Pathophysiology of Increased Intracranial Pressure

Intracranial Pressure Etiology

Increased ICP arises from:

• An increase in cerebrospinal fluid (CSF).
• An increase in the amount of blood or cerebral blood flow.
• An increase in brain tissue, including cerebral swelling or cerebral edema.
• A combination of these etiologies.

The CSF, blood, and brain tissue are the three main elements that occupy the skull. If there is an increase in any of these elements, the ICP will increase.

Respiratory acidosis can eventually lead to increased intracranial pressure.

Changes in these elements occur for different reasons. For example, an increase in CSF is observed in meningitis, an increase in blood or cerebral blood flow is seen during trauma, and an increase in brain tissue is associated with trauma that causes tissue swelling and edema.

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A Case to Illustrate Increased ICP

A patient presents with respiratory acidosis. This presentation is fairly common in patients with emphysema and chronic obstructive pulmonary disease (COPD) and occurs when the amount of carbon dioxide (CO₂) in the body increases.

Because CO₂ is a potent vasodilator and readily crosses the blood-brain barrier, the patient may experience neurological changes. When CO₂ enters the brain, it causes cerebral vasodilation, which increases cerebral blood flow. The increase in blood flow and amount of blood in the brain leads to increased intracranial pressure.

Note: the pathway to increased ICP may differ based on etiology. However, the signs and symptoms will be the same.

Respiratory acidosis can eventually lead to increased intracranial pressure.

Read: Peripheral Vascular Disease

What Are the Signs and Symptoms?

There are both early and late-stage signs and symptoms for increased ICP. It is important for the provider to recognize the early signs and symptoms and correct the underlying etiology. If the underlying etiology is not addressed, it can lead to brainstem herniation. Once the late-stage signs and symptoms present, brainstem herniation is imminent.

The early signs and symptoms of ICP are:

• Restlessness
• Agitation
• Confusion 
• Headache
• Nausea or vomiting

In the early stages of ICP, the patient may seem “off” after previously being calm, alert, and oriented. They will increasingly become restless and agitated and may report a headache (from pressure on the meninges) or nausea and vomiting (from pressure on the medulla — the vomiting center of the brain).

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Cushing Triad: A Unique Set of Symptoms

The Cushing triad is a late but also unique presentation of symptoms caused by increased ICP. The Cushing triad includes an increase in the systolic blood pressure, a decrease in the heart rate (bradycardia), and a decrease in the respiratory rate.

It is important to understand why the symptoms within the triad occur:

• Increase in Systolic Blood Pressure. An increase in ICP creates less room in the brain, which decreases brain perfusion. When the body senses that the brain is not being perfused well, a series of chemical reactions in the brain and body (via norepinephrine) increase perfusion, resulting in increased systolic pressure.

However, as the systolic blood pressure increases, a similar increase will not be seen in the diastolic pressure. Instead, what will result is a widened pulse pressure

• Decrease in the Heart Rate. The increase in systolic blood pressure triggers reactions in the baroreceptors that sense pressure changes. The baroreceptors then prompt a series of reactions to slow the heart rate. The body decreases the heart rate in an attempt to decrease the systolic blood pressure.

The baroreceptors in the carotid artery sense changes in blood pressure.

When the baroreceptors sense increased blood pressure, the body’s response is to slow the heart rate.

• Decreased Respiratory Rate. Increased ICP puts pressure on the brainstem, which controls involuntary breathing. The changes to the brainstem pressure causes decreased or irregular breathing patterns.

The symptoms of the Cushing triad will present in the late stages of ICP when the brainstem is about to herniate. That is why it is essential for the provider to be vigilant of early signs and symptoms of increased ICP to treat the underlying condition and prevent the patient from progressing to the late stages of ICP.

Even though ICP has multiple etiologies, it always presents with the same signs and symptoms. It is important for the provider to recognize and treat the early signs and symptoms to keep the patient from progressing to late-stage signs and symptoms, known as the Cushing triad, and ultimately, brainstem herniation.