STEPTRACKERNERVOUS SYSTEM | PATHOLOGY + PHYSIOLOGY + ANATOMY | HIGH-YIELD REVIEW

Raised ICP, Herniation & Hydrocephalus

A closed skull leaves no room — swelling raises pressure, pressure pushes brain through openings, and blocked CSF piles up.

FOCUS ON
↑ ICP: headache, vomiting, papilledema, CN VI palsy; Cushing triad (hypertension, bradycardia, irregular breathing). Uncal herniation: ipsilateral blown pupil (CN III), contralateral PCA hemianopia, Duret hemorrhages. Tonsillar: medullary compression → death. Hydrocephalus: noncommunicating (blocked ventricles) vs communicating (poor absorption); NPH = wet, wobbly, wacky.
DISORDER
MECHANISM
FEATURES
CLUE
Raised intracranial pressure
Fixed skull volume (brain + blood + CSF); CPP = MAP − ICP
Headache (morning, lying down), vomiting, papilledema, CN VI palsy, ↓ consciousness
Cushing triad: HTN, bradycardia, irregular respirations
Vasogenic edema
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)
Expanding mass, ipsilateral fixed dilated pupil → uncal herniation
Brainstem bleeds after rapid herniation → Duret hemorrhages
Edema around a brain tumor improving on dexamethasone → vasogenic edema
Hydrocephalus with dilated lateral and 3rd but normal 4th ventricle → aqueductal stenosis
Positional headache and sudden death, 3rd ventricle mass → colloid cyst
Hydrocephalus after bacterial meningitis → communicating (arachnoid granulations)
Elderly with incontinence, magnetic gait, dementia → normal pressure hydrocephalus
Obese young woman on isotretinoin with papilledema, normal MRI → idiopathic intracranial hypertension
Big ventricles and wide sulci in Alzheimer disease → hydrocephalus ex vacuo