General Considerations

Historically, one of the most important contributions of Harvey Cushing was defining surgery of posterior fossa tumors. Resection techniques for tumors in this region were further advanced by mandatory preoperative evaluation for selection of appropriate patients, meticulous surgical techniques, and incorporation of methods such as ventricular puncture and electrocautery. Introduction of microsurgical techniques in the 1950s ultimately revolutionized the results of posterior fossa surgery.

Nowadays, the most common workhorse approaches to the posterior fossa include the midline suboccipital and the lateral suboccipital (retromastoid) approaches. The supracerebellar craniotomy is a modification of these approaches. The retromastoid craniotomy and supracerebellar craniotomy are discussed in their own corresponding chapters.

The midline suboccipital craniotomy is the preferred mode of exposure for the majority of vermian, posterior inferior pontine and medullary, foramen magnum, midline cerebellum, and pineal regions. Lesions affecting the fourth ventricle are best approached through the telovelar approach . The most common indications for midline suboccipital craniotomy are:

1. developmental anomalies such as Chiari malformations,
2. posterior fossa tumors such as metastatic tumors, meningiomas, ependymomas, astrocytomas, and medulloblastomas,
3. vascular lesions such as aneurysms, cavernous malformations, and arteriovenous malformations, and
4. pineal region tumors accessed through the supracerebellar corridor.

For lesions extending from the posterior fossa into the middle fossa, a supracerebellar transtentorial or combined middle and posterior fossa craniotomy is recommended.

When compared with its counterpart supratentorial compartment, the infratentorial compartment has limited space and contains vital structures such as brainstem and cranial nerves. These factors make surgery in the posterior fossa risky, so the utmost attention to details during preoperative planning, operative execution, and postoperative care is required.

Because posterior fossa tumors often cause obstructive hydrocephalus, preoperative planning should include methods to relieve cerebrospinal fluid (CSF) pressure before the dural opening. Preoperative shunting is avoided because resection of some tumors resolves CSF obstruction and obviates the need for a permanent CSF diversion procedure.

In patients with communicating hydrocephalus, intraoperative lumbar CSF drainage affords brain relaxation. This drainage is particularly useful in situations when the location of the lesion prohibits early access to the cisterna magna and the cerebellopontine angle cisterns.

Neurophysiologic monitoring is indicated during operations for lesions near the brainstem and cranial nerves. To minimize morbidity, the fourth ventricular floor may be mapped for the facial nucleus.

Careful study of preoperative imaging will warn the operator about risky dissection territories. Displaced cerebrovascular structures should be studied on preoperative imaging and anticipated intraoperatively. Invasion of the brainstem pial surface by the tumor should be evaluated on imaging, and a radical subtotal resection should be planned that leaves a thin sheet of tumor over the brainstem.

Highly vascular tumors of the posterior fossa, including AVMs and hemangioblastomas, require special attention. Large solid hemangioblastomas can be daunting tumors, and aggressive preoperative embolization may decrease the patient’s risk of life-threatening intraoperative blood loss.

The patient may be positioned in either the prone, sitting, or park-bench position. I prefer the park-bench position because gravity will keep the field clear during surgery and I can sit during the microsurgical portion of the operation.

If the patient is placed in the prone position, two surgeons are able to work simultaneously through co-observation with the microscope. The steep angle of the tentorium, however, makes the prone position relatively impractical for the supracerebellar approach, especially if the patient is obese.

The sitting position allows gravity retraction to mobilize the cerebellum away from the tentorium and is advantageous for supracerebellar corridors. However, the risk of venous air embolism and the need for uncomfortable arm postures may motivate some operators to avoid this position.

The dura is primarily closed in a watertight fashion using 4-0 Nurolon running or interrupted sutures. I avoid dural allografts and prefer using pericranium over the occipital bone for dural grafting. A Valsalva maneuver may exclude an obvious source of CSF leakage.

If there is no cerebellar swelling, the bone may be replaced using cranial plates. I avoid placing the deep neck muscles under significant tension and minimize their “strangulation” by the deep sutures, as this maneuver will cause muscle necrosis and uncontrollable postoperative pain. The neck muscles are gently approximated. However, the fascia is closed in a watertight fashion.

The patient is observed in the intensive care unit for a day or two and then transferred to the ward. Steroids are administered prophylactically to minimize the risk of aseptic meningitis. If preoperative hydrocephalus was present and a ventricular catheter was implanted intraoperatively, this catheter should be left in place during surgery and “weaned off” postoperatively.

Aggressive retraction of the cerebellum can lead to retraction edema shown on postoperative imaging that can ultimately cause symptomatic posterior fossa tension and a need for decompression. Therefore, caution should be exercised during dural closure and bone flap replacement. If the brain appears swollen, the dural closure should not cause more tension and the bone flap should not be replaced. This brain swelling is compounded by potential partial transverse sinus thrombosis and vermian veins sacrifice.

• The operator should exercise caution while using monopolar electrocautery at the craniocervical junction and avoid penetrating the dura or injuring the vertebral artery during muscle dissection. The use of careful blunt dissection in the region is advised.
• The risk of air embolism during manipulation of the venous sinuses is significant and the operator should exercise a low threshold of suspicion for this complication.

*Redrawn with permission from Tew JM, van Loveren HR, Keller JT. Atlas of Operative Microneurosurgery, WB Saunders, 2001. © Mayfield Clinic

DOI: https://doi.org/10.18791/nsatlas.v2.ch10