Monitoring intracranial pressure (ICP) is important in a variety of neurologic conditions, including aneurysmal subarachnoid hemorrhage and traumatic brain injury. Monitoring and controlling ICP can mitigate secondary injury of the brain. Of the invasive methods of monitoring ICP, the external ventricular drain is still considered the gold standard. However, microtransducers have been shown to be a reliable option with significantly lower risks of complications. Due to their reproducibility, and their limitations, they are not ready to replace invasive ICP monitoring techniques. This article reviews the commonly used invasive and non-invasive methods of monitoring ICP.
Key points
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Disorders in intracranial pressure are commonly encountered and associated with morbidity and mortality.
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Treating pathologic intracranial pressures requires accurate means of measuring it.
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Intracranial pressure is typically measured via invasive procedures, of which external ventricular drainage is the gold standard.
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Non-invasive means of measuring intracranial pressure are in development and offer the potential for measurement without the risk of surgical procedures.
Introduction
Measuring intracranial pressure is important in various conditions, such as traumatic brain injury (TBI), aneurysmal subarachnoid hemorrhage (aSAH), and other space occupying lesions in the cranium. The Monro-Kellie doctrine states that the total volume of brain, cerebrospinal fluid (CSF), and intracranial blood is constant. , An increase in one component will lead to a decrease in another. This gives the brain a compensatory reserve to maintain intracranial pressure (ICP) when encountering changes in intracranial volume. However, when the reserve is exhausted, intracranial compliance will decrease, and small changes in intracranial volume can result in large increases in ICP, thereby decreasing the cerebral perfusion pressure (CPP) ( Fig. 1 ). This can lead to cerebral ischemia or herniation syndromes, resulting in devastating neurologic consequences.
The relationship between ICP and CPP are described by the following equation:
CPP=MeanArterialPressure−ICP
Under physiologic conditions, cerebral blood flow is independent of CPP due to cerebral autoregulation. Arterioles have the capability to constrict or dilate to maintain constant blood flow when CPP changes. Normal MAP is generally accepted to be between 50 and 150 mm Hg in adults, , thereby maintaining a CPP of greater than 50 mm Hg. However, when the mean arterial pressure is below 50 mm Hg or above 150 mm Hg, or when the brain suffers an insult (ie, TBI or SAH), autoregulation can become dysfunctional. Under these conditions, cerebral blood flow is directly dependent on CPP, highlighting the importance of measuring ICP to be able to determine the perfusion status of the brain.
Measuring ICP can also provide insights into the state of the intracranial compliance, or the amount of reserve the brain has to accommodate additional volume. Under normal circumstances, the ICP waveform consists of 3 peaks ( Fig. 2 A), which are thought to be related to a combination of physiologic parameters. , As cerebral compliance begins to decrease, P2 will become larger than P1 ( Fig. 2 B), and subsequently the peaks will disappear, and the ICP curve will become a single peak with a rounded appearance ( Fig. 2 C). Lundberg A and B waves are another pathologic waveform. Lundberg A waves ( Fig. 3 ) are denoted by a high rise in ICP of greater than 50 mm Hg for 5 to 20 minutes, and are indicative of severe cerebral ischemia and impending herniation. Lundberg B waves occur for 30 seconds to 2 minutes, are associated with a periodic increase in ICP of 20 to 30 mm Hg and are usually associated with respiration. These are related to a brain insult leading to a gradual increase in ICP. Examining ICP waveforms can provide important insights in the management of intracranial hypertension.
Methods of monitoring ICP are one of the cornerstones for neurosurgeons and intensivists caring for patients with neurologic injuries. Here, we review both the invasive, as well as non-invasive methods of monitoring ICP in the clinical setting.
Invasive methods of monitoring intracranial pressure
External Ventricular Drains
There are numerous methods of measuring ICP in various locations of the central nervous system. These include the ventricles, parenchyma, subdural space, and lumbar cistern (via a lumbar puncture).
An external ventricular drain (EVD) is considered the gold standard for measuring ICP due to its accuracy and consistency over time. , EVDs provide the added benefit of being able to treat elevations in ICP with CSF drainage. The main disadvantage of using EVDs for ICP measurement is the higher rate of complications, such as infection and tract hemorrhage, when compared to microtranducers. The rate of hemorrhage and infection associated with EVD placement is 9.2% and 12.2%, respectively. The rate of catheter misplacement is as high as 38.6%. Furthermore, in cases of severe TBI where there is global edema, the ventricles are sometimes effaced, making it technically more challenging for insertion. However, given the reliability of the measurement, as well as its therapeutic potential, EVDs are still widely used to measure ICP.
Microtransducers
Microtransducers are another method of monitoring ICP. They can be divided into fiberoptic devices, strain gauge devices, or pneumatic devices. These are often inserted into the brain parenchyma, although some studies have shown that monitors in the subdural space can be just as accurate at measuring global ICP.
Fiber optic sensors
One of the most widely used fiberoptic devices is the Camino ICP monitor. The Camino ICP monitor works by transmitting light through fiberoptics into the brain tissue. The light is then reflected and displaces a mirror, which is measured and reflects the ICP value. The Camino monitor has been studied extensively in the literature and has been shown to provide reliable ICP data. , The measured ICP does drift over time, with Munch and colleagues showing a 1.4 mm Hg drift with a mean recording time of 49.4 hours. These results are corroborated by a subsequent study, showing a 7.3 mm Hg drift with a mean recording time of 7.3 days. The rate of mechanical failure amongst the Camino devices ranged from 4.5% to 10%, which included defective probe, broken transducer, or screws. , Radiological hemorrhage occurred in 1.1% to 2.5% of patients, while the rate of symptomatic hemorrhage is 0.6%. The rate of infection ranged from 2.1% to 4.8%, with the most common organism being coagulase-negative Staphylococcus . Due to the presence of ferromagnetic substances within the transducer, the Camino monitor is not MRI compatible. Nonetheless, the Camino system provides reliable ICP measurements, and a lower rate of adverse events compared to an EVD.
Pneumatic sensors
The Spielberg sensor is another ICP monitor that has been widely used. It is a pneumatic sensor that has a small balloon on the distal end of the catheter to record pressure changes. Given this design, the Spielberg system can be placed in the epidural space, subdural space, parenchyma, and even in the ventricles. Several studies have shown low rates of complication rates, with very low reported incidence of both hemorrhage, as well as infection. The rate of mechanical failures for these sensors are reported to be 3%.
Strain gauze device
These sensors work using piezoelectric strain gauze technology, and several brands are available including the widely used Codman device and others such as Raumedic Neurovent and Pressio. , These types of monitors have a strain gauze in the tip that bends as a result of increased pressure. When the transducer is bent, the resistance changes and thus, ICP can be calculated. The Codman microsensor has been studied extensively and has shown to be a robust and reliable ICP monitor. , The monitor can be placed in the subdural space or the parenchyma, with both giving reliable values. Several studies have shown good correlations between ICP measured with the Codman compared to the gold standard EVD (r value ranges from 0.79 to 0.97). The measured ICP remained reliable, drifting by less than 2 mm Hg after 7 days of measurements. The rates of clinically significant hemorrhage and infection with the Codman were very low in the published literature. , The Raumedic Neurovent had demonstrated similar safety and reliability profiles. The mean ICP drift was 0.8 mm Hg after 6 days of measurement, with no clinically symptomatic infection, and a 2% rate of radiographic hematoma. Many strain gauze devices, particularly the Codman, are MRI compatible. This is particularly important in patients with TBI, as it allows MRI to be performed for neuro-prognostication while still allowing ICP to be measured.
Non-invasive methods of measuring intracranial pressure
Several non-invasive methods of measuring ICP have been proposed, such as transcranial doppler, optic nerve sheath diameter (ONSD), tympanic membrane displacement (TMD), and imaging. These non-invasive methods do not have the risk profile of traditional ICP monitors, but more work will be needed to validate them in a variety of clinical situations. Here, we will review the commonly used non-invasive methods of measuring ICP.
Transcranial Doppler
Transcranial doppler takes advantage of the Doppler effect and uses ultrasound to measure blood velocity. Subsequently, a measurement called pulsatility index (PI) can be computed with the following equation:
PI=SystolicBloodFlowVelocity−DiastolicBloodFlowVelocityMeanFlowVelocity
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