Positioning

The intended surgical position and the necessary position aids should be agreed on at the outset. The commonly used positions and positioning aids/supports are listed in Table 53-5 .

General Considerations

The prolonged duration of many neurosurgical procedures should be taken into account in all positions. Pressure points should be identified and padded carefully, and pressure and traction on nerves must be avoided. Thromboembolic precautions, including support hose and sequential pneumatic compression devices, are often appropriate. For cranial procedures, almost invariably some component of head-up posturing (e.g., 15 to 20 degrees) will be appropriate to ensure optimal venous drainage. The conspicuous exception occurs with evacuation of a

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Supine

The supine position is used with the head neutral or rotated for frontal, temporal, or parietal access. Extremes of head rotation can obstruct jugular venous drainage, and a shoulder roll can attenuate this problem. The head is usually in a neutral position for bifrontal craniotomies and transsphenoidal approaches to the pituitary. The head-up posture is best accomplished by adjusting the operating table to a chaise lounge (lawn chair) position (flexion, pillows under the knees, slight reverse Trendelenburg). This orientation, in addition to promoting cerebral venous drainage, decreases back strain.

Semilateral

The semilateral position, also known as the Janetta position after the neurosurgeon who popularized its use for microvascular decompression of the fifth cranial nerve, is used for retromastoid access. It is achieved by lateral tilting of the table 10 to 20 degrees combined with a generous shoulder roll. Again, extreme head rotation sufficient to cause compression of the contralateral jugular by the chin should be avoided.

Lateral

The lateral position can be used for access to the posterior parietal and occipital lobes and the lateral posterior fossa, including tumors at the cerebellopontine angle and aneurysms of the vertebral and basilar arteries. A vacuum mattress that can be molded to the patient's anatomy greatly facilitates maintenance of a stable lateral position. An axillary roll is important for preventing brachial plexus injury.

Prone

The prone position is used for spinal cord, occipital lobe, craniosynostosis, and posterior fossa procedures. The prone position has also been referred to, aptly, as the Concorde position because for cervical spine and posterior fossa procedures, the final position commonly entails neck flexion, reverse Trendelenburg, and elevation of the legs, usually with pillows. This orientation serves to bring the surgical field to a horizontal position.

Before turning to the prone position, the anesthesiologist should ensure that the intravenous catheter and endotracheal tube are secure and that appropriate personnel are available to prevent injury during the turn. The anesthesiologist should have a plan for detaching and reattaching monitors in an orderly manner to prevent an excessive monitoring "window." Awake tracheal intubation and prone positioning can be used in patients with an unstable cervical spine, and it should be confirmed that neurologic status is unchanged before induction of anesthesia in the final surgical position. It is also occasionally performed in very obese patients.

The head can be positioned in a pin head holder (applied before the turn), a horseshoe headrest, or a disposable foam headrest. A complication of the prone position to which there must be constant attention is retinal ischemia and blindness caused by occlusion of the central retinal vessel as a result of orbital compression. It must be intermittently confirmed (e.g., every 15 minutes) and after any surgery-related head/neck movement that pressure has not come to bear on the eye. However, it should be understood that not all postoperative visual loss (POVL) is a result of direct orbital compression. Ischemic optic neuropathy (ION) actually appears to be a more frequent cause of POVL than pressure-induced occlusion of the central retinal vessels does. The cause-and-effect relationships associated with ION are uncertain, but low arterial pressure, low hematocrit, and lengthy surgical procedures are statistically associated with the phenomenon.^[65] Ophthalmologists have long recognized the phenomenon of ION in nonanesthetized patients and have identified as risk factors many causes of vascular disease (hypertension, diabetes, smoking, hyperlipidemia) and hypotension, especially when it occurs in hypertensive individuals.^{[66] [67]} It is also suspected that certain normal anatomic and physiologic variations, including poor collateralization or absence of autoregulation of the vasculature of the optic nerve head (or both),^[66] a small and therefore anatomically "crowded" optic nerve head (the "disk at risk"^[68] ), an increase in intraocular pressure that has been observed to occur during prolonged spine surgery in the prone position,^[69] and impaired cerebral venous drainage,^[70] may contribute to the intraoperative occurrence of ION. Note that the notion that ION is merely a function of arterial hypotension is probably an inaccurate oversimplification of a phenomenon that almost certainly involves the interplay of numerous preoperative and intraoperative factors.^{[71] [72]}

Direct pressure can also result in various degrees of pressure necrosis of the forehead, maxillae, and chin, especially during prolonged spinal procedures. The clinician should attempt to ensure that pressure is as evenly distributed as possible over facial structures. Other pressure points to be checked include the axillae, breasts, iliac crests, femoral canals, genitalia, knees, and heels. When the arms are placed in the "stick-em up" position, traction

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An objective during prone positioning, especially for lumbar spine surgery, is avoidance of compression of the inferior vena cava. Impairment of vena caval return diverts blood to the epidural plexus and increases the potential for bleeding during laminectomy. Avoidance of inferior vena cava compression is an objective of all of the spinal surgery frames and is accomplished very effectively by the Relton-Hall, Wilson, and Andrews variants. It does, however, introduce a risk of air embolism, ^{[73] [74]} although clinical occurrences have been very infrequent.^[75]

Attention should be directed at preventing injury to the tongue in the prone position. With both cervical and posterior fossa procedures, it is frequently necessary to flex the neck substantially to facilitate surgical access. Such flexion reduces the anterior-posterior dimension of the hypopharynx, and compression ischemia of the base of the tongue (as well as the soft palate and posterior wall of the pharynx) can occur in the presence of foreign bodies (endotracheal tube, esophageal stethoscope, oral airway). The consequence can be postextubation airway obstruction of rapid onset as a result of "macroglossia" caused by the accumulation of edema after reperfusion of the ischemic tissue (see later). Accordingly, unnecessary paraphernalia in the pharynx should be avoided. Omitting the oral airway entirely is unwise because the tongue may then protrude between and be trapped by the teeth as progressive swelling of facial structures occurs during a prolonged prone procedure. A bite block akin to those used with laryngeal masks will prevent this problem without adding bulk to the hypopharynx.

Sitting

Several reviews of large experiences with procedures performed in the sitting position have been published.^{[76] [77] [78] [79] [80]} All concluded that the sitting position can be used with acceptable rates of morbidity and mortality. However, these reports were prepared by groups that perform 50 to 100 or more of these procedures per year, and the hazards of the sitting position may be greater for teams that have less frequent occasion to use it. With increasing frequency, the sitting position is being avoided through the use of one of its alternatives (prone, semilateral, lateral). However, we are likely to continue encountering it because even surgeons who are inclined to use alternative positions may opt for the sitting position when access to midline structures (the floor of the fourth ventricle, the pontomedullary junction, and the vermis) is required. Nonetheless, alternative positions for posterior fossa surgery exist and should be considered when contraindications to the sitting position exist.

ACHIEVING THE SITTING POSITION.

A properly positioned patient is more commonly in a modified recumbent position as shown in Figure 53-8 rather than truly sitting. The legs should be kept as high as possible (usually with pillows under the knees) to promote venous return and thereby enhance circulatory stability. Ideally, the head holder should be attached to the back portion of the table (see Fig. 53-8A ) rather than to the portions under the thighs or legs (see Fig. 53-8B ). Such attachment will permit lowering of the head and closed-chest massage if necessary without the necessity of taking the patient out of the head holder.

When procedures are performed in the sitting position, the clinician should think in terms of measuring and maintaining perfusion pressure at the level of the surgical field. This objective is best accomplished by referencing transducers to the interaural plane. If a manual blood pressure cuff on the arm is used, a correction ^* to allow for the hydrostatic difference between the arm and the operative field should be applied.

A series of hazards are associated with the sitting position. Circulatory instability, macroglossia, and quadriplegia are discussed in this section. Pneumocephalus is discussed in the section "Pneumocephalus." Venous air embolism and paradoxical air embolism are discussed in the section "Venous Air Embolism." Several of these hazards are also relevant when cervical spine and posterior fossa procedures are performed in nonsitting positions, but they occur with greater frequency in the sitting position.

CARDIOVASCULAR EFFECTS OF THE SITTING POSITION.

Placing an anesthetized patient in the sitting position conveys some risk of impaired cardiovascular function, in particular, hypotension. Pressor administration will be 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. Both aggressive volume loading and the G-suit (a.k.a. pneumatic antishock trousers, MAST suit) have been shown to attenuate the effects of assuming the sitting position. ^{[81] [82] [83] [84]} However, neither of these measures has been widely applied. In most healthy subjects, the hemodynamic changes are of nonthreatening magnitude. In a study of healthy (American Society of Anesthesiologists [ASA] grade 1 and 2) anesthetized adult subjects aged 22 to 64 years, Marshall and coworkers ^[85] observed relatively modest changes. MAP was relatively unaffected, whereas wedge pressure, stroke volume, and the cardiac index decreased, the latter by approximately 15%, although some variation was noted with the anesthetics used. The combination of an unchanged MAP (which in general requires the use of a "light," high-sympathetic tone anesthetic) and a reduced cardiac index implies that systematic vascular resistance (SVR) rose. Their calculations and the observations of other investigators^[86] in fact reveal significant elevations in SVR. Accordingly, for patients in whom an abrupt increase in SVR may be poorly tolerated, the sitting position may represent a physiologic threat and alternative positions should be considered. A pulmonary artery catheter may be warranted in those with clinical or historical evidence of antecedent coronary artery or valvular heart disease and arbitrarily in patients older than 60 to 65 years.

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Figure 53-8 The sitting position. The patient is typically semirecumbent rather than sitting. In A, the head holder support is correctly positioned such that the head can be lowered without the necessity of first detaching the head holder. The configuration in B, with the support attached to the thigh portion of the table, should be avoided. (From Martin JT: Positioning in Anesthesia and Surgery. Philadelphia, WB Saunders, 1988.)

During procedures performed in the sitting position, MAP should be transduced at or corrected to head level to provide a meaningful index of CPP. Specifically, CPP (MAP minus estimated ICP) should be maintained at a minimum value of 60 mm Hg in healthy patients, in whom it is reasonable to assume normal cerebral vasculature. The safe lower limit should be raised for elderly patients, for those with hypertension or known cerebral vascular disease (or both), for those with degenerative disease of the cervical spine or cervical spinal stenosis because they 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.

MACROGLOSSIA.

Sporadic reports have described 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. ^[87] These episodes have been attributed to edema occurring at the time of reperfusion after trauma/prolonged ischemia as a result of foreign bodies (usually oral airways) causing pressure on these structures during 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 fingerbreadths' distance between the chin and the sternum to prevent excessive reduction of the anterior-posterior diameter of the oropharynx. In addition, it is our practice to position patients with the oral airway in place and then, once the final head position is achieved, withdraw it until its tip functions as a bite block between the teeth. Consideration of the macroglossia phenomenon may also be relevant as clinicians contemplate the use of transesophageal echocardiography (TEE) in the neurosurgery suite. For the most part, centers that routinely use TEE in neurosurgery use either pediatric or custom-made small-diameter probes to avoid trauma to the pharyngeal and perilaryngeal structures.

QUADRIPLEGIA.

The sitting position per se has been implicated as a cause of rare instances of unexplained postoperative paraplegia. It has been hypothesized ^[88] that neck flexion, which is a common concomitant of the seated position, may result in stretching or compression of the cervical spinal cord. This possibility may represent a relative contraindication to the use of this position in patients

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