third ventricle

**Anatomy** - Embryology: derived from the prosencephalon → diencephalon (the lumen of the diencephalon). - Location: midline, between the two thalami (lateral walls) and hypothalamus (inferolateral walls/floor). The thalami are connected across it by the interthalamic adhesion (massa intermedia), which crosses the ventricle. - Anterior wall: lamina terminalis, anterior commissure, and columns of the fornix. Lamina terminalis contains the OVLT (organum vasculosum of the lamina terminalis), a circumventricular organ. - Floor: hypothalamic structures (optic chiasm, tuber cinereum/infundibulum, mammillary bodies), plus the tegmentum of the midbrain. Recesses: optic, infundibular, suprapineal, and pineal recesses. - Roof: tela choroidea with the choroid plexus of the third ventricle (CSF production). Lies beneath the fornix and above it the corpus callosum. - Posterior wall: pineal gland and the habenular and posterior commissures, and the opening of the cerebral aqueduct.

**CSF flow (high-yield)** Lateral ventricles → interventricular foramina of Monro → **third ventricle** → cerebral aqueduct of Sylvius → fourth ventricle → foramina of Luschka (lateral, paired) and Magendie (median) → subarachnoid space → arachnoid granulations → dural venous sinuses (superior sagittal sinus).

**Clinical significance** - **Obstructive (non-communicating) hydrocephalus:** - Colloid cyst at the foramen of Monro: classically causes intermittent positional headache, and can cause sudden death or drop attacks. Causes dilated lateral ventricles with a normal-sized third and fourth ventricle. - Aqueductal stenosis: causes dilated lateral and third ventricles with a normal fourth ventricle. May be congenital (X-linked L1CAM mutation) or acquired (infection, hemorrhage). - Pineal region tumors (germinoma, pinealoma): compress the aqueduct and the tectum. Cause Parinaud syndrome (upward gaze palsy, convergence-retraction nystagmus, light-near dissociation) and hydrocephalus. - **Hypothalamic involvement (walls/floor):** Craniopharyngioma (suprasellar, calcified, from Rathke pouch remnants; bitemporal hemianopia, hypopituitarism, diabetes insipidus) and hypothalamic lesions can disrupt temperature, thirst, appetite, and circadian rhythms. - **Wernicke encephalopathy:** Thiamine deficiency causes hemorrhagic lesions of the mammillary bodies, periventricular regions of the third ventricle, and periaqueductal gray (confusion, ophthalmoplegia/nystagmus, ataxia). Can progress to Korsakoff syndrome (anterograde and retrograde amnesia, confabulation). - **Normal pressure hydrocephalus:** a communicating hydrocephalus, with ventricular enlargement including the third ventricle (wet, wobbly, wacky: urinary incontinence, gait apraxia, dementia). - **Treatment of obstructive hydrocephalus:** Endoscopic third ventriculostomy (ETV) makes an opening in the floor of the third ventricle into the interpeduncular/prepontine cistern, bypassing the obstruction. Alternatively, a ventriculoperitoneal (VP) shunt is used. - **Imaging:** Normal third ventricle width is typically <5 mm in young adults (increases with age). Enlargement suggests hydrocephalus or atrophy (including in Alzheimer disease and Huntington disease, which involve the lateral ventricles more prominently).

**Pearls for boards** - Lateral ventricles + third dilated, fourth normal → aqueductal stenosis. - Only lateral ventricles dilated → foramen of Monro obstruction. - All four ventricles dilated → fourth ventricle outlet obstruction or communicating hydrocephalus (e.g., impaired absorption from meningitis or subarachnoid hemorrhage). - Circumventricular organs lacking a blood-brain barrier near the third ventricle: OVLT, subfornical organ, median eminence, pineal gland, and area postrema (fourth ventricle).