cerebral herniation

**Pathophysiology** - The skull is a rigid compartment (Monro-Kellie doctrine): brain + blood + CSF volume is fixed. An expanding mass (tumor, hematoma, abscess, edema, hydrocephalus) first displaces CSF and venous blood. Once compensation is exhausted, ICP rises sharply and tissue shifts toward areas of lower pressure. - Cushing reflex (late sign of raised ICP/brainstem compression): hypertension (widened pulse pressure), bradycardia, and irregular respirations. - Causes: epidural/subdural/intraparenchymal hemorrhage, large MCA infarct with edema, tumor (e.g., glioblastoma), abscess, diffuse cerebral edema, hydrocephalus. Lumbar puncture in the setting of raised ICP/mass lesion can precipitate herniation, so get imaging first.

**Types (high-yield)** 1. **Subfalcine (cingulate) herniation**: cingulate gyrus under the falx cerebri. It can compress the anterior cerebral artery, causing contralateral leg weakness (ACA infarct). 2. **Transtentorial (uncal) herniation**: medial temporal lobe (uncus) through the tentorial notch. - CN III compression causes ipsilateral "blown pupil" (dilated, fixed), "down and out" eye. - Compression of the ipsilateral cerebral peduncle against the tentorium causes contralateral hemiparesis. Compression of the contralateral peduncle against the tentorial edge (Kernohan notch) causes ipsilateral hemiparesis, a false-localizing sign. - Compression of the posterior cerebral artery causes contralateral homonymous hemianopia (occipital infarct). - Duret hemorrhages: midline/paramedian pontine and midbrain hemorrhages from stretching/tearing of basilar artery perforators. They are usually a late, often fatal, finding. 3. **Central (downward transtentorial) herniation**: diencephalon and brainstem pushed caudally. Features include decreasing consciousness, small reactive pupils progressing to fixed midposition pupils, decorticate then decerebrate posturing, and Cheyne-Stokes respirations progressing to apnea. 4. **Tonsillar herniation**: cerebellar tonsils through the foramen magnum. It compresses the medulla, causing cardiorespiratory arrest. It can occur with posterior fossa masses or LP in the setting of raised ICP. Chiari malformation is a distinct condition with chronic tonsillar descent. 5. **Upward (transtentorial) herniation**: posterior fossa mass pushes the cerebellum/midbrain up through the tentorial notch, compressing the midbrain and aqueduct (causing hydrocephalus). 6. **Transcalvarial (extracranial) herniation**: brain extrudes through a skull defect (trauma, craniotomy).

**Clinical features of raised ICP/herniation** - Headache, vomiting, papilledema, altered consciousness, CN VI palsy (false-localizing), Cushing triad, pupillary changes, posturing (decorticate = lesion above red nucleus; decerebrate = lesion at or below red nucleus, brainstem level).

**Diagnosis** - Emergent noncontrast head CT: midline shift, effaced basal cisterns, and loss of gray-white differentiation. MRI is useful later.

**Management (stepwise)** - ABCs, elevate head of bed 30°, and intubate for airway protection. - Brief hyperventilation (target PaCO₂ ~30–35 mmHg) is a temporizing bridge only. Hypocapnia causes vasoconstriction, which lowers cerebral blood volume and ICP, but prolonged use risks ischemia. - Hyperosmolar therapy: mannitol or hypertonic saline. - Sedation, CSF drainage (external ventricular drain), and avoiding hypotonic fluids and hyperthermia. - Definitive treatment: surgical evacuation of the mass lesion, decompressive craniectomy. Dexamethasone helps for vasogenic edema from tumor/abscess but not for traumatic brain injury or stroke.

**Boards pearls** - Blown pupil + contralateral hemiparesis = uncal herniation. - Ipsilateral hemiparesis with a blown pupil = Kernohan notch phenomenon. - Duret hemorrhage = brainstem herniation. - Tonsillar herniation causes respiratory arrest. - Subfalcine herniation affects the ACA (contralateral leg weakness). - Don't perform LP before imaging when a mass lesion is suspected. - Brain death testing includes absent brainstem reflexes (pupillary, corneal, oculocephalic, gag, cough) and apnea test.