F waves are often used to measure nerve conduction velocity, and are particularly useful for evaluating conduction problems in the proximal region of nerves (i.e., portions of nerves near the spinal cord).
It’s called F wave because it was initially recorded in the foot muscles.
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). (These directions are also known as orthodromic and antidromic, respectively.) When the orthodromic stimulus reaches the muscle fiber, it elicits a strong M-response indicative of muscle contraction. When the antidromic stimulus reaches the motor neuron cell bodies, a small portion of the motor neurons backfire and orthodromic wave travels back down the nerve towards the muscle. This reflected stimulus evokes small proportion of the muscle fibers causing a small, second CMAP called the F wave.
Because a different population of anterior horn cells is stimulated with each stimulation, each F wave have a slightly different shape, amplitude and latency.
F wave properties include:
amplitude (µV) – F wave height duration (ms) – length of F wave latency (ms) – period between F wave and initial stimulation F wave measurements Several measurements can be done on the F responses, including minimal and maximal latencies, and F wave persistence.
The minimal F wave latency is typically 25-32 ms in the upper extremities, and 45-56 ms in the lower extremities.
F wave persistence is the number of F waves obtained per the number of stimulations, which is normally 80-100% (or above 50%).
Each skeletal muscle is usually supplied by two or more nerve roots and if one nerve root is affected and the other is spared, the clinically used F wave minimum latency can still be normal 1).
In Guillain Barré syndrome (GBS) with antiganglioside antibodies, isolated absence of F waves is a frequent conduction abnormality especially in the early phase of the disease, and may be caused by axonal dysfunction, such as physiological conduction block or axonal degeneration at the nerve roots 2).
Facial F-wave, blink reflex and facial corticobulbar motor evoked potentials (FCoMEP), are feasible to intra-operatively study changes in excitability of the facial nerve and its nucleus during MVDs. Intra-operative neuromonitoring with the mentioned techniques allows a better understanding of HFS pathophysiology and helps to optimise the MVD 3).
The hypoglossal-facial nerve by “side”-to-side anastomisis with pre-degenerated auto-nerve graft was effective for the treatment of peripheral facial palsy after CPA tumor resection, F wave can be used as one of the objective index for the effect of the operation 4).