NERVE BLOCK

General information

Nerve block refers to the application of chemical agents to a nerve to impair, either temporarily or permanently, the conduction along that nerve. The agents most frequently used are phenol, alcohol, and local anesthetics. Chemical neurolysis is a nerve block that impairs conduction by means of destruction of a portion of the nerve. When these agents are applied to the treatment of spasticity, the intent is to interrupt the stretch reflex arc.

Chemical neurolysis has been performed at every level of the peripheral nervous system from the root to the motor end plate. The therapeutic result achieved and the potential side effects and complications depend to some extent on the anatomic site at which the nerve is injected.

Local neurolysis can be an extremely effective intervention for reducing spasticity. The results are immediate and the clinical descriptions of relief of spasticity have frequently been dramatic and explicit. Many authors have described the sustained elimination of clonuses beginning immediately after nerve block with phenol. Others have described increase in strength or speed of active voluntary movement in antagonists of muscles blocked, and occasionally in the blocked muscle itself. Nerve blocks have also diminished spasticity in opposite extremities, or in synergists not innervated by the nerve that was blocked. Passive range of motion has often been improved when spasticity was contributing to contracture formation, and a reduced need for passive stretching has been seen. In young, pre-ambulatory children, relief of adductor spasticity by chemical neurolysis has facilitated crawling, sitting and standing, and nerve blocks with phenol to the gluteus maximus have improved sitting or allowed the sitting position for the fist time. Improvements in gait have been seen due to a reduction of hip adductor tone, with a resultant decrease in scissoring.

Nerve blocks targeted at spastic ankle plantar flexors have contributed to improved dorsiflexion during gait, and at time allowed the use of orthotics that could not be previously applied. Others have found that nerve block have facilitated proper fitting of an orthotics device. Painful clawing of the toes been relieved by tibial nerve blocks, painful spasms have been relieved, and spasms have been reduced in patients with spinal cord injury. Reduction in hip flexor spasticity by means of chemical neurolysis has been reported to reduce the compensatory lumbar lordosis. Some authors have reported improvements in activities of daily living (ADLs) following chemical neurolysis. Chemical neurolysis has facilitated nursing care, hygiene, positioning and prevention of decubitus ulcers. Perineal nerve block with phenol resulted in a significant reduction of post void residual urine volumes in a man with a spastic external urethral sphincter following spinal cord injury. Several authors have performed selective sacral rhizotomies with phenol or alcohol to reduce detrusor hyperreflexia or external urethral sphincter spasticity.

Although nerve blocks can, in some instances, completely eliminate hyper tonicity, they can also be titrated to a certain extent by adjusting the concentration and quantity of the neurolytic agent injected, by choosing the site along the nerve tree to be injected(and thus the size of the motor branch), and by choosing the number of branch to be injected.

The effect on spasticity can be localized to the specific muscle or group of muscles that are causing a problem. On the other hand, because the result is not generalized, many blocks may have to be performed to affect a widespread problem with spasticity, and some muscles may be inaccessible or difficult to block.

Optimal effect in duration of a nerve block

The duration of the effect is extremely variable. Although, the effect is extremely variable. Although the effect of most neurolytic procedures lasts from several months to a few years, it is usually not permanent. The usual duration is 6-9 months in general. This is in some ways an advantage, because the patient and family, as well as the physician performing the block and the treating team, can be reassured to some degree that if adverse effects result from the block, they are not likely to be permanent .As the duration is limited, this is considered as one of the disadvantages of nerve block. Nerve block can be repeated 3-5 times on the same site.

Nerve blocks can be performed at bedside, or in a clinic or office, and general anesthesia is rarely required.

Mechanism

Nerve blocks are usually performed using a 22-gauge sterile needle that is Teflon-coated except at the bevel, so that a pulsed electrical stimulus can be transmitted to the nerve only when it is in the proximity of the bevel itself. The hub of the needle is connected to a stimulator that delivers a square wave pulse of 0.1 m sec duration once or twice per second, and which contains a rheostat to regulate the current, and, ideally, an ammeter as well. A reference electrode is attached to the surface of the skin. An assistant is often necessary to aid in positioning the patient. The needle is injected, and pulsed stimulation delivered. When a rhythmic response is observed the physician checks to be sure that the appropriate muscle or group of muscles is contracting. Some practitioners use electromyography recording equipment to assist in determining that only the target muscle is contracting. The amperage is than reduced, and with the muscle still contracting, but less vigorously so a search is made by moving the needle. If the injection successful, the contraction will disappear almost immediately or diminish gradually within 1or 2 minutes. At time this response is preceded by an initial increase in the force of contraction as the solution makes contact with the nerve. The use of flexible extension tubing to connect the syringe to the needle helps prevent unwanted movement during injection. The patient can be expected to feel some discomfort during the search; individual tolerance varies greatly. The patient usually feels a burning sensation when phenol or alcohol is injected.

Different agents used for nerve blocks

  • Phenol
  • Alcohol
  • Local anesthetic agents

Phenol nerve block

Phenol (carbolic acid) is a derivative of benzene with one hydroxyl group. It is a colorless crystal that is soluble in water at room temperature at concentration below 6.7%. At higher concentration, it must be warmed to remain soluble. It is also soluble in glycerin and in a number of oils.

In concentrations of 5% or more in water, phenol denatures protein, causing tissue necrosis, and it is this property that makes it valuable as a neurolytic agent. Phenol also has local anesthetic properties, even at concentrations of 1% to 2%, and has been used as a topical local anesthetic agent. The local anesthetic effect is probably responsible for the transient anesthesia and weakness that are commonly seen after nerve blocks.

Phenol is rapidly absorbed through the skin, and in over dosage or high concentration, it causes convulsions, central nervous system depression, and cardiovascular collapse. Chronic exposure to phenol causes rash, gastrointestinal symptoms, and renal toxicity with smoky urine.

Different sites of injection

Mixed sensorimotor peripheral nerve blocks.

Surface stimulation can be used before needle stimulation, if desired, to facilitate localization of the nerve. Fascicles containing fibers innervating specific muscles can often be differentially stimulated with the needle electrode and phenol can be injected in the proximity of the particular fascicles so targeted.

Some physicians use EMG recording electrodes to ensure precise differential localization, although, this can usually be adequately assessed by visual inspection of the contracting limb. Most practitioners have used aqueous phenol in concentrations ranging from 2% to 7%. Although, some authors have found 3% aqueous phenol to be relatively ineffective or to give results of relatively short duration. The quantity injected should depend upon the concentration of the solution, the relative size of the nerve, whether the entire nerve or particular fascicles are targeted, and upon the thoroughness of block desired. Quantities of 5% aqueous phenol, for example, injected onto peripheral nerve, usually vary from one to 10ml. The effect of the block can be titrated either within a session or over multiple sessions by injecting smaller quantities, assessing the effectiveness, and re-injecting, if necessary. If done within a short period of the time, the possible compounding effect of the local anesthetic action of phenol upon assessment should be considered.

Mixed sensorimotor peripheral nerve blocks offer some potential advantages over motor nerve blocks. The major disadvantage of mixed sensorimotor nerve blocks with phenol is the potential for the development of pain in the distribution of the sensory component of the nerve.

Motor nerve blocks.

Largely motor nerve can be blocked at many levels of the peripheral nervous system, and such blocks have been given a variety of names (intramuscular neurolysis, motor point blocks, motor end-plate blocks), depending in part on the anatomic site at which the block takes place. Most frequently, motor nerves are approached within the particular muscle that is targeted.

Motor points can be located by surface stimulation before a needle is inserted. Motor points are usually thought to correspond to the site at which the nerve enters the muscle The phenol can be injected at any of these areas, even at the motor end-plate, which can be definitively identified by the characteristic pattern of EMG activity seen with an EMG needle recording electrode. Motor endplates cluster at characteristic areas within each muscle fiber.

The more peripheral in the nervous system one goes, the smaller the branches, and therefore the more sites that need to be injected for an effective block. The patient then has to suffer more needle sticks and searches, and the physician performing the block becomes more fatigued during a longer, more tedious procedure. The more proximally one moves along the nerve tree, the easier and more effective the procedure becomes.

Lumbosacral Paravertebral Nerve Block

Lumbar or sacral roots or elements of the Lumbosacral plexus can be blocked in the Para vertebral area. Either mixed sensorimotor or motor nerves can be isolated in this area.

The technique is similar to that used for other nerve blocks with phenol, with the addition of precautions related to the risk of accidental intrathecal injection by way of the root sleeves, which are extension of the subarachnoid space.

The major advantage of Lumbosacral Para vertebral blocks is to find out the exact sites for injection because anatomic specificity is not always completely possible as one move more proximally in the peripheral nervous system, although frequently one can satisfactorily isolate fascicles even at this level.

Neuropathic pain is, of course, a possibility when mixed sensorimotor nerves are blocked at the paravertebral level as well.

Possible complications of a nerve block

The potential complication of chemical neurolysis is an unfavorable reversal of the balance of muscle groups about a joint. This is most likely to occur in the context of severe spasticity that is causing or threatening contracture. The nerve(s) supplying the responsible muscle or group of muscles is /are blocked, but if the antagonists to those muscles are also severely spastic, a contracture may begin to develop in the opposite direction.

Another manifestation of this complication might be apparent weakness in the muscle or an increase in spasticity in the antagonist caused by loss of reciprocal inhibition. In either, case, the situation can be remedied by blocking the nerve supplying the antagonist muscle (s). This complication can usually be anticipated during the evaluation preceding the neurolytic procedure, and plans can be made for either careful titration of the effect of the block (over more than one session if necessary) or for blocking the antagonist (s) shortly after the first procedure.

Muscle or ligament sprains can occur after nerve blocks caused by loss of reflexive protection against overstretching the muscle in which spasticity has been relieved, changes in the dynamics of ambulation, or loss of reflexive support for other soft tissues. Therapists should anticipate possible difficulties in ambulation, and should be careful not to overstretch in the first days and weeks following nerve block. Peripheral edema is not uncommon after chemical neurolysis, particularly in the lower extremity. It can be treated with elevation of the extremity and compressive garments or elastic wrapping, and usually resolves within a week two.

Local infection at the site of injection is, of course, always a possibility. Both alcohol and phenol, however, are bacteriocidal at the concentration used for neurolysis, so this is a rare occurrence.

Over dosage with phenol causes convulsions, central nervous system depression, and cardiovascular collapse. The amount of phenol routinely used for nerve blocks, however is well below the lethal range (8.5 grams, or more).

Routine precautions, such as aspiration before injection makes intravascular injection unlikely. Neither over dosage nor accidental intravascular injection has been reported in association with phenol neurolysis.

Mixed nerve blocks with phenol carry additional risk of neuropathic pain, which is dysesthesias in the sensory distribution of the nerve. The pain usually has its onset from a few days to about 2 weeks after the procedure. It is generally experienced as a continuous burning paresthesia exacerbated by light tactile stimulation, so that even a draft or a sheet moving across the affected area can be uncomfortable or even intolerable. The pain is generally experienced in only a small portion of the sensory distribution of the nerve that was blocked.

The majority of patients with neuropathic pain following chemical neurolysis experience a mild to moderate burning sensation that lasts from several days to more than 3 months though several weeks is most typical. Because this complication is usually temporary, treatment is generally aimed at managing the pain until it disappears of its own accord.

Reassurance that the pain is likely to resolve within a few weeks will reduce the intensity of concern on the part of the patient. A uniformly applied compressive garment such as a sock, glove, or elastic wrap minimizes cutaneous stimulation and decreases edema if present. Trans-Cutaneous electrical nerve stimulation (TENS) can be very effective for reducing dysesthesias in some patients. If all else fails and the patient is finding the pain intolerable, re-blocking the nerve with phenol usually brings relief.

The only additional side effect involved with the blocking of motor nerve with phenol is that, if the block is performed at multiple sites within the muscle itself, local pain, swelling, and indurations may be present for a few days or occasionally longer. Tender nodules may appear in the muscle one to three weeks after the injection. The application of ice, acutely, and later heat, elevation of the extremity, and administration of analgesic medications can be helpful in instances in which pain, swelling or nodule formation is prominent.

Lumbosacral Para vertebral nerve blocks with phenol carry the additional risk of accidental intrathecal injection, which could cause cauda equina or even spinal cord injury.

Need of repetition of nerve block

Limited data are available on the effect of repeating nerve blocks with phenol after the recurrence of spasticity. Localization of motor branches by electrical stimulation was more difficult. There may be necrosis of muscle which leads to fibrosis, which protects against complete reinnervation. After several procedures, there is a cumulative effect that is clinically significant.

Alcohol nerve block / Alcohol wash

Ethyl alcohol (Ethanol), a potent neurolytic agent, has not been used as extensively as phenol for the treatment of spasticity. Ethanol has been used mostly for sympathectomy and for the treatment of pain.

Different clinical applications of a nerve block

The decision to perform a nerve block should be based on both a neuromuscular kinesiologic evaluation and an assessment of the functional or potential functional implications of the disordered muscle tone. The possibility of functional gains resulting from chemical neurolysis must be weighed against the potential for functional losses or other complications and side effects. For purposes of evaluation, functional considerations can be classified as involving active motor control or passive positioning. Contractures can play an important role in both.

Spasticity most commonly inhibits active motor control when hyperactive stretch reflexes in antagonist muscle resist the motion of agonists. When the agonists appear to have sufficient for functional purposes if freed from the hyperactive antagonists, nerve blocks are often useful.

For instance, if toe drag or toe-heel gait is resulting from spasticity of the plantar flexors of the ankle in the face of the ability to actively dorsiflexed the ankle, either in an isolated fashion or as part of a flexor synergy, blocking the posterior tibial nerve or its branches to the gastrocnemius and soleus can remedy the problem. The effect of the block on other joints should be considered, in particular the effect of the decreased extension moment on the stability of the knee. If increased inversion of the foot is also present, an attempt should be made to determine whether this is caused by increased tone in the tibialis anterior or in the tibialis posterior muscle. This can be difficult to determine by observation alone and poly ectromyography or diagnostic nerve blocks can be helpful in this situation. Sometime inversion during swing phase results from the inability of the tibialis anterior to "fight "spastic or contracted plantar flexors, so that all of its contractile force goes toward inversion rather than dorsiflexion. In such cases, nerve blocks affecting the plantar flexors or remediation of contractures will resolve the situation. If a spastic tibialis posterior is felt to be responsible for excessive inversion, the branch or fascicles of the posterior tibial nerve helps to support the arch of the foot, the possibility of pes planus resulting from such a block should be considered.

Although spasticity itself is the most likely condition to be affected by a nerve block, other motor disorders associated with the upper motor neuron syndrome, such as co contractions and primitive motor behaviors, can adversely affect function by overpowering an agonist with or without the presence of significant spasticity. If an increase in tone is present only during active motion or is associated with position but not stretch, nerve blocks may still be helpful, although not as consistently. Less frequently, spasticity masks the voluntary strength of the very muscle in which tone is increased. A nerve block then may result in increased strength in the muscle dennervated. Such a result is difficult, if not impossible, to predict, except by the use of a diagnostic block with a local anesthetic agent, for instance.

Although the result of a nerve block is likely to have a greater effect on function when active control is at stake, the reduction of passive posturing because of spasticity or primitive motor behaviors can also have significant functional implications. In the case of increased plantar flexion and inversion tone described above, a good result can be obtained even in some instances where no active dorsiflexion or eversion can be seen. Similarly, if hyper tonicity of the toe flexors is causing a painful grabbing during stance phase, blocking the branch of the posterior tibial nerve to the flexor digitorum muscles can be beneficial, whether active toe extension is present or not. The benefit of an orthosis can be improved by a nerve block that allows it to do its job more effective, for instance, when plantar flexion or inversion tone prevents proper fitting of an ankle-foot orthosis.

Proper passive positioning in a wheel-chair, bed or other device can result in greater patient comfort and can improve communication, use hygiene and skin care. Thus, obturator nerve blocks that alleviate excessive hip adduction can help produce more comfortable sitting or facilitate perineal hygiene. Similarly hygiene of the axilla, palm of the hand, or antecubital fossa can be improved by nerve blocks that relax the muscle that prevents adequate access. Powerful lower extremity extensor synergies that causes a patient to slide out of even well-constructed wheelchairs can be reduced by blocking the innervations of the primary muscle involved, usually the gluteus maximus, quadriceps femoris, and/or spinal extensors. Ankle clonuses that cause positioning problems during transfer, for instance, can be eliminated with blocks of the posterior tibial nerve or its branches. Opisthotonous can be reduced by blocking the innervations of spinal extensor muscles. Hyper tonicity in any accessible muscle or muscle group that results in discomfort or dysfunctional positioning can be considered for treatment with nerve blocks.

Contractures can be prevented with active motion or proper passive positioning and nerve blocks can be used to prevent contractures of potential functional significance when their development is associated with spasticity. If spasticity has already caused contracture, remediation of the contracture by means of range-of -motion exercises, serial casting, or other means can be greatly facilitated by nerve blocks. Recurrence of contracture can be prevented by prophylactic nerve blocks that allow range-of motion exercises to be performed more easily and less painfully, and that permit the proper fitting of bivalved casts or other orthoses. They can be useful for preventing the recurrence of contractures that have been surgically remedied as well.

The clinician considering chemical neurolysis as a treatment alternative must consider the time since onset of the upper motor neuron lesion and the prognosis for improvement. If a nerve block with phenol is performed prematurely, and a patient improves more dramatically than expected shortly after the block, potential function based on primitive motor behaviors may be lost, or strength of already weak muscles reduced. In addition, if the pattern of hyper tonicity changes, contracture or loss of function may develop in the direction opposite that initially anticipated. On the other hand, if a nerve block is avoided because of such risks, and the anticipated contractures develop, the consequences are also likely to be significant. Clinicians should make every attempt possible to prevent contractures if there is even a small possibility of a good outcome.

Alcohol wash represents an alternative approach to nerve blocks with phenol. Although the literature is limited, it indicates that the effect is likely to last only for several weeks after which longer-lasting blocks with phenol can be reconsidered or alcohol wash repeated if necessary. Even when the physician is confident that enough time has elapsed to warrant the use of phenol, the patient or family member may be convinced. In such situations, they may more readily accept an alcohol wash because of the shorter duration of effect, and having seen the benefits, consent to a nerve block with phenol. Local anesthetic blocks can be used to help prevent contractures early after an injury, though they may need to be repeated frequently, and this can be impractical. The benefit of a local anesthetic block can however, be stretched by placing the extremity in a cast that inhibits the return of hyper tonicity and prevents contracture formation. The limb must be carefully observed because the return of forceful spasticity within the cast could cause skin breakdown.

The usual indication for local anesthetic block, however, is, for diagnostic purposes. Local anesthetic can be used to help determine whether or not there is a component of spasticity at end range in the presence of severe contracture. They can be used to determine which muscle is contributing to a pathologic posturing, as in the case of inversion of the foot. They are probably most useful, however, in situations where it is uncertain if a nerve block will cause more harm than good particularly when significant function based on primitive motor behaviors or spasticity is at stake, or when blocks may affect more than one aspect of a function such as ambulation. The clinician evaluating such a situation should keep in mind, however, that the local anesthetic block, although frequently helpful, is not likely to reproduce exactly the effect of a nerve block with phenol. This is particularly true when mixed sensorimotor nerves are blocked with local anesthetics, in which case the resulting sensory loss may have significant functional consequences. Alcohol wash can also be used if several weeks may be needed to evaluate fully the potential effects of a block.

Chemical neurolysis must be considered in light of the other treatment alternatives, and when practical, more conservative approaches tried first. In some instances, spasticity is so powerful, or contracture developing so rapidly, that nerve blocks may of necessity be resorted to before other less invasive treatment is tried. The question of which of the two interventions is more conservative can be a matter of subjective opinion, and judgment must be exercised. For instance, although many clinicians consider medications more conservative than chemical neurolysis, in an elderly person or a practical neurolysis, in an elderly person or a patient with brain damage secondary to trauma, cerebrovascular accident, multiple sclerosis, or other causes, medications that impair cognition can cause significant time loss from a rehabilitation program even when the problem is recognized, and if it is unrecognized, they may permanently impede recovery.

Another important consideration is the possible need to repeat nerve blocks because of the frequently limited duration of their effect. When the time comes to consider repetition, this should not be done reflexively. The possibility that other approaches that were previously unsuccessful might now prove beneficial, or that medications that might previously have been contraindicated might now be acceptable The lack of availability of physicians who perform nerve blocks in the area to which the patient might be discharged may make nerve blocks a poor choice for treatment if it appears that they will need to be repeated. Frequent need for repetition of blocks may also make them less acceptable. In such situations, an attempt to find an appropriate systemic pharmacologic alternative should be pursued, and potentially longer- lasting procedures such as radiofrequency Rhizotomy, selective posterior rhizotomy,or musculotendinous releases with or without neurectomy may be indicated. Chemical neurolysis can be useful to facilitate the effectiveness of musculotendinous procedures performed in the presence of spasticity.

Different anatomic considerations for nerve block

  • OBTURATOR NERVE
  • SCIATIC NERVE
  • FEMORAL NERVE
  • POST TIBIAL
  • NERVE
  • PARAVERTEBRAL NERVES
  • THORQACODORSAL NERVE
  • MUSCULOCUTANEOUS NERVE
  • MEDIAN NERVE