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The
H-reflex and F-wave H-Reflex
The H-reflex is the electrical equivalent of the monosynaptic stretch
reflex and is normally obtained in only a few muscles. It is elicited by
selectively stimulating the Ia fibers of the posterior tibial or median
nerve. Such stimulation can be accomplished by using slow (less than 1
pulse/second), long-duration (0.5-1 ms) stimuli with gradually increasing
stimulation strength.
The stimulus travels along the Ia fibers, through the dorsal root ganglion,
and is transmitted across the central synapse to the anterior horn cell
which fires it down along the alpha motor axon to the muscle. The result
is a motor response, usually between 0.5 and 5 mv in amplitude, occurring
at low stimulation strength, either before any direct motor response (M)
is seen or with a small M preceding it. Understandably, the latency of
this reflex is much longer than that of the M response, and a sweep of
5-10 ms/division is necessary to see it.
The H-reflex can normally be seen in many muscles but is easily obtained
in the soleus muscle (with posterior tibial nerve stimulation at the popliteal
fossa), the flexor carpi radialis muscle (with median nerve
stimulation at the elbow), and the quadriceps (with femoral
nerve stimulation).
Typically, it is first seen at low stimulation strength without any
motor response preceding it. As the stimulation strength is increased,
the direct motor response appears. With further increases in stimulation
strengths, the M response becomes larger and the H-reflex decreases in
amplitude. When the motor response becomes maximal, the H-reflex disappears
and is replaced by a small late motor response, the F-wave.
H-reflex latency can be determined easily from charts, according to
height and sex or from published normal values. Whatever these values however,
the best normal value in localized processes is the patient's asymptomatic
limb. If no facilitation maneuvers are performed, the difference in latency
between both sides should not exceed l ms.
The H-reflex is useful in the diagnosis of S1 and C7 root lesions as
well as the study of proximal nerve segments in either peripheral or proximal
neuropathies.
Its absence or abnormal latency on one side strongly indicates disease
if a local process is suspected. Much controversy remains, however, on
whether its absence bilaterally in otherwise asymptomatic individuals is
of any clinical significance.
F-Wave
The F-wave is a long latency muscle action potential seen after supramaximal
stimulation to a nerve. Although elicitable in a variety of muscles, it
is best obtained in the small foot and hand muscles. It is generally accepted
that the F-wave is elicited when the stimulus travels antidromically along
the motor fibers and reaches the anterior horn cell at a critical time
to depolarize it. The response is then fired down along the axon and causes
a minimal contraction of the muscle. Unlike the H-reflex, the F-wave is
always preceded by a motor response and its amplitude is rather small,
usually in the range of 0.2-0.5 mv.
The F-wave is a variable response and is obtained infrequently after
nerve stimulation. Commonly, several supramaximal stimuli are needed before
an F-response is seen since only few stimuli reach the anterior horn cell
at the appropriate time to depolarize it. With supramaximal stimulation
however, depolarization of the entire nerve helps spread the stimulus to
the pool of anterior horn cells thus enhancing its chances to reach a greater
number of anterior horn cells at the critical time and produce an F-wave.
Because different anterior horn cells are activated at different times,
the shape and latency of F-waves are different from one another. Conventionally, ten to twenty F-waves are obtained and the shortest
latency F-wave among them is used.
The normal values can be determined from charts or published data and, in unilateral lesions, the best normal values remain those
of the patient's asymptomatic limb. The difference between both sides'
shortest latencies should not exceed l ms.
The data obtained from the F-wave have been used in many different ways
to determine proximal or distal pathology. Those include the F-wave chronodispersion
or difference in latency between the F-wave with the shortest and that
with the longest latency, and the F-wave ratio. We find the F-wave
ratio very useful in routine clinical work. It is obtained by dividing
the conduction time of the proximal nerve segment by that of the distal
nerve segment and is performed as follows:
Obtain the F-wave latency from proximal (F
prox) stimulation (knee or
elbow). Obtain the motor response likewise from proximal stimulation (M prox). Then determine the latency of the proximal nerve segment by this
equation:
Proximal latency = (Fprox - Mprox - 1 ms) / 2
where l ms is the estimated delay encountered by the stimulus at the
anterior horn cell.
The latency of the distal segment is none other than the motor response
latency obtained from proximal stimulation (M prox).
The F-ratio is then obtained by dividing the proximal latency by the
distal latency:
F-ratio = (Fprox - Mprox - 1 ms) / 2 x Mprox
Normal F-wave ratios are obtained from published data (Table
XVIII).
The F-wave ratio can be used as follows:
With normal routine conduction velocity:
 | a normal F-ratio indicates normal distal and proximal segments
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| a decreased F-ratio indicates a distal nerve lesion or entrapment (such
as carpal tunnel)
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| an increased F-ratio indicates proximal slowing
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With slowed routine conduction velocity:
| a normal F-ratio indicates equal proximal and distal slowing
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| a decreased F-ratio indicates a normal proximal segment
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| an increased F-ratio indicates a predominant involvement of the proximal
nerve segment
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 Table
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