“F” WAVE: Clinical Importance
B Taksande, A Jain
f wave, latency, nerve conduction
B Taksande, A Jain. “F” WAVE: Clinical Importance. The Internet Journal of Neurology. 2008 Volume 10 Number 2.
F-wave is one of the late responses produced by antidromic activation of
Motoneurons by supramaximal stimulation. They are variable in latency, amplitude, and configuration. Whenever we talk of nerve conduction studies, the importance of F wave are considered less.One should understand the characteristics and the physiology of F wave. This is important since F-waves are one of the most frequently used studies in clinical neurophysiology and much of the controversies surrounding the use of F-waves relates to a failure to adequately consider the requirements of F-wave analysis. These requirements include the number of F-waves that need to be recorded, the parameters that should be evaluated, and the muscle from which the F-waves are recorded. They are recorded over a muscle innervated by the stimulated nerve.F-waves are the only parameter in nerve conduction studies particularly useful for the diagnosis of proximal nerve lesions
Nerve conduction studies are basically performed to study the distal segment involvement. The late responses are preformed to study the proximal segment involvement. There are 3 different late respose: H reflex, F wave and the axon reflex. Out of this H reflex and F wave are preformed to study the proximal regions of nerves (i.e., portions of nerves near the spinal cord. F wave occurs after the CMAP(Compound Muscle Action Potential).It results from antidromic stimulation o motor neurons involving conduction to and from spinal cord and occurs at the interface between peripheral and central nervous system. 1
In a typical F wave study, a strong electrical stimulus (supramaximal stimulation) is applied to the skin surface above the distal portion of a nerve so that the impulse travels both distally (towards the muscle fiber) and proximally (back to the motor neurons of the spinal cord) as shown in figure 1.
(These directions are also known as orthodromic and antidromic, respectively.) When the
The properties of F waves are as follows:
The minimal latency is most reliable and useful measurement. Errors in placing the markers are common which may alter the latency. The markers are best placed at a point where it departs from the baseline. In addition superimposing the traced once all the responses are obtained often is helpful in determining the minimal latency. F latency vary with the height of the patient. F responses are longer in tall patients .The upper limit of minimal F latency is 31ms for female and 34.4 ms in normal males(Nelson et al 1990).The right to left asymmetry of minimal F latency exceeding 2 ms in hand and 4 ms in foot is considered abnormal.
F-waves are required for estimation of chronodispersion (50-60). It basically refers to the difference of maximal and minimal latencies in a series of F waves. I t is highly sensitive for diagonising demyelinating neuropathy.
The ratio of F wave amplitude to the associated M wave (FM ratio) is a measure of proportion of motoneuron pool activated by antidromic stimulation. It helps in diagnosis of axonal neuropathy though not highly sensitive.
It is a measure of antidromic excitability of a particular motor neuron pool. It is decreased in axonal neuropathy. It is calculated by dividing the number of F responses to the number of stimuli.
The “F” Estimate
Whether the prolonged F responses is truly due to a lesion of the proximal nerve segment or merely reflects an abnormal distal motor latency or conduction velocity or an un usually tall patient ,could be estimated by the help of F Estimate. The distal motor latency ,Conduction velocity and the patients limb length are th parameters required for calculating F estimate. It is calculated by the theoretical time it should take for the F response to occur, taking into account these variables. First,to calculate the time it takes for F response to go from stimulation site to the anterior horn cell, one divides the distance between those sites y the motor nerev conduction velocity. Second, there is a brief turnaround time at the anterior horn cell, which has been estimated to be approximately 1ms.Third, the time it takes the F response to travel back down from the anterior horn cell to the stimulation site. Finally, the time it takes from the stimulation site to the muscle is the distant motor latency. 1,6,7
F Estimate=(2D/CV)X10 1 ms DL
Where D is the distance from stimulation site to the spinal cord, Cv is conduction velocity, DL is Distal motor latency and 10 is the conversion factor to milliseconds. The actual measured F response is shorter than F estimate. Therefore, if F response is prolonged as compared to the F estimate it is suggestive of proximal nerve segment injury.
It is a sensitive measure of polyneuropathy and radiculopathy.In Guillain Barré syndrome (GBS) even if the nerve conduction study in terms of distal motor latency and conduction velocity is normal, the F response could be the only parameter which would help in diagnosis. F wave latencies are prolonged in GBS affecting the proximal nerve segment. In amyotrophic lateral sclerosis, reduced persistence of F wave is attributed to the loss of anterior horn cell. 5,8
It checks only the nerve or nerve segments which innervates the muscle being recorded.
Could not check for radiculopathy affecting the sensory nerve root fibers, as F response measures the motor fibres.
Could measure only severe radiculopathy or plexopathy.