Volker K. H. Sonntag

Meningioma of the Foramen Magnum

The second most frequent location for meningiomas of the posterior fossa is on the anterior rim of the foramen magnum. These tumors may extend for a considerable distance up the clivus or down into the spinal canal. These slow-growing tumors may cause remarkable distortion and compression of the cervicomedullary region. The vertebral artery and its branches, as well as the lower cranial nerves, may be completely enveloped by the tumor (Fig. 32-1).

Intra- and Extradural Tumor: Dumbbell Schwannoma

Schwannomas and neurofibromas together comprise one third of primary spinal cord tumors and show a predilection for the thoracic spine over the lumbar and cervical spine. Characteristically, these tumors originate from the sensory nerve root intrathecally. The tumor then may extend along the nerve peripherally through the intravertebral neural foramina. The extradural portion of the well-encapsuled tumor may reach an extreme size. A standard radiograph of the spine may show erosion or complete absence of the pedicles and lamina as well as vertebral scalloping.

Cauda Equina: Conus Medullaris Ependymoma

Tumors of the cauda equina-conus medullaris region are often well-defined, small masses attached to the filum terminale. These lesions are dissected from the nerve roots and the tip of the spinal cord relatively easily. A portion of uninvolved filum terminale is usually present between the tumor and spinal cord. The afferent and efferent segments of the filum must be amputated to remove the tumor. Small and moderate-sized tumors do not require internal decompression. Recurrences after successful en bloc resections are rare.

Combined Occipital-suboccipital Craniotomy

The most frequent location for meningiomas of the posterior fossa, in our experience, is on the posterior surface of the petrous bone (Fig. 28-1). The major blood supply to these very vascular tumors arises from short branches of the internal carotid artery as the blood passes through the carotid canal within the petrous bone. Additional vessels from the posterior branches of the middle meningeal artery and from meningeal branches of the vertebral artery feed the tumor. In medially placed tumors near the clivus, the blood supply may be identical to that for a clivus meningioma. These tumors are attached to the dura over the posterior surface of the petrous bone anterior and superior to the internal auditory meatus and to the undersurface of the tentorium. The tentorium may be perforated by the tumor just as the falx is often penetrated by meningiomas in that location.

Suboccipital Craniectomy: Retromastoid Approach for Acoustic Schwannoma

The most common position for posterior fossa operations in adults is the lateral decubitus or “park bench” position; this approach has largely replaced the use of the sitting position for most procedures. After intubation and placement of a three-point head fixation device, the patient is turned on his side and the shoulder contralateral to the lesion supported by a roll in the axilla; the ipsilateral shoulder is rolled forward and pulled down with tape (Fig. 25-1). The dependent arm can be suspended by a sling in the crook of the Mayfield attachment. All pressure points are carefully padded. In lesions of the cerebellopontine angle, the head is kept in a relatively neutral position and the body is slightly elevated (reverse Trendelenberg). A straight retromastoid incision is used for most lesions centered on the internal acoustic meatus and for exploration of the cranial nerves (see Chapter 26). The incision is usually 8 to 10 cm long and is made one fingerbreadth medial to the mastoid process and digastric groove (Fig. 25-2). The incision extends from a line just above the top of the pinna of the ear to a point just below the mastoid tip. Care in splitting the muscle at the inferior end is important in avoiding an ectactic vertebral artery. For very large lesions, the superior end of the incision can be extended medially to a point 2 cm above the inion and curved toward the midline (inverted hockeystick) (Fig. 25-3). Once the linear retromastoid incision is cleared down to the bone, it can usually be held open by one or two curved cerebellar retractors.

Hypoglossal-Facial Anastomosis

Paralysis of the facial nerve causes a severe cosmetic deformity. Following intracranial operations in which the facial nerve is found to be irreparably damaged, an early nerve-crossing operation should be done to restore tone to the facial muscles as soon as possible. When the status of the nerve lesion intracranially is unknown, several months of clinical observation with electrical testing should be allowed for spontaneous reinnervation to take place.

Suboccipital Craniectomy: Midline and Paramedian Approach

Midline and paramedian suboccipital exposures are used for resection of midline tumors of the posterior fossa, removal of vascular malformations, and treatment of aneurysms and congenital anomalies. Whenever a lesion in the cerebellopontine angle appears to be too large for a routine retromastoid approach (see chapter 25), a paramedian exposure should be employed. In general, these procedures give wide exposure to one or both cerebellar hemispheres, the cerebellar vermis, one or both lateral sinuses and their confluence, the foramen magnum, and one or both mastoid processes. Formerly, midline and paramedian suboccipital craniectomies were carried out in the sitting position; at present they are most frequently performed in a lateral decubitus or prone position (see Chapter 25).

Suboccipital Craniectomy: Retromastoid Approach for Trigeminal Neuralgia

Microvascular decompression of the trigeminal nerve is the most frequently used open surgical procedure for the treatment of trigeminal neuralgia. It is most appropriate for patients with a 5-year expected survival who have had medically refractory trigeminal neuralgia for less than 8 to 10 years. The best results are obtained in patients who are neurologically intact without facial burning or numbness and in those who have not undergone a prior destructive procedure directed at the Vth nerve. The operation is carried out through a retromastoid suboccipital craniectomy, similar in many ways to the procedure described in chapter 25.

Intra- and Extramedullary Tumor: Thoracic Meningioma

Meningiomas of the spinal canal account for approximately 25% of intradural spinal cord tumors in adults. They can arise in any age group but most occur between the fifth and seventh decades of life. Approximately 80% occur in women and approximately 80% are located in the thoracic region. Magnetic resonance imaging is the best diagnostic study; it not only demonstrates the level of the tumor but also its relationship to the spinal cord. Classically, these tumors are located anterolaterally in the intradural space. Most are entirely intradural, but about 10% are both intradural and extradural or entirely extradural. Meningiomas are often covered by a shell-like envelope of spinal cord giving the initial appearance of an intrinsic tumor. Complete surgical removal is the treatment of choice for spinal meningiomas. A meningioma in the upper thoracic area is discussed.

Glomus Jugulare Tumor (Chemodectoma)

Glomus jugulare tumors have their origin from a small (0.25 to 0.5 mm) group of cells in the adventitia of the jugular bulb. These paraganglia have recently been identified in different places in the petrous bone such as in the tympanic branch of the glossopharyngeal nerve, in the canaliculus tympanicus, and in the submucosa of the promentory. Glomus jugulare tumors receive their blood supply from the ascending pharyngeal artery and other small branches of the external carotid artery; branches from the vertebral artery can also contribute to the vascularity of larger tumors. This is important because the majority of these tumors should be considered for preoperative embolization to reduce their vascularity and, frequently, their overall volume. This can make surgery of these tumors considerably safer.