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Reference — Neurology

Brain Herniation Reference

Brain herniation syndromes for nurses — subfalcine, uncal (transtentorial), central transtentorial, and tonsillar herniation: definition, mechanism, key warning signs, clinical findings, and nursing escalation priorities.

Educational use only. This material supports nursing education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional policy, or medical direction. Always follow facility protocols and current provider orders.

Brain herniation is a neurological emergency with minutes to act. Any signs of herniation require IMMEDIATE provider notification and preparation for emergent intervention. Do not delay for further assessment.

Quick Overview

TypeWhat HerniatesHallmark SignUrgency
SubfalcineCingulate gyrus under falxLeg weakness (ACA territory)Urgent
UncalUncus through tentorial notchIpsilateral fixed/dilated pupil (CN III)CRITICAL
CentralBilateral hemispheres downwardProgressive LOC decline; bilateral small pupilsCritical
TonsillarCerebellar tonsils through foramen magnumSudden respiratory arrestCRITICAL — death

Herniation Syndrome Detail

Subfalcine (Cingulate) Herniation

Urgent

Definition: The cingulate gyrus (medial frontal lobe) herniates under the falx cerebri — the rigid dural fold separating the two hemispheres. This is the most common herniation type.

Mechanism: Unilateral supratentorial mass (large stroke, tumor, hematoma) pushes the cingulate gyrus under the falx, compressing the anterior cerebral artery (ACA) and the contralateral hemisphere.

Warning Signs

  • Contralateral leg weakness (ACA territory — leg > arm pattern)
  • Behavioral changes (frontal lobe compression)
  • Headache
  • Often asymptomatic early — detected on CT imaging

Clinical Findings

  • Leg weakness or monoplegia contralateral to herniation side
  • May progress to more dangerous herniation types if untreated
  • ACA infarction if compression persists

Nursing Escalation

  • Report any new leg weakness or behavioral change in at-risk patients
  • Implement ICP reduction measures (HOB 30°, neutral alignment, no Valsalva)
  • Notify provider — imaging may be urgently needed
  • Prepare for emergent neurosurgical intervention (hematoma evacuation, decompressive craniectomy)

Uncal (Transtentorial) Herniation

CRITICAL — Most Dangerous

Definition: The uncus (medial temporal lobe) herniates through the tentorial notch (opening between the tentorium cerebelli and brainstem). This is the most clinically feared herniation syndrome because it directly compresses CN III and the brainstem.

Mechanism: Unilateral mass in the temporal lobe or middle fossa pushes the uncus through the tentorial incisura, progressively compressing CN III (parasympathetics for pupil constriction), the ipsilateral cerebral peduncle (causing contralateral weakness), and ultimately the brainstem.

Warning Signs

  • Ipsilateral pupil dilation — CN III parasympathetics compressed FIRST (earliest sign)
  • Ptosis and eye deviated down and out (complete CN III palsy)
  • Decreasing LOC — lethargy to coma
  • Contralateral hemiparesis (corticospinal tract compression)

Clinical Findings

  • Classic triad: Ipsilateral fixed and dilated pupil + Contralateral hemiplegia + Decreasing LOC
  • Kernohan notch phenomenon (false localizing sign): ipsilateral weakness from contralateral cerebral peduncle being pushed against tentorium
  • Cheyne-Stokes respirations → central neurogenic hyperventilation as brainstem compressed
  • Cushing triad as brainstem herniates further (HTN + bradycardia + irregular respirations)
  • Decorticate → Decerebrate posturing progression

Nursing Escalation

  • CALL CODE — this is a CRITICAL EMERGENCY
  • ANY new unilateral pupil dilation in an at-risk patient = uncal herniation until proven otherwise
  • Immediate provider notification — prepare for emergent intervention
  • Hyperventilate if intubated (target PaCO2 30–35 mmHg briefly) — vasoconstrictive effect buys time
  • Mannitol or 3% NaCl per rapid order to reduce ICP
  • Emergent CT and neurosurgical consultation for hematoma evacuation or craniectomy
  • Do NOT delay for other assessments — every second matters

Central Transtentorial Herniation

Critical

Definition: Bilateral downward displacement of the cerebral hemispheres and diencephalon through the tentorial notch. Unlike uncal herniation, there is no single dominant mass — the entire brain is forced downward by diffuse increased pressure.

Mechanism: Diffuse brain edema, bilateral hemispheric injury, or high ICP causes symmetrical downward displacement of the thalami, hypothalamus, and midbrain through the tentorium. This progressively damages rostral to caudal brainstem structures.

Warning Signs

  • Early: Progressive consciousness impairment (lethargy → stupor → coma)
  • Small, reactive pupils (diencephalic stage) — bilateral symmetric
  • Cheyne-Stokes respirations (early)

Clinical Findings

  • Diencephalic stage: small reactive pupils; Cheyne-Stokes breathing; decorticate posturing
  • Midbrain-pons stage: midsize fixed pupils; central neurogenic hyperventilation; decerebrate posturing
  • Medullary stage: dilated fixed pupils; ataxic breathing; flaccidity — cardiorespiratory arrest imminent

Nursing Escalation

  • Progressive LOC decline is the cardinal sign — monitor GCS continuously
  • Notify provider immediately for any GCS decline ≥2 points
  • ICP reduction measures: HOB 30°, osmotic agents per order, avoid hypercapnia
  • Prepare for ICU-level monitoring and possible surgical intervention
  • Monitor for respiratory pattern changes — may need emergent intubation

Tonsillar Herniation

CRITICAL — Cardiorespiratory Arrest Imminent

Definition: The cerebellar tonsils herniate through the foramen magnum (the opening at the base of the skull through which the brainstem and spinal cord pass). This directly compresses the medulla oblongata — the site of the respiratory and cardiovascular control centers.

Mechanism: Increased posterior fossa pressure (cerebellar hemorrhage, tumor) or diffuse ICP elevation forces the cerebellar tonsils downward into the foramen magnum, compressing the medulla and causing cardiorespiratory collapse.

Warning Signs

  • Neck pain or stiffness (cerebellar tonsils impacting foramen magnum)
  • Occipital headache worsening with position change
  • Sudden respiratory arrest (the medullary respiratory center is compressed)
  • Bradycardia and hypotension (vasomotor center compressed)
  • Loss of consciousness

Clinical Findings

  • Respiratory arrest — often sudden and without warning in rapid onset
  • Cardiac arrest (medullary vasomotor center compressed)
  • Bilateral pinpoint then dilated fixed pupils
  • Flaccid quadriplegia
  • May be preceded by sudden neck pain and opisthotonus

Nursing Escalation

  • IMMEDIATE CODE — this is the most lethal herniation
  • Respiratory arrest may occur within minutes of clinical signs
  • Emergent intubation and ventilation
  • AVOID lumbar puncture in patients with signs of tonsillar herniation or suspected high posterior fossa pressure — LP can precipitate lethal herniation
  • Notify neurosurgery emergently — emergent posterior fossa decompression or VP shunt
  • Prepare for resuscitation

Critical Rules

  • ✦Uncal herniation = ipsilateral fixed dilated pupil. This is a CODE. Every second counts.
  • ✦NEVER perform a lumbar puncture on a patient with signs of herniation or severely elevated ICP — LP can precipitate fatal tonsillar herniation.
  • ✦Cushing's Triad (HTN + bradycardia + irregular respirations) indicates brainstem compression — it is a pre-terminal finding, not an early warning sign.
  • ✦Early herniation is reversible with emergent intervention. Late herniation (decerebrate posturing, fixed dilated pupils bilaterally) often is not.
  • ✦Any unilateral pupil dilation in a brain-injured patient = uncal herniation until proven otherwise. It is NOT benign anisocoria.
  • ✦Position: HOB 30°, neutral neck alignment, no Valsalva — these interventions can slow progression while awaiting emergent treatment.

Related Resources

Increased Intracranial Pressure GuideICP fundamentals, Cushing triad, and nursing interventions to prevent herniation
Open →
ICP Signs and Symptoms ChartEarly and late ICP signs with nursing actions at a glance
Open →
Neurological Assessment FundamentalsDetecting herniation warning signs during bedside neuro assessment
Open →
Glasgow Coma Scale ReferenceGCS decline as herniation progresses
Open →

Standards & sources

Fact-checked Jun 21, 2026

This page is written to align with American Heart Association / American Stroke Association (AHA/ASA) · American Association of Neuroscience Nurses (AANN). It is an educational summary, not a citation of any single document — always verify specific doses, values, and protocols against current guidelines and your facility policy. How we source content →