Supramaximal electrical stimulation to the peripheral nerves evokes antidromic conduction through A motor axons and reaches the ventral horn of the spinal cord. The impulse then travels in an anterograde direction through A motor axons and returns to the muscle. The F-wave represents the resultant muscle contraction. Because F-wave latency represents conduction along the entire length of the motor axon, it has been used as a diagnostic tool in humans for diseases not only of motor nerves but also of the spinal cord, such as amyotrophic lateral sclerosis,1 diabetic neuropathy,2 Guillain-Barré syndrome,3 and polyneuropathies.4
Stimulation techniques for the induction of Fwaves in dogs have been established,5 and F-waves have contributed to the assessment of canine polyneuropathy,6 polyradiculoneuropathy,7 and experimental myelopathy,8 which cannot be diagnosed solely by motor nerve conduction velocity examination. In clinically normal cats, Knecht et al9 determined the mean latency of the recorded response of F and H waves and their latency rates. In that study, the F-wave latency was defined as the time from stimulation of the tibial nerve to contraction of the plantar interosseous muscles; therefore, it included nerve conduction from the stimulation electrode through the lumbosacral junction to the neuromuscular junction and the lag time of muscle contraction. Thus, the F and H latency rates defined in that study9 would be useful for examination of the nerve conduction, neuromuscular junction, and lag time of muscle contraction in combination. In other studies in humans2–4 and dogs,6,7 F-wave latency was determined from the entire length of the motor axon only, and not from the latency of the neuromuscular junction or the lag time of muscle contraction. Thus, the F-wave latency would also be useful for detection of nerve conduction dysfunction in cats. Indeed, the Fwave latency was prolonged in cats with diabetic neuropathy10 and experimentally induced mannosidosis,11 compared with the latencies in the control groups in those studies. Although the usefulness of F-wave latency was suggested in those reports, the F-wave latency and other related parameters have not been determined in clinically normal cats, to our knowledge. For clinical applications, normative values of F-wave latency and related parameters are necessary for identification of abnormalities in cats.
F-wave conduction velocity is calculated by dividing the distance that an impulse travels to elicit F-waves by the latency recorded.3 Because FWCV does not change depending on the nerve length, it is a stable parameter regardless of the size of the animal.12 F-wave conduction velocity has the advantage that it evaluates the conductive function of the proximal portion of motor nerves, whereas the traditional motor nerve conduction velocity examination assesses a limited portion of the distal regions of motor nerves. F-wave conduction velocity has been used in human medicine as a detection tool for nerve diseases.3,13 Because F-waves were absent in cats with lumbar hematomyelia,14 F-wave persistence (occurrence rate) may reflect the state of excitability in the neuronal pool examined. Thus, FWCV and F-wave persistence may be useful in examination of the functions of the motor nerves and spinal cord in felids.
The purpose of the study reported here was to establish a method of F-wave evaluation and to determine normative values of F-wave parameters, including FWCV, persistence, and amplitude for the tibial nerve in cats. Assessment of these parameters may be applicable for the diagnosis of nervous system diseases in cats.
Materials and Methods
Cat selection—Thirty clinically normal clientowned cats (sexually intact males and females of mixed breeds) undergoing neutering procedures were used in the study. The cats were 8 to 22 months old and weighed 2.5 to 5.8 kg. They did not have any evidence of neurologic impairment or hematologic abnormalities. The noninvasive F-wave examination was completed within 5 minutes, prior to commencement of surgery. Informed consent from clients was obtained prior to the participation of cats. All cats were handled in accordance with Azabu University Animal Experiment Guidelines, April 2000.
Procedures—After atropine sulfate was injected SC, anesthesia was induced via IV administration of thiamylal sodium.a Anesthesia was maintained throughout the examination period by the inhalation of isoflurane.b A polygraph systemc was used for electric stimulation and all measurements of evoked potentials.
F-waves of the tibial nerve were measured as described for our previous study,12 except for the method of measuring the distances. The tibial nerve was stimulated immediately proximal to the tarsal joint by use of needle electrodes inserted percutaneously, and the responses of interosseous muscles were recorded via surface disk electrodes. A reference electrode was located on the dorsum of a digit. Electric stimulation was applied as a rectangular wave with duration of 0.2 milliseconds, frequency of 1 Hz, and supramaximal intensity; 32 responses were recorded. Distances (measured in millimeters) from the stimulus point to the cranial border of the spinous process of the L6 vertebral body and from the major trochanter of the femur to the tip of the third digit with limb extension were measured as the nerve length and limb length, respectively.
F-wave analysis—Nerve and limb length have been used to obtain equations of F-wave latency regression or FWCV in dogs.12 In our previous study,12 we determined that F-wave latency was more closely correlated with nerve length than with limb length in dogs. It is not clear whether nerve length or limb length is better correlated with F-wave latency in cats. Thus, the F-wave shortest latency was plotted against nerve length and limb length, and linear regression analysis was performed.
F-wave conduction velocity was calculated by use of an equation12 as follows:
F-wave conduction velocity calculated on the basis of limb length (FWCVlimb) was obtained as follows15:
Results
When supramaximal electrical stimuli were applied to the tibial nerve, F-waves with various shapes and latencies were elicited in all cats examined (Figure 1). F-wave persistence varied from 93.8% to 100% (mean ± SD, 98.7 ± 2.3%). F-wave amplitude varied from 0.23 mV to 3.10 mV (mean, 1.01 ± 0.62 mV).
Recording of M responses (black triangle) and F-waves (white triangle) obtained from the tibial nerve of a clinically normal cat. Negative polarity is indicated by upward deflection. Notice that the recording sensitivity was changed after obtaining the M response (vertical line). SA = Electrical stimulation artifact.
Citation: American Journal of Veterinary Research 69, 2; 10.2460/ajvr.69.2.261
The shortest F-wave latency was closely correlated with nerve length (Figure 2). The correlation coefficient between F-wave latency and nerve length was 0.92. However, correlation coefficient between F-wave latency and limb length was low (0.58; Figure 3). Mean ± SD value for FWCV and FWCVlimb was 97.1 ± 5.0 m/s and 111.4 ± 5.2 m/s, respectively.
Linear regression analysis of F-wave latency and length of the tibial nerve in 30 clinically normal cats. The regression equation is as follows: F-wave latency = 2.60 + (0.02 × nerve length). Correlation coefficient is 0.92. Each circle indicates the value for an individual cat. The solid line is the regression line. Dotted lines represent the 95% confidence intervals (ie, the statistical range for which there is a 95% probability that values in the population will be within the range). Dashed lines present the 95% prediction intervals (ie, the statistical range for which there is a 95% probability that subsequent values will be within the range).
Citation: American Journal of Veterinary Research 69, 2; 10.2460/ajvr.69.2.261
Linear regression analysis of F-wave latency and length of the hind limb in the 30 clinically normal cats in Figure 2. The regression equation is as follows: F-wave latency = 3.42 + (0.02 × the limb length). Correlation coefficient is 0.58. See Figure 2 for key.
Citation: American Journal of Veterinary Research 69, 2; 10.2460/ajvr.69.2.261
Linear regression analysis yielded regression equations as follow: F-wave latency (milliseconds) = 2.60 + (0.02 × nerve length [mm]), and F-wave latency (milliseconds) = 3.42 + (0.02 × limb length [mm]).
Discussion
F-wave latency in cats was closely correlated to tibial nerve length but not to limb length. This characteristic is similar to that in dogs.12 Because the mean value of the nerve length was less than that of the limb length, FWCV was slower than FWCVlimb. The SDs of FWCV and FWCVlimb were similar; however, the correlation coefficient between F-wave latency and nerve length was much higher than that between F-wave latency and limb length. The distance an impulse travels is a determinant of FWCV; thus, an accurate assessment of the distance of impulse conduction was necessary to precisely calculate FWCV. The surface distance from the stimulus point used to evoke F-waves to the cranial border of the spinous process of the L6 vertebra is the distance of impulse conduction through the tibial nerve that results in elicitation of F-waves. Because there is evidence that FWCV is almost identical to tibial motor nerve conduction velocity,16 we propose that nerve length should be used for FWCV evaluation in cats.
F-wave persistence is the number of definable Fwaves divided by the number of stimuli and provides an indication of the state of excitability of the motor neurons examined. F-wave persistence varies depending on the muscle, nerve, and stimulation frequency and is nearly 100% for the tibial nerve in humans.13 F-wave persistence decreases in humans with injuries to the proximal portion of nerves or nerve roots, as does H-reflex persistence.17 We have determined that F-wave is absent in cats with lumbar hematomyelia.14 Therefore, F-wave persistence is a useful parameter for evaluation of nervous system diseases in cats. In other unpublished work performed by our group, F-wave persistence was 100% for the tibial nerve in 30 clinically normal dogs. The range and mean value of F-wave persistence in clinically normal cats was 93.8% to 100% and 98.7 ± 2.3%, respectively; thus, the reference value of F-wave persistence for the tibial nerve in cats may be nearly 100%, which is similar to that in humans and dogs. Because F-wave amplitude reflects the proportion of a motor neuron pool activated by the stimulation, it is influenced by the excitability of the motor neuron in the spinal cord or by the conduction velocity of the impulse in the motor nerve. Thus, F-wave amplitude also is a useful parameter for evaluation of central and peripheral nervous system diseases.13
In the present study, F-waves were obtained via stimulation of the tibial nerve at a location in the distal portion of the limb; thus, the F-wave parameters depended on the conduction velocity of entire length of motor nerve. As the results of our study indicated, this type of F-wave examination may be useful for the detection of lesions that existed diffusely in motor nerves. For subtle abnormalities that are limited to the proximal portion of motor nerves, F-waves should be evoked by stimulation of a nerve that is located more proximally in the limb, such as the sciatic nerve, so that particular portion of the motor nerve is examined. Thus, combined examinations of F-waves evoked with various stimulation points may be useful for evaluation of peripheral nervous diseases, including polyradiculoneuropathy7 and diabetic neuropathy,2 in cats.
To our knowledge, values of F-wave parameters for clinically normal cats, such as FWCV, latency, persistence, and amplitude, have not been reported with the exception of F-wave latency rate. The regression equation for F-wave latency and the normative values of FWCV, F-wave persistence and F-wave amplitude determined in the present study may contribute to the diagnosis of nervous system diseases or injuries such as spinal cord trauma or diabetic neuropathy in cats.
ABBREVIATIONS
| FWCV | F-wave conduction velocity |
| FWCVlimb | F-wave conduction velocity calculated on the basis of limb length |
Isozol, Nichi-iko Pharmaceutical, Osaka, Japan.
Isoflu, Dainippon Pharmaceutical, Tokyo, Japan.
Neuropack MEB-5508, Nihon Koden, Tokyo, Japan.
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