Intracranial hypertension (IH) is a clinical condition that is associated with an elevation of the pressures within the cranium. The pressure in the cranial vault is measured in millimeters of mercury (mm Hg) and is normally less than 20 mm Hg.
The cranium is a rigid structure that contains 3 main components: brain, cerebrospinal fluid, and blood. Any increase in the volume of its contents will increase the pressure within the cranial vault. The Monroe-Kellie Doctrine states that the contents of the cranium are in a state of the constant volume. That is, the total volumes of the brain tissues, cerebrospinal fluid (CSF), and intracranial blood are fixed. An increase in the volume of one component will result in the decrease of volume in 1 or 2 of the other components. The clinical implication of the change in volume of the component is a decrease in cerebral blood flow or herniation of the brain.
CSF is a clear fluid found in the subarachnoid spaces and ventricles that cushions the brain and spinal cord. It is secreted by the choroid plexus in the lateral ventricles, travels to the third ventricle via the foramen of Monroe. From the third ventricle, CSF reaches the fourth ventricle through the aqueduct of Sylvius. From here, it flows into the subarachnoid space via the foramina of Magendie and Luschka and is eventually reabsorbed into the dural venous sinuses by arachnoid granulation.
The causes of increased intracranial pressure (ICP) can be divided based on the intracerebral components causing elevated pressures:
Increase in brain volume
Generalized swelling of the brain or cerebral edema from a variety of causes such as trauma, ischemia, hyperammonemia, uremic encephalopathy, and hyponatremia
Increase in cerebrospinal fluid
Decreased re-absorption of CSF
Increase in blood volume
The true incidence of intracranial hypertension is unknown. The Centers for Disease Control and Prevention (CDC) estimates that in 2010, 2.5 million people sustained a traumatic brain injury (TBI). TBI is associated with increased ICP. ICP monitoring is recommended for all patients with severe TBI. Studies of American-based populations have estimated that the incidence of idiopathic intracranial hypertension (IIH) ranges from 0.9 to 1.0 per 100,000 in the general population, increasing in women that are overweight.
The harmful effects of intracranial hypertension are primarily due to brain injury caused by cerebral ischemia. Cerebral ischemia is the result of decreased brain perfusion secondary to increased ICP. Cerebral perfusion pressure (CPP) is the pressure gradient between mean arterial pressure (MAP) and intracranial pressure (CPP = MAP - ICP). CPP = MAP - CVP if central venous pressure is higher than intracranial pressure. CPP targets for adults following severe traumatic brain injury is recommended at greater than 60 to 70 mm Hg, and a minimum CPP greater than 40 mm Hg is recommended for infants, with very limited data on normal CPP targets for children in between.
Cerebral autoregulation is the process by which cerebral blood flow varies to maintain adequate cerebral perfusion. When the MAP is elevated, vasoconstriction occurs to limit blood flow and maintain cerebral perfusion. However, if a patient is hypotensive, cerebral vasculature can dilate to increase blood flow and maintain CPP.
Clinical suspicion for intracranial hypertension should be raised if a patient presents with the following signs and symptoms: headaches, vomiting, and altered mental status varying from drowsiness to coma. Visual changes can range from blurred vision, double vision from cranial nerve defects, photophobia to optic disc edema and eventually optic atrophy. Infants in whom the anterior fontanelle is still open may have a bulge overlying the area.
Cushing triad is a clinical syndrome consisting of hypertension, bradycardia and irregular respiration and is a sign of impending brain herniation. This occurs when the ICP is too high the elevation of blood pressure is a reflex mechanism to maintain CPP. High blood pressure causes reflex bradycardia and brain stem compromise affecting respiration. Ultimately the contents of the cranium are displaced downwards due to the high ICP, causing a phenomenon known as herniation which can be potentially fatal.
The evaluation of increased ICP should include a detailed history taking, physical examination, and ancillary studies.
A funduscopic exam can reveal papilledema which is a tell-tale sign of raised ICP as the cerebrospinal fluid is in continuity with the fluid around the optic nerve.
Imaging- a computed tomography (CT) of the head or magnetic resonance imaging (MRI) can reveal signs of raised ICP such as enlarged ventricles, herniation, or mass effect from causes such as tumors, abscesses, and hematomas, among others.
Measurement of Opening Pressure with a Lumbar Puncture
In this procedure, a needle is introduced in the subarachnoid space. This can be connected to a manometer to give the pressure of the CSF prior to drainage. A measurement greater than 20 mm Hg is suggestive of raised ICP. Brain imaging should precede an LP because LP can cause a sudden and rapid decrease in ICP and the sudden change in volume can lead to herniation.
Several devices can be used for ICP monitoring.
The procedure involves placement of a fiber optic catheter into the brain parenchyma to measure the pressure transmitted to the brain tissue.
External Ventricular Drain (EVD)
A drain placed directly into the lateral ventricles can be connected to a manometer to give a reading for the pressure in the ventricles.
Assessment and management of airway, specifically breathing and circulation should always be the priority.
Management principles should be targeted toward:
Measures to lower ICP include:
Prognosis depends on the underlying etiology and severity of presentation. Benign intracranial hypertension does not increase the risk of death rate by itself; rather, the death rate is increased by morbid obesity which is a common association with benign intracranial hypertension. Visual loss is a significant morbidity in IIH.
A patient who presents with a headache, vomiting, and blurred vision should be evaluated for neurologic deficits and receive head imaging to rule out causes of intracranial hypertension.
All patients with severe TBI (Glasgow coma scale of 3 to 8 on initial presentation) should follow the latest guidelines on the management of severe TBI that includes monitoring of ICP, maintenance of CPP greater than 60 to 70 mm Hg for adults and treatment of ICP greater than 22 mm Hg.