Nerve conduction studies including F wave studies are used in daily clinical practice. F wave latency is the most sensitive nerve conduction parameter in patients with diabetic neuropathy [9, 10] and F wave abnormalities are often seen early on in neuropathies of other origins as well. In demyelinating polyneuropathy, the F waves are delayed, (i.e., the latencies are increased), and in proximal conduction block they are few in number and delayed. In severe axonal loss with reduced compound muscle action potential (CMAP) amplitudes, the number of F waves is reduced but the F wave amplitudes may be normal. The F waves are also affected by a myopathy with reduced CMAP, where the number of F waves is normal but the F wave amplitudes are reduced. In spastic conditions such as cerebral palsy, F wave amplitudes may be increased, along with the number of F waves showing prolonged duration [11]. The increased F wave amplitude in spasticity was described as early as 1979 [12], and has been observed in patients with konzo which is a progressive spastic disorder that results from consuming bitter (high cyanide) cassava flour (Fig. 7.3) [13].

In the peripheral nervous system, nerve conduction velocities are slow at birth due to immature myelination patterns. The velocities increase as myelination progresses (Fig. 7.4). The conduction velocity varies with the length of the nerve, thickness of the myelin sheath, diameter of the nerve and distance between nodes of Ranvier. Reference studies with nerve conduction velocities from pediatric populations have been performed, and some early studies were published from 1960 to 1993 [14–22]. A comprehensive overview of results of different studies was published in 2002 [23].

In the 1980s, two studies of F waves in children were published [24, 25]. Reference values in healthy children 6 weeks to 14 years of age (n = 49) were collected by the author for use in daily neurophysiological work (Fig. 7.4) [26]. The minimum F-wave latency in children up to 1 year of age correlates more with age than height; between 1 and 2 years of age the reverse becomes true, with height becoming more important than age. Table 7.1 shows minimum F wave latencies in young children. The number of F-waves varies between young individuals (Table 7.2), however, the median values for each age group do not differ significantly from those in older children.

F-wave parameters were studied with linear regression analysis in order to obtain reference values in healthy children and adolescents aged 3–20 years [27]. In 175 subjects (91 boys, 84 girls) 410 nerves were studied (117 median, 102 ulnar, 114 peroneal, 77 tibial). All subjects were studied unilaterally. Linear regression with height as an independent variable generated the best model for the study of minimum F wave latency. The F wave latency increased with height, with 0.12 ms/cm in arm nerves and 0.28 ms/cm in the motor nerves of lower extremity. Height explained 86–93% of the variability of the minimum F wave latency, and age explained 71–87% of the variability. Age and height are closely interrelated in children. There was no correlation between the number of F waves versus height or age. Dispersion of F wave latencies did not correlate with age, height or gender.

Maturation of the nervous system depends heavily on myelination, although several other developmental processes make major contributions as well. The continuous progress and developmental milestones are reflected in the findings of various neurophysiological studies. Knowledge of normality is a prerequisite for identifying abnormalities, rendering diagnostic studies possible, paving the way for adequate care and treatment.

As a motor nerve is stimulated 15–20% above the level for obtaining maximal muscle response (M wave), F responses (F waves) can be obtained, using a sweep speed of 5 ms/D up to 10 ms/D. F waves are action potentials of at least 20 μV. The gain should be set at 0.2–0.5 mV/D for F waves and 2 or 5 mV/D for M responses. By stimulating at 1 Hz for 20 times, the number of F responses from different nerves vary, with the highest number of responses typically obtained from the tibial nerve.