Brain Herniation Syndromes

Increased Intracranial Pressure (ICP) Generation

• Focal mass lesions (hemorrhage, tumor, abscess) create a space-occupying lesion that displaces adjacent brain tissue
• Cerebral edema (cytotoxic from ischemia/hypoxia or vasogenic from blood-brain barrier disruption) increases brain parenchymal volume
• Hydrocephalus impairs CSF circulation and absorption, increasing ventricular pressure
• Increased cerebral blood volume from hypercapnia, hypoxemia, or impaired venous drainage elevates ICP exponentially
• ICP elevation follows exponential pressure-volume curve; once compliance is exhausted (typically ICP >20 mmHg), small volume increases cause disproportionate ICP rises
• Cerebral perfusion pressure (CPP = MAP - ICP) falls as ICP rises, triggering ischemic cascade and further edema

Critical Anatomical Displacement Mechanisms

Cingulate (Subfalcial) Herniation

• Medial frontal lobe herniates under the falx cerebri (rigid dural fold separating hemispheres)
• Results from unilateral supratentorial mass or swelling pushing ipsilateral anterior cingulate gyrus across midline
• Compresses ipsilateral anterior cerebral artery between herniated tissue and falx, causing anterior cerebral artery territory infarction
• Often represents early/warning sign of impending more severe herniation
• Clinical consequence: contralateral lower extremity weakness (motor cortex compression)

Uncal (Transtentorial) Herniation

• Medial temporal lobe (uncus of parahippocampal gyrus) herniates through tentorial notch
• Compresses ipsilateral CN III (oculomotor nerve) against tentorial edge, causing ipsilateral pupillary dilation and "down-and-out" eye positioning (CN III palsy)
• Progressively compresses brainstem, midbrain, and pons
• Shifts contralateral cerebral peduncle against opposite tentorial edge (Kernohan notch phenomenon), producing contralateral motor signs
• Distorts midbrain, causing central midbrain syndrome with bilateral pupillary changes
• Extends downward to compress pons and medulla, compromising vital respiratory and cardiovascular centers

Downward Transtentorial (Central) Herniation

• Bilateral hemispheric swelling or centrally located mass pushes both hemispheres downward through tentorial notch
• Compresses midbrain symmetrically, then pons, then medulla
• Produces characteristic progression: small "pinpoint" pupils (midbrain/upper pons compression) → loss of pupil reactivity → respiratory changes → cardiovascular collapse
• Often triggered by severe diffuse brain edema, extensive hemorrhage, or massive infarction

Upward (Transtentorial) Herniation

• Infratentorial mass (brainstem tumor, hemorrhage, abscess) pushes dorsal midbrain and upper pons through tentorial notch superiorly
• Results in downward compression of tectal structures by free edge of tentorium
• Produces obstructive hydrocephalus via aqueductal compression
• Can cause vertical gaze palsy, pupillary dilation, obtundation

Tonsillar (Foramen Magnum) Herniation

• Cerebellar tonsils herniate through foramen magnum due to infratentorial pathology or severe intracranial hypertension
• Compresses medulla, compromising vital cardiorespiratory centers
• Produces characteristic "Cushing's triad" (hypertension, bradycardia, irregular respirations)
• Often represents terminal event
• May occur with posterior fossa mass or severe diffuse brain edema

Molecular and Cellular Mechanisms

• Cytotoxic edema: Ischemia impairs ATP-dependent Na+/K+-ATPase, causing intracellular sodium accumulation and osmotic water influx
• Vasogenic edema: Blood-brain barrier disruption from inflammation, hemorrhage, or tumor allows fluid extravasation into extracellular space
• Excitotoxicity: Ischemia-induced glutamate release activates NMDA and AMPA receptors, causing calcium influx and cell death
• Oxidative stress: Mitochondrial dysfunction and free radical generation perpetuate cellular injury cascade

Traumatic Causes

• Epidural hemorrhage: Arterial bleeding (usually middle meningeal artery) between dura and calvarium; classic "talk and die" syndrome with lucid interval before deterioration
• Acute subdural hemorrhage: Venous bleeding between dura and arachnoid; high mortality in elderly patients on anticoagulation
• Contusion and diffuse axonal injury: Direct parenchymal injury with traumatic axonal shearing causing immediate brain swelling
• Traumatic subarachnoid hemorrhage: Associated with severe diffuse injury and cytotoxic edema

Vascular Causes

• Intracerebral hemorrhage (ICH): Hypertensive hemorrhage (putamen, thalamus, pons, cerebellum) most common; amyloid angiopathy in elderly; coagulopathy
• Acute ischemic stroke: Large territory infarction with malignant edema (MCA territory most common); space-occupying effect peaks 3-5 days post-stroke
• Subarachnoid hemorrhage: Vasospasm, rebleeding, and hydrocephalus cause secondary edema and ICP elevation
• Venous sinus thrombosis: Impaired venous drainage causes increased venous pressure, edema, and hemorrhage

Mass Lesions

• Primary brain tumors: Glioblastoma, astrocytoma; perilesional edema often exceeds tumor volume
• Secondary metastases: Especially melanoma, renal cell, lung; often multiple lesions with significant edema
• Brain abscess: Pyogenic infection (Staph, Strep) or fungal (Aspergillus, Cryptococcus); perilesional edema proportional to virulence
• Epidural/subdural hematomas: See traumatic causes above

Infectious Causes

• Meningitis: Bacterial (N. meningitidis, S. pneumoniae) > viral; inflammation and vasculitis cause edema and vasogenic hydrocephalus
• Encephalitis: Viral (HSV, VZV, enterovirus) or bacterial; parenchymal inflammation with edema
• Ventriculitis/ventriculoencephalitis: CSF infection with ependymal inflammation and obstructive hydrocephalus

Metabolic/Systemic Causes

• Hypoxemic respiratory failure: CO₂ retention causes cerebral vasodilation and increased ICP; oxygen desaturation impairs metabolism
• Hepatic encephalopathy: Ammonia toxicity causes cytotoxic edema; altered GABA neurotransmission
• Hypertensive emergency: Autoregulation failure with vasogenic edema; posterior reversible encephalopathy syndrome (PRES)
• Hypoglycemia/hyperglycemia: Ischemic edema (hypoglycemia) or osmotic edema (hyperglycemia, hypernatremia, hyperosmolarity)
• Diabetic ketoacidosis: Cerebral edema from osmotic shifts and metabolic acidosis
• Water intoxication/SIADH: Hyponatremia (<120 mEq/L) causes cytotoxic edema
• Anoxic encephalopathy: Cardiac arrest, severe hypoxemia

Obstructive Causes

• Hydrocephalus: Obstruction at cerebral aqueduct, fourth ventricular outlet, or impaired reabsorption; increases pressure throughout ventricular system and parenchyma
• Posterior fossa mass: Tumor (medulloblastoma, ependymoma, pilocytic astrocytoma) compressing fourth ventricle
• Aqueductal stenosis: Congenital or acquired obstruction

Reversible Causes Requiring Specific Management

• Anaphylaxis/angioedema: Medication-induced (ACE inhibitors) or hereditary angioedema; bilateral subcortical edema
• Eclampsia/PRES: Hypertensive emergency in pregnancy; mostly reversible with BP control
• Drug toxicity: Amphetamines, cocaine, isoniazid causing seizures and edema

Risk Factors for Herniation Development

• Age <40 years (worse prognosis paradoxically due to brain swelling vs. atrophy in elderly)
• Bilateral lesions or midline shift
• Obliteration of perimesencephalic cisterns on imaging
• Elevated initial ICP (>25 mmHg)
• Hypoxemia, hypercarbia, hypotension during acute illness
• Delay in definitive treatment (surgical evacuation, external ventricular drain)
• Anticoagulation/coagulopathy in hemorrhagic conditions
• Therapeutic hypothermia withdrawal (causes rebound edema)

Early Signs of Increased Intracranial Pressure (Before Herniation)

• Headache: Often worst of life (especially SAH); progressive, unrelenting
• Altered mental status: Restlessness, confusion, lethargy progressing to obtundation
• Nausea/vomiting: From brainstem compression; often projectile
• Vision changes: Diplopia, photophobia, blurred vision
• Papilledema: Optic disc swelling with blurred margins (takes hours to develop; may be absent in acute ICP rise)

Cingulate (Subfalcial) Herniation

• Contralateral lower extremity weakness: From anterior cerebral artery compression (Broca's area compression may cause expressive aphasia if dominant side)
• Subtle presentation often missed until larger herniation develops
• Abulia: Lack of motivation/initiative from medial frontal involvement
• Altered mental status if bilateral involvement
• Not typically associated with ipsilateral pupil changes (distinguishes from uncal)

Uncal (Ipsilateral Transtentorial) Herniation—Classic Progression

• Ipsilateral pupil dilation (CN III parasympathetic fibers compressed): Initially sluggish reactivity to light, then fixed and dilated ("blown pupil")
• Ipsilateral "down-and-out" eye: CN III motor compression causes eye depression and abduction (CN VI relatively spared initially)
• Contralateral hemiparesis (from contralateral cerebral peduncle compression or ipsilateral medullary tract compression)
• Preserved consciousness or mild obtundation initially
• Ipsilateral motor weakness may appear (Kernohan notch—contralateral peduncle against tentorial edge)

• Both pupils become small (2-4 mm) and react sluggishly to light (mid-position pupils)
• Bilateral pupil dilation may occur with deeper compression
• Decerebrate posturing: Rigid extension of all extremities (flexor and extensor tone imbalance favoring extensors); indicates midbrain/pons damage
• Loss of oculocephalic reflexes ("doll's eyes" - brainstem reflex absent)
• Respiratory changes begin (see below)
• Altered thermoregulation: Poikilothermia (temperature follows environment)

• Pinpoint pupils (1-2 mm): From pons compression with sympathetic fiber loss; reactive but response barely visible
• Decorticate posturing (if present) replaced by flaccidity as pons damaged
• Hyperventilation then Cheyne-Stokes respiration: Irregular breathing pattern with cycles of increasing then decreasing tidal volume
• Loss of corneal reflexes
• Progression to respiratory depression and apnea

• Cardiorespiratory collapse: Hypertension then hypotension, bradycardia then tachycardia, respiratory failure
• Cushing's triad (not pathognomonic): Hypertension + bradycardia + irregular respirations (indicates severe brainstem compression)
• Fixed, dilated pupils
• Complete loss of brainstem reflexes
• Isoelectric EEG

Central (Downward Transtentorial) Herniation

• Bilateral small reactive pupils: "Pinpoint" appearance (2-3 mm) reactive to light
• Bilateral motor findings: Initially hyperreflexia, rigidity, bilateral Babinski signs; progresses to decerebration and then flaccidity
• Yawning and sighing: From brainstem irritation
• Early deep coma without initial focal deficits (unlike uncal)
• Cheyne-Stokes respiration progressing to ataxic breathing
• Rapid progression to medullary compression and cardiovascular collapse
• No single blown pupil (key difference from uncal)

Upward Transtentorial Herniation

• Downward gaze palsy: Loss of vertical eye movements (supranuclear)
• Pupil dilation: Dorsal midbrain compression
• Obtundation progressing to coma
• Hydrocephalus signs: May precede other herniation signs
• Ataxia and gait disturbance (if in posterior fossa mass)

Tonsillar (Foramen Magnum) Herniation

• Cushing's triad: Hypertension, bradycardia, irregular respirations (indicative of medullary compression)
• Neck stiffness: From irritation of meninges/cord
• Ataxia and gait disturbance
• Respiratory depression/apnea: Sudden onset, often at night (sleep apnea-like)
• Loss of gag reflex and other lower CN functions (CN IX, X, XI, XII)
• May present with sudden death from cardiac arrhythmia or apnea

Respiratory Patterns in Herniation (in order of appearance with deepening coma)

1. 1.Cheyne-Stokes respiration: Smooth crescendo-decrescendo cycle (metabolic/bilateral deep structure lesion); seen with uncal and central herniation
2. 2.Neurogenic hyperventilation: Sustained rapid deep breathing (midbrain/upper pons damage); rate 25-40 breaths/min
3. 3.Apneustic respiration: Prolonged inspiration followed by short expiration (mid-to-lower pons); indicates severe brainstem damage
4. 4.Cluster/ataxic respiration: Irregular pattern with irregular intervals (lower pons/upper medulla); immediately preceeds apnea
5. 5.Apnea: Complete cessation of breathing (medullary death)

Physical Exam Findings Summary

• Pupils: Single blown pupil (uncal) vs. bilateral small pupils (central/tonsillar) vs. mid-position sluggish (early transtentorial)
• Eye position: Down-and-out (CN III palsy) vs. convergence (midbrain) vs. divergence (pons/medulla)
• Oculocephalic reflexes ("doll's eyes"): Present initially, lost with brainstem compression; tests CN VI (abducens) intact medial longitudinal fasciculus pathway
• Motor response: Localizes to pain → withdraws → flexor (decorticate)

Initial test

• Noncontrast head CT: fast, detects blood, mass, hydrocephalus, and shift. Obtain emergently in any patient with a falling Glasgow Coma Scale score, new anisocoria, or Cushing reflex.
• Key CT findings: midline shift at the septum pellucidum/pineal (shift beyond roughly 5 mm is generally considered significant and correlates with depressed consciousness); effacement of the basal/perimesencephalic cisterns and sulcal effacement; uncus and hippocampus filling the suprasellar cistern; downward displacement of the brainstem; cerebellar tonsils below the foramen magnum; and Duret hemorrhage — linear midbrain/pontine hemorrhage from stretched perforators, a late and ominous finding.

Confirmatory and adjunctive testing

• MRI: superior for posterior fossa lesions, early infarct, encephalitis, and tonsillar descent, but reserved for stable patients.
• CTA/MRV: if aneurysmal subarachnoid hemorrhage or venous sinus thrombosis is suspected as the driver.
• Invasive ICP monitoring: the reference standard for pressure. The Brain Trauma Foundation recommends monitoring in severe traumatic brain injury (GCS 3–8) with an abnormal CT; an external ventricular drain both measures and treats by draining CSF. BTF supports treating sustained ICP above approximately 22 mmHg and maintaining cerebral perfusion pressure in the roughly 60–70 mmHg range.
• Bedside adjuncts: optic nerve sheath diameter on ocular ultrasound and transcranial Doppler pulsatility suggest raised ICP but do not replace CT or a monitor.

Scoring and pitfalls

• GCS quantifies depth of coma and drives the intubation decision (≤8); FOUR score adds brainstem reflexes and respiratory pattern in intubated patients.
• Lumbar puncture is contraindicated before imaging when focal deficits, papilledema, or depressed consciousness suggest a mass — removing lumbar CSF can precipitate tonsillar herniation.
• Papilledema requires hours to develop and may be absent in hyperacute ICP rise; its absence never excludes herniation.

Immediate stabilization (minutes, before definitive imaging is fully reviewed)

• Airway and oxygenation: intubate for GCS ≤8 or loss of airway reflexes; pretreat to blunt the ICP surge and avoid hypoxia and hypotension, which the Brain Trauma Foundation identifies as major drivers of secondary injury.
• Positioning: head of bed to 30 degrees, head midline, cervical collar loosened — optimizes jugular venous outflow.
• Ventilation: target normocapnia (PaCO₂ roughly 35–40 mmHg). Brief hyperventilation to the low 30s causes cerebral vasoconstriction and can buy minutes during active herniation, but is a bridge only, not a maintenance strategy.

First-line pharmacotherapy — hyperosmolar therapy

• Osmotic diuretic: mannitol (0.25–1 g/kg IV bolus) creates an osmotic gradient drawing water from brain parenchyma; requires an intact blood–brain barrier and a systolic pressure that tolerates diuresis.
• Hypertonic saline: 3% infusion or a 23.4% bolus via central access; the Neurocritical Care Society hyperosmolar therapy guideline favors hypertonic saline in many ICP-crisis scenarios, and it is preferred when the patient is hypotensive or hypovolemic.
• Sedation and analgesia (propofol, fentanyl) reduce cerebral metabolic rate and cough/Valsalva-driven ICP spikes; add neuromuscular blockade if shivering or ventilator dyssynchrony persists.
• Corticosteroid: dexamethasone only for vasogenic edema around tumor or abscess.

Escalation and definitive management

• CSF diversion via external ventricular drain — first move if hydrocephalus is present.
• Surgical evacuation of epidural, subdural, or cerebellar hematoma; suboccipital decompression for posterior fossa mass with brainstem compression.
• Decompressive craniectomy for refractory ICP (RESCUEicp) and, per AHA/ASA stroke guidelines, for malignant middle cerebral artery infarction, with greatest benefit in younger patients treated early.
• Refractory cases: barbiturate coma, targeted temperature management.

Contraindicated

• Steroids in traumatic brain injury — the CRASH trial showed increased mortality; BTF recommends against them. Also not recommended in ischemic stroke edema by AHA/ASA.
• Hypotonic fluids (D5W, half-normal saline) worsen cerebral edema.
• Prolonged or prophylactic hyperventilation — causes ischemia.
• Lumbar puncture with an unevacuated mass.

Complications of herniation itself

• Duret hemorrhage (emergency, usually fatal): downward brainstem displacement shears perforating branches of the basilar artery, producing midbrain/pontine hemorrhage. Signaled by abrupt deep coma with fixed midposition pupils after apparent stabilization.
• Posterior cerebral artery infarction: the PCA is compressed against the tentorial edge during uncal herniation. Survivors show contralateral homonymous hemianopia, often with macular sparing.
• Anterior cerebral artery infarction: from cingulate herniation compressing the ACA against the falx; signaled by contralateral leg-predominant weakness and abulia.
• Kernohan notch phenomenon: contralateral peduncle compressed against the opposite tentorial edge produces hemiparesis ipsilateral to the mass — a classic false localizing sign.
• Obstructive hydrocephalus (emergency): aqueductal or fourth-ventricular compression; worsening consciousness with enlarging temporal horns on CT.
• Central diabetes insipidus: pituitary stalk traction or hypothalamic ischemia; dilute high-volume urine with rising serum sodium — often heralds brain death.
• Neurogenic pulmonary edema and stunned myocardium: catecholamine surge; hypoxemia with bilateral infiltrates, or ECG ST/T changes and troponin rise without coronary disease.
• Brain death with loss of all brainstem reflexes and a positive apnea test.

Complications of treatment

• Mannitol: hypovolemia, hypotension, acute kidney injury, and rebound cerebral edema after prolonged use; a widening osmolar gap signals mannitol accumulation and is the cue to stop.
• Hypertonic saline: hypernatremia, hyperchloremic metabolic acidosis, phlebitis (concentrated solutions need central access), and osmotic demyelination if a chronically hyponatremic patient is corrected too rapidly.
• Sustained hyperventilation: vasoconstriction-induced ischemia in already marginal tissue.
• External ventricular drain: ventriculostomy-related infection and tract hemorrhage; new fever with CSF pleocytosis demands cultures.
• Decompressive craniectomy: subdural hygroma, hydrocephalus, and syndrome of the trephined (sinking skin flap) with positional neurologic decline months later, corrected by cranioplasty.
• Dexamethasone: hyperglycemia, infection risk, and — in traumatic brain injury — excess mortality.

• The blown pupil is ipsilateral to the mass: parasympathetic fibers run superficially on CN III, so uncal compression dilates the pupil before ptosis or the down-and-out eye appears. A unilateral fixed dilated pupil in a comatose trauma patient is uncal herniation until proven otherwise.
• Single best next step in active herniation: elevate the head of bed, secure the airway, give hyperosmolar therapy (mannitol or hypertonic saline) with brief hyperventilation as a bridge, and call neurosurgery — do not wait for an ICP monitor or MRI.
• Kernohan notch is the association examiners love: hemiparesis ipsilateral to the lesion because the contralateral peduncle is crushed against the tentorium. Classic false localizing sign; the dilated pupil, not the weakness, localizes the mass.
• Never LP first in a patient with focal deficits, papilledema, or depressed consciousness — image first. In suspected bacterial meningitis, IDSA advises blood cultures plus empiric antibiotics and dexamethasone before CT so therapy is not delayed.
• Cushing reflex — hypertension with widened pulse pressure, bradycardia, irregular respirations — is a late medullary sign, not an early warning. Bradycardia here should never be treated as a primary arrhythmia.
• Steroids are edema-type specific: dexamethasone helps vasogenic edema around tumor or abscess; the CRASH trial showed steroids increase mortality in traumatic brain injury, and AHA/ASA does not recommend them for ischemic stroke edema. This is the most common distractor.
• Duret hemorrhage in the midbrain/pons is the buzzword for downward central herniation, not for a primary hypertensive bleed.
• Common distractor: absent papilledema does not exclude raised ICP — it takes hours to days to develop and is typically absent in hyperacute epidural hematoma.
• Posterior fossa mass with tonsillar herniation kills by apnea and cardiovascular collapse with little warning; suboccipital decompression, not medical therapy, is definitive.