The cerebrospinal or CSF pressure can be measured spinally (after lumbar puncture of the CSF space) or intracranially (e. g., using a ventricular catheter after neurosurgical intervention).
It would make sense to measure the opening pressure of the CSF with every lumbar puncture, but for pragmatic reasons this is usually not routinely done. The following “special” indications remain:
• If elevated intracranial pressure is suspected but imaging is normal (e. g., pseudotumor cerebri) → confirmation of diagnosis
• If imaging reveals hydrocephalus but no evidence of aqueductal stenosis → confirmation of normal-pressure hydrocephalus
Continuous measurement of intracranial pressure is indicated for the following purposes:
• Prophylactic diagnosis of critical situations.
• Decision–making for surgery (drainage, shunt, removal of hematoma, decompression).
• Evaluation of treatment. Hence, it is often carried out in patients with subarachnoid hemorrhage, craniocerebral trauma, space-occupying cerebral infarction, intracerebral hemorrhage, and severe meningitis.
Technique for Lumbar Measurement of Cerebrospinal Pressure
After puncturing the CSF space, remove the stylet from the puncture needle and immediately connect a sterile rising pipe (or air-filled infusion tube) with its three-way stopcock closed. Fix the infusion tube to a central venous pressure monitor (or a vertical ruler). Zero the monitor on the middle of the vertebra at the level of the puncture site. Now open the passage and wait until the CSF column has reached its maximum.
By applying pressure on both jugular veins for about 10 seconds (Queckenstedt test), the CSF pressure is elevated by 3–10 mmHg. When the compression is released, the pressure rapidly falls back to the original level. The clinical purpose of this maneuver is to check that the spinal CSF has free passage: if the passage is blocked, the rise in pressure will fail to occur. In such cases, the baseline value is often also very low.
After taking the measurement, close the system by means of the three-way stopcock, remove the needle, and transfer the CSF to a tube for analysis.
Lumbar Measurement of Cerebrospinal Pressure
The pressure values obtained by lumbar measurement in a patient lying exactly horizontal and with free passage of spinal CSF correspond to the intracranial pressure (ICP).
Measured values vary slightly depending on heartbeat and respiration. Normal values in adults are between 60 and 200 mmH2O, corresponding to 5–15 mmHg (1 mmHg = 13.62 mmH2O). Values below 5 mmHg suggest a low cerebrospinal pressure (postlumbar puncture syndrome). Values above 15 mmHg indicate an elevated intracranial pressure. These values refer to measurements taken when the patient is positioned absolutely horizontally. Normal values for measurements taken in a seated patient cannot be established, since they differ individually because the water column between the cisterna magna and the puncture site varies depending on body size (Oschmann et al., 2003).
Pitfalls in Lumbar Measurement of Cerebrospinal Pressure
Falsely high values are measured in very tense patients. For this reason, if measured values are greater than 15 mmHg, the first thing is to check whether the patient is relaxed, and perhaps wait for a short time to see whether the CSF pressure falls when the patient relaxes. The pressure may also be slightly elevated in very obese patients (up to 20 mmHg). Shouting and coughing raise the CSF pressure considerably (≥ 20 mmHg and 30–110 mmHg, respectively). In patients ventilated with positive end-expiratory pressure (PEEP), the pressure is expected to increase by 1–2 mmHg per 50 mmH2O of PEEP.
Falsely low values are obtained if the needle tip is partially displaced. If pathologically low values are obtained, therefore, it is therefore advisable carefully turn the needle and check its position by applying abdominal pressure. Low CSF pressure can also occur with forced hyperventilation via reduced partial CO2 pressure.
Measurement of CSF Pressure via a Ventricular Catheter
Determination of intracranial pressure via a ventricular catheter—or via probes directly placed into cerebral tissue, or epidural probes—requires neurosurgical intervention and also calls for considerable experience in assessing the values obtained. The measurement should always be related to a defined reference system; in most cases, the reference system is the foramen of Monro.
Pitfalls in Measuring Cerebrospinal Pressure Intracranially
Since the hydrostatic pressure in a resting fluid depends on the height of the fluid column, the intracranial pressure within the central nervous system varies significantly. The anterior–posterior difference in the brain is 13 mmHg in supine and 10 mmHg in lateral decubitus. Accurate alignment of the measuring point and allowance for the position of the patient are therefore essential. In addition, there are numerous other sources of error that can lead to measurement inaccuracies: errors of the pressure transducer, problems with connecting the pressure transducer, inaccurate measuring because tubes are too long or too short, additive loss of CSF, or motor restlessness of the patient (Aschoff and Steiner, 1999).
Complications with Ventricular Catheters
Common complications with ventricular catheters include infections (6–7%), surgically induced hemorrhage (1%), and malpositioning of the catheter in the basal ganglia or mesencephalon. Particularly when the ventricular catheter remains in place for several days, it makes sense to examine the CSF regularly in order to identify infections early and exchange the catheter system.
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