Neurophysiological changes following traumatic spinal lesions in man

Absence of consistent effects of repetitive transcutaneous electrical stimulation on soleus H-reflex in normal subjects

Our purpose was to determine the effects of transcutaneous electrical nerve stimulation (TENS) on the soleus H-reflex amplitude in normal subjects. Eleven subjects were tested in five experimental sessions, the purpose of which was to compare the effects of 30 minutes of TENS delivered at either 50 or 99Hz (250 microseconds pulses) on a mixed (common peroneal nerve or CPN) versus a sensory (sural) nerve. The soleus H-reflex was elicited according to the classic protocol of Hugon (1973). Control values (Hctrl) were measured for 5 minutes prior to and for 10 minutes after the TENS was administered at twice the sensory threshold. No statistically significant session (treatment) effects (two-way ANOVAs for repeated measures; alpha = 0.05) resulted from the stimulation of the CPN or the sural nerve at 50 or 99Hz. However, although no specific trends were shown across all subjects, there was a definite tendency towards inhibition (> or = 10% Hctrl) of the H-reflex in 63% of the subjects after 30 minutes of TENS at 99Hz over the CPN, and in 50% of the subjects when TENS was applied over the sural nerve at 99Hz. The inherent variability of the H-reflex amplitude in normal subjects as well as the use of different stimulation paradigms and TENS parameters could explain the controversial findings present in the literature.

H-reflex changes during contractions of the ankle extensors in spastic patients

Soleus H-reflexes during tonic contractions and isometric ramp contractions of the ankle extensors in spastic and healthy subjects were measured. During the tonic contractions, the H-reflex increased with the contraction level. The increase was highest in the patients (p < 0.001). The facilitation of the H-reflex during a ramp contraction is due to a static component resulting from the increased excitation level and a dynamic component resulting from the modulation of the H-reflex. The dynamic H-reflex facilitation during the ramp contraction was decreased in the patients (p < 0.05). The findings suggest that there is a decreased H-reflex control in spastic patients and this could be explained by a decreased presynaptic inhibition or by postsynaptic changes.

Stretch responses to ankle rotation in multiple sclerosis patients with spasticity

In 13 spastic patients with multiple sclerosis and 10 control subjects, electromyographic (EMG) and mechanical responses to stretch of the ankle extensors and ankle flexors during maintained contraction were measured. The reflex EMG responses in the extensors were divided into a phasic response (40-140 msec after onset of stretch) and a tonic response (200-400 msec after onset of stretch). In the control subjects, both the onset and peak latency of the phasic EMG response decreased with the contraction level (0.01 < P < 0.02 and P < 0.002 respectively) in the extensors. In the patients the latency of the phasic EMG response in the extensors was independent of the voluntary contraction level. This could be attributed to a disruption of the normal recruitment of the motor units according to the size principle. The phasic EMG response was larger in the patients than in the control subjects (P < 0.01). The tonic EMG response was of equal size in the two groups. The larger phasic EMG response in the patients was not followed by an increase in the reflex mediated mechanical stretch response. This shows that proposed changes in the muscle function in spastic patients based on changes in EMG stretch responses must be made with caution. In the ankle flexors all patients had reduced or absent EMG responses to stretch, consistent with earlier findings of an absent mechanical reflex mediated response.

Soleus H-reflex tests and clinical signs of the upper motor neuron syndrome

Soleus H-reflex tests are used for elucidating pathophysiological mechanisms in motor control. The cumulative vibratory inhibition of the soleus H-reflex, the ratio of the reflex to direct muscle potential (H to M ratio) and the recovery curve of the soleus H-reflex were studied in 38 patients with varying signs of the upper motor neuron syndrome for a possible relation with clinical features. The results were compared with those obtained from a group of healthy volunteers. The magnitude of vibratory inhibition decreased with increase of hypertonia. The H to M ratio increased as the activity of the tendon reflex was enhanced and correlated to a lesser degree with muscle tone. Both the H to M ratio and late facilitation of the soleus H-reflex recovery curve were elevated in clonus. The findings suggest that alterations in the results of soleus H-reflex tests relate to specific clinical features of the upper motor neuron syndrome. Possible pathophysiological implications are discussed.

Evidence that alterations in presynaptic inhibition contribute to segmental hypo- and hyperexcitability after spinal cord injury in man

We examined Hoffmann (H) and tendon (T) reflexes in 3 populations of adult subjects: acute SCI (< 2 weeks post injury), controls, and chronic SCI (> 1 year post injury). We further investigated the effects of continuous tendon vibration and different stimulus rates on the size of evoked H reflexes in these subject populations. All reflex amplitudes were expressed as a function of the maximum direct muscle response (M wave), to allow comparison between subjects. Both H and T reflexes were successfully elicited from all subjects examined, including those in 'spinal shock.' Tendon vibration caused a marked attenuation of H reflexes in acute SCI subjects, intermediate attenuation in controls, and relatively little effect in the chronic SCI group. H reflexes showed greatest attenuation for a given stimulus rate in acute SCI subjects compared to controls (intermediate attenuation) or chronic SCI (limited attenuation) subjects. Both rate sensitivity and vibration influence have been linked to presynaptic inhibitory mechanisms. We suggest that spinal cord injury disrupts the supraspinal influence over segmental interneurons mediating presynaptic inhibition, and that the hyporeflexia associated with 'spinal shock' is due in part to a substantial increase in the efficacy of presynaptic inhibition. Conversely, over time the level of presynaptic inhibition of ankle extensor Ia input in SCI subjects declines to levels less than those of control subjects, contributing to the enhancement of spinal reflexes consistent with the clinical state of 'spasticity' seen in chronic SCI.

Recurrent potentials in human peripheral sensory nerve: possible evidence of primary afferent depolarization of the spinal cord

To study slowly conducted components of the orthodromic compound sensory action potential (CSAP), the response evoked at the lateral malleolus in the sural nerve was recorded through near-nerve needles at two to four sites along the nerve at midcalf. When 500 to 2000 responses were averaged at high gain, components with latencies of 30 to 80 ms were often recorded. In contrast to the main component and late components with latencies of less than 15 to 20 ms, the latencies of these extremely late components diminished the closer to the spinal cord that they were recorded. This suggested that the components were conducted antidromically from proximal to distal. This assumption was supported by abolishing the components by local anesthesia of the nerve proximal to the recording electrodes. These antidromic potentials therefore appear to be due to recurrent discharges in the sural nerve. Recurrent discharges were recorded from 65% of 60 subjects (18 normal subjects and 42 patients with peripheral or central nervous system disorders). The latencies of the recurrent discharges allowed conduction to and back from the spinal cord. Although the origin of these potentials remains unknown, we suggest that they are due to dorsal root reflexes within the spinal cord. In this case, the responses may be a direct expression of primary afferent depolarization (PAD) seen in presynaptic inhibition, and may be of value in further studies on the physiology and pathophysiology of presynaptic inhibition of cutaneous fibers in man.

AAEM Minimonograph #13: H reflexes and F waves: physiology and clinical indications

Motoneurons can be activated both reflexly and antidromically following electrical stimulation of peripheral nerves. These H reflexes and F waves are clinically useful responses which interface at the level of the peripheral nerves and the spinal cord. Because these responses are commonly employed in the electrodiagnostic evaluation of patients, an understanding of their physiology and clinical applications is important. These are reviewed. Reasoning from the physiology, both the value and limitations of H-reflex and F-wave studies are considered for disorders of peripheral nerves, roots, and the central nervous system. Theoretical concepts about the physiology and pathophysiology of the nervous system based on H-reflex and F-wave data are also discussed.

A comparative study of methods for estimation of presynaptic inhibition

The influence of vibration on the H-reflex and on the tendon reflex amplitudes was compared and the efficacy of both methods for the assessment of the presynaptic inhibition was studied. One hundred and twenty patients with post-stroke spastic hemiparesis were investigated. Muscle tone, muscle force and tendon reflexes were assessed. The H-reflex and the Achilles tendon reflex (TA) were recorded under identical experimental conditions. Vibration at a frequency of 100 Hz and an amplitude of 2 mm was applied to the TA. Just after vibration the maximal amplitudes of both reflexes were measured. The ratios of reflex amplitudes after vibration to normal maximal reflex amplitudes (Hvibr/Hmax and TAvibr/TAmax) were evaluated. In all patients with hemiparesis the healthy side was used as a control. Our results revealed significantly increased amplitude ratios on the spastic side. Hence it is concluded that presynaptic inhibition is decreased in spasticity. The amplitude ratios on the healthy and the spastic side were consistent. There was good positive correlation between Hvibr/Hmax and TAvibr/TAmax ratios, suggesting that they provide similar and reliable estimates of presynaptic inhibition.

H-reflex modulation during walking in spastic paretic subjects

Hoffman (H) reflexes were elicited from the soleus muscle during treadmill walking in 21 spastic paretic patients. The soleus and tibialis anterior muscles were reciprocally activated during walking in most patients, much like that observed in healthy individuals. The pattern of H-reflex modulation varied considerably between patients, from being relatively normal in some patients to a complete absence of modulation in others. The most common pattern observed was a lack of H-reflex modulation through the stance phase and slight depression of the reflex in the swing phase, considerably less modulation than that of normal subjects under comparable walking conditions. The high reflex amplitudes during periods of the step cycle such as early stance seems to be related to the stretch-induced large electromyogram bursts in the soleus in some subjects. The abnormally active reflexes appear to contribute to the clonus encountered during walking in these patients. In three patients who were able to walk for extended periods, the effect of stimulus intensity was examined. Two of these patients showed a greater degree of reflex modulation at lower stimulus intensities, suggesting that the lack of modulation observed at higher stimulus intensities is a result of saturation of the reflex loop. In six other patients, however, no reflex modulation could be demonstrated even at very low stimulus intensities.

Cumulative vibratory indices and the H/M ratio of the soleus H-reflex: a quantitative study in control and spastic subjects

Suppression of the soleus (Sol) H-reflex recruitment curve by Achilles tendon vibration and the ratio of maximum Sol H-reflex (Hmax) to maximum M-response (H/M ratio) have been studied by means of computer processing on the basis of peak-to-peak (P-P) and area values in 46 controls and in 16 spastic patients. The 'classical' vibratory index (VI) has been compared with a new cumulative VI (CVI), which is defined as the quotient of the surface under the recruitment curve obtained with vibration and the one obtained without vibration up to an equal intensity level. Statistical analyses were performed from threshold to 3 different intensity levels. Tendon vibration is found to be less effective at higher intensity levels. Mean values with S.D.s of CVIs at Hmax intensity are significantly lower than those of the 'classical' VIs. VIs increase with age while H/M ratios decline. Stepwise discriminant analyses showed that area CVI at Hmax intensity yielded the best distinction between controls and patients. Additional differential contribution is obtained by P-P H/M ratio. A canonic variable derived from these two variables correctly classified 44 out of 46 controls and 15 out of 16 patients. The methods presented improve the specificity of the quantitative neurophysiological measure of spasticity by means of Sol H-reflex recordings.

Reflex changes induced by clonidine in spinal cord injured patients

In a single blind study of 6 spinal cord injured (SCI) men, the effects of clonidine, a selective alpha-2 adrenergic agonist, on spasticity were compared to diazepam and placebo. Since a potential side-effect of clonidine is postural hypotension, a combination of clonidine and desipramine was also tested. Vibration of the leg will inhibit the H reflex in a normal subject; whereas, this inhibition is markedly reduced in SCI patients with spasticity. A vibratory inhibition index (VII) was derived for each treatment. The pre-treatment VII was 92.08 +/- 3.15%; for SCI subjects, compared to 46.5 +/- 7.7% for 6 normal subjects. Clonidine significantly reduced the mean index of SCI patients to 59.42 +/- 3.91% (p less than 0.001). The VII for placebo, diazepam and the clonidine-desipramine combination were not statistically different than the pre-treatment values in SCI patients. In conclusion, clonidine has an anti-spasticity effect in SCI patients, both subjectively, and objectively, in terms of vibratory inhibition of the H reflex.

An investigation of possible transynaptic neuronal degeneration in human spinal cord injury

Neurophysiological studies suggested that transynaptic neuronal degeneration of the anterior horn cells (AHC) may occur after an upper motoneuron lesion as the result of "deafferentation". To test this observation anatomically, patients with spinal cord injury (SCI) who had come to post mortem were investigated. Four patients with longstanding clinically and pathologically "complete" SCI were selected for comparison with 4 age-matched normal controls and with 2 patients who died of motoneuron disease (MND). The total number of AHCs in the L3 spinal cord segment was counted in each of the cases. The lesions in the traumatic group were all above the L3 segment. No significant differences in the number of AHC between the test cases and the normal controls was found. There was, as expected, a highly significant difference between the test cases and those with MND. The conclusion drawn from the study is that transynaptic neuronal degeneration of AHCs does not occur following complete transection of the human spinal cord. Thus the neurophysiological hypothesis is not supported anatomically.

F/M ratios in polyneuropathy and spastic hyperreflexia

The ratios of F to M amplitudes were determined by dividing the mean amplitude of 15 F-waves (mF AMP) by the maximum evoked motor response amplitude recording from the abductor pollicis brevis and calf muscles. Data were compared from age-matched control subjects and patients with polyneuropathies or spastic hyperreflexia. In comparison to normals, mF AMP/M--but not mF AMP--values were increased at statistically significant levels in all patients groups. M amplitudes were decreased (versus normal, P less than 0.005) in the patients with either peripheral or central lesions. In these patients, the normal significant correlation between mF AMP and M amplitudes may be disrupted, whereas the data suggest that increased mF AMP ratios in neuropathies are related to their duration. These results indicate that mF AMP/M ratios can be increased not only with increased central excitability but also in peripheral neuropathies. Although mechanisms may differ, the data are consistent with a similar physiological response to peripheral and central injury and emphasize that M amplitudes may be decreased with central lesions.

Periodicity in the Hoffmann reflex recovery curve

Hoffmann (H) reflex recovery curves were recorded from the soleus muscles of 10 healthy adult subjects with a view to further elucidating the pattern of facilitation of the motoneuron pool. Specific consideration was given to the possible existence of periodicity in facilitation of the motoneuron pool following a subthreshold conditioning H-reflex stimulus. The reliability of the recovery curves was also examined. The recovery curves, which utilized a 50% maximal test response, revealed the well established early facilitation (peak at 10 ms) followed by a later facilitation commencing 50 to 70 ms after the conditioning stimulus. In addition, there was evidence of peaks in excitability of the motoneuron pool occurring at 70 to 75 ms, 125 to 150 ms, and about 250 ms. The reliability of the periodicity in the recovery curve was established by examining the subjects on two separate occasions and investigating the effect of the number of measurements made within each day. Although there was considerable inter- and intrasubject variability in the form of the recovery curves, both within days, and between days, the overall group data showed a remarkably consistent periodicity for the first 350 ms following the conditioning stimulus. Methodological considerations that are important for observing the periodicity in the recovery curve were identified. These included an adequate number of measurements (test reflexes) per data point in the recovery curve, adequate sampling rate, and conditioning and test stimulus intensity. The clinical significance of these results resides in the possible contributions of the peripherally triggered periodic facilitation to rhythmic phenomena such as clonus and physiologic tremor.