Intracranial hypertension is a spectrum of neurological disorders where cerebrospinal fluid (CSF) pressure within the skull is elevated. Normal CSF pressure varies by age. In general, CSF pressure above 250 mm H20 in adults and above 200 mm H2O in children signifies increased intracranial pressure (ICP). It may be idiopathic or arise as a result of neurologic insult or injury.
The human skull is a relatively fixed volume structure of approximately 1400 to 1700 mL. Physiologically its components consist of 80% brain parenchyma, 10% cerebrospinal fluid, and 10% blood. Since the skull is considered an unchangeable volume, any increase in the volume of components within the skull or an addition of a pathologic element will result in increased pressure within the skull. Pathologic structures that can cause increased ICP may include mass lesions, abscesses, and hematomas.
The physiologic volume of the brain parenchyma is a relatively constant value in adults: however, it may be adjusted by mass lesions or in the setting of cerebral edema. Cerebral edema can occur with acute hypoxic encephalopathy, large cerebral infarction, and severe traumatic brain injury. CSF and blood volume in the intracranial space will vary on a regular basis as these are the primary regulators of intracranial pressure. CSF volume is primarily regulated via choroid plexus production at a rate of approximately 20 mL per hour physiologically and through its reabsorption at a similar rate by arachnoid granulations which drain into the venous system of the skull. The control mechanisms for maintaining appropriate CSF pressures may become damaged in neurological injuries such as stroke or trauma. Increased CSF production above the rate at which it can be reabsorbed such as in the presence of a choroid plexus papilloma leads to increased pressure.
A failure to reabsorb at a sufficient rate to match normal secretion rate is another possibility and is seen with arachnoid granulation adhesions after bacterial meningitis. Ventricular obstruction may also induce decreased reabsorption of CSF causing hydrocephalus. The primary regulator of blood volume is via cerebral blood flow. Diseases which obstruct venous outflows such as a venous sinus thrombosis, jugular vein compression, or structural changes due to neck surgery may cause blood congestion within the skull, thus increasing pressure. Idiopathic intracranial hypertension, also known as pseudotumor cerebri, is a term for increased intracranial pressure due to unknown causes with no known structural change.
Etiology of intracranial hypertension can be divided into 2 categories:
Primary or Intracranial Causes
Secondary or Extracranial Causes
The exact epidemiology of intracranial hypertension depends on its etiology. However, of special note is idiopathic intracranial hypertension where up to 90% of affected individuals are women of childbearing age. Individuals with chronic hypertension or obesity are also at an increased risk for developing intracranial hypertension. A frequency of occurrence has been established to be 1.0 per 100,000 in the general population, 1.6 to 3.5 per 100,000 in women, and 7.9 to 20 per 100,000 in women who are overweight.
Anytime there is an elevation in ICP, there is the risk of subsequent injury from direct brainstem compression or from a reduction in cerebral blood flow. Clinically, cerebral blood flow is evaluated via measurement of cerebral perfusion pressure where:
Cerebral perfusion pressure = Mean arterial pressure - Intracranial pressure
Cerebral perfusion pressure in simpler terms is the pressure of blood flowing to the brain and is the driving force for delivery of oxygen necessary for neuronal functioning. Normally, this is a constant value of 50 to 100 mm Hg due to autoregulation. The impact that cerebral perfusion pressure holds is in the concept that blood flow will occur from an area of higher concentration to an area of lower concentration. When ICP becomes elevated, cerebral perfusion pressures decrease, and the net driving force of blood flow to the brain becomes decreased. The physiologic autoregulatory response to a decrease in cerebral perfusion pressure is to increase mean arterial pressures systemically and to vasodilate cerebral blood vessels. This results in increased cerebral blood volume that further increases ICP. Paradoxically, this further reduces cerebral perfusion pressure producing a feedback cycle that results in the total reduction of cerebral flow and perfusion. The result of this feedback loop is cerebral ischemia and brain infarction with neuronal death. In cases where intracranial hypertension is the result of hemorrhage, increased blood pressure will worsen intracranial bleeding, thus worsening intracranial hypertension.
Symptoms of elevated intracranial hypertension are primarily derived from neurological irritation, compression, or displacement and papilledema. Non-specific headaches are recorded in almost all cases and are likely mediated via the pain fibers of the trigeminal nerve in the dura and blood vessels of the brain. Pain is generally diffuse and worse in the mornings with exacerbation by the Valsalva maneuver. Nausea and vomiting are common presentations of elevated ICP. Patients can present with double vision most frequently with horizontal diplopia associated with CN VI palsy from compression. Transient visual abnormalities occur frequently, often described as a gradual dimming of vision in one or both of the eyes. Visual abnormalities worsen with changes in posture. Peripheral visual loss may be reported and most commonly begins in the nasal inferior quadrant with subsequent loss of the central visual field. Alterations in visual acuity with blurring or distortion may occur. Variable degrees of loss of color distinction may occur. In more severe or chronic cases, a sudden visual loss can occur due to intraocular hemorrhage. Tinnitus with a pulsing rhythm exacerbated by supine or bending positions and Valsalva maneuver can occur. Radicular pain, numbness, or paresthesias are possible and most commonly associated with localized compression or possible herniation of the brain. Neurological findings are indications of severe disease. The anatomical locations where herniation is most likely to occur include the subfalcine, central transtentorial, uncal transtentorial, cerebellar tonsillar/foramen magnum, and transcalvarial lobes. These types of changes may lead to decreased consciousness or responsiveness. Focal neurological constellations depend on which region of the brain has herniated. Often this results in a stupor state or more severely with coma due to the local effect of mass lesions or pressure on the reticular formations of the midbrain. It may further lead to respiratory compromise.
Physical exam findings can vary widely depending on etiology. A change in mental status or comatose patient should prompt urgent evaluation. A complete neurological assessment is essential whenever intracranial hypertension is suspected. Cranial nerve assessment is particularly important for identifying lesions. Cranial nerve VI palsy is most common. Blunting of the pupillary reflex with fixed dilation of one pupil is also highly associated with herniation syndromes. Spontaneous periorbital bruising may be present as well. A classic triad of bradycardia, respiratory depression, and hypertension is known as Cushing's triad and is highly indicative of intracranial hypertension. Fundoscopic examination looking for retinal hemorrhages or papilledema is essential. Alterations in respiratory drive and effort may occur leading to failure of respiration and oxygenation.
Infants can have widening of cranial sutures and bulging fontanelle.
Complete blood count (CBC) and complete metabolic panel (CMP) are usually checked in all patients with suspected intracranial hypertension to evaluate for infection, anemia, and electrolyte abnormalities. Initial evaluation should include a head CT scan. CT scan findings of cerebral edema such as compressed basal cisterns and midline shift are predictive of elevated ICP. However, the absence of these findings does not rule out intracranial hypertension. A head MRI is more accurate than head CT in evaluating elevated ICP and to looking for potential etiology. Bedside ultrasonography also can be used to measure the diameter of the optic nerve sheath to determine intracranial hypertension. However, this study is limited by operator skill and not frequently used. A lumbar puncture may sometimes be needed for diagnosis. However, it should be delayed until neuroimaging, especially in those with suspicion of impending herniation. When LP is performed, in addition to measuring opening pressures, CSF should also be tested for infection and other potential etiology. Invasive measurement of ICP is definitive for diagnosis and improves the physician’s ability to maintain adequate cerebral perfusion pressure (CPP). There are 4 main anatomical sites used for clinical measurement of intracranial pressure: intraventricular, intraparenchymal, subarachnoid, and epidural. Ventriculostomy catheter is preferred device for ICP monitoring and can be used even for therapeutic CSF drainage to lower ICP. When ventricles cannot be cannulated, intraparenchymal devices using microsensor and fibreoptic transducer may be used. Subdural and epidural monitors are not as accurate as ventriculostomy and parenchymal monitors.
Treatment of chronic intracranial hypertension is mainly focused on treating and reversing the etiology.
A sudden increase in ICP is a neurosurgical emergency, requiring close monitoring in an intensive care unit (ICU) setting. For acute intracranial hypertension, a patient should first be stabilized with healthcare professionals aiming for hemodynamic stability, and preventing and treating factors that may aggravate or precipitate intracranial hypertension. These patients should have close monitoring of heart rate, blood pressure, body temperature, ventilation and oxygenation, blood glucose, input and output, and ECG. Patients with suspected intracranial hypertension, especially with severe traumatic brain injury, should also have ICP monitoring.
It is vital to prevent and treat factors that may aggravate or precipitate intracranial hypertension. These interventions are used to buy time until the underlying etiology is identified and corrected.
For patients with sustained intracranial hypertension, additional measures are needed to control the ICP.
Mannitol is commonly used as a hyperosmolar agent and is usually given as a bolus of 0.25 to 1 g/kg body weight. Serum osmolality should be kept less than 320 mOsm to avoid side effects of therapy like renal failure, hypokalemia, and hypo-osmolarity.
Hypertonic saline can also create an osmotic shift from the interstitial space of brain parenchyma into the intravascular compartment in the presence of an intact blood-brain barrier. Hypertonic saline has an advantage over mannitol for hypovolemic and hypotensive patients. Adverse effects of hypertonic saline administration include hematological and electrolyte abnormalities. Hyponatremia should be excluded before administering hypertonic saline to reduce the risk of central pontine myelinolysis.
Prognosis is highly variable depending on etiology and varies from benign to lethal. Children usually can tolerate higher intracranial pressure (ICP) for a longer period.
The management of intracranial hypertension is with a multidisciplinary team consisting of a neurologist, neurosurgeon, intensivist, ICU nurses, internist, and a pulmonologist. Treatment of chronic intracranial hypertension is mainly focused on treating and reversing the etiology. These patients need ICU admission and continuosu monitoring. In addition, the patients should have close monitoring of heart rate, blood pressure, body temperature, ventilation and oxygenation, blood glucose, input and output, and ECG. Patients with suspected intracranial hypertension, especially with severe traumatic brain injury, should also have ICP monitoring. In patients in whom the intracranial pressure is short lived and treated the prognosis is good but in those patients with delay in treatment or a malignant cause, the prognosis is abysmal. even those who survive develop permanent neurolgocial deficits.(Level V)