Overview of Intracranial Pressure Monitoring
Normal pressure inside the skull, also known as intracranial pressure (ICP), should usually be between 7 and 15 mm Hg. When a person stands up, this pressure should still not go above 15 mm Hg. The best way to check a person’s ICP is by monitoring them overnight while they are awake.
When this pressure goes beyond 20 to 25 mm Hg, it can cause problems and doctors must work to lower it. If it stays too high for too long, it can prevent blood flow to the brain, causing damage and potentially leading to serious situations like brain herniation, which can eventually cause death.
For individuals with severe brain injuries, carefully monitoring their ICP is critical for managing their condition. This allows doctors to intervene when necessary with treatments that are aimed at a specific ICP target. This approach can help avoid overly aggressive treatments that might be harmful. Following guidelines from the Brain Trauma Foundation when caring for the patient has proven to be effective in improving outcomes and reducing costs for acute care, or immediate emergency care.
Anatomy and Physiology of Intracranial Pressure Monitoring
The Monro-Kellie doctrine or theory is a concept in medicine that simplistically means if one substance increases in the skull, another one must decrease to maintain the same pressure. Our skull is seen as a rigid box that contains three things: brain tissue, blood, and a fluid called cerebrospinal fluid (or CSF), which cushions the brain and spinal cord. If the volume of one of these increases, like in the case of a brain tumor, the volume of others must decrease to compensate.
The doctrine was originally proposed by Alexander Monro Secundus in 1783 who believed that the amount of brain tissue and blood volume in the skull remained constant. Later, George Kellie reinforced the assumption that the total amount of blood flowing within the skull stays consistent. Unfortunately, during their time, more information on the nervous system was still needed to fully substantiate the theory, especially considering the role cerebrospinal fluid (CSF) – the clear fluid around the brain and spinal cord – plays in controlling pressure in the skull.
It was only in the 20th century that Harvey Cushing established the importance of CSF in maintaining pressure inside our skull. He found that these three components – brain tissue, blood, and CSF, work together to compensate for any increase or decrease in volume within the skull.
For example, when the pressure increases inside the skull – called intracranial hypertension – the body tries to remove some of the blood and CSF to keep the pressure stable. If the increased pressure is due to a growth such as a tumor, the blood and CSF have to decrease even more to avoid further pressure increase.
On the other hand, in people with low pressure in the skull – called intracranial hypotension – the amount of brain tissue stays the same, but the volume of blood increases, especially in the veins, as they can expand more than arteries. This can cause certain changes that can be observed in an MRI scan, such as expansion of veins in the brain, brain tissue being pushed down, and enlargement of the pituitary gland – a small organ at the base of the brain.
In simpler terms, think of the brain as a balloon that can expand or contract (its ‘compliance’) depending on how much air (or in this case, volume) you put into it. Initially, the brain can accommodate changes in volume without a significant increase in pressure. But as the volume continues to increase, the brain will find it progressively difficult to adapt, leading to a significant increase in pressure. This is, in fact, the limit of the doctrine – it doesn’t consider the role of CSF in the brain’s ability to adapt, the effect of gravity on the volume and pressure in the skull, and it is not applicable for children with still developing skulls.
Why do People Need Intracranial Pressure Monitoring
If someone suffers a traumatic brain injury, certain steps and procedures are recommended by the Brain Trauma Foundation to manage the condition, based on the severity of the situation.
One of these key procedures is the use of an ICP monitor. ICP stands for Intracranial Pressure, which simply means the pressure inside your skull. This is crucial if a patient scores lower than 8 on the Glasgow Coma Scale (a test used to evaluate the level of consciousness after a brain injury), has a peculiar brain scan, or if they have at least two of these features: over 40 years old, having irregular limb movements, or having a lower than normal blood pressure.
Other situations where ICP monitoring might be needed include: patients who initially have a normal brain scan but then show signs of worsening conditions on a re-examination; if there is evidence of swelling in the brain; in patients with significant bruises on the front of the brain regardless of their mental state; when it’s found to be unsafe to stop sedation to check a patient’s neurological function, for instance when the lung is bruised or has failed, causing difficulty in breathing; or if a reliable neurological test can’t be done due to other injuries, like facial or spinal cord traumas.
Another measure, known as a decompressive craniectomy, may be performed as a back-up option when the pressure inside the skull continues to rise despite medical treatment. This procedure involves removing a part of the skull to make room for the swollen brain and to help lower the pressure inside the skull.
After a craniotomy (a surgery to open the skull), ICP monitoring is also recommended for patients who hold certain risk factors that can cause brain swelling, such as low oxygen levels, low blood pressure, weird pupil reactions, or significant shift of brain structure.
When a Person Should Avoid Intracranial Pressure Monitoring
There are several reasons that might prevent a doctor from using invasive methods to monitor a person’s intracranial pressure (ICP), which is the pressure inside the skull and thus in the brain tissue and cerebrospinal fluid:
First, if a person is already taking anticoagulant drugs, which are medications that prevent blood clotting, it might not be safe to use these monitoring techniques.
Second, if a person has bleeding disorders — conditions that affect the body’s ability to control bleeding — these techniques might also be too risky.
Third, if a person has a scalp infection, it could potentially cause complications when trying to use invasive ICP monitoring methods.
Lastly, if a person already has a brain abscess, which is a collection of pus in the brain usually caused by a bacterial infection, this could complicate the procedure and make it unsafe.
Equipment used for Intracranial Pressure Monitoring
When preparing for the procedure, the following items should be ready:
Items like non-sterile gloves, soap, a brush, a hand towel, a razor, and a marker pen are used to prepare and mark the area where the monitoring devices will be placed.
During the procedure, the healthcare provider will wear a face mask, sterile gown, and gloves. They will also use an antiseptic solution to clean the area, a drape to cover you, a local anesthetic to numb the area, a 5-mL syringe, a surgical blade (number 11 or 15), and a special kit for monitoring intracranial pressure (ICP).
Some procedures will require a drill with a drill bit, a bolt, an ICP sensor, and a transducer, depending on the methods utilized. Once the procedure is done, suture material is used to close the wound and a sterile dressing is applied to keep it clean.
It’s important to monitor the pressure inside your skull (intracranial pressure or ICP) during the procedure. This can be done in different ways such as through a ventriculostomy, an intraparenchymal strain gauge, or a fiber-optic monitor. These monitoring devices should be readily accessible.
Who is needed to perform Intracranial Pressure Monitoring?
Your treatment will be taken care of by a team of healthcare professionals. This team includes:
A neurosurgeon, a specialized doctor who performs surgeries on the brain and the nerves.
A qualified assistant, who aids the neurosurgeon during the operation.
An attending nurse, who provides overall care before, during, and after the operation.
And finally, an anesthetist who is responsible for making you comfortable by administering medication that puts you to sleep so you don’t feel pain during the operation.
You can rest assured as each member of the team is there to ensure the operation goes smoothly, and you’re taken care of before, during, and after the surgery.
Preparing for Intracranial Pressure Monitoring
Before any medical treatment, the doctor will explain in detail why the treatment is necessary and any possible risks. It’s also important for the patient or their representative to give written permission. To prevent any infections during the process, the doctor will strictly follow clean and sterile procedures. Antibiotics would also be given at the start of the treatment to further reduce the risk of infection. The success of the treatment largely depends on it being performed very carefully and accurately.
The doctor will make sure that the patient is comfortable and relaxed. They will ensure that the patient can breathe easily and comfortably. Then, a local anesthetic, which is a kind of medication to reduce pain and discomfort, will be given at the particular area where the procedure will be done, such as a ventriculostomy (which is a procedure to drain excess liquid from the brain) or when inserting devices to monitor intracranial pressure (pressure within the skull).
How is Intracranial Pressure Monitoring performed
The process of placing an intracranial pressure (ICP) monitoring device involves creating a small hole in the skull using either a burr hole or twist drill technique. This hole allows the doctor to insert the device into the brain. The preferred location for this procedure is the Kocher point, which is slightly to the side of the center of the skull and slightly forward of a line that runs from ear to ear. Other possible locations for the procedure include the Keen, Dandy, and Frazier points.
Once the device is in place, it allows doctors to closely monitor the pressure and pulsation pattern of the patient’s brain waves. This provides more useful information than just measuring the height of the cerebral spinal fluid (CSF), which is the liquid that bathes the brain and spinal cord.
The brain wave pattern typically shows three peaks. The P1 peak, or percussion wave, is caused by the pulsing of arteries. The P2 peak, or tidal wave, tells us how well the brain is managing pressure changes, also known as brain compliance. The P3 peak, or dicrotic wave, occurs when the main heart valve closes.
As the brain’s ability to absorb pressure changes decreases, the amplitude (height) of the wave pattern will increase, causing P2 to become higher than either P1 or P3. This can lead to the appearance of plateau waves, and Lundberg B and A waves, which are specific patterns in the wave signal.
Using a ventricular catheter (a tube placed in the hollow spaces within the brain) is the most reliable method for ICP monitoring as it provides an overall picture of the brain pressure. It also allows for removal of CSF if pressure becomes too high, and the delivery of certain types of medication directly to the brain.
Another type of ICP monitoring device uses strain gauge or fiber-optic systems, which can provide a more accurate reading. These are particularly useful if there is a lot of brain swelling or bleeding. These monitors do have some limitations – they can only measure pressure in the immediate area, they cannot be recalibrated, and they may give less accurate results if used for a long time.
Brain scans such as computed tomography (CT) or magnetic resonance imaging (MRI) only provide a single image of the brain at a point in time. Other non-invasive techniques, such as transcranial Doppler, near-infrared spectroscopy, and assessments of the optic nerve sheath diameter, are not accurate enough to replace traditional invasive techniques, as there is an error margin of +/- 10 to 15 mm Hg.
Possible Complications of Intracranial Pressure Monitoring
When doctors want to measure the pressure inside your brain, known as intracranial pressure (ICP), they usually take a reading from a certain spot and assume that this reading can tell them about the pressure in the whole brain. However, this may not be the best approach. This is because the pressure in the brain can vary in different parts, specifically within the ventricular system (a set of fluid-filled cavities inside the brain) and where it meets the solid brain tissue. There are also concerns about whether the equipment to measure ICP is always accurate, and whether it can provide correct readings over time.
If there is severe swelling in the brain which narrows the ventricles, it can be difficult for doctors to correctly place a catheter (a thin tube) to monitor ICP. This in turn can lead to further problems.
Here are some possible complications that can occur when using ventricular catheter-based ICP monitoring:
- Internal bleeding within the brain and along the path of the catheter – happens in around 10% of cases.
- Infection, specifically ventriculitis (inflammation of the ventricles) – occurs in about 20% of cases.
- Technical problems, like not being able to tap into the ventricle or accidentally placing the catheter in the wrong place – seen in about 5% of cases.
- There’s a risk that too much fluid might drain out, which can complicate conditions like hydrocephalus (a condition where fluid builds up in the brain) or even cause an aneurysm (a bulge in a blood vessel) to bleed again.
- It’s fairly common for the catheter to develop kinks or blockages from things like air, blood, or debris. This can hinder or even distort the ICP readings.
- Sometimes, the pressure in a specific part of the brain can increase due to a mass growth like a tumor.
What Else Should I Know About Intracranial Pressure Monitoring?
Intracranial pressure (ICP) is the level of pressure in your brain and is often monitored to assess conditions such as brain hemorrhage or brain trauma. There are various ways to monitor ICP and each approach has its strengths and weaknesses.
One method is a fluid-based system, which assesses pressure using a ventricular catheter (a thin tube) placed in the brain. However, this method can have false results caused by air bubbles, movement, and blockages, and it carries a risk of misplacement and infection. Alternatively, implantable ICP sensors can be used, which are special devices placed into the brain tissue, under the skull, or outside the lining of the brain. These also carry risks, such as causing bleeding or infection, and their accuracy can vary.
Different systems can be specifically used to monitor ICP including a fiber-optic system (Integra), a strain-gauge system (Codman), Raumedic (Neurovent P), and a pneumatic system (Spiegelberg).
In terms of what to monitor, one common measure is the cerebral perfusion pressure, which is how much blood flow is reaching the brain. The ICP mean wave amplitude, a value that shows the average pressure wave in the brain, is also often checked. A value of less than 4 mm Hg is considered normal.
Observing the way pressure changes in different parts of the brain over time can also provide important information. There are different areas of focus such as respiratory waveforms and pulse pressure waveforms, which reflect different facets of how the brain is functioning.
In terms of measuring the accuracy of ICP monitoring systems, it’s recommended that the reading difference between invasive and noninvasive systems should not be greater than 2 mm Hg when the ICP is between 0 to 20 mm Hg and less than 10% when the ICP is between 20 to 100 mm Hg.
However, simply measuring ICP as a single number can have limitations as it doesn’t take into account the various interconnected processes in the brain. This is why the RAP (a correlation coefficient between amplitude and pressure) or PVI (pressure-volume index) methods have been developed to provide a more individualized way of measuring and managing ICP. These methods check the tolerance of the brain to changes in pressure and blood flow.
It’s worth noting that according to a South American study, managing patients based on ICP outcomes did not show significant improvements compared to management based on regular computer scans and clinical examinations. However, this study had limitations and the results were met with some skepticism.
Furthermore, all invasive ICP monitoring methods, which involve placing a device or sensor within the brain, come with some limitations. For example, they heavily rely on technology and are susceptible to various kinds of inaccurate results. There is also uncertainty about the accuracy of ICP readings in different parts of the brain.
One ultimate goal in the field is to develop a noninvasive ICP monitoring technique capable of real-time monitoring. Current efforts in ICP monitoring mostly measure the average ICP over a short while, rather than real-time changes in pressure. Therefore, expecting real-time monitoring without any distortions, processed via a robust machine learning algorithm, is an aspirational goal in neurosurgery.