[Somatosensory evoked potentials and Hoffmann reflex in acute spinal cord lesions; physiopathological and prognostic aspects]

Functional plasticity following spinal cord lesions

Spinal cord injury results in marked modification and reorganization of several reflex pathways caudal to the injury. The sudden loss or disruption of descending input engenders substantial changes at the level of primary afferents, interneurons, and motoneurons thus dramatically influencing sensorimotor interactions in the spinal cord. As a general rule reflexes are initially depressed following spinal cord injury due to severe reductions in motoneuron excitability but recover and in some instances become exaggerated. It is thought that modified inhibitory connections and/or altered transmission in some of these reflex pathways after spinal injury as well as the recovery and enhancement of membrane properties in motoneurons underlie several symptoms such as spasticity and may explain some characteristics of spinal locomotion observed in spinally transected animals. Indeed, after partial or complete spinal lesions at the last thoracic vertebra cats recover locomotion when the hindlimbs are placed on a treadmill. Although some deficits in spinal locomotion are related to lesion of specific descending motor pathways, other characteristics can also be explained by changes in the excitability of reflex pathways mentioned above. Consequently it may be the case that to reestablish a stable walking pattern that modified afferent inflow to the spinal cord incurred after injury must be normalized to enable a more normal re-expression of locomotor rhythm generating networks. Indeed, recent evidence demonstrates that step training, which has extensively been shown to facilitate and ameliorate locomotor recovery in spinal animals, directly influences transmission in simple reflex pathways after complete spinal lesions.

Spinal shock revisited: a four-phase model

Spinal shock has been of interest to clinicians for over two centuries. Advances in our understanding of both the neurophysiology of the spinal cord and neuroplasticity following spinal cord injury have provided us with additional insight into the phenomena of spinal shock. In this review, we provide a historical background followed by a description of a novel four-phase model for understanding and describing spinal shock. Clinical implications of the model are discussed as well.

Transient spinal inhibition induced by electrical stimulation of the rat brain

Intracortical electrical stimulation of the rat brain using single pulse induced motor evoked potentials (MEPs) with shorter onset latencies in the bilateral extremity muscles. The MEPs appeared in the stimulation of cortical areas outside the motor cortex (MI) and subcortical areas. Train-pulse stimulation of the MI at a stimulus intensity just above the threshold induced MEPs with longer onset latency in muscles corresponding to the somatotopy of the MI stimulated. This implies that the potential characterizing shorter onset latency is equal to responses induced via the extrapyramidal tract, and responses with longer onset latency originate in the pyramidal tract. Corresponding to MEPs, we recorded two types of spinal potentials (SPs) via extrapyramidal and pyramidal tracts. In paired-pulse stimulation of the extrapyramidal tract, second MEPs showed a long-lasting inhibition up to 3 s after the first MEPs, while the changes in second SPs were not remarkable. Extrapyramidal tract stimulation inhibited H-reflex in the same manner as MEPs. These results suggest that the electrical stimulation of rat brain has a long-lasting effect in inhibiting lower motoneuron excitabilities. Our method may be a useful experimental model to induce the transient inhibition of spinal motoneuron excitabilities caused by supraspinal structures.

Incomplete spinal cord injury: neuronal mechanisms of motor recovery and hyperreflexia

OBJECTIVE

To understand neuronal mechanisms of motor recovery and hyperreflexia after incomplete spinal cord injury (SCI), and their role in rehabilitation.

DESIGN

Reviewed and compared clinical, neurophysiologic, and neuropathologic data from human SCI patients with behavioral, neurophysiologic, and neuroanatomic data from animals to postulate underlying neuronal mechanisms.

OUTCOME

A postulation that two neuronal mechanisms-receptor up-regulation and synapse growth-act sequentially, to explain the gradual appearance of motor recovery after incomplete SCI. These same mechanisms may also act in spinal reflex pathways to mediate hyperreflexia caudal to SCI.

RESULTS

After incomplete SCI, walking ability and hyperreflexia often develop. Initially, cord neurons are hyperpolarized and less excitable because of loss of normal descending facilitation; this is spinal shock. Then, gradually, voluntary movement recovers and hyperreflexia develops. Early (hours to days), these changes develop simultaneously, suggesting a common postsynaptic mechanism-likely, an increase in postsynaptic receptor excitability, possibly receptor up-regulation. Late (weeks to months), recovery and reflex changes occur at a slow rate, are no longer simultaneous, and are long-lasting, which suggests a presynaptic mechanism, such as local synapse growth in spared descending pathways and in reflex pathways. This presumed synapse growth is seemingly enhanced by active use of the growing pathway. Also, developing hyperreflexia appears to limit motor recovery.

CONCLUSIONS

These observations suggest that rehabilitation for incomplete SCI should (1) increase activity in spared descending motor pathways, (2) initially use reflex facilitation or central nervous system stimulants to assist spared descending inputs in depolarizing cord neurons, and (3) later minimize reflex input, when spared descending inputs can depolarize cord neurons without reflex facilitation. Better understanding of neuronal mechanisms that underlie motor recovery after incomplete SCI promises better outcomes from rehabilitation.

Behavior of the H-reflex in humans following mechanical perturbation or injury to rostral spinal cord

In humans H-reflexes are suppressed during early spinal shock. In animals rostral cord injury results in loss of segmental reflexes within seconds. If H-reflexes persist under general anesthesia, can they be used to monitor the integrity of the rostral cord? In part I of this study, we recorded H-reflexes intraoperatively in 25 patients to elucidate general anesthesia effect. In 23 subjects, H-reflexes were consistently elicited, and within +/- 13% of the normalized group mean amplitude. In part II, we recorded H-reflexes in 31 patients during spinal cord surgery to elucidate H-reflex behavior immediately following rostral spinal cord injury. In 6, abrupt suppression of the H-reflex coincided with cord injury. In 4 of 6, suppression was transient and less than 50% of baseline; none of these patients developed neurological deficits. In 2, suppression exceeded 90% and persisted throughout surgery; both patients developed profound deficits. We conclude that (1) the H-reflex can be consistently elicited under general anesthesia in most patients, (2) rostral cord injury rapidly suppresses the H-reflex, and (3) the degree and duration of H-reflex suppression reflects the severity of the injury.

Changes in spinal reflex excitability in brain-dead humans

The excitability of proprio- and exteroceptive spinal reflexes was monitored electrophysiologically and clinically during the occurrence of brain death (BD) in 8 patients. After a period of total reflex unresponsiveness, the soleus H reflex attained a steady-state excitability level in 2-6 h. The recovery cycle of this response regained its normal shape at 10-20 h. The threshold of the cutaneous reflex evoked in the biceps femoris by electrical stimulation of the sural nerve had become normal in 4-13 h, although the response displayed an abnormal multi-component pattern. Digital responses to mechanical stimulation of the foot sole were evident after 6-8 h. Knee and ankle jerks were never evoked during the time of monitoring. The time-courses of the changes in excitability were not directly correlated with the fall in the blood pressure which may occur during BD. It is concluded that the human spinal cord reacts to BD with a spinal shock, characterized by sequential recovery of reflex transmission. The overall timing of this process appears to be much shorter than that previously described for the spinal shock following traumatic transection of the cord, but the latter was never studied in the earliest phases.

Cortical evoked potentials and somatosensory perception in chronic spinal cord injury patients

The correlation between somatosensory evoked potentials (SEPs) and sensory perception was studied in 110 patients with traumatic chronic spinal cord lesions. Perception thresholds over the legs for light touch, vibratory sensibility, temperature and thermal pain were tested together with recordings of tibial and peroneal SEPs. Tibial nerve SEPs correlated better with sensory perception than peroneal nerve SEPs. Normal tibial nerve SEPs were not present with absent or trace vibratory sensibility and vice versa. However, we found many exceptions to the correlation between temperature and pain perception and SEPs. Light touch, vibratory sensibility, and SEPs were highly correlated between each other, while temperature and pain perception correlated poorly to these other modalities. This represents an evident segregation of touch perception, vibratory sensibility and SEPs, which are thought to share dorsal columns as a common ascending pathway, and temperature and pain perception known to be related to the spinothalamic system.

Somatosensory evoked potentials to posterior tibial nerve stimulation in children

The somatosensory evoked potentials (SEPs) to stimulation of the tibial nerve were studied in 88 children ranging in age from 1 day to 16 years. SEPs were not evidenced in 10 out of 44 infants less than 1 year old. In others it was a major positive wave (P) with a variable topographic distribution on the midline. The onset and peak latencies of this P were highly variable in different subjects of the same age or body-size, and in the same subject with the active electrode placed in different locations. The lowest values for latency were in subjects about 3 years old. The ascending time of P was the only parameter strictly correlated with age. The results are compared with SEPs to upper limb stimulation, which are constant and more reliable. These results indicate: that the maturation of the peripheral somatosensory pathway proceeds at a faster rate than that of the central somatosensory pathway; that the maturation of the somatosensory pathway of the upper limb precedes that of the lower limb; and that the ascending time of P is a good index of thalamo-cortical maturation. The clinical utility of these SEPs in pediatrics is discussed.

Effect of epidural morphine on the Hoffman-reflex in man

Four healthy, young subjects undergoing knee surgery with epidural anaesthesia were given epidural morphine for the relief of post-surgical pain. The Hoffman (H)-reflex was tested before and after the injection of morphine in the epidural space. The reflex was chosen as a monitor of the function of monosynaptic pathways in the spinal cord. Analogue scales for the measurement of pain revealed a satisfactory decrease of nociception after morphine injection, while the H-reflex was not significantly affected. This study may confirm that monosynaptic reflexes are not affected by epidural administration of morphine and that they are probably not involved in pain transmission.