Which of the following correctly explains the Monro-Kellie doctrine and its application to intracranial hypertension?
- A The cranial vault is a fixed-volume rigid container; total volume of brain parenchyma, blood, and CSF is constant; an increase in any one compartment must be compensated by a decrease in at least one other compartment to maintain constant ICP, until compliance is exhausted ✓
- B Cerebral blood flow is constant regardless of perfusion pressure between MAP 60–150 mmHg due to myogenic autoregulation, independent of Monro-Kellie
- C The blood-brain barrier prevents entry of albumin, which is why ICP cannot be influenced by plasma osmolality
- D CSF is produced by the ependyma lining the cerebral ventricles and its rate is directly proportional to ICP
Explanation
The Monro-Kellie doctrine (refined by Kellie and later by Burrows) states that within the rigid cranial vault, the sum of volumes of brain (~80%), blood (~10%), and CSF (~10%) is constant. An expanding lesion (e.g., hematoma, tumor, edema) must be compensated by displacement of CSF into the spinal subarachnoid space and/or compression of venous blood, maintaining ICP in a range. Once compliance is exhausted (the compensatory reserve of CSF displacement is used up), even small volume increases cause exponential ICP rises. Option B describes cerebrovascular autoregulation, which is a separate concept but related in clinical practice. Option C incorrectly states plasma osmolality doesn't affect ICP; mannitol and hypertonic saline work precisely because BBB impermeability to osmoles allows osmotic water extraction. Option D: CSF production rate (~500 mL/day, ~0.35 mL/min) is independent of ICP within physiological ranges.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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