Temporal profile of the effects of regional anesthesia on the cutaneous reflexes of foot muscles

Evaluation of the Cutaneous Silent Period in a Healthy Brazilian Population

PURPOSE

To establish cutaneous silent period (CSP) normative values and investigate the variables that may influence them.

METHODS

We tested 41 healthy subjects. All subjects underwent nerve conduction studies, and we evaluated the CSP in both arms.

RESULTS

Four subjects did not have CSP and were excluded. The analyses were performed in the healthy group composed of 23 women and 14 men, with a mean age of 35 (range, 19-64) years. The CSP median duration was 23.2 milliseconds (ms), with 2 to 98th percentile at 11.3 and 48.7 ms. The median onset latency was 87.9 (range, 72.9-109) ms, and the median end latency was 112 (range, 93.8-138) ms. The CSP onset latency positively correlated with height, whereas CSP end latency and duration were weakly but significantly associated with age. Some measurements of ulnar nerve conduction study also correlated with CSP measures. The interrater coefficients for the primary measures of onset and end latency demonstrates the reproducibility of the method.

CONCLUSIONS

The CSP with the fifth digit stimulation and recording from the abductor digiti minimi muscle is a valid diagnostic tool that can be used in clinical practice.

Modulation of the excitatory phase following the cutaneous silent period by vibration

INTRODUCTION

The post-inhibition excitatory phase (E3) of the cutaneous silent period (CSP) is attributed to the resynchronization of motoneuron activity following the inhibitory period but there is also evidence that a somatosensory startle reflex may contribute to this phase. We hypothesized that the startle reflex component contained in E3 will decrease during vibration.

METHODS

Sixteen healthy individuals were included in the study. CSP was recorded from slightly contracted right thenar muscles after painful index finger stimulation, before, during, and immediately after vibration. The values of the percentage change of E3 relative to pre-stimulus baseline (E3%) were compared before, during, and after vibration for each individual.

RESULTS

There was a reduction in E3% during vibration and the values returned to normal immediately after vibration (153.1 ± 43.5%, 115.2 ± 30.2%, 154.9 ± 68.2%, respectively; p = 0.030).

DISCUSSION

E3 is reduced during vibration in healthy individuals, presumably due to suppression of a reflex component, which is superimposed upon the known resynchronization of motoneurons.

Exteroceptive suppression of voluntary activity in thenar muscles by cutaneous stimulation: How many trials should be averaged?

OBJECTIVE

To determine a minimum number of trials that preserve input-output (I-O) properties of duration and magnitude of exteroceptive EMG suppression (eEMGs).

PATIENTS AND METHODS

eEMGs was recorded in 16 healthy subjects from thenar muscles following index finger stimulation at 2.5, 5, 10, and 20 times sensory threshold (xST). Individual trials were rectified and incrementally averaged in blocks of 5, 10, 20, 30, 40, 50, and 60. To determine if the block size affects I-O properties, the goodness of curve fit parameter R² for each block was compared to R² of the global function across all blocks combined.

RESULTS

eEMGs was found in all subjects at 10xST and 20xST (100%, respectively) but less often at 5xST (63-75%) and 2.5xST (25-56%). A quadratic function best described both duration and magnitude of eEMGs. The quadratic R² did not significantly differ between any individual block function (5-60) and the global function (eEMGs duration 0.647-0.704 vs 0.679; magnitude 0.525-0.602 vs 0.560, respectively).

CONCLUSIONS

Averaging 5 trials consistently shows eEMGs at and above 10xST. I-O properties of eEMGs do not differ whether 5 or up to 60 trials are averaged. Clinical studies of eEMGs in thenar muscles are possible with as few as 5 trials averaged.

Effects of vibration on cutaneous silent period

Suppression of an ongoing muscle contraction following noxious digital stimulation is called cutaneous silent period (CSP) which is under the influence of several physiological factors. In this study, we aimed to evaluate the influence of group Ia afferents on the cutaneous silent period (CSP) by applying 2-min vibration. CSP was obtained from abductor pollicis brevis muscle after stimulating index finger. The recordings were repeated three times-before, during and after vibration-which was applied over the tendon of flexor carpi radialis muscle. Onset latency, duration and magnitude of total CSP, inhibitory phases I1 and I2, and of the long-loop reflex were measured and compared. Suppression indices of CSP, I1 and I2 increased significantly during and after vibration, indicating significantly less exteroceptive EMG suppression outlasting the time of vibration. Vibration also caused mild shortening of I2 end latency (p = 0.048) and I2 duration (p = 0.019). Our findings indicate that vibration exerts a powerful influence on CSPs and causes reduction in the magnitude of exteroceptive EMG suppression during and after vibration. Although vibration is known to activate Ia afferents, we cannot exclude contribution of other afferents, e.g. mechanoreceptors, as well as pre- or postsynaptic inhibitory effects on ensuing interneurons, or enhanced vibration-related excitatory influence.

Effect of clonidine on the cutaneous silent period during spinal anesthesia

AIM

To investigate the effect of clonidine on the cutaneous silent period (CSP) during spinal anesthesia.

METHODS

A total of 67 adult patients were included in this randomized, prospective, single-center, double-blind trial. They did not have neurological disorders and were scheduled for inguinal hernia repair surgery. This trial was registered on ClinicalTrials.gov (NTC03121261). The patients were randomized into two groups with regards to the intrathecally administered solution: (1) 15 mg of 0.5% levobupivacaine with 50 µg of 0.015% clonidine, or (2) 15 mg of 0.5% levobupivacaine alone. There were 34 patients in the levobupivacaine-clonidine (LC) group and 33 patients in the levobupivacaine (L) group. CSP and its latency were measured four times: prior to the subarachnoid block (SAB), after motor block regression to the 0 level of the Bromage scale, with ongoing sensory blockade, and both 6 and 24 h after SAB.

RESULTS

Only data from 30 patients in each group were analyzed. There were no significant differences between the groups investigated preoperatively and after 24 h. The CSP of the L group at the time point when the Bromage scale was 0 was 44.8 ± 8.1 ms, while in the LC group it measured 40.2 ± 3.8 ms (P = 0.007). The latency in the L group at the time point when the Bromage scale was 0 was 130.3 ± 10.2 ms, and in the LC group it was 144.7 ± 8.3 ms (P < 0.001). The CSP of the L group after 6 h was 59.6 ± 9.8 ms, while in the LC group it was 44.5 ± 5.0 ms (P < 0.001). The latency in the L group after 6 h was 110.4 ± 10.6 ms, while in LC group it was 132.3 ± 9.7 ms (P < 0.001).

CONCLUSION

Intrathecal addition of clonidine to levobupivacaine for SAB in comparison with levobupivacaine alone results in a diminished inhibitory tonus and shortened CSP.