Increased Intracranial Pressure

Etiology

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

Mass Effect

• Hematoma
• Tumor
• Abscess
• Infarct

Increase in Cerebrospinal Fluid

• Increased production of CSF
• Choroid plexus tumor

Decreased Reabsorption of CSF

• Obstructive hydrocephalus
• Meningeal inflammation or granulomas

Increase in Blood Volume

• Increased cerebral blood flow during hypercarbia, aneurysms
• Venous stasis from
• Venous sinus thromboses,
• Elevated central venous pressures, e.g., heart failure

Other Causes

• Idiopathic or benign intracranial hypertension
• Skull deformities such as craniosynostosis
• Hypervitaminosis A, tetracycline use

Epidemiology

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.[2]

Pathophysiology

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).[3] CPP = MAP - CVP if central venous pressure is higher than intracranial pressure. CPP target 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.

History and Physical

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.[4]

Evaluation

The evaluation of increased ICP should include detailed history taking, physical examination, and ancillary studies.

It is extremely important to identify increased ICP as early as possible to prevent herniation and death. For example malignant middle cerebral artery stroke presenting with increased ICP. Malignant middle cerebral artery stroke is seen more commonly in the younger population. Usually, these patients are admitted to the ICU setting. Following the neurological exam closely is very important. Usually, there is an altered mental status and development of a fixed and dilated pupil. Patients presenting with findings suggestive of cerebral insult should undergo computed tomography (CT) scan of the brain; this can show the edema, which is visible as areas of low density and loss of gray/white matter differentiation, on an unenhanced image. There can also be an obliteration of the cisterns and sulcal spaces. A CT scan can also reveal the cause in some cases. If flattened gyri or narrowed sulci, or compression of the ventricles, is seen, this suggests increased ICP. Serial CT scans are used to monitor the progression or improvement of the edema.[5]

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.

ICP Monitoring[6]

Several devices can be used for ICP monitoring.

The procedure involves the 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.

Optic Nerve Sheath Diameter (ONSD)[7]

The use of ultrasound to measure the diameter of the optic nerve sheath has been recently identified as a method to indicate raised ICP. This is usually measured 3 mm behind the globe with 2–3 measurements taken in each eye. The threshold for denoting elevated ICP usually ranges from 0.48 cm to 0.63 cm.

Treatment / Management

Assessment and management of the airway, specifically breathing and circulation should always be the priority.[8]

Management principles should be targeted toward:

• Maintenance of cerebral perfusion pressure by raising MAP 
• Treatment of the underlying cause.
• Lowering of ICP.[9]

Measures to lower ICP include:[10]

• Elevation of the head of the bed to greater than 30 degrees.
• Keep the neck midline to facilitate venous drainage from the head.
• Hypercarbia lowers serum pH and can increase cerebral blood flow contributing to rising ICP, hence hyperventilation to lower pCO2 to around 30 mm Hg can be transiently used.
• Osmotic agents can be used to create an osmotic gradient across blood thereby drawing fluid intravascularly and decreasing cerebral edema. Mannitol was the primary agent used at doses of 0.25 to 1 g/kg body weight and is thought to exert its greatest benefit by decreasing blood viscosity and to a lesser extent by decreasing blood volume. Side effects of mannitol use are eventual osmotic diuresis and dehydration as well as renal injury if serum osmolality exceeds 320 mOsm.[11] Steroids are indicated to reduce ICP in intracranial neoplastic tumors, but not in traumatic brain injury.
• Three percent hypertonic saline is also commonly used to decrease cerebral edema and can be administered as a 5 ml/kg bolus or a continuous infusion, monitoring serum sodium levels closely. It is considered relatively safe while serum sodium is < than 160mEq/dl or serum osmolality is less than 340 mOsm.[12]
• Drugs of the carbonic anhydrase inhibitor class, such as acetazolamide, can be used to decrease the production of CSF and is used to treat idiopathic intracranial hypertension.
• Lumbar punctures, besides being diagnostic, can be used to drain CSF thus reducing the ICP. The limitation to this is raised ICP secondary to mass effect with a possible risk of herniation if the CSF pressure drops too low.
• Similar to a lumbar puncture, an EVD can also be used to not only monitor ICP but also to drain CSF.
• Optic nerve fenestrations can be performed for patients with chronic idiopathic hypertension at a risk of blindness. Neurosurgical shunts such as ventriculoperitoneal or lumbar-peritoneal shunts can divert CSF to another part of the body from where it can be reabsorbed.[13]
• Intravenous (IV) glyburide is being investigated in the prevention of hemispheric stroke edema. It acts by inhibiting SUR1 receptors.[14]
• Barbiturates can be considered in cases where sedation and usual methods of treatment are not successful in reducing the ICP.[15]
• Therapeutic hypothermia to 32-35 degrees Celcius can be used in a refractory rise in ICP not responding to hyperosmolar therapy and barbiturate coma. But its use has been questioned in recent days.
• A decompressive craniectomy is a neurosurgical procedure wherein a part of the skull is removed, and dura lifted, allowing the brain to swell without causing compression.[16] It is usually considered as a last resort when all other ICP lowering measures have failed.
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Differential Diagnosis

• Acute nerve injury
• Blood dyscrasias and stroke
• Hydrocephalus
• Intracranial hemorrhage
• Intracranial epidural abscess
• Lyme disease
• Meningioma
• Migraine variants
• Subarachnoid hemorrhage

Prognosis

Prognosis depends on the underlying etiology and severity of the 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 significant morbidity in IIH.