Judy McDanal

Relief of sciatic radicular pain by sciatic nerve block

Many diseases and injuries produce low back pain that radiates into the peripheral sensory distribution of a nerve root. It is commonly believed that root damage resulting from compression and/or inflammation leads to the generation of impulses i n the sensory fibers of the injured roots which are perceived as peripheral pain (1,2). In the radicular pain of sciatica, for example, ectopic impulses arising at the site of root injury are perceived as pain in the peripheral distribution of this nerve. We have seen, however, several cases of low back pain with sciatica i n which block of the sciatic nerve distal to the site of the lesion with a local anesthetic produced complete and often long-lasting relief of pain in the distribution of the sciatic nerve. We present here cases demonstrating this phenomenon. These cases are not selected according to the response to the block of the sciatic nerve; they represent our outpatient population as a whole.

Does antidromic activation of nociceptors play a role in sciatic radicular pain

We describe a case where transcutaneous electrical stimulation of the right sciatic nerve in a patient with right L5 radiculopathy reproduced the patients pathological pain in the leg. Following a right ankle block with 0.5% bupivacaine, the sciatic nerve stimulation induced pain in the thigh and the calf but not in the foot. Despite an increase in the magnitude of stimulation by 50% (compared with the stimulation before the block) the pain was not perceived below the level of blockade. We suggest that in this case the electrical stimulation generated impulses propagated antidromically into the leg and activated nociceptors in it. The bupivacaine blockade prevented antidromic propagation of impulses into the foot, therefore pain in this region was not perceived.

Sympathetic Blockade Increases Tactile Sensitivity

To determine whether tactile sensitivity of the normal skin is altered by suppression of sympathetic efferent activity, the effect of stellate ganglion block and epidural sympathetic block on touch threshold was studied. The study was performed on ten individuals with various chronic pain syndromes. Tactile sensitivity was measured in the normal skin area with the use of von Frey filaments and a two-alternative forced-choice procedure with a staircase presentation of touch stimuli. With stellate ganglion block, touch threshold decreased on the side of the block by 48.8 ± 8.% (P = 0.002) without any significant change in the threshold on the healthy, nonblocked side (P = 0.003 for the difference between the sides). With epidural sympathetic block, touch threshold decreased to the same extent on the diseased and healthy sides, which were both affected by the block (46.2 ± 11.4%, P = 0.027 and 47.7 ± 12.5%, P = 0.032, respectively). The results show that sympathetic blockade increases tactile sensitivity. They also suggest that sympathetic efferent activity modulates the function of tactile receptors. It is hypothesized that the sympathetic modulation makes tactile receptors less sensitive to touch, less specific, and probably more prone to code tactile stimuli in such a way that the brain recognizes this code as pain.

Blockade of Sciatic Nerve Branches Relieves Sciatic Radicular Pain

We thank Drs. Durrani, Ogin, and Vandam for responding to our recently published report (1). Their explanations for the phenomenon of sciatic radicular pain relief by sciatic nerve block are based on the proximity of the site of blockade to the piriformis muscle (incidental piriformis block in piriformis syndrome) or the injured root (retrograde block). However, we have found that blockade of the branches of the sciatic nerve at a significant distance from both the piriformis muscle and the injured root also produces relief of the pain in sciatica. Table 1 presents five cases of lumbar radiculopathy (confirmed by the EMC study) in which we examined the effect of blockade of the sciatic nerve branches. I n two patients, the tibial nerve was blocked at the ankle, and in three other patients, the tibial and common peroneal nerves were blocked in the popliteal fossa. With the tibial nerve block at the ankle, 7 rnl of 1% lidocaine solution was used, whereas 30 ml of 0.5% bupilracaine was administered for the tibial and common peroneal block in the popliteal fossa. Change in sensitivity to pinprick was used as an indicator of the blockade. The effect of the block on pain intensity was assessed with the use of the visual analog scale in the low back and in the distribution of the sciatic nerve at three different levels: thigh, calf, and foot. One would not expect the block (distal to the site of tlic root injury) to abolish the perception of pain rtwlt1tiK frorn ortlzodrornic traiisviissioii of ectopic impulses arisin<? in the injured root. Hozcvver, like it was with sciatic nerve ldock (1 ), bloclirrdr of tlie sciatic ncriw branches resulted in iornpletc~ p i n rc/ief Iielozii the site of injection. It is highly unlikely that the local anesthetic could produce the effect by reaching the injured root through proximal spread of the drug along the tibial (peroneal) and the sciatic nerves from the ankle level. A direct effect of the anesthetic on the piriformis muscle is also excluded. Not less interesting was the finding of pain relief above the level of blockade. In all three cases with the blockade at the knee level, the pain disappeared not only below the knee, but also in the thigh area. With blockade at the ankle level, in one case, there was a complete pain relief in the sciatic nerve distribution in the foot, the calf, and the thigh. In the other case, the pain was completely relieved in the foot (from 6 to 0), decreased in intensity in the calf (from 6 to 2), and was not changed in the thigh. In all five cases there was no significant change in the low back pain. The observation that block-induced pain relief in one area results in pain relief in the adjacent nonblocked areas suggests central nervous system involvement in the mech-

Barbiturates inhibit stress-induced analgesia

The effect of pentobarbitone and thiopentone on stress-induced analgesia was studied in 40 male Sprague-Dawley rats. Antinociception was determined by measuring motor reaction threshold to the noxious pressure on the tail with the use of an “Analgesymeter.” Stress was induced by placement of a clamp on the hind paw. The stress procedure was found to cause an increase in reaction threshold, which was partially suppressed by naloxone 0.5 mg-kg~’. Pentobarbitone in a subanaesthetic dose of 25 mg-kg~’, SC, almost completely abolished the stress-induced increase in the reaction threshold (an increase in reaction threshold from 329 ± 33 g to 486 ±62 g in control group, and from 250 ± 26g to 273 ± 35 g in pentobarbitone group, p < 0.02 for the difference in the threshold changes). Thiopentone used in a dose of 25 mg·kg-1, IV, caused a loss of the righting reflex for 37 ± 10 minutes; stress procedure applied ten minutes after regaining the righting reflex did not cause any increase in the reaction threshold (with an increase in the reaction threshold in control group from 355 ± 50 g to 540 ±26 g, p < 0.001 for the difference between the groups). The results suggest that the barbiturates in subanaesthetic doses inhibit stress-induced analgesia. Thiopentone used in an anaesthetic dose has the potential for inhibition of stress-induced analgesia in the period of recovery from anaesthesia.RésuméĽeffet du pentobarbitone et thiopentone sur ľanalgésie induite par le stress est étudié chez 40 rats Sprague-Dawley. Ľantinociception fut déterminée par la mesure du seuil de la réaction motrice suite à une pression nocive appliquée sur la queue utilisant un “Analgesy-meter”. Le stress était induit en plaçant une pince sur la patte arrière. La man œuvre stressante occasionna une augmentation du seuil de réaction qui était partiellement supprimée par le naloxone 0.5 mg·kg-1 . Le pentobarbitone administré à des doses inférieures aux doses anesthésiantes (25mg·kg-1) en sous cutané a presque complètement aboli ľaugmentation du seuil de réaction suite au stress (ľaugmentation du seuil de réaction était de 329 ± 33 g à 486 ± 62 g pour le groupe contrôle, et de 250 ±26 g à 273 ± 35 g pour le groupe pentobarbitone, p < 0.02 quant à la différence des variations du seuil). Le thiopentone utilisé à des doses de 25mg·kg-1 par voie intraveineuse a provoqué une perte du “righting reflex” pour 37 ± 10 minutes. La man œuvre stressante appliquée dix minutes après le regain du “righting reflex” n’a pas provoqué une augmentation du seuil de réaction (ľaugmentation du seuil de réaction dans le groupe contrôle était de 355 ± 50g à 540 ± 26 g, p <0.001 concernant la différence entre les groupes). Ces résultats suggèrent que les barbituriques à des doses inférieures aux doses anesthésiantes inhibent ľanalgésie induite par le stress. Le thiopentone utilisé à des doses anesthésiques a le potentiel d’inhiber ľanalgésie induite par le stress en période de réveil de ľanesthésie.

Mechanism of pain caused by the nerve‐root tension test in patients with sciatica

We describe 2 instances where sciatic nerve block prevented pain in the sciatic nerve distribution caused by the nerve-root tension test in patients with radiculopathy. We hypothesize that antitoxic activation of nociceptors from the injured root is a mechanism underlying sciatica.