Dr. Milhorat said it was time to test out the
ideas about surgical treatment which really hadn't changed in the
last 40 years, despite significant advances in diagnosis such as
MRI. The hypothesis that formed the basis of the traditional type
operation has been strongly influenced by the theories of Dr.
Gardner and Dr. Bernard Williams. The idea was that the Chiari I
Malformation, with the plug of the cerebellar tonsils through the
foramen magnum, causes fluid that is in the last ventricle of the
brain to be diverted directly down the central canal of the spinal
cord. Instead of this fluid coming out and exiting around the
cerebellum, it was forced to dilate and distend the central canal.
Surgical treatments were aimed at unplugging the foramen magnum by
a variety of means.
The standardized operation, which really isn't
standardized, was developed based on these theories and involved
suboccipital craniectomy (with varying sizes of openings). Most
surgeons remove the first or second laminae (C-1 or C-2
laminectomy), some open the dura and do lysis of adhesions, and
some plug the obex. Some amputate the tonsils, some put in fourth
ventricle stent, and all patients undergoing the standard
operation will have a patch of something- either tissue from a
patient or synthetic material- to produce an enlargement of the
CSF spaces behind the cerebellar tonsils. The procedure has been
done for 40 years and many do well with it, but many do not.
Dr. Milhorat was curious why an operation that had
such fine theoretical basis wouldn't work most of the time. It
became evident with the advent of MRI that the idea of fluid
coming down from the fourth ventricle of the spinal cord might not
be how syrinxes were formed. Syrinxes were seen via MRI at a
considerable distance from the fourth ventricle where there were
normal intervening segments of the spinal cord. This was practical
evidence that maybe the way these cavities filled was not by the
Gardner technique.
An autopsy study was done by Milhorat and
associates and published in Journal of Neurosurgery in 1995. They
collected 175 spinal cords of SM patients; 105 were suitable for
complete study. They found most syrinxes associated with Chiari
Malformation were not in continuity with the fourth ventricle.
They were separated from it by stenosis of the central canal, and
there was an isolated cavity in the spinal cord that is closed at
the top and closed at the bottom. The proposition of fluid coming
down and filling this cavity was null and void. Another thing
learned was that you could have a syrinx that distends the central
canal of the spinal cord in an impressive way and have little or
nothing to show for it in the way of symptoms as long as it
remains a central cavity. If the cavity should expand
paracentrally, should it rupture or burst or move into one of the
quadrants of the spinal cord, as many do, dissecting up and down
into the parechemal tissues of the cord, it produces a very
specific neurological deficit as related to the area of injury. If
it expands into the left hemicord, there will be symptoms on the
left side of the body. If it expands into the left side
anteriorally, where the motor fibers are found, it would produce
weakness and atrophy of muscles. Many syrinxes can extend from the
top of the spinal cord to the base and be very large yet be
associated with surprisingly few symptoms.
Microscopic studies were done which showed that
dyes placed into the spinal cord tissues or into the spinal fluid
around it quickly move to this maze of interconnected cavities and
enter the central canal of the spinal cord. This made it clear
there was an anatomical, microscopic pathway between the
subarachnoid space that houses the spinal fluid around the spinal
cord through the tissues of the spinal cord directly to the
central canal. The central canal communicates with the outside,
picks up ingredients, blood products and things we inject (like
dyes) and they move directly up the central canal and drain out
the top. A group in Australia extended this work. Using these
anatomical pathways they demonstrated that pulsations of the
arteries help to drive fluid from the subarachnoid space outside
of the spinal cord, through these interweaving spaces in the
spinal cord into the central canal. So the route of fluid appears
not to be from the fourth ventricle down, but there is now
scientific proof that there is an anatomical pathway between the
outside and the inside of the spinal cord. The tonsils pound down
with each heartbeat which drives the pressure up, and there is
evidence that fluid compressed out of the posterior compartment
into the spinal canal is also a contributing factor. So increased
pressure and increased fluid in the subarachnoid space in a
certain number of patients with Chiari, will allow the central
canal to dilate.
Based on the results of this study, a prospective
study of patients was begun to see if a simple operation could
improve outcome. Dr. Milhorat said he does not wish to discuss
results until a minimum of 3-year follow up is done.
Dr. Milhorat has strict requirements for surgery.
He does not recommend surgery for patients unless they meet one of
the following criteria. First, evidence of progressive clinical
deterioration. Two, the current symptoms, while not progressive,
have reached a point of being unbearable or disabling. It is the
patient who is in the best position to make that determination,
put into the context of their own lives. Dr. Milhorat also uses a
scale of 0 to 100. 100 means you're asymptomatic, 0 would mean
you're dead. He does not operate on patients unless they are 70 or
below. He does not recommend surgery for nuisance symptoms. These
are complaints you wish you didn't have but you do; nevertheless
you get through the day all right and they do not really affect
your way of life. At the present time, he does not recommend
surgery to prevent problems from occurring in the future because
the natural history of Chiari I and SM is not understood. With the
advent of MRI we are able to see vast numbers of patients with a
wide spectrum of complaints ranging from nothing at all, to a
little something, to moderate, to severe. There are no longer any
rules about progression. If a patient is getting worse after
you've followed it for a while, it's likely it will continue. But
if a patient is stable and remains so month after month and year
after year, there is no way to determine whether it's going to get
worse, and probably as time passes it is increasingly likely it
will remain the same forever.
With one exception. Trauma is a big problem.
Patients with Chiari with or without a syrinx are much more likely
to have symptoms begin to progress after an accident. Patients
with a diagnosis of Chiari or SM that is not producing much in the
way of symptoms should be on notice to keep themselves out of
harm's way. Avoid activities that have a high risk. Dr. Milhorat
believes that the Duke Study may identify many people who are
asymptomatic. He believes these people can become symptomatic if
they have an accident, fall, trauma or so forth.
Dr. Milhorat's surgical procedure does not include
lamenictomy, opening of the dura or duraplasty and no chopping of
the tonsils. What he does do is a take off a large panel of the
skull, trying to make the posterior fossa optimally decompressed.
He is not sure how important the duraplasty is but he is trying to
keep it simple and just provide maximum decompression. Many
symptoms related to CSF phenomenon are improved at the time of
surgery such as pounding headache, pressure behind the eyes, sense
of dizziness and tinnitus. Dr. Milhorat says some patients opt for
an optional procedure in which a titanium mesh plate is fashioned
and a putty like acrylic is added and is screwed in with titanium
screws which are MR compatible. He is not convinced this is
necessary but some patients are more comfortable with it. It will
take more time to see whether the simple act of decompressing the
posterior fossa will cause spontaneous shrinkage of syrinxes as
well as doing a duraplasty. If it doesn't then it may be desirable
at some point to consider adding duraplasty for patients with
certain types of syrinxes.
Some problems can occur with traditional
decompressions when the decompression is too small and another
problem can occur when the duraplasty is too big. Another problem
is when the duraplasty leaks or is purposely left open and the
patient has a very small craniectomy and no spinal fluid behind
the cerebellum.
Patients experience various kinds of pain,
headaches due to Chiari Malformation, neurogenic pain, dull pain,
shooting pain, pain in the face. Then there is the terrible
dysethetic pain characterized by a burning quality often
associated with a hypersensitivity of the skin where even the
touch of clothing can cause waves of intractable pain. Sometimes
it is associated with pins and needles, sweating problems, glossy
skin, coldness of the extremities and so on. Dysethetic pain seems
to be related to special centers in the spinal cord that have to
do with the modulation of pain. This type of pain responds
unpredictable to surgical treatment. Patients with SM seem to have
an accumulation of Substance P in the spinal cord (sometimes
referred to as the bad peptide). There is a drug company working
on an anti-substance P drug. Some patients do improve with time,
and in severe cases, sympathetic nerve fibers can be cut.
Dr. Milhorat does not believe that you will become
addicted to pain drugs if you have severe, legitimate pain. Go up
the ladder of treatment beginning with medicines, continuing to
narcotics, dorsal column stimulation, tens, implantation of pumps
before getting to the last step of "cutting things". Dr.
Milhorat said, "There is treatment for pain; it's just a
question of how severe the treatment will be. You can continue to
get narcotics to the point of sedation, you can cut things to the
point of numbness, and one should always try to get off the step
ladder of treatment at the lowest rung possible."
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