Late responses: F waves and H reflex

Generally speaking, the late responses of H-reflexes and F-wave latencies provide information about the proximal segments of nerves, while typical NCS studies distal nerves.

F waves

What is it and why is it relevant?

F-waves reflect conduction along the entire length of the peripheral motor nerve and does not involve any synapses. 

The F wave makes use of the antidromic direction of conduction (conduction going proximally towards the spinal cord) and can give us information about the entirety of the motor nerve from the point of stimulation all the way up to the anterior horn cell and back to the recording site. Once the action potential gets to the anterior horn cells it causes a proportion of them to fire again i.e. to "backfire", and the electrical impulse actually travels back distally and is recorded as the F wave. It is important to note that the F wave involves only motor nerves (not sensory).

We can use F waves to test the otherwise inaccessible proximal motor nerves and their roots. For example, F wave studies help in the evaluation of demyelinating disease like AIDP or CIDP, radiculopathies, and motor neuron disease.  If F waves latency is prolonged more than 130% of upper normal limit, that is highly suggestive of demyelination, since loss of fast fibers affects F wave more than axonal loss or anterior horn cell damage. If a patient's neuropathy is so severe that you are unable to elicit any CMAP on nerve conduction, then it is essentially useless to test F waves since it is obvious that those motor nerves are not conducting. 

The main caveat with F wave studies is that sensitivity is limited since the current travels via a number of motor roots, not just one. So, even if there are severe lesions of some nerve roots but adjacent ones are unaffected, the F wave may remain relatively unaffected.

Figure 7.1: Circuit travelled by F wave. Stimulation leads to action potential travelling proximally towards the anterior horn cells which then "rebounds" back distally towards the muscle, i.e. the F wave recording site.

How do we measure F wave latency and what does it look like?

• Set up is the same as for normal motor NCS at each site, EXCEPT the simulator is placed in the opposite orientation to typical NCS studies (cathode which is black facing towards spinal cord i.e. proximally) and a series of supramaximal stimuli is applied. Common sites of stimulation for F waves include median, ulnar, peroneal, and tibial nerves. 

• We stimulate a motor nerve of our choice, and first see the routine CMAP that we record in motor NCS. In F wave studies, the CMAP is known as the "M-wave".

• This is followed by a small amplitude response which occurs considerably later. This is the F-wave. Now you see why it is also called a "late motor response".

• It is common practice to measure a series of F wave responses (at least ten) because normal F-waves show slight variation in amplitude and latency on repeated stimulation.  Out of the series of responses, we determine minimum latency (the most commonly used parameter in clinical practice), persistence, and other parameters.

Figure 7.2: Example of F wave study results in a normal patient.

Note: this F wave tracing also has an "A wave" just after the vertical yellow dotted line. The most important thing is not to confuse A waves with F waves. A waves are relatively easily identified since they always appear before F waves and are identical in latency and waveform on each successive stimulation. The interpretation of A waves is beyond the scope of this website, but they are briefly mentioned here to avoid confusion with F waves.

H reflex

What is it and why is it relevant?

The H reflex is the electrophysiologic correlate of a spinal reflex, specifically the Achilles reflex (S1, ankle jerk). The H reflex study is important and helpful in evaluating L5-S1 radiculopathies, peripheral neuropathies like diabetes, early demyelinating neuropathy (AIDP), and conditions like Miller Fisher syndrome.

Stimulation at the tibial nerve produces a sensory axon potential that travels orthodromically (in the normal direction of sensory nerves, i.e. proximally and towards the spinal cord). This acts as part of a reflex arc and activates anterior horn cells in the cord, producing a potential in the motor nerve that travels orthodromically (in the normal direction of motors nerves, i.e. distally and away from the spinal cord) back to the muscle. The potential reaching the muscle is recorded as the H reflex (wave). See Figure 7.3 below.

Figure 7.3: Circuit travelled by H wave. Stimulation of the tibial nerve results in an action potential along the reflex arc, i.e. the sensory axon, through the spinal cord, and out to the motor nerve, where it is recorded at the soleus as the H wave. Stimulation of the tibial nerve also results in an action potential that travels along the motor nerve back to the recording electrode, reaching it before the H wave. This is basically the CMAP and again termed the M wave.

Figure 7.4: Example of H wave study results in a normal patient.

How do we measure the H reflex and what does it look like?

• The direction of the stimulating electrode is reversed so that the cathode (black) is oriented proximally and the anode (red) is oriented distally. This placement of stimulating electrode is the opposite of what we do in all other NCS studies.

• A long duration stimulus is applied typically to the tibial nerve. 

• The stimulus intensity is gradually increased until the H wave is recorded. As the intensity of the stimulus is increased further, the amplitude of the H response will increase until eventually an M wave appears before H wave. 

• These M waves represent MUAPs. Once generated, MUAPs travel antidromically along motor axons toward the spinal cord. If the stimulus intensity continues to be increased, these MUAPs (M waves) will begin to cancel out the H reflex's MUAPs that are travelling orthodromically. This can cause the H reflex to decrease in amplitude and ultimately disappear.

Figure 7.5: Set-up for H reflex study. Stimulator electrode (with anode pointed proximally) is placed in the popliteal fossa, the recording electrode on the soleus muscle, and the ground electrode in between.

F waves and H reflex: Self-test