Intercranial Pressure: A Silent Stroke Trigger?

Intracranial pressure (ICP) is the pressure exerted by the brain, blood, and cerebrospinal fluid within the skull. The Monroe-Kellie Doctrine states that the skull is a closed system with a constant volume, meaning that an increase in the volume of one component will result in a decrease in the volume of one or two of the other components.

Increased ICP can be caused by a variety of factors, including head trauma, ischemia, brain tumours, meningitis, and stroke. It can lead to dangerous complications such as brain herniation and long-term brain damage.

While the exact mechanisms are still being investigated, studies suggest that ICP elevation after a stroke may be caused by factors other than cerebral edema, such as changes in cerebrospinal fluid volume or cerebral blood volume.

The treatment of increased ICP focuses on lowering intracranial pressure and addressing the underlying cause, which could include high blood pressure, infection, or stroke.

Intracranial hypertension (IH)

IH can occur suddenly, as a result of a severe head injury, stroke, or ruptured brain aneurysm. This is known as acute IH. It can also develop as a complication of another condition, such as high blood pressure. In some cases, the cause of IH is unknown, and this is referred to as idiopathic IH or benign IH.

The cranium is a rigid structure that contains the brain, cerebrospinal fluid, and blood. An increase in the volume of any of these components will lead to a rise in intracranial pressure. This can result in a decrease in cerebral blood flow or herniation of the brain.

The symptoms of IH include constant throbbing headaches, temporary loss of vision, nausea, drowsiness, and problems with coordination and balance. It can also cause permanent vision loss due to swelling of the optic nerve.

The treatment for IH depends on the underlying cause. For idiopathic IH, treatment options include weight loss, discontinuation of certain medications, diuretics to remove excess fluid, medication to reduce cerebrospinal fluid production, and steroids to relieve headaches and reduce the risk of vision loss. Surgery may be considered if other treatments are ineffective, particularly if vision is deteriorating.

Cerebrospinal fluid (CSF)

Functions of CSF

CSF has several functions, including:

• Buoyancy: The brain weighs around 1400-1500 grams, but its weight suspended in CSF is equivalent to just 25-50 grams. This neutral buoyancy allows the brain to maintain its density without being impaired by its own weight, which would cut off the blood supply and kill neurons in the lower sections.
• Protection: CSF acts as a shock absorber, providing a fluid buffer that protects the brain from some forms of mechanical injury.
• Prevention of brain ischemia: By decreasing the amount of CSF in the limited space inside the skull, CSF helps to reduce total intracranial pressure and facilitates blood perfusion.
• Regulation: CSF allows for the homeostatic regulation of the distribution of substances between the brain's cells and neuroendocrine factors. For example, high glycine concentration disrupts temperature and blood pressure control, and high CSF pH causes dizziness and fainting.
• Waste clearing: CSF allows for the removal of waste products from the brain and is critical to the brain's lymphatic system, known as the glymphatic system. When this process breaks down, as in amyotrophic lateral sclerosis, CSF can become toxic.

CSF and Intracranial Pressure

CSF is one of the three main components of the cranium, the others being the brain and blood. According to the Monroe-Kellie Doctrine, the contents of the cranium are in a state of constant volume. Therefore, any increase in the volume of one component will result in a decrease in volume in one or two of the others. An increase in CSF volume can lead to a decrease in cerebral blood flow or herniation of the brain.

Intracranial hypertension (IH) is a build-up of pressure around the brain and can be caused by a variety of factors, including head injury, stroke, ruptured brain aneurysm, brain abscess, brain tumour, meningitis, or encephalitis. IH can also develop as a complication of another condition, such as high blood pressure.

Diagnosis and Treatment

A lumbar puncture, or spinal tap, can be used to test the intracranial pressure, as well as indicate diseases including infections of the brain or meninges. A lumbar puncture is carried out by inserting a needle into the subarachnoid space, usually between the third and fourth lumbar vertebrae. The procedure is relatively safe, with serious complications being rare. However, it should not be performed if increased intracranial pressure is suspected due to certain situations, such as a tumour, as it can lead to fatal brain herniation.

Treatment for IH depends on the cause and may include losing weight, stopping certain medications, diuretics to remove excess fluid, or medication to reduce CSF production. Surgery may be considered if other treatments are ineffective, particularly if the patient's vision is deteriorating.

Collateral blood flow

The brain's collateral circulation consists of arterial anastomotic channels that provide nutrient-rich blood to regions whose normal sources of flow have been compromised. These collateral pathways can be classified into three main types:

• The circle of Willis, located at the base of the brain, connects the anterior and posterior circulatory systems, as well as the left and right hemispheres. It serves as a primary collateral pathway that can immediately reroute blood flow in the event of an acute occlusion. However, it is important to note that the circle of Willis exhibits significant variation among individuals, with less than 50% of people having a complete and symmetrical configuration.
• Leptomeningeal (pial) collaterals, on the other hand, are small arteriolar connections (approximately 50-400 µm) found at the distal branches of large cerebral and cerebellar vessels. These collaterals can provide bidirectional blood flow, depending on the metabolic and hemodynamic needs of the territories they connect.
• Extracranial sources of collateral circulation arise from branches of the external carotid artery or ascending and deep cervical arteries. They can provide blood flow in the event of an internal carotid artery occlusion by supplying retrograde flow through the circle of Willis or directly to the terminal internal carotid artery.

The extent and functionality of the collateral circulation are influenced by both genetic factors and various vascular risk factors. Aging, for example, leads to a decrease in the number and diameter of leptomeningeal collaterals, as well as increased vascular resistance. Additionally, vascular risk factors such as chronic hypertension, diabetes, and smoking can negatively impact collateral flow.

The role of the collateral circulation in ischemic stroke is crucial. It helps maintain blood flow to the affected tissue, slowing down the progression of penumbra to infarct and increasing the therapeutic window for effective treatment. A functional venous drainage system also plays a complementary role by enhancing the function of arterial collaterals.

The assessment of collateral circulation is now a routine part of neuroimaging protocols for acute ischemic stroke. Various imaging techniques, such as CT angiography, digital subtraction angiography, and magnetic resonance angiography, are used to visualize and evaluate the collateral pathways.

The collateral circulation has important clinical implications. It is a prognostic factor for both clinical and imaging outcomes in ischemic stroke. Poor collaterals on admission are associated with larger ischemic cores and worse clinical outcomes. Additionally, the collateral status may provide insights into the etiology of large vessel occlusions and can be used to select or exclude patients from acute reperfusion therapies.

Therapeutic strategies to enhance the collateral circulation have been explored, but none have demonstrated clear benefits to recommend their regular use. However, approaches such as remote ischemic conditioning, sphenopalatine ganglion stimulation, and the use of nitric oxide donors are showing promising results in clinical trials.

Intracranial pressure (ICP) monitoring

There are three main methods for ICP monitoring:

• Intraventricular catheter: This is the most accurate method, where a catheter is inserted through a hole drilled in the skull and into the lateral ventricle, which contains cerebrospinal fluid (CSF). This method can also be used to drain excess CSF.
• Subdural screw (bolt): This method is used when immediate monitoring is required. A hollow screw is inserted through the skull and the membrane that protects the brain and spinal cord (dura mater). This allows the sensor to record from inside the subdural space.
• Epidural sensor: This procedure is less invasive but cannot remove excess CSF. A sensor is inserted between the skull and dural tissue through a hole drilled in the skull.

ICP monitoring is crucial for guiding interventions and avoiding potentially harmful aggressive treatments. It allows for the judicious use of interventions with a defined target point. Additionally, ICP monitoring is essential for managing severe traumatic brain injuries and can improve patient outcomes and minimize the cost of acute care.

ICP monitoring devices can also be used to drain CSF or administer medications such as antibiotics and thrombolytic agents. However, there are risks associated with ICP monitoring, including brain herniation, brain tissue damage, and technical difficulties. Therefore, strict adherence to aseptic guidelines and prophylactic antibiotics are crucial to preventing infection.

ICP treatment

Intracranial hypertension (IH) is a clinical condition characterised by elevated pressure within the cranium. The cranium is a rigid structure that contains three main components: the brain, cerebrospinal fluid, and blood. Any increase in the volume of these components will increase the pressure within the cranial vault. This can lead to a decrease in cerebral blood flow or herniation of the brain.

The treatment of IH depends on the underlying cause. The priority is to assess and manage the airway, specifically breathing and circulation. Management principles should be targeted towards maintaining cerebral perfusion pressure by raising the mean arterial pressure and treating the underlying cause.

Measures to lower ICP include:

• Elevating the head of the bed to more than 30 degrees
• Keeping the neck midline to facilitate venous drainage from the head
• Hyperventilation to lower carbon dioxide levels to around 30 mm Hg
• Using osmotic agents to create an osmotic gradient across the blood, thereby drawing fluid intravascularly and decreasing cerebral oedema
• Lumbar punctures to drain cerebrospinal fluid and reduce ICP
• Neurosurgical shunts to divert cerebrospinal fluid to another part of the body for reabsorption
• Intravenous glyburide to inhibit SUR1 receptors
• Barbiturates in cases where sedation and other methods are unsuccessful in reducing ICP
• Therapeutic hypothermia to 32-35 degrees Celsius for a refractory rise in ICP
• Decompressive craniectomy as a last resort when all other ICP-lowering measures have failed

Frequently asked questions