Local anesthesia

Local anesthesia is any technique to render part of the body insensitive to pain without affecting consciousness.

• Local anesthesia, in a strict sense, is anesthesiaof a small part of the body such as a tooth or an area of skin.
• Regional anesthesia is aimed at anesthetizing a larger part of the body such as a leg or arm.
• Conduction anesthesia is a comprehensive term which encompasses a great variety of local and regional anesthetic techniques.

Conduction anesthesia allows patients to undergo many surgical procedures without significant pain or distress. In many situations, such as cesarean section), conduction anesthesia is safer and therefore superior to general anesthesia. In other situations, either conduction or general anesthesia are suitable. Anaesthetistssometimes combine both techniques.

Conduction anesthesia is also used for relief of non-surgical pain, also to enable diagnosis of the cause of some chronic pain conditions.

The commonest form of conduction anaesthesia is probably local anaesthesia to enable dentalprocedures.

Techniques

To achieve conduction anesthesia a local anestheticis injected or applied to a body surface. The local anesthetic then diffuses into nerves where it inhibits the propagation of signals for pain, muscle contraction, regulation of blood circulation and other body functions. Relatively high drug doses or concentrations inhibit all qualities of sensation (pain, touch, temperature etc.) as well as muscle control. Lower doses or concentrations may selectively inhibit pain sensation with minimal effect on muscle power. Some techniques of pain therapy, such as walking epidurals for laborpain use this effect, termed differential block.

Anesthesia persists as long as there is a sufficient concentration of local anesthetic at the affected nerves. Sometimes a vasoconstrictor drugis added to decrease local blood flow, thereby slowing the transport of the local anesthetic away from the site of injection. Depending on the drug and technique, the anesthetic effect may persist from less than an hour to several hours. Placement of a catheterfor continuous infusion or repeated injection allows conduction anesthesia to last for days or weeks. This is typically done for purposes of pain therapy.

Local anesthetics can block almost every nerve between the peripheral nerve endings and the central nervous system. The most peripheral technique is topical anesthesia to the skin or other body surface. Small and large peripheral nerves can be anesthetized individually (peripheral nerve blocks) or in anatomic nerve bundles (plexus anesthesia). Spinal anesthesiaand epidural anesthesiaare applied near the spinal cord where the peripheral nervous system merges into the central nervous system.

Clinical techniques include:

• Surface anesthesia - application of local anesthetic spray, solution or cream to the skin or a mucous membrane. The effect short lasting and is limited to the area of contact.
• Infiltration anesthesia - injection of local anesthetic into the tissue to be anesthetized. Surface and infiltration anesthesia are collectively topical anesthesia.
• Field block - subcutaneous injection of a local anesthetic in an area bordering on the field to be anesthetized.
• Peripheral nerve blocks - injection of local anesthetic in the vicinity of a peripheral nerve to anesthetize that nerve´s area of innervation.
• Plexus anesthesia - injection of local anesthetic in the vicinity of a nerve plexus, often inside a tissue compartment that limits the diffusion of the drug away from the intended site of action. The anesthetic effect extends to the innervation areas of several or all nerves stemming from the plexus.
• Epidural anesthesia - a local anesthetic is injected into the epidural spacewhere it acts primarily on the spinal nerveroots. Depending on the site of injection and the volume injected, the anesthetized area varies from limited areas of the abdomen or chest to large regions of the body.
• Spinal anesthesia - a local anesthetic is injected into the cerebrospinal fluid, usually at the lumbar spine (in the lower back), where it acts on spinal nerveroots and part of the spinal cord. The resulting anesthesia usually extends from the legs to the abdomen or chest.
• Intravenous regional anesthesia (Bier's block) - blood circulation of a limb is interrupted using a tourniquet (a device similar to a blood pressure cuff), then a large volume of local anesthetic is injected into a peripheral vein. The drug fills the limb´s venous system and diffuses into tissues where peripheral nerves and nerve endings are anesthetized. The anesthetic effect is limited to the area that is excluded from blood circulation and resolves quickly once circulation is restored.
• Local anesthesia of body cavities (e.g. intrapleural anesthesia, intraarticular anesthesia)

Uses in surgery

Virtually every part of the body can be anesthetized using conduction anesthesia. However, only a limited number of techniques are in common clinical use. Sometimes conduction anesthesia is combined with general anesthesiaor sedationfor the patient´s comfort and ease of surgery. Typical operations performed under conduction anesthesia include:

• Eye surgery (surface anesthesia with topical eye anesthetics, retrobulbar block)
• ENT operations, head and neck surgery, dental surgery (infiltration anesthesia, field blocks, peripheral nerve blocks, plexus anesthesia)
• Shoulder and arm surgery (plexus anesthesia, intravenous regional anesthesia)
• Heart and lung surgery (epidural anesthesiacombined with general anesthesia)
• Abdominal surgery (epidural/spinal anesthesia, often combined with general anesthesia)
• Gynecological, obstetrical and urological operations (spinal/epidural anesthesia)
• Bone and joint surgery of the pelvis, hip and leg ((spinal/epidural anesthesia, peripheral nerve blocks, intravenous regional anesthesia)
• Surgery of skin and peripheral blood vessels (topical anesthesia, field blocks, peripheral nerve blocks, spinal/epidural anesthesia)

Uses in acute pain

Acute painmay occur due to trauma, surgery, infection, disruption of blood circulation or many other conditions in which there is tissue injury. In a medical setting it is usually desirable to alleviate pain when its warning function is no longer needed. Besides improving patient comfort, pain therapy can also reduce harmful physiological consequences of untreated pain.

Acute pain can often be managed using analgesics. However, conduction anesthesia may be preferable because of superior pain control and fewer side effects. For purposes of pain therapy, local anesthetic drugs are often given by repeated injection or continuous infusion through a catheter. Low doses of local anesthetic drugs can be sufficient so that muscle weakness does not occur and patients may be mobilized.

Some typical uses of conduction anesthesia for acute pain are:

• Labor pain (epidural anesthesia)
• Postoperative pain (peripheral nerve blocks, epidural anesthesia)
• Trauma (peripheral nerve blocks, intravenous regional anesthesia, epidural anesthesia)

Uses in chronic pain

Generally, a certain risk of local tissue damage is involved when using local anesthetics often and regularly during a longer period at a specific location of your body. For example, the chronic use of topical eye anestheticsin chronic eye pain would almost certainly and relatively quickly lead to serious eye damage (corneal damage). Before using local anesthetics, chronic local pain should not only carefully be assessed with medical specialists for the local problem (for example an ophtalmologist, dermatologist, dentist or a neurologist/specialist for peripheral nerve blocks), but also by specialists for centrallyactive pain medication.

• Medical specialists (ophtalmologists, dermatologists, dentists or neurologists for peripheral nerve blocks) for the local problem can assess how big the risk of chronically using local anesthetics at that particular location is.
• In most cases of chronic pain, it is not the peripheral nervous systembut the central nervous systemthat is not able to cope with the pain impulses coming from the location in the peripheral nervous system where the damage that causes the pain is actually located. Therefore, medical specialists for the central nervous system (usually a neurologist or pain specialist) can assess if and what kind of centrally acting medication can or should be taken. Very often, it is less harmful and risky to use centrally active substances (especially when they are only taken in low doses) against chronic pain in the peripheral nerve system than chronically using local anesthetics at a particular location of the body. Relatively risk free, especially when taken in small doses, and at the same time often effective against chronic peripheral pain are antidepressantsof the SSRIclass and the stronger tricyclic antidepressantslike for example amitriptyline. Furthermore, modern anticonvulsantslike pregabalinor gabapentinare relatively risk free and at the same time relatively effective against chronic pain, at least when taken in very low or low doses. One of the strongest and more risky options in the reduction of peripheral pain are anticonvulsants like carbamazepine. In many cases patients make use of opioidmedication to manage their chronic pain and improve their quality of life. The consensus amongst medical professionals, the World Health Organisation and the UN is that the benefits of opioidsfar outway any potential risks if prescribed by a responsible physician and/or pain specialist. There are some who consider the use of opioidsfor chronic pain is risky, in particular that it can lead to dependence and addiction. Most sufferers of chronic pain (and their carers) consider this to be a minor issue compared to the economic and social costs of chronic pain however.

Miscellaneous uses

Topical anesthesia, in the form of EMLAcream is most commonly used to enable relatively painless venipuncture(bloodcollection) and placement of intravenous cannulae(or 'drip') in children.

It may also be suitable for other kinds of punctures such as ascitesdrainage and amniocentesis.

Surface anesthesia also facilitates some endoscopicprocedures such as bronchoscopy(visualization of the lower airways) or cystoscopy(visualization of the inner surface of the bladder).

History

The leaves of the coca plantwere traditionally used as a stimulant in Peru. It is believed that the local anesthetic effect of coca was also known and used for medical purposes. Cocainewas isolated in 1860 and first used as a local anesthetic in 1884. The search for a less toxic and less addictive substitute led to the development of the aminoester local anesthetic procainein 1904. Since then, several synthetic local anesthetic drugs have been developed and put into clinical use, notably lidocainein 1943, bupivacainein 1957 and prilocainein 1959.

Shortly after the first use of cocaine for topical anesthesia, blocks on peripheral nerves were described. Brachial plexus anesthesia by percutaneous injection through axillary and supraclavicular approaches was developed in the early 20th century. The search for the most effective and least traumatic approach for plexus anesthesia and peripheral nerve blocks continues to this day. In recent decades, continuous regional anesthesia using catheters and automatic pumps has evolved as a method of pain therapy.

Intravenous regional anesthesia was first described by August Bierin 1908. This technique is still in use and is remarkably safe when drugs of low systemic toxicity such as prilocaine are used.

Spinal anesthesia was first used in 1885 but not introduced into clinical practice until 1899, when August Bier subjected himself to a clinical experiment in which he observed the anesthetic effect, but also the typical side effect of postpunctural headache. Within few years, spinal anesthesia became widely used for surgical anesthesia and was accepted as a safe and effective technique. Although atraumatic (non-cutting-tip) cannulas and modern drugs are used today, the technique has otherwise changed very little over many decades.

Epidural anesthesia by a caudal approach had been known in the early 20th century, but a well-defined technique using lumbar injection was not developed until the 1930s. With the advent of thin flexible catheters, continuous infusion and repeated injections have become possible, making epidural anesthesia a highly successful technique to this day. Beside its many uses for surgery, epidural anesthesia is particularly popular in obstetrics for the treatment of labor pain.

External links

• New York School of Regional Anesthesia
• General information and tutorials in peripheral regional anesthesia

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It uses material from the http://en.wikipedia.org/wiki/Local+anesthesia Wikipedia article Local anesthesia.