Clinical Signs of Brainstem Herniation
The cardinal signs of brainstem herniation include pupillary abnormalities (ipsilateral mydriasis progressing to bilateral fixed dilated pupils), altered level of consciousness, abnormal motor responses (extensor posturing), and late respiratory pattern changes (irregular breathing, apnea), with bradycardia occurring as a terminal sign. 1
Supratentorial Herniation Patterns
Brainstem herniation from supratentorial lesions presents in two distinct clinical patterns 1:
Gradual Rostrocaudal Deterioration
- Pupillary changes: Development of midposition pupils → bilateral fixed pupils
- Motor deterioration: Progressive worsening of motor responses → extensor posturing
- Respiratory dysfunction: Progression to irregular breathing patterns → death
- Level of consciousness: Gradual decline in arousal
Acute Sudden Deterioration
- Pupillary changes: Unilaterally dilated pupil (ipsilateral to lesion) → rapidly progressing to bilateral pupillary dilatation
- Motor response: Rapid decline from localization → flexion rigidity
- Consciousness: Abrupt loss of responsiveness
Specific Clinical Signs by System
Ocular/Pupillary Signs
- Ipsilateral pupillary dysfunction with varying degrees of mydriasis 1
- Adduction paralysis 1
- Fixed, dilated pupils (bilateral in advanced stages) 2
- Loss of oculocephalic responses 1
Motor Signs
- Worsening limb power progressing to extensor posturing 1
- Contralateral Babinski sign (from brainstem notching against tentorium) 1
- Flexor or extensor posturing 2
Respiratory Signs (Late Findings)
These signal lower brainstem dysfunction and typically occur late 1:
- Central neurogenic hyperventilation
- Ataxic respiratory patterns
- Periodic breathing
- Sudden apnea (terminal event) 1
Cardiovascular Signs
- Bradycardia (late sign indicating further brainstem compression) 1
- Hypertension during acute episodes 2
Level of Consciousness
Cerebellar Herniation Pattern
Cerebellar infarcts with swelling cause brainstem compression with distinct features 1:
- Pupillary anisocoria
- Pinpoint pupils (distinguishing feature from supratentorial herniation)
- Loss of oculocephalic responses
- Bradycardia, irregular breathing, sudden apnea with further compression
- Obstructive hydrocephalus (secondary manifestation)
Critical Monitoring Recommendations
Clinicians should frequently monitor level of arousal and ipsilateral pupillary dilation in patients at high risk for deterioration (Class I recommendation) 1. The gradual development of midposition pupils and worsening motor response indicate deterioration.
Important Clinical Caveats
Timing Matters
Absent brainstem responses at presentation carry less prognostic weight than absent responses at 12-24 hours 3. Early findings may be reversible with aggressive intervention, particularly if modifiable conditions (seizures, hydrocephalus, electrolyte abnormalities) are present.
EEG Findings
During herniation episodes, EEG shows abrupt background rhythm slowing to 2-3 Hz delta range without epileptiform discharges, with diffuse voltage attenuation 2. QEEG demonstrates decreasing frequencies, worsening asymmetry, decreasing amplitude, and increasing burst suppression ratio—these changes may precede clinical signs by up to 1 hour 4.
Distinguishing from Seizures
Fixed dilated pupils, posturing, and unresponsiveness WITHOUT epileptiform activity on EEG should raise suspicion for herniation rather than seizure, particularly in patients with ventricular shunts or known intracranial pathology 2.
Reversibility
While historically considered terminal, Duret hemorrhage and brainstem herniation are not always irreversible 5. Aggressive decompression and ICP management can result in good outcomes in selected patients, though this requires early recognition and intervention.