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Knowing how different pathological
processes affect nerve conduction studies underlies
the understanding and interpretation of nerve conduction
findings. The following changes will be discussed:
Demyelination
As a rule, latencies and conduction
velocities are affected most. With few exceptions, the
sensory fibers are affected first. The sensory nerve action
potential's duration is increased, resulting in a low
amplitude and prolonged distal latency. At a later stage,
the motor fibers are affected essentially in the same
fashion with decreased conduction velocities, usually
50 percent below normal values.
In advanced demyelination,
sensory responses may be altogether absent.
In the entrapment
or pressure neuropathies, the demyelination is focal,
with the nerve remaining normal both above and below
the lesion. When the nerve is stimulated above the entrapment
or pressure area, the conduction velocity is slowed.
Stimulation below the lesion, however, results in a
normal velocity.
In distal entrapments,
where stimulation below the lesion is either impossible
or technically difficult, the findings are limited to
a prolonged distal latency and a reduction of the sensory
amplitude and, in time, of the motor response
In polyneuropathies, the above changes are present diffusely,
although they may be more severe at or about potential
pressure and entrapment points.
Conduction
Block
The cause of conduction blocks
is unclear. Such blocks can arise from a severe focal
demyelinating lesion, making impulse propagation through
the area of demyelination impossible; or from physiological
interruption of conduction without detectable abnormalities
histologically. Below the lesion, the nerve conducts the
impulse normally.
A
partial block is one in which only a few fibers are
affected. The nerve can still be stimulated above
the lesion, but, since only a few fibers conduct, a
low amplitude response is obtained.
When the block is
complete, no response can be obtained on stimulation
above the lesion. When stimulation below the lesion
is possible, a normal response is seen.
Sometimes partial
conduction blocks are seen along with focal demyelinating
lesions. Stimulation above the lesion yields a low amplitude
response with a slowed conduction velocity along the
involved segment. Stimulation below the lesion, when
feasible, results in a normal amplitude and conduction
velocity.
Axonal
Loss
In contrast to the effects of
myelin-sheath lesions, loss of axons results primarily
in decreased amplitudes. The sensory fibers are affected
first with a resulting decreased in amplitudes but relatively
preserved distal latencies. As the lesion becomes more
severe, motor amplitudes are decreased and sensory potentials
may even become unobtaibnable.
In
advanced disease, the motor amplitudes may be so depressed
that motor distal latencies are prolonged and conduction
velocities slowed, though usually the slowing
does not fall below 30 percent of the expected normal
value. These effects result from a dropout of the fastest
conducting fibers.
In contrast to amplitude
effects in lesions with conduction block, low amplitude
from axonal loss cannot be corrected by stimulation
below the lesions. In conduction blocks, the nerve segment
below the lesion is normal, whereas in axonal loss it
undergoes Wallerian degeneration and does not conduct
normally distal to the lesion. An evoked response can
still be obtained however, from stimulation below the
level of the injury, up to ninety-six hours after total
nerve transection.
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