1.1.1 Basic Physiology of Cerebral Blood Flow

The normal cerebral blood flow (CBF) of 50 mL/100g/min^-1 is dependent on cerebral perfusion pressure (CPP), i.e., the difference between mean arterial pressure and intracranial pressure (MAP − ICP).

Three main principles regulate CBF: (1) flow-metabolism coupling, (2) autoregulation, and (3) carbon dioxide (CO₂) reactivity. In regions of increased metabolic activity the local CBF is increased by vasodilation of arterioles to deliver more oxygen and glucose, whereas vasoconstriction is encountered in phases of diminished activity.

In healthy adults, cerebral autoregulation keeps CBF constant within blood pressure ranges between 50 and 150 mm Hg, thus preventing cerebral ischemia. Cerebral vessels react to arterial partial pressure of carbon dioxide (PaCO₂) by responding to hypercapnia with vasodilation and vice versa.

As described in the Monro-Kellie doctrine, the intracranial volume is the sum of brain tissue, intracranial blood volume, and cerebrospinal fluid and is limited by the non-expandable skull.

The ICP-volume curve is nonlinear and shows the relationship between intracranial volume and ICP. When the initial intracranial volume is low and compensatory mechanisms are not exhausted, an increase in intracranial volume produces a small change in ICP. On the steep part of the curve a similar increase of intracranial volume results in a large increase of ICP, resulting in a decrease of CPP, respectively CBF (Fig. 1‑1).

1.1.2 What Is Different in Patients with Moyamoya Disease?

Moyamoya is characterized by chronic progressive stenotic to occlusive changes in the terminal parts of the intracranial internal carotid arteries including the proximal parts of anterior and middle cerebral arteries. A compensatory fine vascular network is developed. Classically, moyamoya disease (MMD) is present bilaterally, but may also develop unilaterally. In these compromised areas, the cerebrovascular reactivity and the cerebral hemodynamic reserve capacity are impaired, causing transient ischemic attacks (TIAs) or strokes. ¹ The risk of impaired autoregulation may be even higher in pediatric patients. ²

Furthermore, the fragile moyamoya vessels are prone to hemorrhage. Typically, CBF shows a paradoxic reactivity to a vasodilatory stimulus in the altered areas. The altered moyamoya vessels are already maximally dilated to provide adequate oxygen supply and perfusion to the brain tissue. These vessels cannot react to a stimulus like hypercapnia the way normal vessels do. Thus, in a hypercapnic state flow will increase in brain areas of preserved normal vasculature and decrease in moyamoya affected vessels, leading to insufficient perfusion. This regional redistribution of blood flow to healthy areas is called “steal phenomenon” ³ and might clinically present as a neurologic deficit.

1.2.1 Choice of Anesthesia Technique

The superior aim of anesthesia for revascularization procedures is to ensure adequate perfusion and oxygenation of the brain and to avoid ischemic episodes. The ideal anesthetic agent should deliver smooth and hemodynamically stable anesthesia, good operating conditions (“slack brain”), and a smooth and rapid emergence to allow early neurological assessment. Cerebral perfusion pressure should be maintained, autoregulation and CO₂ reactivity should be preserved.

There are some studies that have investigated propofol-maintained versus inhalational-maintained anesthesia in adult patients undergoing elective craniotomy. Both strategies were associated with similar brain relaxation, although mean ICP values were lower and CPP values higher with propofol-maintained anesthesia. The recovery profiles, e.g., eye opening, tracheal extubation, obeying verbal commands, and orientation varied only in the range of minutes without clinical significance. Also the incidence of postoperative pain, seizures, and agitation were similar with both techniques. Nevertheless, the incidence of postoperative nausea and vomiting (PONV) was significantly lower during propofol-maintained anesthesia. ⁴

Concerning moyamoya patients, both anesthetic concepts are established and no significant differences in patient outcome were noted. Rather the carefully titrated induction drugs and good control of blood pressure, oxygenation, and stability of CO₂ level are determinative. ⁵

In authors’ opinion, there are some important arguments in favor of total intravenous anesthesia: the lower rate of PONV, ⁶ the better preservation of the regional cortical blood flow in the frontal lobe in comparison to sevoflurane, ⁷ the occurrence of steal phenomenon with inhalational anesthesia, ⁸ and finally, a positive practical experience with this technique for intracranial surgery over the past 20 years in their center.

1.2.2 Preoperative Evaluation and Premedication

Patients with MMD often present with many other medical conditions, which may impact anesthetic management. Therefore, a profound preoperative anesthetic assessment is necessary and special attention should be paid to the preexisting neurologic deficits and the neurologic physical status. Motor deficits or epilepsy are signals of chronic ischemia. A history of frequent TIAs, prolonged intermittent neurologic deficits, or stroke should draw attention to an already impaired cerebral blood supply in these patients, and has been identified as a significant risk factor for perioperative complications. ⁹ Preoperative evaluation must also include the determination of the individual baseline blood pressure, which involves several measurements before the day of surgery. A comparative blood pressure measurement on both arms is recommended to exclude falsely low blood pressure measurement intraoperatively due to, for example, subclavian artery stenosis.

Hypertension is found in some patients as a compensatory mechanism for cerebral vascular insufficiency. Caution is necessary when attempting to treat an elevated blood pressure in these patients.

Special attention has to be paid to the patient’s chronic medication. Anticonvulsive and antihypertensive medication should be continued until the day of surgery.

Regarding the antiplatelet-medication in MMD patients, the practice of continuing the medication varies among centers. The perioperative application of aspirin and the postoperative antiplatelet therapy have become controversial. Some centers are giving antiplatelet-medication while others have abandoned them. In our center we determine the effectiveness of aspirin in each patient through a platelet-function test. Thereby detected aspirin nonresponders receive alternative antiplatelet agents. ¹⁰

Premedication should be prescribed carefully. Anxiolysis may be necessary and beneficial in children with MMD, as crying should be strictly avoided, because the resultant hyperventilation may lead to hypocapnia and consecutively to cerebral vasoconstriction, resulting in cerebral ischemia. Vice versa oversedation followed by hypoventilation should also be avoided.

Midazolam is most often used for premedication, but other drugs can also be used. ¹¹

1.2.3 Monitoring

The American Society of Anesthesiologists (ASA) standard monitoring should be extended to invasive arterial blood pressure monitoring and urine output measurement. Anesthesiologists should consider placing the arterial line prior to induction, especially if preexisting medical conditions prompt it, and if the procedure is not considered too stressful for the patient.

Continuous arterial blood pressure monitoring intra- and postoperatively is the key for keeping the blood pressure within a predefined range (see Chapter 1.2.4).

Adequate venous access is essential and can be established by two “well-running” intravenous lines. A central venous catheter is not mandatory but should be considered in patients with very poor venous access or severe coexisting medical conditions.

Cerebral function can be monitored in various ways. Most reliable techniques are the combined transcranial motor-evoked potentials (MEP) and sensory-evoked potentials (SEP) monitoring. Cerebral function monitoring is of crucial importance especially in pediatric patients and in unstable adult patients, because they may experience strokes even after short-term blood pressure drops. Electroencephalography can help identify focal slowing, indicating a compromise CBF. Although near-infrared spectroscopy (NIRS) is only validated for measurement of cerebral oxygen saturation on the forehead, it has been shown that a sustained drop in regional oxygen saturation is closely related to the occurrence of neurological events following surgery, ¹² and thus NIRS may provide useful information intraoperatively.