Inpatient neurologic consultations usually involve questions regarding specific disease processes or prognostication after various cerebral injuries. Common reasons for neurologic consultation include stroke (Chap. 446), seizures (Chap. 445), altered mental status (Chap. 34), headache (Chap. 21), and management of coma and other neurocritical care conditions (Chaps. 328 and 330). This chapter focuses on additional common reasons for consultation that are not addressed elsewhere in the text.
CONSULTATIONS REGARDING CENTRAL NERVOUS SYSTEM DYSFUNCTION
HYPERPERFUSION STATES LEADING TO POSTERIOR REVERSIBLE ENCEPHALOPATHY SYNDROME
A group of neurologic disorders shares the common feature of hyperperfusion, probably related to endothelial dysfunction, playing a key role in pathogenesis. These seemingly diverse syndromes include hypertensive encephalopathy, eclampsia, postcarotid endarterectomy syndrome, and toxicity from calcineurin-inhibitor and other medications. Modern imaging techniques and experimental models suggest that vasogenic edema is typically the primary process leading to neurologic dysfunction; therefore, prompt recognition and management of this condition should allow for clinical recovery as long as superimposed hemorrhage or infarction has not occurred.
The brain’s autoregulatory capability successfully maintains a fairly stable cerebral blood flow in adults despite alterations in systemic mean arterial pressure (MAP) ranging from 50 to 150 mmHg (Chap. 330). In patients with chronic hypertension, this cerebral autoregulation curve is shifted, resulting in autoregulation working over a much higher range of pressures (e.g., 70–175 mmHg). In these hypertensive patients, cerebral blood flow is kept steady at higher MAP, but a rapid lowering of pressure can lead to ischemia on the lower end of the autoregulatory curve, even at values typically thought of as normotensive. This autoregulatory phenomenon is achieved through both myogenic and neurogenic influences causing small arterioles to contract and dilate. When the systemic blood pressure exceeds the limits of this mechanism, breakthrough of autoregulation occurs, resulting in hyperperfusion via increased cerebral blood flow, capillary leakage into the interstitium, and resulting edema. The predilection of all of the hyperperfusion disorders to affect the posterior rather than anterior portions of the brain may be due to a lower threshold for autoregulatory breakthrough in the posterior circulation or a vasculopathy that is more common in these blood vessels.
Although elevated or relatively elevated blood pressure is common in many of these disorders, some hyperperfusion states such as calcineurin-inhibitor toxicity occur with no apparent pressure rise. In these cases, vasogenic edema is likely due primarily to dysfunction of the capillary endothelium itself, leading to breakdown of the blood-brain barrier. It is useful to separate disorders of hyperperfusion into those caused primarily by increased pressure and those due mostly to endothelial dysfunction from a toxic or autoimmune etiology (Table 463e-1). In reality, both of these pathophysiologic processes likely play some role in each of these disorders.