The brain sits inside a rigid skull and has very limited room to expand. Any process that adds volume — mass lesion, edema, hemorrhage, hydrocephalus — runs up against the Monro-Kellie doctrine: the sum of brain, blood, and CSF inside the cranium is constant, so an increase in any component must be matched by a decrease in another or by an increase in intracranial pressure (ICP). When the compensation runs out, ICP rises, blood flow falls, brain shifts, and structures herniate. The neuropathology of cerebral edema, raised ICP, and herniation is among the most important to recognize at autopsy and on imaging because the findings often explain the patient’s clinical decline and death. This page covers the major types of edema, the patterns of herniation, their gross findings, and the secondary injuries they produce.

Cerebral Edema

Three classical types of cerebral edema, often overlapping in real disease:

Vasogenic Edema

Increased permeability of the blood-brain barrier with extravasation of plasma fluid into the extracellular space. The classical edema around:

  • Brain tumors (especially metastases and high-grade gliomas).
  • Brain abscesses.
  • Hemorrhage.
  • Contusion.
  • Demyelinating lesions (acute).

Vasogenic edema preferentially affects white matter because the loose extracellular matrix of white matter is more easily expanded than the tightly packed gray matter. On imaging: T2/FLAIR hyperintensity around the lesion, often finger-like extension along white matter tracts. Responds to steroids (which restore BBB integrity), especially in tumor edema.

Cytotoxic Edema

Intracellular swelling of neurons, astrocytes, or both, from cellular energy failure (sodium-potassium ATPase failure) and accumulation of intracellular water. The BBB is intact. Classical of:

  • Acute ischemic stroke (the substrate of restricted diffusion on DWI).
  • Severe hypoxia.
  • Reye syndrome.
  • Hyponatremia.
  • Some toxic encephalopathies.

Cytotoxic edema affects both gray and white matter. Does not respond to steroids (no BBB to repair). On imaging: restricted diffusion on DWI within minutes of onset; T2/FLAIR changes follow over hours.

Interstitial (Transependymal) Edema

Periventricular accumulation of CSF from acute hydrocephalus — CSF crosses the ependymal lining into the periventricular white matter. On imaging: T2/FLAIR hyperintensity surrounding the lateral ventricles in a smooth band (“transependymal flow”). Resolves with ventricular decompression.

Hydrostatic Edema

From rapid acute hypertension (hypertensive emergency, PRES). The autoregulation of cerebral vessels is overwhelmed; fluid extravasates through stretched endothelial junctions. Predilection for posterior circulation (parietal-occipital region) — the substrate of posterior reversible encephalopathy syndrome (PRES).

The Monro-Kellie Doctrine and ICP

The intracranial volume is fixed at approximately 1400-1700 mL in adults, distributed:

  • Brain parenchyma: ~80%.
  • Blood: ~10%.
  • CSF: ~10%.

When a mass is added, compensation occurs initially by:

  1. CSF displacement: CSF is pushed from the cranial vault into the spinal subarachnoid space.
  2. Venous blood displacement: blood is squeezed out of compressible cerebral veins.
  3. Once these reservoirs are exhausted, ICP rises sharply.

The pressure-volume curve is initially flat (compensation), then becomes very steep (decompensation). A small additional volume at the steep portion produces a large rise in ICP.

Cerebral Perfusion Pressure (CPP)

CPP = MAP − ICP. When ICP rises, CPP falls. Ischemic damage from inadequate perfusion follows. Treatment of raised ICP aims simultaneously to lower ICP and to support MAP.

Cushing Reflex

Severely raised ICP triggers a brainstem response: bradycardia + hypertension + irregular respirations. This is the classical Cushing triad — a sign of impending brainstem herniation and a neurosurgical emergency.

Patterns of Herniation

Subfalcine (Cingulate) Herniation

The cingulate gyrus is forced under the falx cerebri from one hemisphere into the contralateral compartment. Caused by unilateral supratentorial mass effect. Findings:

  • Midline shift.
  • Compression of the ipsilateral ACA against the falx → ACA territory infarct (medial frontal, paracentral) → contralateral leg weakness.
  • Contralateral ventricular enlargement from contralateral foramen of Monro obstruction.

Subfalcine herniation is one of the most common patterns and is often the first to appear with a large unilateral mass.

Transtentorial (Uncal) Herniation

The medial temporal lobe (uncus and parahippocampal gyrus) is forced through the tentorial notch into the posterior fossa. The classic and most clinically important pattern. Findings:

  • CN III compression: ipsilateral fixed dilated pupil — often the earliest sign.
  • PCA compression: occipital infarct → contralateral hemianopia, sometimes cortical blindness if bilateral.
  • Cerebral peduncle compression on the ipsilateral side: contralateral hemiparesis (expected).
  • Kernohan notch: the contralateral cerebral peduncle is forced against the opposite tentorial edge, producing ipsilateral hemiparesis — a famous “false localizing sign.”
  • Midbrain compression: progressive deterioration of consciousness, decerebrate posturing, midposition fixed pupils.
  • Duret hemorrhages: small linear hemorrhages in the central midbrain and upper pons from stretching/rupture of paramedian perforating arteries. Often a terminal finding.

Central (Transtentorial) Herniation

Bilateral or symmetric downward displacement of the diencephalon and midbrain through the tentorial notch from generalized swelling or bilateral mass effect. The classic “rostral-caudal deterioration” of clinical neurology:

  1. Diencephalic stage: small reactive pupils, Cheyne-Stokes breathing, decorticate posturing.
  2. Midbrain-upper pontine: midposition fixed pupils, decerebrate posturing.
  3. Lower pontine: loss of oculocephalic, ataxic breathing.
  4. Medullary: apnea, hemodynamic collapse, death.

Pathology shows symmetric downward displacement with Duret hemorrhages in the brainstem.

Tonsillar (Cerebellar) Herniation

Cerebellar tonsils forced through the foramen magnum, compressing the medulla. Causes:

  • Cerebellar mass effect (tumor, infarct with edema, hemorrhage, abscess).
  • Chronic descent (Chiari I malformation; usually asymptomatic at modest descent).
  • Acute decompression of supratentorial pressure (e.g., after lumbar puncture in patient with raised ICP) — the feared LP-induced herniation.

Acute tonsillar herniation:

  • Cerebellar tonsils visibly displaced caudally on imaging (and at autopsy, with “pressure cones” — molded shape).
  • Medullary compression → sudden apnea, cardiovascular collapse, death.

Upward (Transtentorial) Herniation

Posterior fossa mass forces the cerebellum and brainstem upward through the tentorial notch. Less common. Compression of midbrain, posterior cerebral arteries; obstruction of aqueduct → hydrocephalus.

External (Transcalvarial) Herniation

Brain protrudes through a skull defect — surgical decompression, craniotomy defect, fracture. Important to recognize because the extruded brain is at risk of injury, infection, and continued swelling.

Secondary Injuries from Herniation

Duret Hemorrhages

Small linear or “flame-shaped” hemorrhages in the central midbrain and upper pons, from stretching and rupture of paramedian perforating arteries during caudal brainstem displacement. Often bilateral, midline. Classical of severe central herniation. Often a terminal pathology — the patient dies of brainstem failure shortly after their appearance.

PCA Territory Infarcts

From uncal herniation compressing the PCA at the tentorial edge. Bilateral occipital infarcts can produce cortical blindness — sometimes seen as a survivor of severe central herniation.

ACA Territory Infarcts

From subfalcine herniation compressing the ACA. Contralateral leg weakness as a late finding.

Hydrocephalus

Mass effect can obstruct CSF pathways:

  • Foramen of Monro obstruction → ipsilateral lateral ventricle hydrocephalus.
  • Aqueductal compression → obstructive hydrocephalus involving both lateral and third ventricles.
  • Fourth ventricle obstruction → hydrocephalus of lateral, third, and fourth ventricles.
  • Subarachnoid space scarring → communicating hydrocephalus.

Hydrocephalus

Communicating Hydrocephalus

CSF can circulate through the ventricular system but cannot be absorbed normally. Causes:

  • Post-meningitic scarring of subarachnoid space.
  • Post-hemorrhagic (especially after SAH).
  • Idiopathic / age-related (normal-pressure hydrocephalus).
  • Carcinomatous meningitis blocking absorption.

Non-Communicating (Obstructive) Hydrocephalus

Block in the ventricular system itself:

  • Aqueductal stenosis (congenital or acquired).
  • Tumor obstructing third or fourth ventricle.
  • Colloid cyst at foramen of Monro.
  • Chiari I/II malformations.
  • Posterior fossa mass effect.

Pathologic Findings

  • Ventricular enlargement.
  • Periventricular interstitial edema.
  • Thinning of corpus callosum and septum pellucidum.
  • Effacement of cortical sulci.
  • Eventually atrophy of the cortex.

Normal Pressure Hydrocephalus (NPH)

Communicating hydrocephalus with normal opening pressure but enlarged ventricles. Clinical triad: gait apraxia (“magnetic gait”) + cognitive decline + urinary incontinence. Pathology: ventricular enlargement disproportionate to cortical atrophy (“hydrocephalus ex vacuo” pattern excluded); may show signs of chronic transependymal flow. Some patients respond dramatically to ventriculoperitoneal shunt.

Hydrocephalus ex Vacuo

Apparent ventricular enlargement due to underlying cerebral atrophy (Alzheimer disease, vascular dementia, etc.). Not true hydrocephalus — the CSF expands to fill the space left by atrophic brain. Pressure is normal. Distinguish from NPH where there is genuine fluid pathology.

Gross Pathology of Herniation at Autopsy

Classical findings to look for at brain examination:

  • Pressure cones: groove or notch on the medial temporal lobe (uncus) from forcing against the tentorium.
  • Tonsillar pressure cones: notched, mushroom-shaped tonsils from foramen magnum compression.
  • Midline shift: visible on coronal sections.
  • Duret hemorrhages: small punctate hemorrhages in central pons and midbrain.
  • Secondary infarcts: PCA, ACA, medial occipital, paracentral.
  • Mass lesion: tumor, hemorrhage, infarct, abscess.
  • Edema appearance: gyri flattened against the dura, sulci effaced.
  • Hydrocephalus: ventricular enlargement; site of obstruction if non-communicating.

Imaging Correlates

  • Midline shift: measured at septum pellucidum on axial CT/MRI.
  • Effaced sulci, compressed ventricle: signs of mass effect.
  • Loss of basal cisterns: severe global edema.
  • Uncal effacement: ipsilateral ambient cistern lost.
  • Tonsillar herniation: tonsils descend below foramen magnum line.
  • Duret hemorrhages: visible on SWI or GRE as low signal in central brainstem.
  • Hydrocephalus: enlarged ventricles + transependymal edema (T2/FLAIR hyperintense rim).

Management Principles (Brief)

  • Identify and treat the underlying cause.
  • Head elevation to 30°.
  • Optimize CPP (treat both ICP and MAP).
  • Osmotic therapy (mannitol 0.25-1 g/kg, hypertonic saline).
  • Hyperventilation (brief, controlled — caps PCO2 ~30-35; vasoconstriction lowers ICP transiently).
  • Steroids for vasogenic edema (tumor) — NOT for stroke or trauma.
  • External ventricular drain for hydrocephalus.
  • Surgical decompression (hemicraniectomy for malignant MCA infarct; suboccipital for cerebellar swelling).

🔍 Did You Know?

The classical “Kernohan notch” phenomenon — ipsilateral hemiparesis from a unilateral supratentorial mass — is one of the most clinically consequential false localizing signs in neurology. The mechanism is unusual: a large mass on one side (say, the left) pushes the brain to the opposite side, jamming the contralateral (right) cerebral peduncle against the tentorial edge. The right peduncle carries fibers that have not yet decussated — they will eventually control the left side of the body. Compression of the right peduncle therefore produces weakness on the left side of the body, which is the SAME side as the original supratentorial mass. The clinical implication is sobering: a patient with a left-sided mass who develops left-sided hemiparesis may be presumed to have a new lesion on the right — and the surgeon may operate on the wrong side. Recognition of Kernohan notch is one of the most important reasons that careful clinical-radiologic correlation is mandatory before any neurosurgical procedure. The phenomenon was first described by James Kernohan and Henry Woltman in 1929; the notch on the contralateral peduncle is visible at autopsy and on detailed imaging. The lesson generalizes: when the bedside findings don’t match the expected lateralization of an imaged lesion, consider that the displacement caused by the mass is producing secondary effects on adjacent structures.

Pitfalls and Pearls

  • Cerebral edema types: vasogenic (white matter, BBB breakdown, responds to steroids), cytotoxic (gray + white, intact BBB, restricted diffusion), interstitial (periventricular, hydrocephalus), hydrostatic (PRES).
  • Monro-Kellie doctrine: fixed cranial volume; small added volume eventually produces large ICP rise.
  • Cushing triad (bradycardia + hypertension + irregular respiration): impending herniation.
  • Uncal herniation: ipsilateral CN III palsy + contralateral hemiparesis + PCA infarct. Emergency.
  • Kernohan notch phenomenon: contralateral peduncle compression → ipsilateral hemiparesis (same side as the mass). Don’t operate on the wrong side.
  • Subfalcine herniation: cingulate forced under falx; ACA compression → contralateral leg weakness.
  • Tonsillar herniation: cerebellar mass → medullary compression → sudden apnea, cardiac arrest.
  • LP in raised ICP can precipitate fatal tonsillar herniation: image first.
  • Duret hemorrhages: central brainstem from severe herniation. Usually terminal.
  • Central herniation: rostral-caudal deterioration; sequential brainstem failure.
  • Steroids help vasogenic edema (tumor); do NOT help cytotoxic edema (stroke).
  • Cerebellar infarct can swell catastrophically. Suboccipital decompression is life-saving.
  • NPH: gait apraxia + cognitive decline + urinary incontinence; shunt-responsive in selected patients.
  • Hydrocephalus ex vacuo: atrophy filling with CSF; NOT true hydrocephalus.
  • Hyperventilation lowers ICP only briefly (CO2 buffering compensates within hours); use as bridge to definitive treatment.

References

  1. Love S, Budka H, Ironside JW, Perry A, eds. Greenfield’s Neuropathology. 9th ed. CRC Press; 2015.
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  3. Wijdicks EFM. The Practice of Emergency and Critical Care Neurology. 2nd ed. Oxford University Press; 2016.
  4. Kernohan JW, Woltman HW. Incisura of the crus due to contralateral brain tumor. Arch Neurol Psychiatry. 1929;21:274-287.
  5. Marmarou A. A review of progress in understanding the pathophysiology and treatment of brain edema. Neurosurg Focus. 2007;22(5):E1.
  6. Stocchetti N, Maas AI. Traumatic intracranial hypertension. N Engl J Med. 2014;370(22):2121-2130.