What is the intracranial pressure (ICP)?
There are three compartments in the cranium:
- The brain, i.e. the parenchymal compartment;
- The blood, i.e. the vascular compartment;
- The Cerebro-Spinal Fluid, i.e. the liquid compartment.
As the cranium is an inextensible cavity for the adult, Intra-Cranial Pressure (ICP) is then the combination of the pressures exerted by these three components.
If the volume of one of those three components increases, the volume of one of the two others at least must decrease, or the Intra-Cranial Pressure will increase as a mechanical consequence
Potential causes for an increase in volume are:
- for the brain: a tumor, an oedema, an abcess...
- for the blood: an hematoma or a brain haemorrage;
- for the CSF: hydrocephalus (= excess of liquid in the brain).
Cerebral compliance corresponds to the « elasticity » of the brain. The brain has the capacity to behave like a sponge which will absorb a variation in volume of one of the compartment without increasing pressure. In other words, compliance is the capacity of the three brain components (cerebral tissues, CSF, blood) to maintain a normal ICP. The better the compliance is, the lower the pressure variation for a given variation in volume and the lower the pathology progression. Compliance decreases with age.
Cerebral compliance allows a compensation phase during which brain adapts itself to variations of volume in one or the other of the compartments. For instance, blood volume can vary by distension or contraction of blood vessels, and volume of CSF by the regulation of its production and re-absorption cycle.
Cerebral compliance has its limits though. After some time, the de-compensation phase is reached, which provokes a rapid increase in ICP, with serious complications for the patient. ICP monitoring is used to avoid the switch from the compensation phase to the de-compensation phase.
Symptoms of a rise in ICP are headaches, vomiting, visual troubles, consciousness troubles.
The reference measurement of CSF pressure is measured at the level of the cerebral ventricles: this is the intra-ventricular pressure. It is often also called intra-cranial pressure (ICP).
The figures hereafter are indicative values which show that ICP is lower among children than among adults. Indeed, in the infant, since the cranium bones are not knit together, a deformation is still possible, that will compensate part of the variation in volume of one of the compartment avoiding an increase in pressure.
Normal ICP value (adult lying on his side) is comprised between 6 and 15 mmHg. The pressure is almost zero or even negative when standing up. Above 20 mmHg, the situation is pathological. Between 15 and 20 mmHg, symptoms will vary among patients. Some will not develop any symptom, even with ICP superior to 15 mmHg. Others will not withstand it and will develop some or all of symptoms described above (headaches, vomiting, visual or consciousness troubles…).
The scheme below give ICP values on two different scales: mmHg and mmH2O. Most of the time, ICP is given in mmHg.
The objective of ICP monitoring is to follow the trend of intracranial pressure, because the pressure values determine what interventions are necessary to avoid any additional cerebral lesion, which could be irreversible and lethal.
ICP monitoring gives access to the Cerebral Pressure Perfusion (CPP) value, which is very important, because CPP is the pressure allowing a correct brain perfusion, and then a correct oxygenation.
Should CPP be insufficient, ischemia would appear, leading to a decrease of blood-brain barrier efficiency.
In parallel, excessive ICP can lead to brain herniation.
That is the reason why ICP is monitored to maintain CPP at 60/70 mmHg to avoid to reach the de-compensation phase and to avoid ischemia.
A brief compensation is possible = the CSF is quickly reabsorbed or less produced; there is less blood in cerebral vessels. However, the next phase is a sudden and strong decompensation which will lead to a quick and important ICP increase.
The higher the ICP increases, the harder the blood is pumped until the head to oxygenate the brain. Indeed, when the ICP increases, the blood has difficulties to irrigate the brain.