J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.50.4.453 on 1 April 1987. Downloaded from

Journal of Neurology, Neurosurgery, and Psychiatry 1987;50:453-459

F wave size as a monitor of motor neuron excitability: the effect of deafferentation

J E FOX, E R HITCHCOCK From the Department ofNeurosurgery, University of Birmingham, Birmingham, UK.

SUMMARY F waves have been recorded from hand or leg muscles following surgical section or traumatic avulsion of dorsal roots. It has been found that under these conditions F wave size can still be changed by manoeuvres which influence motor neuron excitability but that the size and direction of the changes are variable and may be dependent on the resting level of that excitability.

The F wave is a late response recorded in the electro- neurons,2-6182526 some experiments indicate that, myograph (EMG) of a muscle following stimulation especially under conditions of motor neuron of its nerve supply.1 Although the response was origi- excitation, the response may also include reflex com- nally thought to be a reflex, subsequent work showed ponents.3 25 27 28 Any increase in the size of the late that it can result from a recurrent discharge in motor response might then result from increased reflex activ- - neurons following their antidromic activation.2 6 ity rather than from changes in the size of the F wave Protected by copyright. Although its most frequent clinical use has been in per se. determining conduction velocity in the axons of We report two groups of observations on the F motor neurons,7 -15 there has also been some interest wave. The first results, which were obtained from in the possibility of using F wave size as a method of patients who had extensive dorsal root sections, show measuring changes in motor neuron excitability. In that F wave size can still change during voluntary contrast to the results of Mayer and Feldman5 both muscle activation, even in the absence of a sensory McComas et al16 and Sica et all7 reported that, in the input from the stimulated nerve. The nature of the normal human subject, F wave size increases during relationship between F wave size and motor neuron voluntary manoeuvres which increase motor neuron excitability was then investigated further in a group of excitability and Trontelj'7 reported that the occur- experiments on decerebrate cats. Some ofthese results rence of F waves recorded in single muscle fibres have been briefly reported.2930 could be facilitated by Jendrassik's manoeuvre or by weak voluntary motor neuron activation. There have Material and methods been numerous reports that F wave size increases in spasticity'9 20 and decreases when tone is patholo- Patients gically reduced;2' attempts have also been made to The first patient was a 41 year old woman in whom posterior http://jnnp.bmj.com/ rhizotomies of C5-T2 had been carried out for the relief of use the F wave to monitor procedures carried out to intractable pain in the left arm after a brachial plexus injury. relieve spasticity.22-24 Examination revealed complete anaesthesia from C5-T2 and Despite the development of these practical applica- grade 4 weakness throughout the left arm. During the tions there are conflicting reports about the nature of investigation the patient sat with the forearm and hand the relationship between F wave size and motor neu- lightly fixed, with the elbow semiflexed and the hand held in ron excitability5 16 and there is still uncertainty about a vertical plane. A ring around the thumb was connected to the physiological basis of the observed changes. a strain gauge which was arranged to measure abducting Although it has been established that F waves can be forces. The output from the strain gauge was displayed on on September 25, 2021 by guest. purely the result of a recurrent discharge in motor an oscilloscope which could be seen by the patient. EMG recordings were made using surface electrodes placed over the thenar eminence; the activity was amplified and displayed in the conventional way and recorded on mag- Address for reprint requests: Dr JE Fox, Midland Centre for Neurosurgery and Neurology, Holly Lane, Smethwick, Warley, West netic tape for subsequent analysis. F waves were evoked by Midlands B67 7JX, UK. stimulating the median nerve at the wrist, using O I ms pulses at an intensity 1-25 times that which gave a maximum M Received 31 January 1986 and in revised form 21 July 1986. response: F waves were identified by eye and the size of each Accepted 26 July 1986 response was measured by electronic integration. Average F 453 J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.50.4.453 on 1 April 1987. Downloaded from

454 Fox, Hitchcock wave size was calculated from the responses evoked by 50 delivered at a rate of 1 Hz. The right sciatic nerve was stimu- consecutive stimulus presentations in the control and each lated at a rate between 10 and 50 Hz The intensity was experimental situation. Control F wave size was measured adjusted until a crossed extensor response was observed and with the hand as relaxed as possible. The patient was then subsequent stimulation was carried out at an intensity just asked to make and maintain a weak abduction movement of below this level. The cerebellum was stimulated by a pair of the thumb. The force level was displayed to the patient on silver ball electrodes, separated by a distance of 3 mm. Stim- the oscilloscope and this enabled her to maintain a constant ulus rate varied between 30 and 200 Hz. Intensity was force despite the absence of a sensory input from the hand. adjusted until a reduction in extensor tone could be detected This level of muscle contraction induced only a very small by manual examination. amount of background EMG activity: in order to ensure, F waves were identified and average size calculated in the however, that any apparent change in F wave size was not manner described for the two patients above. In order to caused by the recording of background EMG activity from ensure that repeated antidromic activation did not itself lead the hand muscles, the following procedure was adopted in to a significant change in F wave size,31 the mean size of the control and experimental records: after the onset and end of response evoked by the first 10 stimuli was compared with the F wave had been identified and its size measured by that evoked by the last 10 stimuli in each of the control integration, the activity in the same period before the stimu- groups of 50 stimuli: there was no significant difference lus was measured in the same way and subtracted from the between the two. measured F wave size. F wave size is extremely variable and even the mean size, The second patient was an otherwise healthy 22 year old calculated from 50 stimulus presentations, can change dur- man who had suffered a unilateral traumatic avulsion of the ing the course of an experiment (vide infra). For this reason, third, fourth and fifth lumbar posterior roots and all sacral in the present experiments, control F wave size was mea- posterior roots with preservation of the ventral roots. The sured before and after each experimental procedure: results extent of the injury was confirmed at laminectomy. There were only included if the two values differed by not more was total sensory loss in the left leg, extending from L3 than 7%. downwards and grade 4 weakness of the left ankle. For this investigation the patient sat comfortably in a chair with the Results foot supported. F waves were recorded using surface elec- Protected by copyright. trodes placed over tibialis anterior following stimulation of the lateral popliteal nerve at the popliteal fossa. Recordings Patients with dorsal root section were made with the leg at rest and during a weak, sustained In the patient with cervical dorsal root sections, F dorsiflexion movement at the ankle. In order to maintain waves were recorded from the left (deafferented) hand constant muscle tension, in this instance the movement was with a latency of 27-32ms. Mean F wave size was made against an inflated sphygmomanometer cuff. The determined in the left hand, (a) with the hand relaxed patient could see the column of mercury and was asked to and (b) with the thumb a maintain it at a constant level. Measurement of average F exerting minimal abduction wave size was then made in a manner similar to that used for force. Mean F wave size, with the hand at rest, was the first patient. defined as 100%; weak abduction of the thumb increased mean F wave size by 38% (p < 0-05). Fol- Decerebrate animals lowing relaxation of the hand F wave size returned to Results are reported from experiments on 11 cats weighing the control level (fig la). The patient was then asked between 2-7 kg and 5 4 kg. The cats were anaesthetised with to make a fist with her right hand, contracting the halothane and made decerebrate at the level of the superior hand and forearm muscles as vigorously as possible: colliculus. The cerebellum was exposed by a further crani- at the same time she was asked to make no voluntary ectomy. In those experiments in which dorsal root section movements with her left hand (absence of detectable http://jnnp.bmj.com/ was carried out, a brief paralysis was induced with iv voluntary activity in muscles of the left thenar emi- suxamethonium and positive pressure ventilation carried nence was confirmed by inspection of EMG. out. The spinal cord and spinal roots were exposed and dor- the sal roots L2 to S5 were cut on the left side. records). During this manoeuvre mean F wave size Stimulating electrodes were placed on the right sciatic increased by 67% (p < 0 05) and returned to control nerve and the nerve was cut distal to the electrodes. Stimu- level following relaxation (fig lb). lating electrodes were also placed on the left tibial nerve at F waves were also recorded from the right the level of the ankle. Wire electrodes, insulated except for (unaffected) hand. During minimal abduction of the the final 3 mm, were inserted into the left plantar muscles right thumb mean F wave size increased by 81% (p < on September 25, 2021 by guest. and used to record EMG activity. After all surgery was com- 0 05). pleted, halothane was discontinued and an interval of In the patient with lumbosacral root sections, F approximately 2 hours was allowed for decerebrate rigidity to develop. EMG activity was amplified using conventional waves were recorded with latencies of 32-36 ms. circuitry, displayed on an oscilloscope and recorded on mag- Mean F wave size in the anterior crural muscles was netic tape for subsequent analysis. measured with the leg relaxed and during weak F waves were evoked by 0 1 ms stimuli applied to the left dorsiflexion of the ankle. During dorsiflexion mean F tibial nerve at an intensity which was at least 1-25 times that wave size increased by 87% (p < 0 01) (fig lc). necessary to evoke a maximum M response. Stimuli were The procedure was repeated on the unaffected side. J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.50.4.453 on 1 April 1987. Downloaded from

F wave size as a monitor ofmotor neuron excitability: the effect ofdeafferentation 455 During a weak dorsiflexion movement mean F wave 200 size increased by 23% (p < 0.01). Decerebrate animals At low stimulus intensity a small M wave was recorded together with a variable late response with a latency of 8-10 ms. With increased stimulus intensity the M wave became larger and reached a maximum 1- T size. Two late responses could then be seen, the first ^ had a latency of 8-10 ms and the second had a latency of 12-15ms: we identify the first of the late com- ponents as the F wave and the later as a polysynaptic reflex response (fig2) Spontaneous changes in F wave size A oJ -P.,JU _ _ _ Although in some animals mean F wave size (esti- mated from 50 consecutive stimulus presentations) stayed reasonably constant, in others the value changed during the course of the experiment. In order to measure this F wave size was measured 200 variability 26 in each of five animals on eight occasions over a 2-4 1 1 hour period under constant experimental conditions. I The maximum change which any single value showed --from the mean of the estimates varied between 12% .2 Protected by copyright. I and 78% (mean = 33%). Because of this variability it was important to N ensure that following each experimental procedure ux 100- the mean F wave size returned to the control level (see G) Methods). O. The effect ofcerebellar stimulation LE- Stimulation of the anterior vermis of the cerebellum Co at 50Hz and 2 5 volts induced a reduction in the extensor rigidity. At the intensity at which a change in B - clinically observed tone was noted, there was a reduction in F wave size (mean reduction = 58%, p < 0-01). The effect ofcontralateral sciatic nerve stimulation The contralateral sciatic nerve was stimulated at a http://jnnp.bmj.com/ 20072001~~~~ rate and at an intensity just below that which would Fig 1 Changes in F wave size during voluntary muscle contraction. In A and B, F waves were recordedfrom the thenar eminence in a patient with section ofcervical dorsal .A.eTr roots. (a) represents mean F wave size (calculatedfrom 50 stimulus presentations) with the muscle at rest (defined as 100 100%); (b) shows mean F wave size during weak voluntary on September 25, 2021 by guest. contraction ofthe ipsilateral thenar eminence (A) and strong contraction ofthe contralateral arm and hand and arm muscles (B); (c) shows that mean F wave size returned to control levelfollowing muscle relaxation. In (C), F waves were recordedftom tibialis anterior in a patient with section oflumbosacral dorsal roots. (a) represents control F wave size; (b) shows mean F wave size during a weak dorsiflexion 0- - ofthe ankle and (c) shows that mean F wave size returned to C a b c the control levelfollowing muscle relaxation. J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.50.4.453 on 1 April 1987. Downloaded from

456 Fox, Hitchcock Recordings were also made in another animal in 0 which "resting" F wave size changed during the course of the experiment. The effect of contralateral sciatic nerve stimulation in this animal differed at different times. In the early part of the experiment, contralateral sciatic nerve stimulation evoked a con- sistent decrease in mean F wave size (mean reduction of 29% on four separate tests, carried out over a 4 hour period). "Resting" F wave size then fell by 43% and contralateral sciatic nerve stimulation then evoked a 14% decrease in F wave size. A further reduction in F wave size (to 7% of the initial value) then occurred: contralateral sciatic nerve stimulation (identical stimulus parameters to those used in the earlier part of the experiment) then resulted in a facil- itation of mean F wave size (mean increase = 47%). The effect was consistent with repeat testing over a 2 hour period. Figure 3 illustrates this result. These results suggest that when resting F wave size was small, contralateral sciatic nerve stimulation induced an increase in F wave size, but when resting F wave size was larger, contralateral sciatic nerve stimulation Fig 2 EMG responses recordedftom the plantar muscles of a F wave size. the cat hindlimbfollowing stimulation ofthe tibial nerve. In induced decrease in

A, weak nerve stimulation evoked a small M wave,followed Protected by copyright. by a prominent H wave with a latency of8-10 ms. In B, ._Q~ stronger nerve stimulation gave a maximum M wavefollowed by an F wave and a polysynaptic reflex response. Following E dorsal root section, C, nerve stimulation again evoked a maximum M wave,followed by an F wave but no ;n polysynaptic reflex could be seen. C') C 3 give a crossed extensor reflex (typically 15-30 Hz, 0- 1-0-4 volts). At this intensity of stimulation there C Ln was an increase in F wave size (mean increase = 72%, > p < 0-01). a z-s1-X 1-.1 a ii Deafferentation >0 I It may be seen from fig 2 that following tI deafferentation the M wave and F responses are still clearly present, but that the 12ms (polysynaptic) http://jnnp.bmj.com/ response can no longer be seen. The mean F wave size following deafferentation was smaller than the size - i, - estimated immediately prior to deafferentation (mean reduction = 57%, p < 0.01). DE0 ' , The effects ofcontralateral sciatic nerve stimulation F wave size following deafferentation Fig 3 Changes in F wave size induced by stimulation ofthe The effects of contralateral sciatic nerve stimulation contralateral sciatic nerve at three different levels ofcontrol F on September 25, 2021 by guest. were variable. In six experiments increases in F wave wave size. In this experiment "resting" F wave size varied size were seen during stimulation of the contralateral spontaneously during the course ofthe experiment. When which "resting" F wave size was small (a), stimulation ofthe sciatic nerve at an intensity just below that opposite sciatic nerve consistently induced an increase in F evoked a crossed extensor reflex (mean increase in F wave size. When "resting" F wave size was larger (b, c), wave size = 47%, p < 0.01). In three experiments, however, identical stimulation ofthe opposite sciatic nerve however, stimulation of the opposite sciatic nerve, at induced a decrease in F wave size. If "resting" F wave size in the equivalent intensity, induced a decrease in F wave (c) is defined as 100%; "resting" F wave size in (a) and (b) size (mean decrease = 26%, p < 0 01). was 7% and 56%, respectively. J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.50.4.453 on 1 April 1987. Downloaded from

F wave size as a monitor ofmotor neuron excitability: the effect ofdeafferentation 457 Discussion recorded in our experiments reflects both the fre- quency of recurrent discharge following antidromic The F wavefollowing deafferentation invasion in individual neurons and the total number of Our results confirm the observations that F waves can individual neurons giving a recurrent discharge, then be recorded following deafferentation.452528 This our results are consistent with those obtained by indicates that the F wave can be the result of a purely Schiller and Stalberg32 and can again be explained on recurrent discharge following antidromic activation the basis of Eccles' observations; that is, when activa- of motor neurons. Under resting conditions, tion of the motor neuron pool results in an increased relatively few neurons give this recurrent number of motor neurons reaching the critical level of discharge3 16 31 32 and hence the F wave is very much depolarisation, there is an increase in mean F wave smaller than the M wave. size; when motor neuron activation occurs against a Our results also confirm previous background of already existing depolarisation then observations'6 17 33 that when dorsal roots are intact many neurons which previously gave a recurrent dis- F wave size increases during manoeuvres which charge will fail to give recurrent discharges and mean increase motor neuron excitability. In addition, how- F wave size falls. Schiller and Stalberg32 suggest that ever, we have shown that, following deafferentation, although individual neurons respond very infre- F wave size still changes when motor neuron quently (so that F wave size is only a small percentage excitability is changed by weak voluntary contraction of M wave size) F waves, over a period of time, arise of the muscle under investigation, by vigorous con- from a large part of the motor neuron pool. This traction of contralateral muscles or by synaptic acti- implies that the whole motor neuron pool may behave vation following stimulation of sensory nerves from in a relatively homogeneous way so far as F wave the contralateral limb. Since the sensory input has generation is concerned and hence an increase in been interrupted there can be no reflex contribution mean F wave size (recorded from a whole muscle)

to the late response and therefore the changes in may occur over only a relatively small range of syn- Protected by copyright. response size reflect changes in the size of the F wave aptic activation applied to the motor neuron pool as itself. a whole. Changes in mean F wave size, as recorded in our The dependence of the direction of size change on experiments, imply that individual motor neurons the number of neurons entering a small, critical level change their frequency of recurrent discharge follow- of depolarisation could also explain the fact that ing antidromic invasion and/or that there is a change Upton et a128 saw no change in F wave size during in the total number of motor neurons giving a recur- voluntary activation in patients who had undergone rent discharge. posterior root section, while in our similar It follows from the earlier work carried out in the investigation F wave size increased. According to the lumbosacral cord of the cat that the occurrence of a hypothesis under consideration F wave size may or recurrent discharge31 in an individual neuron may not change, depending on the background level depends on the level of motor neuron depolar- of depolarisation in the motor neurons comprising isation:34 (a) when the neuron is relatively hyper- the motor neuron pool. polarised, the antidromic action potential fails to pass from the axon hillock into the soma (and there can Combination ofF waves and reflex components when therefore be no recurrent discharge); (b) over a small dorsal roots are intact http://jnnp.bmj.com/ range of depolarisation, soma invasion occurs after In our experiments, acute deafferentation was associ- an interval which is sufficiently long for the initial seg- ated with a reduction in mean (resting) F wave size. ment to have passed through its refractory period and Deafferentation is followed by complex effects on hence a recurrent discharge occurs; (c) further depo- motor neuron excitability5 and there are many factors larisation of the soma allows antidromic invasion to that could contribute to our observed reduction in F occur very rapidly (that is, at a time when the initial wave size (for example, a reduction in ongoing syn- segment is still refractory) and no recurrent discharge aptic activation of motor neurons or unintended is seen. damage to the cord and ventral roots). However, the on September 25, 2021 by guest. Using the single fibre EMG technique, Schiller and observation would be consistent with the removal of a Stalberg32 studied the responses of individual motor reflex component to the late response. units in normal and spastic man; they showed that the It is likely that when dorsal roots are intact some H frequency of F wave occurrence in individual units is waves will be recorded at the same time as F consistent with its dependence on soma depolar- waves.25 27 28 Stimulation of a mixed nerve (supra- isation, as observed by Eccles34 in his direct record- maximal for motor fibres) results in activation of all ings from motor neuron cell bodies. alpha motor neurons and of the large fibre afferents. If we assume that the mean size of the F wave Antidromic conduction in motor fibres reaches the J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.50.4.453 on 1 April 1987. Downloaded from

458 Fox, Hitchcock initial segment and some impulses will reach the think it is likely that a reflex discharge contributes to soma: some will induce recurrent discharges (vide the overall size, so that the observed increase in this supra). Orthodromic conduction in very fast sensory situation reflects an increase in the F-H complex. fibres arrives at the soma (via synapses) before any antidromic impulses. In some motor neurons this We are grateful to William Mitchell, Senior Tech- leads to an action potential: under conditions of rest, nician, Department of Neurosurgery, for assistance in however, this will not lead to an H wave because the the animal experiments and Veronica Turner for typ- action potential will collide with the antidromic ing and correction of manuscripts. This work was action potential travelling towards the cord. How- supported by funds from Research grants of the Fac- ever, an H wave will occur in two circumstances: (a) ulty of Medicine and Department of Neurosurgery, when there is ongoing activity in the motor neuron; in University of Birmingham. this situation the "spontaneous" action potential will collide with the antidromic action potential, enabling References the orthodromic H wave to pass through the whole length of the axon and (b) when orthodromic sensory 1 Magladery JW, McDougal DB. Electrophysiological activation is via medium-fast sensory studies of nerve and reflex activity in normal man. I. fibres (that is, Identification of certain reflexes in the EMG and the slower than the fastest conducting motor fibres).35 conduction velocity of peripheral nerve fibres. Bull Synaptic activation via these synapses arises at the Johns Hopkins Hosp 1950;86:265-90. soma after any antidromic impulse (which in many 2 Dawson GD, Merton PA. "Recurrent" discharges from instances will terminate at the initial segment): there- motoneurones. 20th International Congress on Physi- fore, if synaptic activation is sufficiently intense an ology. Brussels. Abstracts of Communications orthodromic action potential will be initiated in the 1956:221-2. motor neuron, giving rise to an action potential in the 3 Thorne J. Central responses to electrical activation ofthe muscle fibres at approximately the time at which an F peripheral nerves supplying the intrinsic hand muscles. wave J Neurol Neurosurg Psychiatry 1965;28:482-95. Protected by copyright. would be expected. 4 McLeod JG, Wray SH. An experimental study of the F For these reasons, when the dorsal roots are intact, wave in the baboon. J Neurol Neurosurg Psychiatry it might be more appropriate to describe the late 1966;29: 196-200. response evoked by supramaximal stimulation of 5 Mayer RF, Feldman RG. Observations on the nature of a peripheral nerve as an F-H complex rather than the F wave in man. Neurology 1967;17:147-156. simply as an F wave, particularly under conditions of 6 Miglietta OE. The F response after transverse partial motor neuron activation. myelotomy. In: Desmedt JE, ed. New Developments in EMG and Clinical Neurophysiology. Basel: Karger, Change in F wave size in spasticity 1973;3:323-7. 7 Eisen A, Schomer D, Melmed C. The application of F Since, under appropriate conditions, motor neuron wave measurements in the differentiation of proximal activation may lead to an increase, a decrease-or no and distal upper limb entrapment. Neurology change in mean F wave size, it is at first sight a little 1977;27:662-8. surprising that the F wave should have gained some 8 Kimura J. F wave velocity in the central segment of the acceptance as an indicator of spasticity'o 22-24 and median and ulnar nerves. A study in normals and in be capable of being used to monitor changes in spas- Charcot-Marie-Tooth disease. Neurology ticity. We believe that there are two explanations for 1974;24:539-46. http://jnnp.bmj.com/ this. First, it has been shown that even in moderately 9 Kimura J. Proximal versus distal slowing of motor nerve severe spasticity very little ongoing EMG activity can conduction velocity in the Guillaine-Barre syndrome. Ann Neurol 1978;3:344-50. be recorded.32 This suggests that, in these patients, 10 Kimura J, Butzer JF. F wave conduction velocity in large numbers of motor neurons may be partially Guillaine-Barre syndrome. Assessment of nerve seg- activated without reaching the level of generating ment between axilla and spinal cord. Arch Neurol action potentials. In this situation it seems not 1975;32:524-9. unlikely that large numbers of motor neurons may be 11 Young RR, Shahani BT. Clinical value and limitation of within the critical level of depolarisation. Thus, mean F wave determination. Muscle Nerve 1978;1:248-50. on September 25, 2021 by guest. F wave size would be higher in the spastic limb than 12 King D, Ashby P. Conduction velocity in the proximal in the control limb and operative procedures to segments of a motor nerve in the Guillaine-Barre syn- reduce tone would also reduce the number of neurons drome. J Neurol Neurosurg Psychiatry in 1976;39:538-44. the critical depolarisation range. In addition, as we 13 Panayiotopoulos CP. F wave conduction velocity in the have discussed above, in patients with intact posterior proximal segment of the deep peroneal nerve: roots, the observed increase in "F wave" size may be Charcot-Marie-Tooth disease and dystrophia myo- only in part due to an increase in the number of neu- tonica. Muscle Nerve 1978;1:37-44. rons giving a recurrent discharge: in addition, we 14 Panayiotopoulos CP, Scarpalezos S, Nastas PE. 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