In
routine clinical work, certain sets of nerve-conductions
must be studied in evaluating peripheral nervous system
function. These sets depend on the nature of the patient's
problem and their referral diagnosis.
General guidelines can usually
be prepared for performing a given work-up for a specific
group of diseases. These guidelines need to meet two
requirements:
- General enough to include most
of the abnormalities seen in this group
- Flexible enough to allow adjustments as needed.
Five general work-ups are described:
routine upper extremity, routine lower extremity, generalized
neuropathic process, myopathy, and neuromuscular junction.
Routine
Upper Extremity
This work-up is for the study
of root or plexus lesions and compression/entrapment or
traumatic neuropathies of the upper extremity.
The work-up
consists of:
- Median sensory and motor studies with F-waves
- Ulnar sensory and motor studies with F-waves
- Radial sensory study
Routine Lower Extremity
This work-up is for the study
of root or plexus lesion and compression/entrapment or
traumatic neuropathies of the lower extremity.
The work-up
consists of:
- A sural sensory study
- A superficial peroneal sensory study
- A peroneal motor study with F-waves
- A posterior tibial motor study with F-waves
- H- refelx studies in peripheral neuropathies and suspected
lumbosacral root lesions
Generalized
Neuropathic Process
This work-up is for the study
of generalized sensory/motor peripheral neuropathies and
disease processes involving the anterior horn cell.
The work-up
consists of:
- A routine upper extremity (see above)
- A routine lower extremity (see above)
- H-reflex studies
Myopathy
This work-up is for the study
of the muscle diseases and myotonias.
The work-up
consists of:
- A limited routine upper extremity work-up (median
sensory and motor studies)
- A limited routine lower extremity work-up (sural sensory
and peroneal motor studies)
Only limited studies are performed
because the proximal nature of the disease results in
a low yield on nerve conduction studies.
NeuromascularJunction
This work-up is divided into
presynaptic (for diseases such as Lambert-Eaton, botulism)
and postsynaptic (for diseases such as myasthenia gravis).
Presynaptic
neuromuscular junction work-up: Limited routine
upper and lower extremity work-ups are done. In Lambert-Eaton
syndrome, low motor amplitudes are present diffusely.
A muscle with a particularly low amplitude is chosen
and a postexercise (post-tetanization) study is performed:
ask the subject to exercise the affected muscle against
resistance for ten seconds. Then stimulate the
nerve once. Typically the pre-exercise response has
an extremely low amplitude, in the order of .5 to l
mv. Immediately after exercise the amplitude is significantly
increased, at least by 100 percent over the pre-exercise
level, and commonly by 200 to 300 percent. The facilitation
thereafter decreases slowly and the response regains
its pre-exercise level in about three minutes. Slow
(2-3 Hz) repetitive stimulation causes a small decrement
of the response.
Postsynaptic neuromuscular junction
work-up: Limited routine upper and lower extremity
work-ups are done and slow repetitive stimulation
(2 pulses per second x 4 or 9 depending on equipment
used) are done on a distal foot or hand muscle at first,
and, if negative, on a proximal upper extremity muscle.
Take extra care to ensure that the temperature of the
limb under study is no less than 35 degrees; cooler
temperatures may artificially repair a decrement on
repetitive stimulation.
Slow
2-3 Hz repetitive stimulation is performed before and
after exercise during a three-minute period. Adequate
immobilization of the limb under study is essential
as minimal displacement of the baseline may give a false
decrement.
To begin,
stimulate the nerve under study four times in a row
at a frequency of 2 pulses per second. The baseline
of all four potentials must be strictly superimposed.
A decrease in the amplitude or area of more than 10
percent between the first and fourth potential is interpreted
as a positive decrement (Figure 63).
Next, ask the subject to exert the muscle under study
against resistance for 30 seconds. Immediately thereafter,
stimulate the nerve repetitively four times. Perform
these repetitive stimulations at 30-second intervals
for three minutes. Typically a myasthenic response will
show a pre-exercise decrement between the first and
fourth response exceeding 10 percent. This decrement
is partially and at times totally corrected immediately
after exercise. Gradually, however, it reappears and
becomes maximal after two minutes, at which point it
exceeds the pre-exercise level.
When a distal muscle shows no decrement with the pre-
and postexercise repetitive stimulations, a proximal
muscle study is mandatory. The study can be performed
on the deltoid, biceps, or trapezius. Proximal nerve
stimulation requires the use of higher stimulation strengths
and longer stimulus duration. They cause a good deal
of discomfort and most produce excessive limb displacement.
Adequate baseline superimposition is difficult under
these circumstances. In about 20 or 30 percent of myasthenics
with general symptoms, both distal and proximal slow
repetitive stimulation studies may be normal.
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