How to read: coronal slice — falx between the hemispheres, tentorium above the cerebellum, brainstem passing down through the foramen magnum; gray = ventricles. Red = what moves, swells or blocks; dark circle = mass.
Blood-brain barrier breakdown → extracellular fluid, mainly white matter
Around tumors, abscesses, hemorrhage, trauma
Responds to dexamethasone
Cytotoxic edema
Na⁺/K⁺ pump failure → intracellular swelling (gray + white matter)
Ischemia, hypoxia, hyponatremia, toxins
Steroids don't help
Subfalcine herniation
Cingulate gyrus slides under the falx
Compresses ACA → contralateral leg weakness
Most common type
Uncal (transtentorial) herniation
Medial temporal uncus over the tentorium
Ipsilateral CN III (blown pupil, down and out); PCA → contralateral hemianopia; contralateral peduncle (Kernohan notch) → ipsilateral hemiparesis
Duret hemorrhages (stretched basilar branches in pons)
Tonsillar herniation
Cerebellar tonsils through the foramen magnum
Medullary compression → respiratory and cardiac arrest
Why no LP with a mass lesion
Noncommunicating hydrocephalus
Obstruction inside the ventricular system: aqueductal stenosis, colloid cyst (3rd ventricle), posterior fossa tumor, Chiari II, Dandy-Walker
Ventricles dilated proximal to the block
Colloid cyst → positional headache, sudden death
Communicating hydrocephalus
↓ absorption at arachnoid granulations (after meningitis, SAH) or ↑ production (choroid plexus papilloma)
All ventricles dilated; infants: macrocephaly, bulging fontanelle, "sunset" eyes
VP shunt
Normal pressure hydrocephalus
Idiopathic communicating hydrocephalus in the elderly; periventricular fibers stretched
Urinary incontinence, magnetic gait, dementia; enlarged ventricles out of proportion to atrophy
Improves after large-volume LP → shunt
Hydrocephalus ex vacuo
Brain atrophy (Alzheimer, HIV, frontotemporal dementia) → ventricles look large
Normal ICP; not a CSF flow problem
Sulci widen too
Idiopathic intracranial hypertension
↑ ICP without a mass; obese women of childbearing age; vitamin A, isotretinoin, tetracyclines, OCPs
Headache, papilledema, transient visual obscurations, pulsatile tinnitus, CN VI palsy; normal imaging; ↑ opening pressure
Acetazolamide, weight loss · threatens vision
Mechanism — the CSF path
Choroid plexusMakes CSF
→
Lateral ventricles
→
Foramina of Monro
→
3rd ventricle
→
Cerebral aqueductNarrowest point
4th ventricle
→
Luschka (lateral) + Magendie (median)
→
Subarachnoid space
→
Arachnoid granulationsDural venous sinuses
Herniation signs
TYPE
STRUCTURE COMPRESSED
SIGN
Subfalcine
ACA
Contralateral leg weakness
Uncal
CN III
Ipsilateral dilated fixed pupil
Uncal
PCA
Contralateral hemianopia
Uncal
Opposite cerebral peduncle
Ipsilateral hemiparesis (false localizing)
Central
Midbrain, pons
Duret hemorrhages, posturing
Tonsillar
Medulla
Apnea, cardiovascular collapse
How it's tested
COMMON CONFUSIONNPH and Alzheimer both cause dementia with big ventricles. NPH adds gait and incontinence early and responds to CSF removal; ex vacuo dilation from atrophy does not.
THE TRAPA lumbar puncture in a patient with a mass lesion or obstructive hydrocephalus can trigger tonsillar herniation — image first when focal signs or papilledema are present.
YOUR APPROACHSigns of ↑ ICP → edema type or mass → herniation signs by side → ventricles (which are dilated?) → block level → treatment (steroids, shunt, acetazolamide, surgery).
If you see… think…
Hypertension, bradycardia, irregular breathing in head injury → Cushing reflex (↑ ICP)