Cerebral autoregulation maintains constant cerebral blood flow (CBF) over a mean arterial pressure (MAP) range of approximately 60–160 mmHg. A patient with chronic hypertension (MAP 130 mmHg for years) undergoes antihypertensive therapy, rapidly lowering MAP to 90 mmHg. The patient develops confusion and visual disturbances. The explanation is:

• A Chronic hypertension shifts the autoregulatory curve rightward; an MAP of 90 mmHg may now be below the lower limit of autoregulation, causing ischemia ✓
• B Rapid normalization of MAP causes hyperemia due to reset autoregulatory curve lower limit being higher than 60 mmHg
• C Antihypertensive drugs have direct neurotoxic effects on cortical neurons
• D The drop in MAP reduces cerebral venous drainage causing intracranial hypertension

Explanation

In chronic hypertension, the cerebral autoregulatory curve shifts rightward: the lower limit of autoregulation rises from ~60 mmHg to approximately 100–120 mmHg (and upper limit also rises accordingly). This rightward shift occurs due to adaptive hypertrophic changes in cerebral arteriolar walls. When MAP is rapidly reduced to 90 mmHg—which is within the normal autoregulatory range for healthy individuals—it may fall below the lower limit for a chronically hypertensive patient's shifted curve. Below the lower limit, CBF becomes pressure-dependent and falls, causing ischemic symptoms (confusion, visual changes—watershed zone ischemia). This is why aggressive acute BP lowering in hypertensive patients is dangerous; target reductions should be gradual (no more than 25% in first 24h in hypertensive urgency).

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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