Although neurolytic blocks are an important tool in the management of pain in patients with terminal cancer, certain neuralgias, and vascular occlusive disorders, only about 30% of cancer patients with intractable pain require neurolysis as a means to obtain effective analgesia (49). A diagnostic or prognostic block prior to neurolysis may be helpful to evaluate the patient and to help familiarize the patient with the possible side effects, but does not predict the exact outcome of neurolysis.
Mechanism of Action The peripheral nerve lacks lymphatic innervation, which can lead to increases in endoneural fluid pressure as a result of any toxic, metabolic, or traumatic insult. This initiates mast cell degen eration and the release of vasoactive substances, increasing the permeability of the blood-nerve barrier and subsequent accumulation of fluid in the endoneurial space. The elevated endoneurial pressure results in stretching of the perineurium and compression of perineurial vessels, leading to nerve fiber ischemia. The entire process peaks in 6-7 d and reverts to normal in about 30 d.
Although a wide variety of compounds can produce neurolysis, only absolute alcohol and phenol remain useful in clinical practice. Increasing efforts at neurolysis are being performed by mechanical means such at cryoanal-gesia and radiofrequency ablation. Absolute alcohol, in a commercially available concentration of >95%, remains the mainstay of chemical neurolysis. Alcohol is also a local irritant and can cause considerable pain during injection, which can be limited with a preceding injection of a local anesthetic. Alcohol extracts cholesterol, phospholipids, and cerebrosides from nerve tissue, thereby causing precipitation of lipoproteins and mucoproteins (50). The topical application of alcohol to peripheral nerves produces changes characteristic of wallerian degeneration. When injected near the sympathetic chain, alcohol destroys the ganglion cells and thus blocks all postgangli-onic fibers to all effector organs (51).
Phenol has the unique property of acting as a local anesthetic at lower concentrations and as a neurolytic agent at higher concentrations. Thus, a potential advantage is that phenol produces minimal pain on injection. Phenol causes nonselective neurolysis by denaturing proteins of axons and perineural blood vessels (52). The degeneration process characteristically occurs in about 14 d, and regeneration is completed in about 14 wk after injection
Administration Neurolytic blocks with alcohol are commonly used for cranial neuralgias with blocks of the trigeminal and glossopharyngeal nerves, epidural and intrathecal interruption of neuraxial transmission, lumbar sympathetic block, and celiac plexus neurolysis. There is a 95% dehydrated absolute alcohol solution commercially available for medical use. The volume of injectate is often small; thereby, none of the effects of ingested ethanol are seen. Extreme caution is taken at the time of injection to avoid local tissue necrosis and cellulitis. After the therapeutic injection, the needle should be first flushed with remaining local anesthetic or saline prior to removal to avoid the application of residual alcohol along the needle tract.
Phenol is less commonly used in injection procedures as it is not commercially available in an injectable form. A phenol solution can be prepared by the hospital pharmacy. Phenol is a potent neurolytic in its aqueous form at concentrations usually of 6-8%. Phenol in glycerine diffuses out very slowly. Compared to alcohol, phenol pro duces a shorter lived and less intense blockade. The typical duration of pain relief lasts for 2-6 mo in patients with chronic pain.
Adverse Effects Despite adequate and effective analgesia, many patients experience painful, annoying, and psychologically distressing neuralgias following alcohol neurolysis. The neuralgia is most commonly a dull to severe pain, which can occasionally result in a burning sensation or even a sharp, shooting pain sensation. Recovery from the pain can occur as soon as a few weeks or may take many months to resolve. The incidence of this complication is higher with a thoracic paraverte-bral sympatholytic injection than in a lumbar injection, possibly owing to the closer proximity of the somatic fibers of the sympathetic chain in the thoracic region. The dermatomal distribution of hypesthesia or anesthesia of the nerve roots treated is a rare but distressing complication. Fortunately, the recovery from this symptom is usually quick. Lumbar or sacral neurolysis can result in loss of bowel or bladder sphincter tone and thus bowel or urinary incontinence. Celiac plexus neurolysis can result in increased gastrointestinal peristalsis, which may lead to the development of diarrhea. Lumbar sympathetic neu-rolysis has been associated with the development of severe groin pain secondary to genitofemoral neuralgia. The referred pain is related to degeneration of the L2 nerve root, which gives rise to the genitofemoral nerve (53). A rare complication is paraplegia, which can occur if the alcohol injection results in significant vasospasm in the artery of Adamkiewicz.
Phenol has demonstrated a high affinity for vascular tissues, generating concerns regarding its use in celiac plexus neurolysis, where major vessels are in close proximity to the injection site. Neuritis is uncommon with phenol injection. In patients with intractable pain, both alcohol and phenol have equivalent analgesic efficacy
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