Post cranial fossa surgery and anesthesia considerations

Editor's Notes

• #3: Largest and deepest of the three cranial fossa.
• #4: Applied anatomy: fracture of post cranial fossa causes bruising over mastoid region extending down over the sternocleidomastoid muscle
• #5: 3rd to 12th cranial nerves nuclei and many efferent and afferent fiber tracts that connect the brain with the rest of the body
• #7: Foramen magnum See main article at Foramen magnumThe most conspicuous, large opening in the floor of the fossa. It transmits the medulla, the ascending portions of the spinal accessory nerve (XI), and the vertebral arteries. [edit]Internal acoustic meatus Lies in the anterior wall of the posterior cranial fossa. It transmits the facial (VII) and vestibulocochlear (VIII) cranial nerves into canal in thepetrous temporal bone. [edit]Jugular foramen Lies between the inferior edge of the petrous temporal bone and the adjacent occipital bone and transmits the internal jugular vein (actually begins here), the glossopharyngeal (IX), thevagus (X) and the accessory (XI) nerves. [edit]Anterior condylar (hypoglossal) canal Lies at the anterolateral margins of the f. magnum and transmits the hypoglossal (XII) nerve. [edit]Other Also visible in the posterior cranial fossa are depressions caused by the venous sinuses returning blood from the brain to the venous circulation: Right and left transverse sinuses which meet at the confluence of sinuses (marked by the internal occipital protuberance). The transverse sinuses pass horizontally from the most posterior point of the occiput. Where the apex of the petrous temporal meets the squamous temporal, the transverse sinuses lead into sigmoid (S-shaped) sinuses (one on each side). These pass along the articulation between the posterior edge of the petrous temporal and the anterior edge of the occipital bones to the jugular foramen where the sigmoid sinus becomes the internal jugular vein. Note that a superior petrosal sinus enters the junction of the transverse and sigmoid sinuses. Also an inferior petrosal sinus enters the sigmoid sinus near the jugular foramen. The posterior cranial fossa is formed in the endocranium, and holds the most basal parts of the brain.
• #8: Tumors that arise in this depression are of special concern because the posterior fossa is a small enclosed space near critical brain structures, including the brain stem, the cerebellum, and cranial nerves, and the tumors often can be difficult to treat. Most pediatric brain tumors, 55 to 70 percent, arise in the posterior fossa, compared with 15 to 20 percent of adult tumors. The tumors that occur in the posterior fossa are more likely to be primary brain tumors, meaning that they arise from the tissues of the brain, rather than metastatic tumors, which arise in a different part of the body and spread to the brain. Posterior fossa tumors may be almost any type of primary brain tumor, including gliomas, astrocytomas, or hemangioblastomas, tumors that arise from glial cells, the supportive cells of the brain; ependymomas, tumors of the lining of the cavities in the brain; acoustic neuromas, benign tumors that grow from the sheath of a cranial nerve; meningiomas, tumors of the protective cover of the brain; medulloblastomas, malignant tumors that arise from incompletely developed cells; pineoblastomas, malignant tumors of the pineal gland; or other types of primary brain tumor. Symptoms Because of the location of posterior fossa tumors, they often grow to block the flow of cerebrospinal fluid, the fluid that bathes the brain and spinal cord, causing hydrocephalus, an increase in pressure inside the skull. Increased cranial pressure causes headaches, nausea, and vomiting. The posterior fossa is a small space surrounded by important functional structures. As the tumor grows to fill the space, it will begin to exert pressure on these structures, causing a range of other symptoms, including disturbances in vision and hearing, weakness or numbness in the face, and problems with balance and walking. Diagnosis Imaging studies are the key component in the diagnosis of posterior fossa tumors. Magnetic resonance imaging (MRI) is the best available imaging modality for these tumors. For this study, an agent that provides contrast in the image is administered intravenously so neurological surgeons can visualize the tumor against the normal brain in the background. In some cases, neurological surgeons may employ an MRI scan with frameless stereotactic guidance. For this study, a contrast MRI is performed after special markers (called fiducials) are placed on the patient's scalp. The fiducials are processed by a computer, which calculates the location of the tumor and creates a three-dimensional reconstruction. This image then is used at the time of surgery to help locate the tumor precisely, maximize tumor removal, and minimize injury to the surrounding brain. Treatment Surgery usually is the first treatment for tumors of the posterior fossa. Because of the proximity of these tumors to critical brain structures, even benign ones will be removed. The goal of surgery is to remove the entire mass to alleviate the pressure exerted on surrounding tissue. In addition, surgery is done to retrieve a biopsy to provide an accurate tumor diagnosis. Complete resection may be difficult because of the critical structures in the area that must be preserved. Depending on the patient and the extent of the resection, surgery may be followed by radiation therapy. In addition, some smaller tumors may be treated effectively with stereotactic radiosurgery, which involves the use of a highly focused beam of radiation to target the cancer cells specifically and leave the surrounding brain unaffected.
• #12: Posterior Fossa Tumors The posterior fossa is a region near the base of the skull. Tumors that arise in this depression are of special concern because the posterior fossa is a small enclosed space near critical brain structures, including the brain stem, the cerebellum, and cranial nerves, and the tumors often can be difficult to treat. Most pediatric brain tumors, 55 to 70 percent, arise in the posterior fossa, compared with 15 to 20 percent of adult tumors. The tumors that occur in the posterior fossa are more likely to be primary brain tumors, meaning that they arise from the tissues of the brain, rather than metastatic tumors, which arise in a different part of the body and spread to the brain. Posterior fossa tumors may be almost any type of primary brain tumor, including gliomas, astrocytomas, or hemangioblastomas, tumors that arise from glial cells, the supportive cells of the brain; ependymomas, tumors of the lining of the cavities in the brain; acoustic neuromas, benign tumors that grow from the sheath of a cranial nerve; meningiomas, tumors of the protective cover of the brain; medulloblastomas, malignant tumors that arise from incompletely developed cells; pineoblastomas, malignant tumors of the pineal gland; or other types of primary brain tumor. Symptoms Because of the location of posterior fossa tumors, they often grow to block the flow of cerebrospinal fluid, the fluid that bathes the brain and spinal cord, causing hydrocephalus, an increase in pressure inside the skull. Increased cranial pressure causes headaches, nausea, and vomiting. The posterior fossa is a small space surrounded by important functional structures. As the tumor grows to fill the space, it will begin to exert pressure on these structures, causing a range of other symptoms, including disturbances in vision and hearing, weakness or numbness in the face, and problems with balance and walking. Diagnosis Imaging studies are the key component in the diagnosis of posterior fossa tumors. Magnetic resonance imaging (MRI) is the best available imaging modality for these tumors. For this study, an agent that provides contrast in the image is administered intravenously so neurological surgeons can visualize the tumor against the normal brain in the background. In some cases, neurological surgeons may employ an MRI scan with frameless stereotactic guidance. For this study, a contrast MRI is performed after special markers (called fiducials) are placed on the patient's scalp. The fiducials are processed by a computer, which calculates the location of the tumor and creates a three-dimensional reconstruction. This image then is used at the time of surgery to help locate the tumor precisely, maximize tumor removal, and minimize injury to the surrounding brain. Treatment Surgery usually is the first treatment for tumors of the posterior fossa. Because of the proximity of these tumors to critical brain structures, even benign ones will be removed. The goal of surgery is to remove the entire mass to alleviate the pressure exerted on surrounding tissue. In addition, surgery is done to retrieve a biopsy to provide an accurate tumor diagnosis. Complete resection may be difficult because of the critical structures in the area that must be preserved. Depending on the patient and the extent of the resection, surgery may be followed by radiation therapy. In addition, some smaller tumors may be treated effectively with stereotactic radiosurgery, which involves the use of a highly focused beam of radiation to target the cancer cells specifically and leave the surrounding brain unaffected.
• #15: Pre-cordial doppler and TEE if available. IBP and pre-cordial doppler are said to be minimum requirement for sitting craniotomies
• #16: From the surgical aspect, the sitting position gives good surgical access to the operative site, improves venous drainage, gives a better view of facial area for monitoring evoked responses from cranial nerve stimulation and allows for better ventilation, decreased blood loss, decreased ICP by promoting cerebral venous drainage. Conversely, the sitting position can present complications such as air emboli, postural hypotension and serious cardiac arrhythmias due to surgical stimulation of cranial nerves and brainstem. Abstract A new head-prone position is described for posterior cranial fossa surgery on infants whose cerebral cortical mantle has been markedly thinned by severe hydrocephalus. The new position furnished a direct line of sight to the apex of the IV ventricle corresponding to that provided by the classic high sitting position, without the latter's risks of air embolism and of acute subdural hematoma secondary to tearing of corticodural bridging vessels due to escape of gravity-impelled CSF from the large ventricles. The anesthesiologic technique, the positioning of the patient, and the surgeon's posture presented no unusual problems. ADVANCES IN OPERATIVE TECHNIQUES New head-prone position for posterior fossa surgery in infants with severe hydrocephalus Mark Rayport and John T. Martin
• #17: The patient is actually semirecumbent in the standard sitting position (Figure 26–1); the back is elevated to 60°, and the legs are elevated with the knees flexed. The head is fixed in a three-point holder with the neck flexed; the arms remain at the sides with the hands resting on the lap.
• #18: sitting position gives good surgical access to the operative site, improves venous drainage, gives a better view of facial area for monitoring evoked responses from cranial nerve stimulation and allows for better ventilation, decreased blood loss, decreased ICP by promoting cerebral venous drainage. Conversely, the sitting position can present complications such as air emboli, postural hypotension and serious cardiac arrhythmias due to surgical stimulation of cranial nerves and brainstem.
• #19: Arterial line should be Zeroed at cerebral level not heart since the cerebral perfusion differs greatly from heart level during sitting position. TED- ThromboEmbolism Deterrent. Maintain 1 inch space between the chin and chest to prevent cervical cord stretching and obstruction of venous drainage from the face and tongue
• #27: Vigilance for blood loss
• #29: Placing the anesthetized patient in the sitting position conveys some risk of impaired cardiovascular function, in particular, hypotension. Pressor administration is required in some patients. Measures to avoid hypotension include prepositioning hydration, wrapping of the legs with elastic bandages to counteract gravitational shifts of blood, and slow, incremental adjustment of table position. Aggressive volume loading and the G suit (also known as pneumatic antishock trousers or military antishock trousers) attenuate the effects of assuming the sitting position. [77] [78] [79] Neither of these measures has been widely applied, however. MAP was relatively unaffected, whereas wedge pressure, stroke volume, and cardiac index decreased—the last-mentioned by approximately 15%—although there was some variation with the anesthetic drugs employed. The combination of an unchanged MAP (which generally requires the use of a “light,” high sympathetic tone anesthetic) and a reduced cardiac index implies that systematic vascular resistance increased. Their calculations and the observations of other investigators[81] reveal significant elevation of systematic vascular resistance. For patients in whom an abrupt increase in systematic vascular resistance may be poorly tolerated, the sitting position may represent a physiologic threat, and alternative positions should be considered. A pulmonary arterial catheter may be warranted when there is clinical or historical evidence of antecedent coronary artery or valvular heart disease and arbitrarily in patients older than 60 to 65 years. During procedures performed in the sitting position, MAP should be corrected to head level to obtain a meaningful index of CPP. Specifically, CPP (MAP - estimated ICP) should be maintained at a minimum value of 60 mm Hg in healthy patients in whom it is reasonable to assume a normal cerebral vasculature. The safe lower limit should be increased for elderly patients, for patients with hypertension or known cerebrovascular disease, and for patients with degenerative disease of the cervical spine or cervical spinal stenosis who may be at risk for decreased spinal cord perfusion, and in the event that substantial or sustained retractor pressure must be applied to brain or spinal cord tissue.
• #30: Operations in the posterior fossa can injure vital circulatory and respiratory brain stem centers as well as cranial nerves or their nuclei. Such injuries occur as a result of direct surgical trauma, retraction, or ischemia. Damage to respiratory centers is said to be nearly always associated with circulatory changes, so that abrupt changes in blood pressure, heart rate, or cardiac rhythm should alert the anesthesiologist to the possibility of such an injury. Communication of such changes to the surgeon is critical. Rarely, isolated damage to respiratory centers without premonitory circulatory signs has occurred during operations in the floor of the fourth ventricle; historically, spontaneous ventilation has been used during these procedures to offer an additional monitor of function. At completion of the surgery, brain stem injuries often present as an abnormal respiratory pattern or as an inability to maintain a patent airway following extubation. Monitoring brain stem auditory evoked potentials may be useful in preventing eighth nerve damage during resections of acoustic neuromas. Electromyography is also used to avoid injury to the facial nerve, but the latter requires incomplete neuromuscular blockade intraoperatively.
• #31: monitoring can be a challenge for the anesthesiologist because muscle relaxants complicate interpretation of the EMG, and N2O and high-dose inhalation anesthesia may interfere with SSEPs. The BAEPs(brainstem Auditory evoked potentials) are robust and minimally influenced by anesthetics
• #32: Air bubbles entering the venous system ordinarily lodge in the pulmonary circulation, where their gases eventually diffuse into the alveoli and are exhaled. Small bubbles are well tolerated by most patients. When the amount entrained exceeds the rate of pulmonary clearance, pulmonary artery pressure progressively rises. Eventually, cardiac output decreases in response to increases in right ventricular afterload. Preexisting cardiac or pulmonary disease enhances the effects of venous air embolism; relatively small amounts of air may produce marked hemodynamic changes.
• #34: ?? Increase in EtN2 (nitrogen) due to air entrainment that is finally absorbed and exhaled.
• #35: Deliberate hypoventilation?? For prevention of VAE ?. Higher levels of PEEP >10 is required to increase venous pressure and decrease VAE,but it can itself have hemodynamic consequences
• #36: These monitors can detect air bubbles as small as 0.25 mL. TEE has the added benefit of detecting the amount of the bubbles and any transatrial passage, as well as evaluating cardiac function. Doppler methods employ a probe over the right atrium (usually to the right of the sternum and between the third and sixth ribs). Interruption of the regular swishing of the Doppler signal by sporadic roaring sounds indicates venous air embolism.
• #37: The Doppler probe should be placed after the patient is in the operative position. The probe is usually positioned at the middle third of the sternum on the right side but, because the position of the right atrium varies with the patient's position, proper placement must be confirmed. Hearing heart tones is not enough. To test for proper placement of the probe, agitated saline is injected through a right atrial catheter or peripheral intravenous line; alternatively, the injection of 0.5 to 1 mL of air, carbon dioxide (CO2), or circuit gas is acceptable. The probe is properly placed if this maneuver produces characteristic Doppler sounds signaling air embolism. Because of its sensitivity, a properly positioned Doppler, combined with end-tidal gas monitoring, is essential for all posterior fossa procedures in the seated position.
• #41: Confirmation of correct catheter positioning is important and is accomplished by intravascular electrocardiography or by transesophageal echocardiography (TEE). During intravascular electrocardiography, a high atrial position is indicated by the appearance of a biphasic P wave. If the catheter is advanced too far, the P wave changes from a negative to a positive deflection, and a right ventricular waveform may also be observed when the pressure is transduced
• #42: If the filtration rate of pulm. artery air by lungs is exceeded the bubbles can even pass into the systemic circulation
• #43: addition, damage to or edema of the respiratory centers from intraoperative manipulation can result in hypoventilation or erratic respiratory patterns. Therefore, longer-term ventilation and airway protection might be required in some patients
• #45: There have been sporadic reports of upper airway obstruction after posterior fossa procedures in which swelling of pharyngeal structures, including the soft palate, posterior wall, pharynx, and base of the tongue, has been observed.[82] These episodes have been attributed to edema formation at the time of reperfusion after trauma or prolonged ischemia occurring as the result of foreign bodies (usually oral airways) causing pressure on these structures in the circumstances of lengthy procedures with sustained neck flexion (which is usually required to improve access to posterior structures). It is customary (and we think ideal, although there is no science to support the practice) to maintain at least two fingerbreadth's distance between the chin/mandible and the sternum/clavicle to prevent excessive reduction of the anterior-posterior diameter of the oropharynx. In addition, we position patients with the oral airway in place and then, after the final head position is achieved, withdraw it until its tip functions as a bite block between the teeth. The macroglossia phenomenon also may be relevant as clinicians contemplate the use of transesophageal echocardiography (TEE) in the neurosurgery suite. The centers that routinely employ TEE in neurosurgery, for the most part, use either pediatric or custom-made small-diameter probes to avoid trauma to pharyngeal and perilaryngeal structures.
• #47: Tension pneumocephalus is one of the causes of delayed awakening or nonawakening after posterior fossa and supratentorial procedures ( Fig. 63-9 ). [89] [90] It occurs because air enters the cranium while the patient is in a head-up position at a time when the volume of the intracranial contents has been reduced as a result of some combination of hypocapnia, good venous drainage, osmotic diuresis, and CSF loss from the operative field. When the cranium is closed, and the patient is returned to the near-supine position, CSF, venous blood volume, and extracellular fluid return or reaccumulate, and the air pocket becomes an unyielding mass lesion (because of the very slow diffusion of nitrogen). This mass may cause delayed recovery of consciousness or severe headache. Among supratentorial craniotomies, the largest residual airspaces occur after frontal skull base procedures in which energetic brain relaxation measures are used to facilitate subfrontal access (see Fig. 63-9 ). At the end of these procedures, typically done in a supine/brow-up position, it is not feasible to fill the intracranial dead space with normal saline as is commonly done with smaller craniotomy defects, and there may be a large residual pneumatocele. We doubt that the possible occurrence of this phenomenon represents a contraindication to N2O. Withdrawal of N2O may be appropriate at the time of cranial closure, however. The diagnosis of pneumocephalus is confirmed by a brow-up lateral x-ray or CT scan. The treatment is a twist-drill hole followed by needle puncture of the dura.
• #50: http://www.frca.co.uk/article.aspx?articleid=100662