Late response
is,
“a general term used to describe an evoked potential having
a longer latency than the M
wave” (AAEE, See August 2004 Special Feature for definition
of M wave). Three commonly encountered late responses are the
F wave, H wave (or reflex) and A wave. Many recording systems
have programs designed for late response testing which allow
the operator to split the sensitivity on each tracing. The M
wave can be displayed at the same setting used for the MNCV,
whereas the latter part of the signal is displayed in the microvolt
range. The sweep duration
must be long enough to record these potentials (50 msec
is usually sufficient). Numerous
(a minimum of 10) individual responses are displayed. F waves
are the result of antidromic propagation
of action potentials up the motor nerve to the cell body, which
then initiates an additional volley in the orthodromic
direction. Compared to the M wave, it has a lower amplitude
and variable configuration (often polyphasic).
The latencies are also variable.
F wave latencies are inversely related to the M wave
latencies, stimulation at a distal site will have relatively
short M wave latencies but long F wave latencies and proximal
stimulation will result in relatively long M wave and short
F wave latencies (Figs.
1A and 1B). H waves represent the reflex arc, whereby the sensory component of
the nerve is stimulated and synapses on the motor neuron in
the spinal cord, triggering an additional motor volley.
This potential is best recorded with fairly low stimulus
intensity and can frequently be found below that which elicits
an M wave (Figs. 2A and 2B). They are
often blocked at higher stimulus intensities, a feature that
helps to distinguish them from F waves.
A waves, like F waves, are the result
of antidromic motor potentials. However,
in this case the volley travels up to collateral branches of
the nerve and back down these fibers so their latencies tend
to be shorter. When present, they also tend to have consistent
latencies (Fig. 3).
A. |
B. |
Fig 1A. Normal canine
F waves recorded following peroneal n. stimulation
at the hock. Recordings
are the result of 32 superimposed individual tracings.
Fig 1B. Normal canine
F waves recorded following peroneal n. stimulation at the hip.
A. |
B. |
Fig 2A. Normal canine
H waves recorded following peroneal n. stimulation
at the hip using low stimulus intensity (0.6 mA). No M waves are present.
Fig 2B. The
same as in Fig. 2A after a slight increase in stimulus intensity
(to 1.0 mA). M waves
are now present.
Fig 3. Possible A waves
(early peaks) and F waves following peroneal n. stimulation
at the hock. These tracings are from the same dog whose SEPs are shown in Fig. 4A in the second article on this topic
(August 2004).
One advantage of late waves, as compared
to MNCVs, is they can provide information
on the status of the most proximal segment of a nerve. A complete absence, or the loss of individual
potentials, can be seen in some cases.
Minimum F wave latency determination, ratio calculations
and chronodispersion, the difference between minimum and maximum
F wave latencies, can be analyzed and compared with reference
values. Minimum F wave latencies are considered the most sensitive
and reproducible measure of conduction slowing in people with
diabetes mellitus (Kimura). Variability can be quite dramatic in cases with
advanced disease (Fig.
4). If desired, collision techniques can be used
to isolate individual waves (i.e. by eliminating M waves after
proximal stimulation). In human medicine, A
waves are thought to occur primarily in patients with peripheral
neuropathies (Kimura).
Fig 4. F waves and probably several A waves in a cat with diabetic
neuropathy following peroneal
n. stimulation at
the hock.
Repetitive nerve stimulation (RNS) is “the technique of repeated supramaximal
stimulation of a nerve while recording M waves from muscles
innervated by the nerve” (AAEE). The technique is similar
to that previously described for MNCV but trains of stimuli
at various frequencies are employed. Analysis of the individual M waves both by amplitude
and area under the curve can be performed by most modern systems.
Percentage decrement (or increment)
of subsequent potentials, as compared to the initial one of
the series, is calculated (Fig.
5). As with NCVs, maintaining the patient’s body temperature is critical.
A decremental response can
be masked if the animal’s temperature is low. When performing this test, it is important to
allow at least one minute of recovery time between the trains
of stimuli. Frequencies tested generally range from 0.5
Hz to 50 Hz. The normal
physiologic response must be considered when interpreting
the results, as a small amount of decrement (<10%) can
be seen in normal animals, particularly at rates of 5 per
second or greater. At
very high rep rates (20 Hz and above) a normal response known
as pseudofacilitation can be observed.
This is identified by an increase in amplitude with
no change in the area under the curve (the M wave becomes
taller and narrower, Fig.
6).
Fig 5. Normal feline repetitive nerve stimulation study following 1 Hz stimulation
of the peroneal n. at the hock.
Fig 6. Pseudofacilitation in a dog following 50 Hz peroneal n. stimulation
at the hock. Negative
values indicate an increment in the response.
RNS testing is useful in identifying disorders
involving the neuromuscular junction (junctionopathies)
such as myasthenia gravis and botulism.
Results may vary between different sites, so testing
multiple nerve/muscle combinations may be indicated. In patients
with myasthenia gravis, either acquired or congenital, a decremental
response of over 10% is typical (Fig.
7). This can be seen even with low rates of stimulation.
Normal responses can be observed in cases with focal disease
(i.e. myasthenia gravis affecting the esophagus only). Botulism cases often have a decremental response at low repetition rates but an incremental
response at higher ones. In
true facilitation
there is an increase in area, as well as, amplitude (Fig.
8).
Fig 7. Decremental response
in a cat following 3 Hz peroneal n. stimulation
at the hock. This
patient was suspected of having either congenital or seronegative
myasthenia gravis.
Fig 8. Facilitation in a dog following stimulation of the peroneal n. (30 Hz) at the hock. Note: both area and amplitude values are increased
(as opposed to those in Fig. 6).
No etiology could be determined in this case. The dog presented for exercise induced weakness
which occurred in warm weather.
In
conclusion, electrodiagnostic testing
can provide the clinician with valuable information on the
functional status of a patient’s neuromuscular system.
Together with histologic
examination of muscle and nerve biopsy specimens (collected
from the opposite side as that used to perform these studies),
a more complete diagnostic picture can emerge.
References
Cuddon PA. Electrodiagnosis in Veterinary Neurology: Electromyography,
Nerve Conduction Studies, and Evoked Responses. Loveland,
CO, 2000.
Dumitru D. Electrodiagnostic Medicine 2nd editon,
Hanley and Belfus, Inc. Philadelphia, PA 2002.
Kimura J. Electrodiagnosis in Diseases of muscle and Nerve:
Principles and practices 3rd edition. Oxford University
Press, New York, New York, 2001.
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