Nerve blocks.

Nerve blocks produce analgesia by interrupting the nervous transmission of pain signals either by temporary inhibition of conduction or by destruction of the nerve. Nerve blocks may be used alone or with analgesics in the management of acute or chronic pain associated with a well-defined anatomical site, especially when the pain is unresponsive to or not adequately controlled by conventional therapy. The route of administration and method employed depend on the site to be blocked but may include peripheral nerve block, autonomic nerve blocks such as sympathetic nerve blocks and coeliac plexus block, and central nerve blocks such as epidural (including caudal) and spinal block. Local anaesthetics are used when a temporary effect is required. Neurolytics such as phenol or alcohol or freezing of the nerve (cryoanalgesia) produce more prolonged block, but even so the effects may last no more than a few months, and the variable and non-selective neural damage produced correlates poorly with pain relief; some consider the risk of complications to outweigh the benefits obtained.

The use of nerve blocks in the management of cancer has declined following the refinement of the use of conventional analgesics. Some consider that their value may be limited to patients with a life expectancy of 3 months or less and that the main benefit of nerve blocks in cancer is to produce maximum pain relief rapidly. However, others consider that chemical and thermal neurolysis can provide long-term control of severe cancer pain without a substantial incidence of adverse effects. Neurolytic blocks may be of particular value in cancer pain syndromes involving the viscera or the torso, but are rarely applicable in the management of extremity pain. Neuropathic pain is rarely helped by somatic neural block and may even be aggravated, but block of the splanchnic nerves or coeliac plexus with alcohol or phenol is reputed to be effective in relieving severe intractable pain caused by cancer of the pancreas, stomach, small intestine, gallbladder, or other abdominal viscera, especially when the cancer has not spread to the parietal peritoneum.

Similar neurolytic blocks preceded by a local anaesthetic have also been used in patients with severe intractable pain of chronic pancreatitis, postcholecystectomy syndrome, or other chronic abdominal visceral diseases unrelieved by medical or surgical therapy.

Central nerve blocks using local anaesthetics with or without opioids are used for the management of acute pain such as labour pain and postoperative pain including that in children; they are also sometimes used for cancer pain.

Sympathetic nerve blocks using repeated injections of local anaesthetics or neurolytics have been used for sympathetically maintained pain. Intravenous regional sympathetic block is an alternative when a single limb is involved; guanethidine is one of the drugs that has been used.

Injections of local anaesthetics with or without corticosteroids are often used for blocks of localised painful joints. Nerve blocks are also used to block localised painful trigger areas such as postoperative or post-traumatic neuroma formation and for focal muscle pain.