of visual field defects and partial or total loss of vision due to upward compression of the optic structures. Ophthalmoplegia, Horner syndrome, or stroke may be seen due to compression of cranial nerves, the sympathetic branch, or the internal carotid artery within the cavernous sinus. These features help differentiate pituitary apoplexy from meningitis or aneurysmal rupture, wherein fever and meningismus develop fairly early. Hypopituitarism, usually that of the anterior pituitary hormones, often results in significant morbidity and mortality, especially adrenocorticotropic hormone deficiency that leads to acute adrenal insufficiency [54]. It is important to perform visual acuity and visual field examinations as early as possible if the patient is alert, and changes should be monitored regularly. Cerebrospinal fluid studies are not conclusive. MRI is the technique of choice, particularly in determining optic apparatus or cavernous sinus extension. It is also