Transtentorial Herniation

Transtentorial herniation is displacement of brain tissue through the tentorial notch from a pressure gradient between supratentorial and infratentorial compartments. It includes descending (central) herniation and uncal herniation, and is a neurosurgical emergency with high mortality.

Epidemiology. It complicates large hemispheric lesions: intracerebral and subdural haematoma, extensive infarction, tumour with oedema, abscess, severe traumatic brain injury and diffuse cerebral oedema. Mortality is high once brainstem signs appear.

Causes. Mass effect from haematoma, malignant middle cerebral artery infarction, tumour or abscess, diffuse oedema (hepatic failure, hypoxic injury, meningitis), hydrocephalus and cerebral venous thrombosis.

Pathophysiology. Rising supratentorial pressure pushes the diencephalon and midbrain downward, compressing the brainstem, kinking or distorting penetrating basilar branches (causing Duret haemorrhages), and compressing the posterior cerebral artery and oculomotor nerve. Brainstem function fails in a rostral to caudal sequence.

Early diencephalic stage: reduced consciousness, small reactive pupils, Cheyne-Stokes respiration, yawning, and decorticate posturing. Midbrain stage: mid-position fixed pupils, hyperventilation, decerebrate posturing. Pontine and medullary stage: loss of corneal reflexes, ataxic or gasping breathing, hypotension and death. Cushing response (hypertension, bradycardia and irregular breathing) is a late sign.

Red flags.

• Falling Glasgow Coma Scale score by 2 or more points.
• Pupil dilatation or asymmetry.
• Extensor posturing, Cushing response or respiratory irregularity.

Diagnosis is clinical, and treatment must not wait for imaging. CT head shows mass effect, midline shift, effaced basal cisterns, and the cause. Optic nerve sheath ultrasound (over 5 to 6 mm) and transcranial Doppler assist. Check glucose, sodium, osmolality, coagulation and gas. Do not perform lumbar puncture.

Differential diagnosis. Metabolic coma, sedative drug effect, nonconvulsive status, brainstem stroke, basilar artery occlusion and psychogenic unresponsiveness.

Act in minutes. The goals are to reduce intracranial pressure, restore cerebral perfusion pressure and remove the cause.

Immediate measures.

• Secure the airway: intubate (induction with minimal rise in pressure, such as ketamine 1 to 2 mg/kg or etomidate 0.3 mg/kg with rocuronium 1.2 mg/kg) and ventilate to normocapnia.
• Raise the head of the bed to 30 degrees with the neck neutral and no tight neck ties.
• Maintain systolic pressure at least 110 mmHg and oxygen saturation over 94%. Avoid hypotension and hypoxia.
• Treat seizures with levetiracetam 60 mg/kg IV (maximum 4500 mg).

Hyperosmolar therapy (bolus, then repeat as needed).

• 3% sodium chloride 250 mL IV over 15 to 20 minutes (2 to 5 mL/kg), or 23.4% sodium chloride 30 mL IV over 10 to 20 minutes through a central line. Aim for sodium 145 to 155 mmol/L.
• Mannitol 20% 0.5 to 1 g/kg IV over 15 to 20 minutes, with serum osmolality kept under 320 mOsm/kg. Avoid in hypovolaemia, and watch renal function.

Brief hyperventilation. Reduce PaCO2 to 4.0 to 4.5 kPa (30 to 35 mmHg) as a bridge for active herniation only, for minutes to hours, with a return to normocapnia after treatment (risks include cerebral ischaemia).

Sedation and analgesia. Propofol 1 to 4 mg/kg/h with fentanyl, and neuromuscular blockade for dyssynchrony.

Corticosteroid. Dexamethasone 10 mg IV then 4 mg every 6 hours only for vasogenic oedema from tumour or abscess. Not for traumatic brain injury, stroke or haemorrhage.

CSF drainage. External ventricular drain for hydrocephalus.

Definitive treatment. Surgical removal of haematoma or mass, decompressive craniectomy (for malignant infarction, traumatic brain injury or refractory intracranial hypertension), and treatment of the cause (drainage of abscess, antibiotics).

Refractory. Thiopental 3 to 5 mg/kg IV bolus then 1 to 3 mg/kg/h with EEG and haemodynamic monitoring.

Monitoring. Intracranial pressure (target under 20 to 22 mmHg), cerebral perfusion pressure (60 to 70 mmHg), pupils hourly, sodium every 4 to 6 hours, osmolality and CT repeat after any deterioration.

Referral and escalation. Neurosurgery and neurocritical care at once. Transfer without delay if neurosurgery is not available.

Special groups. In children, 3% sodium chloride 2 to 5 mL/kg and mannitol 0.5 to 1 g/kg. In pregnancy, left lateral tilt and obstetric input.