Monro-Kellie Doctrine — FRCA Primary MCQ
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Correct answer: C — The combined volumes of brain tissue, cerebrospinal fluid and intracranial blood remain constant
The correct answer is C. The classical Monro-Kellie doctrine treats the adult cranial vault as a rigid compartment containing brain tissue, cerebrospinal fluid and blood. Their combined volume is fixed, so expansion of one component must initially be offset by displacement or reduction of another; once compensation is exhausted, intracranial pressure rises. Cerebral blood volume is not independent of PaCO2 because carbon dioxide alters cerebral vascular tone. Cerebral perfusion pressure is approximately mean arterial pressure minus intracranial pressure, rather than mean arterial pressure alone. CSF production and absorption need not be instantaneously equal. Intracranial pressure is therefore not always constant: it changes with intracranial volume according to cranial compliance.
Reference: Benson JC et al. The Monro-Kellie Doctrine: A Review and Call for Revision. American Journal of Neuroradiology, 2022. https://pubmed.ncbi.nlm.nih.gov/36456084/