Know the early signs of brain herniation

Know the early signs of brain herniation: Excerpt from Avoiding Common Pediatric Errors

Author: Megha Shah Fitzpatrick, MD

What to Do - Interpret the Data

Herniation occurs when the brain shifts across structures within the skull, from one intracranial compartment to another, as a result of pressure gradients created by high intracranial pressure (ICP). Although the brain has considerable elasticity, the arteries and veins responsible for its blood supply are relatively fixed in space, leading to the risk that a shifting brain will cause moving portions to lose their blood supply. It is essential to be able to recognize the early clinical manifestations of herniation syndromes and to rapidly institute therapies to decrease ICP in order to reverse the process and maintain viability of the patient. Herniation syndromes of the brain can be classified as follows: central, uncal, cerebellar tonsillar, subfalcine, and transcalvarial.

Central Herniation Syndrome

Central (transtentorial) herniation syndrome occurs as a result of diffuse brain swelling due to trauma or a centrally located mass. This causes the diencephalon (thalamus, hypothalamus, epithalamus, subthalamus, and pre- tectum) to move caudally through the tentorial notch. The resulting alteration of consciousness is thought to be caused by cerebral hypoperfusion secondary to increased ICP as well as dysfunction of the reticular formation. The reticular formation is involved in stereotypical actions such as walking, sleeping, and lying down. It is absolutely essential for the basic functions of lifeandisevolutionarilyoneoftheoldestportionsofthebrain.Consequently, an initial presenting sign of potential central herniation is a decreased level of alertness, which later progresses to stupor and coma.

Other key early features of the central herniation syndrome include meiotic but reactive pupils secondary to loss of sympathetic output from the hypothalamus; decorticate or flexor posturing that can be elicited spontaneously or via noxious stimuli; and Cheyne-Stokes respiration, an abnormal pattern of breathing characterized by periods of breathing with gradually increasing and decreasing tidal volumes scattered with periods of apnea. It is crucial to recognize this constellation of symptoms in the face of severe traumatic brain injury or a known central intracranial mass, because at this stage, herniation is potentially reversible.

In addition to these early signs of central herniation, it is crucial to be able to recognize signs and symptoms of Parinaud syndrome. Children with midbrain or pineal tumors, as well as direct or compressive trauma to the midbrain often present with Parinaud syndrome, also known as dorsal midbrain syndrome or pretectal syndrome. This syndrome consists of paralysis of up gaze, Pseudo-Argyll Robertson pupils (light-near dissociation), convergence-retraction nystagmus, and eyelid retraction (Collier sign). If this group of symptoms is present or even just a limitation of upward gaze is present, it behooves the physician to obtain neuroimaging to rule out an intracranial process.

With progression of the central herniation syndrome, there is a marked decrease in the likelihood of reversibility. As the failure progresses to the midbrain from the diencephalon, the pupils enlarge to midposition and posturing becomes decerebrate or extensor. Attempts to elicit horizontal eye movements via the cerebro-ocular reflex (commonly known as doll's eyes) or the cerebrovestibular reflex (also known as the cold calorics test) fail, respiratory patterns continue to become more irregular, and the patient becomes overtly comatose. At this point, the patient will also display signs of Cushing triad (hypertension, bradycardia, and alteration in respiratory pattern) and will likely continue to complete loss of all brainstem reflexes and death.

Uncal Herniation Syndrome

Uncal herniation (or lateral mass herniation) syndrome occurs when a lateral expanding cerebral mass pushes the uncus and the hippocampal gyrus over the lateral edges of the tentorium. Initially, prior to the herniation syndrome developing, signs and symptoms are often due to the mass itself (i.e., contralateral hemiparesis). With shifting of the diencephalon away from the mass, the initial two signs of impending herniation are alteration in consciousness and an ipsilateral third nerve palsy, which is present in about 85%ofpatients.Asthethirdcranialnerveiscompressedagainstthetentorial notch, the papillary fibers—which are located most peripherally within the nerve—are damaged and cause a dilated pupil. This unilateral dilated pupil, along with contralateral hemiparesis with or without significant impairment in consciousness, are the hallmarks of the uncal herniation syndrome.

As the lateral displacement of the midbrain continues, an ipsilateral hemiplegia is produced secondary to compression of the contralateral corticospinaltract within thecerebral peduncle againstthe edge of thetentorium. Thishemiplegiaisalsoreferred toasKernohan notch phenomenon.Patients may also have bilateral papillary dilatation secondary to distorted cranial nerve three anatomy and midbrain ischemia. After the initial triad of symptoms, uncal herniation begins to affect the midbrain and pons, producing bilateral fixed pupils and decerebrate posturing. Symptoms are often rapidly progressive, and at this point, the signs and symptoms of uncal herniation are no longer distinguishable from central herniation.

Cerebellar Tonsillar Herniation Syndrome

Cerebellar tonsillar herniation is seen in patients with posterior fossa masses, causing brainstem compression, cranial nerve dysfunction, and obstructive hydrocephalus.Thecerebellartonsilsarepushedintoandeventuallythrough the foramen magnum as the pressure gradient across the foramen increases. Patients can initially present with neck pain prior to losing consciousness. In addition, compression of the medulla affects respiratory centers within this structure and results in apnea.

Conversely, patients undergoing ventriculostomy for relief of obstructive hydrocephalus can have a caudal or upward transtentorial herniation of the posterior fossa into the diencephalic region. Consequently, neurosurgeons performing a ventriculostomy also prepare thepatientfor an emergent posteriorfossadecompressionbeforebeginningthe procedure,giventherisk of caudal herniation of the posterior fossa.

As discussed earlier, it is crucial as a physician to be able to identify the signs and symptoms of brain herniation syndromes. Often, patients are asymptomatic in their presentation of an intracranial mass until increased ICP is above a threshold necessary to cause pressure gradients that lead to shifting of parts of the brain between compartments and ischemia secondary tocompressionofthevasculature.Oncethiscascadeofsymptomshasbegun, there is often very limited time to initiate treatment to decrease intracranial pressure and eliminate the pressure gradients preventing complete herniation and death.

Suggested Readings

Furhman BP, Zimmerman J, eds. Pediatric Critical Care. 3rd ed. Philadelphia: Mosby Elsevier; 2006.
Goetz CG, Pappert EJ, eds. Textbook of Clinical Neurology. 2nd Ed. Philadelphia: WB Saunders Company; 2003.

Book Source Details

• Book Title: Avoiding Common Pediatric Errors
• Author(s): Anthony D Slonim MD, DrPH; Lisa Marcucci MD
• Year of Publication: 2008
• Copyright Details: Avoiding Common Pediatric Errors, Copyright © 2008 Lippincott Williams & Wilkins.

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Copyright notice for book excerpts: Copyright © 2008 Lippincott Williams & Wilkins. All rights reserved.

More About This Book: Title: Avoiding Common Pediatric Errors Authors: Anthony D Slonim MD, DrPH; Lisa Marcucci MD Publisher: Lippincott Williams & Wilkins Copyright: 2008 ISBN: 0-7817-7489-6

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