A spinal tap (also known as a lumbar puncture, or LP) is an outpatient
procedure in which a needle is inserted into the lumbar region of the spine and used to remove cerebral spinal fluid (CSF). Because the
needle is inserted in the lumbar region of the spine, well below the spinal
cord, there is very little risk associated with the procedure. (see Anatomy section)
The consensus among CMI experts is that spinal taps may be risky for patients
who have CMI. The reason being that, as fluid is drawn, a downward
suction occurs which may cause further herniation of the cerebellar
tonsils. In addition, it is not uncommon for people with CMI to have
elevated intracranial pressure (ICP) which places increased pressure on the hole created
by the needle, and thereby increases the risk of leaking after the procedure.
The needle used has a specially designed tip that can be used
to measure pressure of the CSF (as an analogy, think of how a tire gauge measures air pressure).
A local anesthetic is first administered to the area and, shortly after,
the spinal tap is performed. The patient remains awake and in the fetal
position during the procedure. Most patients report no pain during the
procedure but do feel pressure from the needle.
Before any fluid is removed, the needle is used to measure the "opening
pressure". Although many sources vary on what "normal" CSF pressure
should be, most respectable sources seem to agree that normal CSF
pressure ranges between 12mmHg and 15mmHg (or, 120mmH2O and 150mmH2O).
Once the opening pressure is noted, some fluid is drawn and pressure is
noted again. This process is repeated until the desired amount of fluid
has been removed. At some point during the process, the patient may
be asked to "valsalve", or push down as if giving birth, so that this pressure
can also be noted. Finally, just before the needle is removed, the "closing
pressure" is noted.
Most CMI patients do not feel well at all after having a spinal tap.
However, there are some who actually report great relief. Regardless of
whether the patient feels better or worse as a result, it usually
takes 3 to 9 days for the patient to feel "normal" again. If the
spinal tap brings relief, it generally does not last. Much depends
on the opening pressure, closing pressure, and how extreme the difference
is between the two. How a person feels afterwards is largely unpredictable.
As an example, a patient with an opening pressure of
15 and a closing pressure of 8 may have no problems, while a patient
with an opening pressure of 23 and a closing pressure of 8 may feel
awful for several days (because of the radical change in pressure).
Likewise, a patient with an opening pressure of 15 and a closing
pressure of 8 may also feel awful. It's a good idea to aim for a closing
pressure that is considered in the range of normal (between 12mmHg and 15mmHg).
