Summary
Intracranial pressure (ICP) is the pressure that exists within the skull and all of its compartments (e.g., the subarachnoid space and the ventricles). ICP varies with the relative position of the head towards the rest of the body and is periodically influenced by normal physiological factors (e.g., cardiac contractions). Adults in supine position have a physiological ICP of 15 mm Hg or less while a pressure of 20 mm Hg or more indicates pathological intracranial hypertension.
Elevation of ICP may occur in a variety of conditions (e.g., intracranial tumors) and can result in a decrease in cerebral perfusion pressure (CPP) and/or herniation of cerebral structures. Symptoms of raised ICP are generally nonspecific (e.g., impaired consciousness, headache, vomiting). However, more specific symptoms may be present depending on the affected structures (e.g., Cushing triad if the brainstem is compressed). Brain imaging (e.g., showing a midline shift) and physical examination (e.g., papilledema) can detect ICP elevation, but not necessarily rule it out. Therefore, ICP monitoring and quantification is vital in at-risk patients. Management usually involves osmotic diuretics such as mannitol or hypertonic saline. Further therapeutic options include controlled hyperventilation, removal of CSF, and decompressive craniectomy.
Etiology
- Idiopathic intracranial hypertension
- CNS inflammation, infection, and/or abscess
- Space-occupying lesions
- Elevated venous pressure (e.g., as a result of heart failure)
- Increased CSF (hydrocephalus)
- Metabolic disturbances; (e.g., hyponatremia, hepatic encephalopathy)
- Epilepsy and seizures
References:[1][2][3][4][5][6][7]
Pathophysiology
Physiology
- Physiological ICP is ≤ 15 mm Hg in adults (in supine position), children generally have a lower ICP
- ICP varies with the relative position of the head towards the rest of the body and is influenced by certain physiological processes (e.g., cardiac contractions, sneezing, coughing, Valsalva maneuver).
- Expansion of either blood, CSF, or tissue within the skull → limited capacity for the intracranial volume to increase within the rigid skull → increase in intracranial pressure
Consequences of elevated ICP
-
Decreased cerebral perfusion pressure (CPP)
- CPP is the effective pressure that delivers blood to the cerebral tissue.
- CPP = mean arterial pressure - ICP
- Therefore, if the ICP rises, the CPP diminishes (as long as the arterial pressure remains constant).
-
Brain tissue herniation
- As a bony structure, the skull is rigid and can not expand to compensate elevated internal pressure.
- Increased pressure gradient within the skull in the presence of inflexible brain structures (e.g., tentorium cerebelli) → flexible brain tissue shifts → possible brain tissue herniation
- This may result in direct physical damage or in blocking of cerebral vessels and subsequent ischemia.
-
Cushing triad
- ↑ Intracranial pressure → ↓ perfusion pressure within the brain → compensatory activation of the sympathetic nervous system to maintain cerebral perfusion → ↑ systolic blood pressure → stimulation of aortic arch baroreceptors → activation of the parasympathetic nervous system (vagus) → bradycardia
- ↑ Pressure on brainstem → dysfunction of respiratory center → irregular breathing
References:[3][8][9]
Clinical features
-
Global
- Cushing's triad: irregular breathing, widening pulse pressure and bradycardia
- Reduced levels of consciousness
- Headache
- Vomiting
- Papilledema
- Psychiatric changes
- In infants: macrocephaly, bulging fontanel, sunset sign
-
Focal
- Diplopia
- Cerebral herniation syndrome → see “Subtypes and variants” below
References:[3][8][10][11]
Subtypes and variants
-
Subfalcine herniation: cingulate gyrus of one hemisphere is compressed and herniates under the falx cerebri → compression of:
- Contralateral hemisphere → obstruction of the foramen of Monro → hydrocephalus
- Pericallosal arteries → hemiparesis (predominantly lower limbs)
-
Uncal herniation: medial temporal lobe (the uncus) herniates at the tentorial incisure
- → Compression of:
- Ipsilateral oculomotor nerve palsy → fixed and dilated pupil
- Ipsilateral posterior cerebral artery → cortical blindness with contralateral homonymous hemianopia
- Contralateral cerebral peduncle → ipsilateral paralysis + Kernohan phenomenon: a paradoxical ipsilateral weakness (due to contralateral cerebral peduncle compression). This is unusual because commonly, an ipsilateral brain lesion results in contralateral motor symptoms. It occurs in patients with increased ICP caused by intracranial hemorrhage or cerebral edema.)
- → Downward shift of the brainstem → brainstem hemorrhages → focal deficits, impaired consciousness, death
- → Compression of:
- Foramen magnum herniation: structures of the posterior fossa (e.g., cerebellar tonsils, medulla) herniate at the foramen magnum → impaired consciousness, decerebrate posturing, apnea, impaired circulation, death
References:[8][12][13][14][15][16][17]
Diagnostics
Imaging
- CT/MRI: may indicate mass lesions, midline shift, or effacement of the basilar cisterns
- Ultrasound (e.g., ocular sonography): measures the optic nerve sheath diameter
Clinical examination and imaging may indicate elevated ICP, but cannot rule it out! Additionally, these tests do not allow quantification of intracranial pressure, which is necessary to determine CPP!
Invasive ICP monitoring
-
Indications
- Patients at risk of elevated ICP
- Closed head trauma
- Patients that have a non-surgical intracranial hemorrhage and undergo or have undergone major nonneurological surgery and can not be properly evaluated.
- Patients suffering from moderate head injury.
-
Placement of monitors
-
Intraventricular (gold standard)
- Technique: implantation of monitoring device directly into the ventricles
- Advantages: highest accuracy, allows for treatment of elevated ICP and/or diagnostic collection of CSF samples via drainage system
-
Intraventricular (gold standard)
- Analysis: > 20 mmHg indicates elevated intracranial pressure that requires treatment
References:[3][8]
Treatment
Acute stabilization and treatment
- Resuscitation and emergency measures (head elevation, controlled hyperventilation and IV mannitol)
- Cardiopulmonary support
- Sedation, analgesia, antipyretic therapy, antiseizure medication
ICP management
- General approach
- Goal of ICP management is generally to keep ICP < 20 mm Hg.
- Positioning : e.g., head elevation (about 30 degrees), avoiding neck flexion/rotation or circumstances that may provoke Valsalva responses
- Fluid management: patients should be euvolemic, blood hypoosmolarity should be avoided
- Hyperventilation: up to a pCO2 of 26–30 mm Hg
- Hypothermia
- Causal treatment (e.g., removal of brain tumor) if possible
- Medical therapy
-
Osmotic diuretics
- IV mannitol: can generally be administered every 6–8 hours, effects last for up to 24 hours
- IV hypertonic saline: particularly for short-term treatment
-
Osmotic diuretics
- Removal of CSF via an intraventricular monitor with drainage system (e.g., external ventricular drain or lumbar drain) or a cerebral shunt (e.g., in hydrocephalus patients)
- Decompressive craniectomy: removal of part of the skull, allowing the brain to expand and reduces ICP.
References:[8][18]
Complications
Irreversible loss of brain function (brain death)
- Definition: irreversible, complete loss of function of the entire brain (including the brainstem), even if cardiopulmonary functions can be upheld by artificial life support.
- Practical steps for determination of brain death: The American Academy of Neurology has published a practical guide that consists of four steps. It cites specific measures and interpretations (e.g., limits of body temperature) that can be used to determine brain death, although not all of them are evidence-based
-
Management
- If brain death is proven, no consent is required to remove life support or other forms of treatment (e.g., antibiotic therapy).
- If the surrogate decision-maker disagrees with the physician's decision, it is judicious to consult a hospital's ethical committee
Requirements for the diagnosis of brain death
-
Clinical setting
- Loss of brain function must be attributable to a specific cause (e.g., clinical or radiologic evidence of acute, severe damage to the CNS that is consistent with brain death).
- Irreversible loss of brain function
- Factors that may impede proper clinical judgment must be absent.
-
Neurological examination
- Coma (with a known cause)
-
Absence of brainstem reflexes
- Pupillary light reflex (shining of light into the eye normally causes pupils to constrict in adaptation to bright light)
-
Vestibuloocular reflex (VOR): eye movement that is ellicited by activating the semicircular canals of the vestibular system and mediated by the afferent sensory pathway of CN VIII and the efferent motor pathway of the contralateral CN VI and the ipsilateral CN III
-
Oculocephalic maneuver: used to test VOR by observing the patient's eye movement while stimulating the vestibular system
- Rapid rotation of the head to one side normally elicits eye movement in the opposite direction to stabilize the image in the center of the visual field.
- This test should not be performed in patients with injuries to the cervical spine, since it may cause further damage!
-
Caloric test: used to test VOR by stimulating the vestibular system
- The test is performed by irrigating warm water (∼44 °C) into the external auditory canal to stimulate the vestibular system and observing the patient's eye movement.
- The patient's gaze will normally move away from the stimulus i.e. towards the contralateral ear
-
Oculocephalic maneuver: used to test VOR by observing the patient's eye movement while stimulating the vestibular system
- Corneal reflex (touching of the cornea, e.g., with a sterile cotton swab, normally triggers blinking)
- Gag reflex (touching of either side of the pharynx, e.g., with a sterile tongue depressor, normally triggers gagging)
- Cough reflex (stimulation of the larynx or the respiratory epithelium normally provokes coughing)
- No reaction to irritation of trigeminal nerve branches (normally painful)
- Deep tendon reflexes are occasionally seen in brain-dead patients who have intact spinal cords
- Apnea (absence of breathing drive)
- Ancillary brain death tests: only to be performed if clinical examination and/or apnea testing are inconclusive, or if patient is < 1 year
If spontaneous breathing is present, the medulla is intact! If the corneal reflex is present, the pons is intact! If the pupillary light reflex is present, the midbrain is intact!
Cerebral edema
-
Definition: excess accumulation of fluid within the brain parenchyma as a result of damage to the blood-brain barrier and/or the blood-CSF barrier [19]
- Vasogenic: extracellular accumulation of fluids as a result of impaired capillary permeability secondary to breakdown of endothelial tight junctions
- Cytotoxic: intracellular accumulation of fluids as a result of impaired Na+/K+-ATPase function
-
Etiology
- Cerebral infarction (stroke)
- Iatrogenic (Rapid lowering of glucose or rapid correction of hypernatremia)
- Trauma (particularly in closed head injury)
- Toxic (e.g., lead intoxication)
- Inflammatory (e.g., meningitis)
- Space-occupying lesions (e.g., brain tumors, intracranial hemorrhage)
- Management: treatment of raised ICP
References:[18][19][20][21][22][23][24][25]
We list the most important complications. The selection is not exhaustive.