(cerebrovascular reserve capacity, CVRC, expressed as a percentage; also known as the vasomotor reserve) is calculated. For direct measurements of rCBF, methods that can be used include PET, stable Xe-CT, and quantitative SPECT; transcranial Doppler (TCD) ultrasonography only allows indirect estimates.
In physiological conditions, rCBF stimulation can be expected to produce at least a 30% increase in rCBF (CVRC > 30%—i.e., normal). Depending on the extent of hemodynamic restriction, there is then a reduction in reactive vasodilation, so that restricted CVRC is described as being present at a CVRC < 30%. At values < 10%, CVRC is regarded as having been eliminated. When there is a paradoxical reduction in rCBF (CVRC < –5%)—i.e., maximum vasodilation before stimulation and consequent nonreaction of the resistance vessels—it is assumed that an intracranial steal phenomenon is present. This is the most severe grade of hemodynamic impairment and is associated with the highest risk of secondary ischemia (Fig. 1.2-16).
Indications for revascularization
The following criteria generally arise:
With regard to the underlying pathology, there is considerable variability in the pathogenesis. In most cases, the patients have localized or systemic atherosclerosis. In a far smaller proportion of the patients, there is an indication for surgery due to inadequate collateral supply after carotid dissection or progressive tumor growth with resulting constriction or occlusion of the internal carotid artery, mostly in the area of the skull base. Patients with moyamoya disease or moyamoya syndrome represent a special case, which is discussed separately below.
Surgical technique
The aim of the surgical procedure is to normalize the CVRC and thus to “restore” physiological perfusion conditions. The technique used is a standard extracranial–intracranial (EC/IC) bypass. A donor vessel in an extracranial site, usually the superficial temporal artery (STA), is anastomosed with a cortically located branch of the middle cerebral artery (MCA) in the area of the lateral sulcus (sylvian fissure). The procedure is carried out with the patient under general anesthesia, with endotracheal intubation and neuroprotective measures to prevent ischemic complications. After preparation of the donor vessel (the STA), a craniotomy with a diameter of approximately 3 cm is carried out in the area of a defined target point over the lateral sulcus. After opening of the dura mater and exposure of a suitable recipient vessel (the middle cerebral artery in the M2/M3 segment), the standard bypass is placed using an end-to-side technique with 10–12 interrupted sutures (Fig. 1.2-17). The patency of the bypass that has been created can usually be immediately documented intraoperatively using indocyanine green (ICG) video angiography. Thanks to improved perioperative treatment, more sophisticated surgical techniques, and the ability to check the success of the procedure immediately, it is now possible to carry out this procedure with only minor surgical morbidity (< 5%). The operation is only carried out after adequate inhibition of platelet aggregation (100 or 325 mg ASA/d p.o.). Among other things, this makes it possible to achieve a bypass patency of > 98%. Platelet aggregation inhibition starts before the operation and continues on a lifelong basis.
Fig. 1.2–16a, b A standard extracranial–intracranial (EC/IC) bypass in a patient with atherosclerotic occlusion of the right internal carotid artery. (a) Conventional angiography reveals insufficient collateral supply to the right hemisphere. Angiographic demonstration of the STA-MCA bypass using the right superficial temporal artery (STA), which was anastomosed with a distal branch of the middle cerebral artery (MCA). (b) Anteroposterior view.
In addition, selective DSA and MRI are carried out postoperatively. Specific follow-up treatment for the patients is not necessary. When the indications described above are observed, the principles of microsurgery are rigorously applied, and the relevant expertise is present, it is possible to reduce the risk of secondary ischemia significantly in comparison with conservative treatment.
Special case: moyamoya disease and moyamoya syndrome
This is a rare, progressive steno-occlusive disease in which, due to unexplained causes, slowly progressive occlusion of the cerebral arteries in the region of the circle of Willis occurs. In parallel with this, spontaneous intracerebral and also extracranial–intracranial formation of collateral vessels is seen. These are regarded as outstanding examples of the way in which complex natural collaterals can develop on the basis of chronic ischemia. Both steno-occlusive changes with hemodynamically significant impairment of the cerebral blood supply along with spontaneous compensation mechanisms in the form of neoangiogenesis and arteriogenesis are therefore seen in patients with moyamoya disease. In addition to the impressive angiographic findings, the disease is distinct from other steno-occlusive diseases in relation to its epidemiology and clinical course (Fig. 1.2-18). The diagnostic characteristics of moyamoya disease (bilateral lesions, spontaneous collateral network) are more frequently associated with other diseases, sometimes systemic ones. In these cases, the condition is known as moyamoya syndrome. However, this does not generally lead to any changes in the treatment strategy. In central Europe, moyamoya disease and moyamoya syndrome occur sporadically and affect both children and adults. In contrast to the Asian form, which usually becomes manifest in adults in the form of intraparenchymal hemorrhage, cerebral ischemia is the major symptom in both age groups in Europe. Although it is mainly adult patients who suffer TIAs or stroke, the disease can also occur in children, often in the context of focal epilepsy or a cerebral organic psychological syndrome, resulting in inadequate formation of spontaneous collaterals and cerebral compensation mechanisms.
Fig. 1.2–17a, b Intraoperative view. (a) Positioning of the patient for planned placement of a superficial temporal artery (STA)–middle cerebral artery (MCA) anastomosis. The planning incision line (blue) is shown, along with the course of the superficial temporal artery (red) and the planned craniotomy. (b) Completed bypass in the area of the lateral sulcus (sylvian fissure).
Fig. 1.2–18a-c Typical angiographic findings in adult moyamoya disease. There are bilateral steno-occlusive changes in the area of the intracranial carotid bifurcation, with simultaneous formation of extensive basal collaterals.
Indications for revascularization
In view of the poor results of conservative therapy and the underlying pathogenesis of the disease, revascularization surgery
