Insertion of an external ventricular drain (EVD) is a common neurosurgical procedure, for
example in patients with intracranial bleeding, head trauma of tumours. The aim of an
external ventricular drain is either to drain cerebrospinal fluid (CSF) or to measure
intracranial pressure. In patients with external ventricular drains, it is a common
practice in many centres, including UZ Leuven, to routinely sample CSF two or three times
a week. The following tests are routinely performed: protein level, glucose level,
lactate level, white cell count, red cell count, and cultures. Gram stain is performed in
case white blood cell count exceeds 5 cells/mm³. This regular sampling (1) permits early
detection of ventriculitis, which may be rather asymptomatic, especially in sedated
patients,and (2) may guide neurosurgeons to the ideal timeframe to either taper the
external ventricular drainage, or replace it by a ventriculoperitoneal shunt.
A typical external ventricular drainage system consists of a ventricular catheter, which
is nowadays usually impregnated with antibiotics (e.g. Codman Bactiseal - coated with
clindamycin and rifampicin), an extension tube, a drip chamber, and a collection bag.
Multiple three-way taps and access ports along its course allow for CSF sampling (or
injection of therapeutics, such as tissue plasminogen activator, or saline to flush the
catheter in case of blockage).
From a practical perspective, CSF sampling from the external ventricular drain is
possible at all of these three-way taps and access ports. Broadly, one may sample CSF as
proximally (close to the ventricle) as possible (e.g. by using the access port), or
distally (at the level of the drip chamber, e.g. by using the three-way tap between the
drip chamber and the collection bag). There is no consistency in for CSF collection site
amongst different institutions. Traditionally, in UZ Leuven, we have been sampling CSF
proximally until now.
The main theoretical advantage of proximal sampling is that actively aspirated "fresh"
CSF close to the ventricle likely closely resembles the true characteristics of our study
object, i.e. intraventricular CSF. Nevertheless, this approach is not without risk.
First, it requires to directly access the sterile inside of the extension tube. This
inherently comes with a risk of iatrogenic infection. Secondly, by aspirating the
external ventricular catheter, the neurosurgeon may induce an iatrogenic haemorrhage in
the brain parenchyma or the ventricle, and/or direct trauma to the brain parenchyma, as
in suboptimally positioned catheters or small ventricles, white matter or choroid plexus
may be aspirated along with CSF. Because of the risk of iatrogenic infections,
haemorrhage or trauma, it is our institutional policy that CSF sampling from external
ventricular drains is performed by neurosurgeons only. This also poses practical problems
and may delay sampling, especially in patients who are not admitted to neurosurgical
wards (e.g. paediatric cases).
From a theoretical perspective, distal CSF sampling, at the level of the drip chamber,
has multiple advantages. First, the risk of iatrogenic infections induced by CSF sampling
is likely to be lower, as there is no direct contact between the CSF in the drip chamber
and the intraventricular CSF. Secondly, there is no risk of inducing haemorrhages or
brain trauma, as no "active" aspiration is required for CSF sampling. Hence, distal CSF
sampling can perhaps also be carried out by physicians from other fields than
neurosurgery, or by trained nurses. However, theoretically, it is possible that the
characteristics of CSF sampled distally do not sufficiently resemble those of
intraventricular CSF.