tonsillar herniation

**Definition:** Caudal displacement of the cerebellar tonsils through the foramen magnum. This compresses the medulla oblongata and upper cervical cord.

**Pathophysiology:** - The posterior fossa is a small, rigid compartment bounded by the tentorium cerebelli above and the foramen magnum below. - Increased pressure in the posterior fossa, or a supratentorial mass with diffuse raised ICP, pushes the tonsils downward. - The tonsils wedge into the foramen magnum ("coning"). This compresses the medullary cardiorespiratory centers, obstructs CSF outflow and worsens hydrocephalus, and can compress the posterior inferior cerebellar arteries (PICA), causing cerebellar and medullary infarction.

**Causes:** - Posterior fossa mass: cerebellar hemorrhage, tumor (medulloblastoma, hemangioblastoma, metastasis), abscess, or cerebellar infarct with edema. - Supratentorial mass or diffuse cerebral edema: large intracerebral hemorrhage, hematoma, or malignant MCA infarct, often after downward transtentorial herniation. - Iatrogenic: **lumbar puncture** performed with raised ICP or a mass lesion. The sudden drop in spinal CSF pressure creates a pressure gradient that drives the tonsils downward. - Congenital: **Chiari I malformation** (tonsillar ectopia ≥5 mm below the foramen magnum). This is a structural anomaly, not acute herniation, though it can present with similar compression.

**Clinical features:** - Neck stiffness, head tilt, and occipital headache. - Decreased level of consciousness, then coma. - **Cushing triad:** hypertension (widened pulse pressure), bradycardia, and irregular respirations. - Respiratory arrest, apnea, and death if untreated. - Quadriparesis and lower cranial nerve dysfunction. - Pupils are often midsize and may be preserved early, in contrast to uncal herniation, which causes a blown pupil.

**Diagnosis:** - Emergent non-contrast head CT or MRI shows tonsils below the foramen magnum and effacement of the cisterns around the brainstem. - Do NOT perform LP when herniation risk is present (focal deficits, papilledema, altered mental status, or a mass on imaging).

**Management (emergency):** - ABCs, intubation, and mild hyperventilation (PaCO₂ 30–35 mmHg) as a temporizing measure. This causes cerebral vasoconstriction. - Osmotherapy (mannitol or hypertonic saline) and head-of-bed elevation to 30°. - Corticosteroids (dexamethasone) for vasogenic edema from tumors or abscess. They are not used for traumatic or hemorrhagic causes. - Definitive treatment: neurosurgical intervention such as an external ventricular drain (EVD) for hydrocephalus, or suboccipital decompression and hematoma evacuation.

**Comparison with other herniation syndromes (high-yield):** - **Subfalcine (cingulate):** the cingulate gyrus herniates under the falx and may compress the ACA, causing contralateral leg weakness. - **Uncal (transtentorial):** the medial temporal lobe herniates and compresses CN III, causing a "blown pupil" (ipsilateral dilated pupil, "down and out" eye). It also compresses the PCA (contralateral homonymous hemianopia with macular sparing) and the contralateral cerebral peduncle, causing ipsilateral hemiparesis (Kernohan notch phenomenon, a false-localizing sign). Duret hemorrhages in the pons and midbrain are a late sign. - **Central transtentorial:** downward displacement of the diencephalon and brainstem. - **Tonsillar:** downward herniation through the foramen magnum, causing medullary compression and death from respiratory and cardiac arrest. - **Upward (reverse) herniation:** a posterior fossa mass pushes the cerebellum up through the tentorium, which can occur with overly rapid ventricular drainage.

**Board pearls:** - A patient with headache and papilledema who gets an LP and then suddenly becomes bradycardic, hypertensive, and apneic has tonsillar herniation. Get imaging before LP. - Cerebellar hemorrhage in a hypertensive patient with sudden vomiting, ataxia, and gaze palsy can progress rapidly to tonsillar herniation and requires emergent decompression. - Chiari I presents in adolescents and adults with occipital headache worsened by Valsalva and may be associated with syringomyelia (cape-like loss of pain and temperature sensation with preserved touch). Chiari II is associated with myelomeningocele and hydrocephalus and displaces the vermis and medulla.