A 50-year-old with chronic meningitis is found to have communicating hydrocephalus. Which arachnoid structures are responsible for CSF absorption into the venous sinuses, and where are they maximally concentrated?

• A Ependymal cells lining the choroid plexus in lateral ventricles
• B Arachnoid granulations (Pacchionian bodies) concentrated in the superior sagittal sinus and lateral lacunae ✓
• C Lymphatics draining along cranial nerve sheaths through the cribriform plate
• D Perivascular Virchow-Robin spaces surrounding cortical vessels

Explanation

Arachnoid granulations (Pacchionian bodies) are invaginations of the arachnoid mater into the walls of the superior sagittal sinus and its lateral lacunae. They act as one-way pressure-dependent valves, absorbing CSF from the subarachnoid space into venous blood when CSF pressure exceeds venous pressure by approximately 5 cm H2O. In communicating hydrocephalus (normal pressure hydrocephalus, meningitis), obstruction of CSF reabsorption at the granulations — rather than obstruction within the ventricular system — causes ventriculomegaly with patent aqueduct (communicating). The choroid plexus is the site of CSF production (0.35 mL/min), not absorption.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.