Christopher R. Newey, DO

Joao Gomes

DESCRIPTION

• The cranial vault is a fixed area composed of parenchyma (~1,200 cc or 80%), CSF (~150 cc or 10%) and blood (~150 cc or 10%). The Monroe–Kelli doctrine states that an increase in one of these constituents must be balanced by a decrease in another constituent. Herniation is the shift of brain tissue from its proper location to a new location. The cerebral hemispheres can absorb some degree of distortion from mass effect and increases in intracranial pressure (ICP) as long as blood and CSF can be displaced extracranially. ICP can be reflected as changes in compliance. Compliance is defined as the change in volume divided by the change in pressure. This is not a linear relationship. When compensatory mechanisms are exhausted, brain tissue may herniate to an area of lower pressure/volume. The location of the herniated tissue determines the type of clinical syndrome. Several herniation syndromes have been described:
  • Subfalcine herniation: Lateral supratentorial lesions herniating the cingulate gyrus under the falx cerebri. This may compromise blood flow to the anterior cerebral artery resulting in leg weakness and or mutism due to anterior cerebral artery infarction.
  • Uncal (also known as lateral) herniation: Mesial temporal lobe lesions herniating medially displacing the midbrain resulting in pupillary dilation from compression of the third nerve and contralateral hemiparesis from ipsilateral cerebral peduncle. Ipsilateral hemiparesis may result if contralateral cerebral peduncle is compressed (i.e., Kernohan's phenomenon).
  • Central transtentorial herniation: Supratenorial mass causing downward pressure on midbrain and diencephalon resulting in altered level of consciousness, posturing, and respiratory compromise.
  • Cerebellar (tonsillar) herniation: Mesial aspect of cerebellum (tonsils) into the foramen magnum resulting in alterations in respiration, autonomic functions, vertigo, skew deviation, vomiting, and coma/death.
  • Upward transtentorial herniation: Upward herniation of infratentorial compartment (cerebellum/brainstem) through tentorium resulting in pupillary dysfunction, vertical gaze paresis, decerebrate posturing, respiratory changes, and coma.
  • Transcalvarial herniation: Herniation of cortex through skull defect.

PATHOPHYSIOLOGY

• Brain herniation can cause injury in several ways.
  • When there is little compensatory room for brain tissue to continue herniating, the diencephalon and midbrain can be affected—either laterally, rotationally, or downward. The degree of displacement corresponds to the level of impaired consciousness.
  • Compression of arterial supply (e.g., posterior cerebral artery (PCA) at the edge of the tentorium or anterior cerebral artery at the edge of the falx cerebri) during herniation can cause infarction of the brain parenchyma.
  • Increasing intracranial CSF due to outflow blockage (obstructive hydrocephalus). For example, blockage of the aqueduct of Sylvius may occur with posterior fossa herniation syndromes.

RISK FACTORS

• The etiology of brain herniations can be grouped as follows:
  • Masses: Tumors, hematomas, abscess, air, foreign body
  • Vascular: Increased cerebral blood flow/cerebral blood volume (CBV) from exhausted autoregulation, cerebral venous thrombosis
  • Edema: Cytotoxic (ischemia, ATPase pump failure), vasogenic (vessel injury from tumor, abscess), hydrostatic (transmural pressure from hydrocephalus), hypo-osmolar (hyponatremia), hyper-ammonemia (liver failure)
  • CSF excess: Hydrocephalus (obstructive or nonobstructive), overproduction (papilloma)

HISTORY

• The key to diagnosis is recognizing that cerebral herniation is a possibility.
• The history should document time of onset, prior cancer, recent stroke, coagulopathy, infectious history, trauma, preceding headache, nausea/vomiting, and hiccuping.

PHYSICAL EXAM

First, assess the airway, breathing, and circulation. Identify the Cushing's response if present. This is an increase in blood pressure and fall in heart rate with respiratory abnormalities. Make note of the breathing patterns given the localizable information they provide (e.g., Cheyne–Stokes (cortex), hyperventilation (midbrain), apneustic (pons), cluster (pons), and ataxic (medulla). Perform a Glasgow Coma Scale. Decorticate posturing localizes from the cortex to the red nucleus. Decerebrate posturing localizes from the red nucleus to the vestibular nucleus. Pupils can be small and reactive (diencephalic), fixed and dilated (third nerve palsy from compression of the uncus of the temporal lobe or the PCA), midposition and fixed (midbrain), pinpoint and reactive (pons), large and fixed with hippus (tectal). Papilledema and absent venous pulsation may be found on fundoscopy examination. Weakness is typically contralateral but can be ipsilateral to the herniation if the contralateral cerebral peduncle is also involved (Kernohan's notch).

DIAGNOSTIC TESTS AND INTERPRETATION

Lab

Initial Lab Tests

Complete blood count (CBC), complete metabolic panel (CMP), partial thromboplastin time (PTT), prothrombin time (PT)/international normalized ratio (INR), arterial blood gas (ABG), serum osmolality, type and cross

Follow-Up & Special Considerations

Serial neurological examinations with a focus on level of consciousness and changes in neurological function are critical in monitoring for cerebral herniation.

Imaging

Initial Approach

CT head without contrast is necessary to rapidly identify type and extent of herniation, treatable underlying causes of herniation (e.g., bleeds or large infarcts), and secondary pathologies. Look for midline shift, degree of pineal shift, mass effect, effacement of basal cisterns and sulci, global or focal edema, and obstructive hydrocephalus. In traumatic brain literature, it is estimated that 10% of patients with increased ICP will have normal head CT.

Follow-Up & Special Considerations

Serial neuroimaging (either MRI or CT) can be useful in confirming herniation as well as further identifying the underlying etiology.

Diagnostic Procedures/Other

• Measurement of ICP can be accomplished via several anatomic spaces including:
  • Intraventricular (gold standard, but with highest risk of hemorrhage and/or infection)
  • Intraparenchymal (lower rates of hemorrhage and infection, but inability to drain CSF as a therapeutic intervention)
  • Subdural
  • Subarachnoid
  • Epidural (typically used in patients with liver failure)

These locations are in reference to the foramen of Monro which is estimated by the external auditory meatus. Normal ICP is typically defined as <15–20 mm Hg. ICP waveforms have three components. P1 is the arterial wave, P2 is the rebound wave, and P3 is the venous outflow. An elevated P2 waveform indicates poor compliance.

Pathological Findings

• The emergence of elevated ICP (20–100 mm Hg) for several minutes to hours is called a plateau wave (Lundberg A wave). Lundberg B waves are an elevation of ICP (10–20 mm Hg) that lasts a few minutes and are thought to be related to respiratory fluctuations in PaCO₂. Lundberg C waves are rapid sinusoidal fluctuations corresponding to arterial pressure.

DIFFERENTIAL DIAGNOSIS

Cerebral herniation can result from any intracranial process causing mass effect. Diffuse processes are less likely to cause cerebral herniation.

• Seizure
• Metabolic coma with altered mental status
• Mydriasis with normal examination such as from beta agonists or anticholinergic medications or physiological anisocoria
• Meningeal irritation
• Migraine

MEDICATION

• Osmotic agents:
  • Mannitol 20%: Typically loaded at 1–1.5 g/kg followed by maintenance of 0.5 g/kg every 4–6 h if needed. The goal is titrate to serum osmolarity of 300–320 mOsm. The half life is 0.16 h but is efficacious up to 1.5 to 6 h. Use in caution in patients with renal disease or patients who are volume depleted and/or hypernatremic.
  • Hypertonic saline (23%): Typically given at as a 30 cc bolus over 10–15 minutes via central access followed by 3% hypertonic saline infusion at 0.5–1 mL/kg/h with goal Na of 155–160. Be careful to not infuse 23% too rapidly as pulmonary edema and hypotension can occur.
  • Mannitol has a reflection coefficient of 0.9 compared to hypertonic saline of 1. Thus, mannitol is more likely to cross blood brain barrier resulting in gradient degradation.

ADDITIONAL TREATMENT

General Measures

• Optimize cerebral perfusion pressure (CPP = MAP-ICP) to >60–65 mm Hg
• Head of bed 30–45 degrees
• Neutral neck angle. Lateral rotation may collapse venous outflow and increase ICP.
• Treat agitation and pain
• Treat fever with acetaminophen and/or cooling devices (either surface or invravascular)
• Normocarbia
• Avoid hyper- or hypo-glycemia
• Minimize shivering
• Prompt nutritional support
• Prophylactically treat for seizures

Additional Therapies

• Hyperventilation to PaCO₂ of 25–30 mm Hg can reduce CBV and ICP. If actively herniating and on mechanical ventilation, disconnect patient from ventilation and manually bag. Transient benefit only from hyperventilation.
• Barbiturates: Pentobarbital at 10–20 mg/kg bolus followed by 1–4 mg/kg/h titration can reduce metabolic demand and thus decrease ICP.
• Induced hypothermia to 32–34°C can reduce cerebral oxygen metabolism and reduce inflammation.
• Paralytics can reduce metabolic demand but carry risk of intensive care units (ICU) myopathy/neuropathy.
• Steroids have been shown effective in vasogenic cerebral edema but not in other forms of cerebral edema (e.g. cytotoxic).

SURGERY/OTHER PROCEDURES

Neurosurgical evaluation is imperative in management of cerebral herniation syndromes. Surgical decompression may be an option for malignant cerebral edema in selecting patients with large infarcts. Patients are selected based on age, timing of surgery, and neuroimaging findings. Additionally, debulking surgery may be an option for tumors. Lastly, placement of ventricular pressure monitoring devices or ventricular drainage devices should be considered.

IN-PATIENT CONSIDERATIONS

Admission Criteria

Patients with signs and symptoms of increased ICP and brain herniation should be admitted to the ICU of the hospital. Discharge will be based on stabilization of the underlying cause.

Nursing

• Serial neurological examinations are necessary. These should be performed every hour.
• Cardiac and respiratory monitoring
• Strict ins and outs must be maintained

Discharge Criteria

Discharge will be determined upon stabilization of the underlying cause of the cerebral herniation.

FOLLOW-UP RECOMMENDATIONS

Patients should have follow up after discharge with appropriate departments. For example, ischemic strokes will need to be seen in stroke clinics, tumors will need to be seen in neuron-oncology clinics.

DIET

Prompt nutritional support should occur with either nasogastric tube or percutaneous endoscopic gastrostomy (PEG) tube.

PATIENT EDUCATION

ICUs are necessary for monitoring patients with cerebral herniation. Aggressive cardiac, respiratory, and ICP monitoring along with serial neurological examinations are necessary.

PROGNOSIS

The prognosis of cerebral herniation depends on the course and extent of the herniation, secondary injuries, and the primary pathology underlying the cerebral herniation.

COMPLICATIONS

• Patients need to be monitored for complications of prolonged bedrest and malnutrition. Patients should have serial duplex scans for Deep-vein thrombosis (DVTs). Prophylaxis for DVTs should be initiated if not contraindicated. Serial chest x-rays need to occur to evaluate for pneumonia, particularly in those patients who are intubated. Nutritional markers will need monitoring to ensure adequate nutrition.
• Once patients are able, physical and occupational therapy evaluations are recommended.

• Pinskey MR, Brochard L, Mancebo J, et al., eds. Applied physiology in intensive care medicine. Berlin: Spring-Verlag, 2009.
• Posner JB, Saper CB, Schiff ND, et al., eds. Plum and Posner's diagnosis of stupor and coma. 4th ed. New York, NY: Oxford Press, 2007.
• Ropper AH. Lateral displacement of the brain and level of consciousness in patients with acute hemispheral mass. New Engl J Med 1986;314: 953–958.

SEE-ALSO

• Cerebral herniation
• Increased ICP
• Hydrocephalus

ICD9

348.4 Compression of brain

The key is to recognize patients at risk of herniating and also recognizing the clinical syndromes that occur with herniation. Once recognized, prompt treatment should be initiated along with neurosurgical consultation.