Peripheral
nerve surgery:
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The
human nervous system
consists of four parts: the brain,
spinal cord, autonomic nervous
system, and peripheral nerves.
Peripheral nerves are cord-like
structures containing bundles
of nerve fibers that carry information
from regions of the body to the
spinal cord (and vice versa).
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What
are peripheral nerves?
Constructed in
a fashion similar to telephone cables,
peripheral nerves have a tough outer layer
of connective tissue that surrounds discrete
groups of miniscule fibers, called "axons,"
each originating from its own nerve cell.
Peripheral
Nerve Function
The primary function
of a peripheral nerve is to transmit signals
from the spinal cord to the rest of the
body, or to transmit sensory information
from the rest of the body to the spinal
cord. For example, to flex a muscle, the
brain sends a signal to the spinal cord
that is received by a motor neuron in
the spinal cord. This neuron sends the
signal down an axon, which may be up to
3 feet long. This axon terminates in a
muscle, where the impulse to flex is received.
Once the muscle receives this signal,
it flexes. The whole process requires
only a fraction of a second to occur.
Some nerves in the body, such as the sciatic
nerve, are quite large and can be over
1 centimeter in diameter. Other peripheral
nerves are so small that they cannot be
readily seen without magnification.
Peripheral
Nerve Injury And Healing
The central nervous
system includes the brain and spinal cord.
Injuries in these locations have only
a very limited capacity to heal, because
nerve regeneration tends not to occur.
In contrast, peripheral nerves have a
striking capacity for regeneration. Even
completely severed peripheral nerves,
if repaired in a timely fashion, can regrow,
allowing the patients to enjoy complete,
or nearly complete recovery in many cases.
When a peripheral
nerve is cut, the axon segments distal
to the injury (furthest away from the
spinal cord) die off in a process called
"Wallerian degeneration." When
the nerve is repaired, the axons in the
proximal segment (closest to the spinal
cord) regrow into the distal, denervated
segment. This growth occurs at a rate
of about 1 mm per day. This translates
roughly to 1 inch per month, or 1 foot
per year. Once the axons regrow back into
the denervated muscles, the muscles will
begin to function again. During the time
it takes for the axons to regrow into
the muscles, a process that can take many
months, or even years, the muscles will
be paralyzed and will atrophy.
This is an important
point when dealing with peripheral nerve
injuries. The healing process almost invariably
requires an extensive amount of time to
occur. It is important for patients not
to lose hope during this time. It is vital
that they participate in their exercises,
keeping the affected muscles and joints
flexible and ready to be used once again
when the axons regrow into them. It is
not unusual for patients to undergo a
lengthy, complex, peripheral nerve reconstruction
procedure, only to see no evidence of
recovery for a year or more. This can
be immensely frustrating for the patient.
Unfortunately, there is nothing in medical
science currently that can make these
axons grow any faster. Perhaps it is best
to think of this delay all as part of
the healing process, paving the road to
getting better!
How
are peripheral nerve problems diagnosed?
Electrodiagnostic
Studies
The most important
diagnostic study in the evaluation of
peripheral nerve problems is electromyography
(EMG) with nerve conduction studies (NCS).
This study provides accurate, quantitative
information on the function of nerves
and muscles and helps to more precisely
localize the region of injury, and to
pick up more subtle signs of injury and/or
recovery that are otherwise undetectable
to the examiner.. Sometimes it is the
only way to reliably localize a lesion,
quite crucial if surgery is being contemplated.
Unfortunately, this study can make some
patients uncomfortable, as it involves
placement of tiny needles into the muscles.
Most patients, however, tolerate the procedure
just fine.
Sometimes intraoperative
nerve action potential recordings (NAPs),
which measure the function of nerves during
surgery, are necessary to establish whether
a nerve must undergo repair or if it can
heal on its own.
Imaging
Studies
Imaging studies
can be quite helpful in patients with
peripheral nerve problems. For example,
patients who have peripheral nerve tumors
almost invariably require an MRI. This
study uses harmless magnets (NOT ionizing
radiation) to create highly detailed pictures
of the relevant anatomy, including the
tumor. Such detailed pictures facilitate
the planning of surgery to treat peripheral
nerve tumors.
CT myelography
is used to evaluate brachial plexus traumatic
injuries. The brachial plexus represents
the network of neural structures that
connects the nerve roots exiting the spinal
cord in the neck to the individual nerves
in the shoulder and arm. Patients who
have suffered severe brachial plexus trauma
may actually have one or more avulsion
injuries, in which the nerve roots are
actually pulled out from the spinal cord
in the neck. A CT myelogram is really
the only way to reliably document this
type of injury. To perform a CT myelogram,
contrast dye is injected along into the
spinal canal. Then spinal X-rays and a
spine CT are performed.
Preparing
for Your Surgery
If
you and your physician decide together
that surgery is the best option for you,
then here are some useful things to keep
in mind:
