Basic Principles of Cranial Surgery for Brain Tumors

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CHAPTER 115 Basic Principles of Cranial Surgery for Brain Tumors

Preoperative Imaging Interpretation

Interpretation of the preoperative imaging studies is important in terms of planning the surgery, setting surgical goals, and educating the patient. Most patients have already had gadolinium-enhanced magnetic resonance imaging (MRI) scans when they are seen by the neurosurgeon. When evaluating the MRI scan, answers to the following questions are invaluable:

Answering these six questions allows the surgeon to identify the most likely pathology, assess the urgency of the problem, plan the surgical approach, and schedule any additional preoperative imaging studies that are needed.

While studying the MRI findings and answering the six questions, surgeons should begin to organize their thoughts about the biology of the tumor. For example, is this a slow-growing tumor, such as an intrinsic low-grade astrocytoma, or a more aggressive intrinsic brain tumor? The presence of brain edema, mass effect, and contrast enhancement argues against a slow-growing process and favors a high-grade, fast-growing tumor. Surgeons must be aware, however, that there are always exceptions to such rules. The surgical issues and approach will differ, depending on the probable diagnosis, and surgeons must constantly test their diagnostic hypothesis by looking for clues that suggest a different diagnosis. For example, meningiomas are enhancing lesions that can reach a large size and have a significant mass effect, but the surgical goals differ from those in patients with high-grade gliomas. Although the extradural location leads the surgeon to the diagnosis of meningioma, the pace and severity of symptoms provide important clues about the biology of the tumor. Slow-growing tumors can reach very a large size and cause impressive shifts in normal brain structures without causing many symptoms. Growth of the tumor over a long period allows the brain to accommodate and adjust and maintain its function at a normal or nearly normal level. Symptoms that arise over a short period and increase in severity suggest a more ominous pathology and a fast-growing lesion. Correlating the pace and severity of symptoms with the radiologic findings is critical to formulating a surgical treatment plan and its timing.

Timing of Surgery

The timing of surgery is dictated by the type of symptoms and their pace. Progressive symptoms over a short period indicate that the tumor and edema are expanding faster than the brain is able to compensate. Recognition of this situation is critical because such patients are at risk for rapid deterioration. The Monro-Kellie principle, introduced to neurosurgery by Cushing, states that changes in brain volume cause reciprocal changes in the blood and cerebrospinal fluid (CSF) compartments within the rigid cranial sphere to maintain intracranial pressure (ICP) within a normal range.1 As brain volume increases secondary to a mass such as a tumor, a compensatory decrease in CSF or blood volume occurs to maintain ICP within the normal range. As the tumor expands, ICP increases slowly until there is no further displaceable volume (i.e., CSF or blood). At this point, further increases in brain volume result in an accelerated increase in ICP. The relationship between volume and pressure is depicted by the volume-pressure curve in Figure 115-1. As a tumor grows and expands, a patient moves along the curve from point A to point B. During this time, compensatory changes occur, including displacement of CSF and blood, as well as compression of normal brain tissue, to maintain ICP in the normal range. At point B, the ability of the brain to further compensate is limited. Thus, a small additional increase in volume results in a larger increase in pressure. It is important to recognize patients who are approaching point B on the volume-pressure curve because they are at risk for a rapid increase in ICP with small additional increases in volume. This rapid increase in ICP can lead to rapid neurological decline.

It is unusual for a patient with a brain tumor to need emergency surgery. The two exceptions are patients with markedly increased ICP secondary to the tumor mass and brain edema and those with acute or subacute obstructive hydrocephalus. In both circumstances, ICP is elevated. Patients with increased ICP can have marked alterations in mental status. Patients who prefer to keep their eyes closed or sleep are nearing a critical point on the volume-pressure curve. Surgeons should be wary of a patient who can perform a neurological examination adequately when stimulated but who falls back to sleep when unstimulated. A patient in this condition may need emergency surgery, especially if the source of the increased ICP is the tumor mass itself rather than brain edema. Patients with obstructive hydrocephalus need either a temporary ventriculostomy before a planned resection or a ventriculoperitoneal shunt. A third option is to proceed directly to tumor resection, with the goal of relieving the hydrocephalus. Patients with marked brain edema require high doses of steroids, which usually results in neurological improvement.

Surgical Planning: Imaging Studies

After deciding that the patient needs surgery, the surgeon must determine whether additional imaging studies are needed to better define the relationship between the tumor and the patient’s normal anatomy. The studies that might be obtained are (1) angiography or magnetic resonance angiography or venography to assess the relationship of the tumor to the blood vessels, (2) MRI for use with a frameless stereotactic system, (3) functional MRI to assess the tumor’s relationship to areas controlling eloquent brain functions, and (4) diffusion tensor imaging to assess the relationship of critical cortical pathways to the tumor.image

The use of angiography for the diagnosis and evaluation of brain tumors has diminished significantly with the advent of MRI. The location of large vessels within the subarachnoid space is well seen on T2-weighted and fluid-attenuated inversion recovery (FLAIR) MRI scans. On both sequences the vessels are black. The displacement and orientation of these vessels with respect to the tumor can usually be well seen on MRI. An important use for angiography or magnetic resonance angiography or venography is evaluation of the venous sinus system in patients with extra-axial dural tumors, particularly meningiomas. These tumors can often partially or completely occlude the large dural sinuses. This information can be important in planning the surgical exposure and resection. Frequently, magnetic resonance venography provides the information, thus making angiography unnecessary. If the dural sinus has been occluded by the tumor, magnetic resonance venography will show the cortical draining veins entering the sinus in front of or behind the tumor. Those entering anterior to the tumor drain anteriorly, and those posterior drain posteriorly. If the image on magnetic resonance venography is unclear and the surgical plan includes removal of a segment of a dural sinus, angiography should be performed. Finally, preoperative angiographic embolization can be beneficial with certain dural tumors. If the tumor has a significant dural blood supply, embolization can decrease the vascularity of the tumor and make surgery safer. Preoperative embolization represents another indication for preoperative angiography.

Frameless stereotaxy has become an important part of intracranial tumor surgery. The intraoperative correlation of anatomy with the preoperative MRI is helpful when planning the skin incision, bone flap, cortical incision, and approach to the tumor, as well as to maximize resection of the tumor. Typically, routine enhanced MRI is the imaging modality used interactively in the operating room. However, additional imaging modalities can now be placed onto the navigation system and used to increase the likelihood of a successful operation (i.e., maximal tumor resection with no neurological sequelae). Such modalities include functional MRI, diffusion tensor imaging with fiber tract images, positron emission tomography, and ultrasonography. It is critical to understand the limitation of this technology in terms of the brain shifts that occur during surgery, as well as the accuracy and reliability of using the different imaging modalities. Although most useful with intra-axial tumors, we have found it valuable for extra-axial tumors also, particularly meningiomas. image

For intra-axial tumors, frameless stereotaxy provides feedback to the surgeon about the degree of tumor resection. The extent of tumor resection as judged by postoperative imaging studies is greater when frameless stereotaxy is used. An invaluable use of frameless stereotaxy is for resection of low-grade astrocytomas. These tumors typically do not enhance and can be very difficult to grossly distinguish from normal brain. Furthermore, the extent of the tumor is often well defined by the high signal seen on T2-weighted MRI. These T2-weighted images can be loaded onto the frameless stereotactic system to allow the surgeon to map out the extent of the tumor on the cortex. The depth of the tumor can be defined by using the navigation probe as a biopsy needle and passing it through the middle of the tumor until the tip is at the deepest edge of the tumor. By leaving the tip at this point, the surgeon resects tissue around the probe until the tip is reached. With this technique, the deep edge is reached before any significant brain shift, and the subsequent resection is carried to that depth. By defining the extent of resection before removing any tissue, the probability of gross total resection of the tumor, as defined by the change in signal on T2-weighted images, is much greater. As tumor and CSF are removed during the resection, the brain drops away from the skull, and this must be taken into account when interpreting the data provided by the stereotactic system. The surgeon must use other sources of information in these circumstances, including (1) the appearance of the cavity wall in comparison to the tumor’s gross appearance, (2) the relationship of the cavity margins to surrounding normal sulci, and (3) how far the cortical surface has dropped away from the inner table.

The natural evolution from frameless stereotaxy is intraoperative MRI. Intraoperative MRI allows the surgeon to use an updated MRI scan obtained during surgery to determine whether additional resection is necessary. The usefulness of this tool is limited by the quality of the images, which is dependent on the strength of the magnet. The added benefit in terms of improved tumor resection and better outcomes remains to be determined.

Finally, an emerging imaging technology is localization of function on brain MRI scans, known as functional MRI. Functional MRI is becoming easier to obtain and thus more commonly used. Its value resides in its ability to identify the relationship of eloquent brain functions, speech and motor, to the tumor. This information is used to assess the patient’s risk with surgery and to assist the surgeon in approaches to resection. There are limitations in functional MRI that involve the specificity of the data. The study is based on changes in blood flow that occur when carrying out a task, so the accuracy of the result depends on the assumption that the measured increased blood flow identifies the area of the brain responsible for the activity being measured. This assumption may not be correct in the setting of certain tumors in which there is increased blood flow and perfusion such that there is an uncoupling of the relationship between blood flow and function around the tumor. It is imperative that the surgeon discuss the functional MRI results with an experienced radiologist.

Surgical Preparation

We prefer to pretreat patients with steroids for several days before surgery when there is symptomatic brain edema or mass effect. The typical dose is 16 mg/day or 4 mg four times a day. In patients with high ICP, higher doses of steroids are used, up to 120 mg/day (20 mg every 4 hours). The upper limit of the dosage of dexamethasone (Decadron), at least in the short term, has not been established. In patients with significant brain edema, there appears to be a dose threshold that must be surpassed before symptomatic benefit is derived. Therefore, dexamethasone should be started at a high dose in patients with large amounts of brain edema and then reduced after neurological improvement has occurred.

In general, patients respond within 24 hours of beginning steroid treatment. This clinical improvement parallels the increased compliance measured in patients 24 hours after beginning steroids. Investigative studies in patients and animals indicate that dexamethasone decreases tumor capillary permeability and tumor blood volume.24 These actions alter the configuration of the volume-pressure curve such that further increases in tumor volume result in a smaller increase in ICP.

In addition to steroids, an anticonvulsant is started, usually phenytoin (Dilantin) or levetiracetam (Keppra). Patients with high-grade lesions are operated on within 7 to 10 days of initial evaluation. Dexamethasone is generally started after the first imaging study and maintained until surgery. In patients undergoing more elective surgery, such as those with schwannomas and meningiomas, surgery may not be performed for several weeks or longer. These patients may be given steroids beginning 3 days before surgery. However, if there is only minimal edema, steroids are typically first given at time of surgery. In addition to clearing any cardiac or pulmonary issues before surgery, the surgeon should review the patient’s medications to check for any that might be contraindicated for surgery, particularly aspirin and some of the antipsychotics.

Once the patient is under general anesthesia, an arterial line and Foley catheter are placed. For many but not all craniotomies, central venous access is indicated. In particular, central venous access is important when the patient’s head is going to be significantly higher than the heart. There is a real risk of venous air embolism when the head is positioned significantly higher than the heart, and the central venous line is helpful in treating this problem. In addition to anesthetic needs, neuromonitoring electrodes for evoked potentials, electromyography, and electroencephalography are placed on the patient. Somatosensory evoked potentials (SEPs) are the most common neurophysiologic monitoring tool used in craniotomies for brain tumors. One derivation of SEPs is to localize the motor strip with a strip electrode. This is valuable when planning where to make the cortical incision for an intrinsic tumor located near the central sulcus. If frameless stereotaxy is to be used, registration of the machine to the patient is carried out after the patient’s head has been positioned in the head-holding device.