Intracranial and cerebrovascular disease

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What is normal cerebral blood flow (CBF)? At what level is CBF considered ischemic?

Normal CBF in humans is 40 to 60 mL/100 g/min (15% of cardiac output). The cerebral metabolic rate for oxygen (CMRO ₂ ) in adults is 3 to 4 mL/100 g/min (20% of whole-body oxygen consumption). The CBF at which ischemia becomes apparent on electroencephalogram (EEG) is 18 to 20 mL/100 g/min. The threshold between reversible and irreversible ischemia depends on many factors, but most importantly on how low CBF goes and how long it stays low.

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Define cerebral autoregulation. How is it affected in cerebrovascular disease?

Cerebral autoregulation is the ability of the brain vasculature to maintain CBF in a normal range over varying mean arterial pressures (50–150 mm Hg). To illustrate in a patient with carotid stenosis, the obstruction in the internal carotid artery causes a pressure drop beyond the obstruction. In an effort to maintain CBF, the cerebral vasculature dilates. Conversely, in a patient with uncontrolled hypertension, arteries in the brain vasoconstrict to limit blood flow. Importantly, in both cases, once compensatory mechanisms reach their limits, CBF becomes passive and is linearly correlated with systemic blood pressure. Patients with cerebrovascular disease likely have dysfunctional cerebral autoregulation and blood pressure control is critically important.

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What is cerebrovascular insufficiency?

Cerebrovascular insufficiency refers to any condition resulting from inadequate blood supply to the brain. Inadequate blood flow is most often from partial or complete arterial obstruction. This leads to inadequate delivery of essential oxygen and nutrients placing the affected tissue at risk of ischemia. Cerebrovascular accidents (CVA) or transient ischemic attacks (TIA) are common outcomes and the severity depends on the extent and location of the obstruction, as well as the duration of insufficient blood flow.

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What are the symptoms of cerebral ischemia?

Cerebral ischemia presentation depends on whether the injury is global or focal in nature. Global ischemia occurs over a wide area and is severe, often leading to loss of consciousness and widespread neurologic dysfunction. Cardiac arrest is the most common cause of global ischemia. Other causes include respiratory failure, heart failure, and congenital heart defects. In contrast, focal ischemia presents in numerous ways and neurologic dysfunction is much more limited. Focal insults can impair vision, motor function, and the ability to understand or speak language. Thrombotic and embolic phenomena are the most common causes of focal ischemia. Other causes include hemorrhage, vasospasm, and trauma.

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What are the factors that increase the risk for cerebral ischemic events?

The leading cause of a CVA is uncontrolled hypertension. Other risk factors include a previous CVA or TIA, diabetes, hypercholesterolemia, sickle cell disease, coronary artery disease, atrial fibrillation, and cardiac valvular disease. Lifestyle choices also play a significant role in the long-term CVA risk. High fat and salt diets, physical inactivity, obesity, excessive alcohol intake, and tobacco use are important and modifiable risk factors. Age, gender, ethnicity, and genetics are nonmodifiable risk factors. The elderly, women, and African descent and Latino populations are at higher risk of a CVA.

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