Local anesthetic


A local anesthetic is a medication that causes absence of pain sensation. When it is used on specific nerve pathways, paralysis also can be achieved.
Clinical LAs belong to one of two classes: aminoamide and aminoester local anesthetics. Synthetic LAs are structurally related to cocaine. They differ from cocaine mainly in that they have a very low abuse potential and do not produce hypertension or vasoconstriction.
They are used in various techniques of local anesthesia such as:

Acute pain

may occur due to trauma, surgery, infection, disruption of blood circulation, or many other conditions in which tissue injury occurs. In a medical setting, pain alleviation is desired 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, LA drugs are often given by repeated injection or continuous infusion through a catheter. LA drugs are also often combined with other agents such as opioids for synergistic analgesic action. Low doses of LA 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:
is a complex and often serious condition that requires diagnosis and treatment by an expert in pain medicine. LAs can be applied repeatedly or continuously for prolonged periods to relieve chronic pain, usually in combination with medication such as opioids, NSAIDs, and anticonvulsants. Though it can be easily performed, repeated local anaesthetic blocks in chronic pain conditions are not recommended as there is no evidence of long-term benefits.

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 anesthesia or sedation for the patient's comfort and ease of surgery. However, many anaesthetists, surgeons, patients and nurses believe that it is safer to perform major surgeries under local anesthesia than general anesthesia. Typical operations performed under conduction anesthesia include:
Diagnostic tests such as bone marrow aspiration, lumbar puncture and aspiration of cysts or other structures are made to be less painful upon administration of local anesthetic before insertion of larger needles.

Other uses

Local anesthesia is also used during insertion of IV devices, such as pacemakers and implantable defibrillators, ports used for giving chemotherapy medications and hemodialysis access catheters.
Topical anesthesia, in the form of lidocaine/prilocaine is most commonly used to enable relatively painless venipuncture and placement of intravenous cannulae. It may also be suitable for other kinds of punctures such as ascites drainage and amniocentesis.
Surface anesthesia also facilitates some endoscopic procedures such as bronchoscopy or cystoscopy

Side effects

Localized side effects

Edema of tongue, pharynx and larynx may develop as a side effect of local anaesthesia. This could be caused by a variety of reasons including trauma during injection, infection, an allergic reaction, haematoma or injection of irritating solutions such as cold-sterilisation solutions. Usually there is tissue swelling at the point of injection. This is due to puncturing of the vein which allows the blood to flow into loose tissues in the surrounding area. Blanching of the tissues in the area where the local anaesthetic is deposited is also common. This gives the area a white appearance as the blood flow is prevented due to vasoconstriction of arteries in the area. The vasoconstriction stimulus gradually wears off and subsequently the tissue returns to normal in less than 2 hours.
The side effects of inferior alveolar nerve block include feeling tense, clenching of the fists and moaning.
The duration of soft tissue anaesthesia is longer than pulpal anaesthesia and is often associated with difficulty eating, drinking and speaking.

Risks

The risk of temporary or permanent nerve damage varies between different locations and types of nerve blocks.
There is risk of accidental damage to local blood vessels during injection of the local anaesthetic solution. This is referred to as Haematoma and could result in pain, trismus, swelling and/or discolouration of the region. The density of tissues surrounding the injured vessels is an important factor for Haematoma. There is greatest chance of this occurring in a posterior superior alveolar nerve block or in a pterygomandibular block.
Giving local anaesthesia to patients with liver disease can have significant consequences. Thorough evaluation of the disease should be carried out to assess potential risk to the patient as in significant liver dysfunction, the half-life of amide local anaesthetic agents may be drastically increased thus increasing the risk of overdose.
Local anaesthetics and vasoconstrictors may be administered to pregnant patients however it is very important to be extra cautious when giving a pregnant patient any type of drug. Lidocaine can be safely used but bupivacaine and mepivacaine should be avoided.  Consultation with the obstetrician is vital before administrating any type of local anaesthetic to a pregnant patient.

Recovery

Permanent nerve damage after a peripheral nerve block is rare. Symptoms are likely to resolve within a few weeks. The vast majority of those affected recover within four to six weeks; 99% of these people have recovered within a year. An estimated one in 5,000 to 30,000 nerve blocks results in some degree of permanent persistent nerve damage.
Symptoms may continue to improve for up to 18 months following injury.

Potential side effects

General systemic adverse effects are due to the pharmacological effects of the anesthetic agents used. The conduction of electric impulses follows a similar mechanism in peripheral nerves, the central nervous system, and the heart. The effects of local anesthetics are, therefore, not specific for the signal conduction in peripheral nerves. Side effects on the central nervous system and the heart may be severe and potentially fatal. However, toxicity usually occurs only at plasma levels which are rarely reached if proper anesthetic techniques are adhered to. High plasma levels might arise, for example, when doses intended for epidural or intrasupport tissue administration are accidentally delivered as intravascular injection.

Emotional reactions

When patients are emotionally affected in the form of nervousness or fear, it can lead to vasovagal collapse. This is the anticipation of pain during administration that activates the parasympathetic nervous system while inhibiting the orthosympathetic nervous system. What results is a dilation of arteries in muscles which can lead to a reduction in circulating blood volume inducing a temporary shortness of blood flow to the brain. Notable symptoms include restlessness, visibly looking pale, perspiration and possible the loss of consciousness. In severe cases, clonic cramps resembling an epileptic insult may occur.
On the other hand, fear of administration can also result in accelerated, shallow breathing, or hyperventilation. The patient may feel a tingling sensation in hands and feet or a sense of light-headedness and increased chest pressure.
Hence, it is crucial for the medical professional administrating the local anaesthesia, especially in the form of an injection, to ensure that the patient is in a comfortable setting and has any potential fears alleviated in order to avoid these possible complications.

Central nervous system

Depending on local tissue concentrations of local anesthetics, excitatory or depressant effects on the central nervous system may occur.
Initial symptoms of systemic toxicity include ringing in the ears, a metallic taste in the mouth, tingling or numbness of the mouth, dizziness and/or disorientation.
At higher concentrations, a relatively selective depression of inhibitory neurons results in cerebral excitation, which may lead to more advanced symptoms include motor twitching in the periphery followed by grand mal seizures. It is reported that seizures are more likely to occur when bupivacaine is used, particularly in combination with chloroprocaine.
A profound depression of brain functions may occur at even higher concentrations which may lead to coma, respiratory arrest, and death. Such tissue concentrations may be due to very high plasma levels after intravenous injection of a large dose.
Another possibility is direct exposure of the central nervous system through the cerebrospinal fluid, i.e., overdose in spinal anesthesia or accidental injection into the subarachnoid space in epidural anesthesia.

Cardiovascular system

Cardiac toxicity can result from improper injection of agent into a vessel. Even with proper administration, it is inevitable for some diffusion of agent into the body from the site of application due to unforeseeable anatomical idiosyncrasies of the patient. This may affect the nervous system or cause the agent to enter into general circulation. However, infections are very seldom transmitted.
Cardiac toxicity associated with overdose of intravascular injection of local anesthetic is characterized by hypotension, atrioventricular conduction delay, idioventricular rhythms, and eventual cardiovascular collapse. Although all local anesthetics potentially shorten the myocardial refractory period, bupivacaine blocks the cardiac sodium channels, thereby making it most likely to precipitate malignant arrhythmias. Even levobupivacaine and ropivacaine, developed to ameliorate cardiovascular side effects, still harbor the potential to disrupt cardiac function. Toxicity from anesthetic combinations is additive.

Endocrine

Endocrine and metabolic systems only have slightly adverse effects with most cases being without clinical repercussions.

Immunologic allergy

Adverse reactions to local anesthetics are not uncommon, but legitimate allergies are very rare. Allergic reactions to the esters is usually due to a sensitivity to their metabolite, para-aminobenzoic acid, and does not result in cross-allergy to amides. Therefore, amides can be used as alternatives in those patients. Nonallergic reactions may resemble allergy in their manifestations. In some cases, skin tests and provocative challenge may be necessary to establish a diagnosis of allergy. Also cases of allergy to paraben derivatives occur, which are often added as preservatives to local anesthetic solutions.

Methemoglobinemia

is a process where iron in hemoglobin is altered, reducing its oxygen-carrying capability, which produces cyanosis and symptoms of hypoxia. Exposure to aniline group chemicals such as benzocaine, lidocaine, and prilocaine can produce this effect, especially benzocaine. The systemic toxicity of prilocaine is comparatively low, but its metabolite, o-toluidine, is known to cause methemoglobinemia.

Second-generation effects

Application of local anesthetics during oocyte removal during in vitro fertilisation has been up to debate. Pharmacological concentrations of anesthetic agents have been found in follicular fluid. Clinical trials have not concluded any effects on pregnant women. However, there is some concern with the behavioral effects of lidocaine on offspring in rats.
During pregnancy, it is not common for local anesthetics to have any adverse effect on the fetus. Despite this, risks of toxicity may be higher in pregnancy due to an increase in unbound fraction of local anesthetic and physiological changes increase the transfer of local anesthetic into the central nervous system. Hence, it is recommended that pregnant women use a lower dose of local anesthetic to reduce any potential complications.

Treatment of overdose: "Lipid rescue"

This method of toxicity treatment was invented by Dr. Guy Weinberg in 1998, and was not widely used until after the first published successful rescue in 2006. Evidence indicates Intralipid, a commonly available intravenous lipid emulsion, can be effective in treating severe cardiotoxicity secondary to local anesthetic overdose, including human case reports of successful use in this way. However, the evidence at this point is still limited.
Though most reports to date have used Intralipid, a commonly available intravenous lipid emulsion, other emulsions, such as Liposyn and Medialipid, have also been shown effective.
Ample supporting animal evidence and human case reports show successful use in this way. In the UK, efforts have been made to publicise this use more widely and lipid rescue has now been officially promoted as a treatment by the Association of Anaesthetists of Great Britain and Ireland. One published case has been reported of successful treatment of refractory cardiac arrest in bupropion and lamotrigine overdose using lipid emulsion.
The design of a 'homemade' lipid rescue kit has been described.
Although lipid rescue mechanism of action is not completely understood, the added lipid in the blood stream may act as a sink, allowing for the removal of lipophilic toxins from affected tissues. This theory is compatible with two studies on lipid rescue for clomipramine toxicity in rabbits and with a clinical report on the use of lipid rescue in veterinary medicine to treat a puppy with moxidectin toxicosis.

Mechanism of action

All LAs are membrane-stabilizing drugs; they reversibly decrease the rate of depolarization and repolarization of excitable membranes. Though many other drugs also have membrane-stabilizing properties, not all are used as LAs.
LA drugs act mainly by inhibiting sodium influx through sodium-specific ion channels in the neuronal cell membrane, in particular the so-called voltage-gated sodium channels. When the influx of sodium is interrupted, an action potential cannot arise and signal conduction is inhibited. The receptor site is thought to be located at the cytoplasmic portion of the sodium channel. Local anesthetic drugs bind more readily to sodium channels in an activated state, thus onset of neuronal blockade is faster in rapidly firing neurons. This is referred to as state-dependent blockade.
LAs are weak bases and are usually formulated as the hydrochloride salt to render them water-soluble. At a pH equal to the protonated base's pKa, the protonated and unprotonated forms of the molecule exist in equimolar amounts, but only the unprotonated base diffuses readily across cell membranes. Once inside the cell, the local anesthetic will be in equilibrium, with the formation of the protonated form, which does not readily pass back out of the cell. This is referred to as "ion-trapping". In the protonated form, the molecule binds to the LA binding site on the inside of the ion channel near the cytoplasmic end. Most LAs work on the internal surface of the membrane - the drug has to penetrate the cell membrane, which is achieved best in the nonionised form.
Acidosis such as caused by inflammation at a wound partly reduces the action of LAs. This is partly because most of the anesthetic is ionized and therefore unable to cross the cell membrane to reach its cytoplasmic-facing site of action on the sodium channel.
All nerve fibers are sensitive to LAs, but due to a combination of diameter and myelination, fibers have different sensitivities to LA blockade, termed differential blockade. Type B fibers are the most sensitive followed by type C, type A delta, type A gamma, type A beta, and type A alpha. Although type B fibers are thicker than type C fibers, they are myelinated, thus are blocked before the unmyelinated, thin C fiber.

Techniques

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 or in anatomic nerve bundles. Spinal anesthesia and epidural anesthesia merge into the central nervous system.
Injection of LAs is often painful. A number of methods can be used to decrease this pain, including buffering of the solution with bicarbonate and warming.
Clinical techniques include:
Dental-specific techniques include:

Vazirani-Alkinosi Technique

The Vazirani-alkinosi technique is also known as the closed-mouth mandibular nerve block. It is mostly used in patients who have limited opening of the mandible or in those that have trismus; spasm of the muscles of mastication. The nerves which are anesthetised in this technique are the inferior alveolar, incisive, mental, lingual and mylohyoid nerves.
Dental needles are available in 2 lengths; short and long. As Vazirani-akinosi is a local anaesthetic technique which requires penetration of a significant thickness of soft tissues, a long needle is used. The needle is inserted into the soft tissue which covers the medial border of the mandibular ramus, in region of the inferior alveolar, lingual and mylohyoid nerves. The positioning of the bevel of the needle is very important as it must be positioned away from the bone of the mandibular ramus and instead towards the midline.

Intraligamentary Infiltration

Intraligamentary infiltration, also known as periodontal ligament injection or intraligamentary injection, is known as “the most universal of the supplemental injections”. ILIs are usually administered when inferior alveolar nerve block techniques are inadequate or ineffective. ILIs are purposed for:
1. Single-tooth anesthesia
2. Low anesthetic dose
3. Contraindication for systemic anesthesia
4. Presence of systemic health problems
ILI utilization is expected to increase because dental patients prefer fewer soft tissue anesthesia and dentists aim to reduce administration of traditional inferior alveolar nerve block for routine restorative procedures.
Injection methodology: The periodontal ligament space provides an accessible route to the cancellous alveolar bone, and the anesthetic reaches the pulpal nerve via natural perforation of intraoral bone tissue.
Advantages of ILI over INAB: rapid onset, small dosage required, limited area of numbness, lower intrinsic risks such as neuropathy, hematoma, trismus/jaw sprain and self-inflicted periodontal tissue injury, as well as decreased cardiovascular disturbances. Its usage as a secondary or supplementary anesthesia on the mandible has reported a high success rate of above 90%.
Disadvantages: Risk of temporary periodontal tissue damage, likelihood of bacteriemia and endocarditis for at-risk populations, appropriate pressure and correct needle placement are imperative for anesthetic success, short duration of pulpal anesthesia limits the use of ILIs for several restorative procedures that require longer duration, postoperative discomfort, and injury on unerupted teeth such as enamel hypoplasia and defects.
Technique description:
Syringes:
Things to note:
Gow-Gates technique is used to provide anesthetics to the mandible of the patient's mouth. With the aid of extra and intraoral landmarks, the needle is injected into the intraoral latero-anterior surface of the condyle, steering clear below the insertion of the lateral pterygoid muscle. The extraoral landmarks used for this technique are the lower border of the ear tragus, corners of the mouth and the angulation of the tragus on the side of the face.
Biophysical forces and gravity will aid with the diffusion of anesthetic to fill the whole pterygomandibular space. All three oral sensory parts of the mandibular branch of the trigeminal nerve and other sensory nerves in the region will come in contact with the anesthetic and this reduces the need to anesthetise supplementary innervation.
In comparison to other regional block methods of anestheising the lower jaw, the Gow-Gates technique has a higher success rate in fully anesthetising the lower jaw. One study found that out of 1,200 patients receiving injections through the Gow-Gate technique, only 2 of them did not obtain complete anesthesia.

Types

Local anesthetic solutions for injection typically consist of:
Esters are prone to producing allergic reactions, which may necessitate the use of an amide. The names of each locally clinical anesthetic have the suffix "-caine".
Most ester LAs are metabolized by pseudocholinesterase, while amide LAs are metabolized in the liver. This can be a factor in choosing an agent in patients with liver failure, although since cholinesterases are produced in the liver, physiologically or pathologically impaired hepatic metabolism is also a consideration when using esters.
Sometimes, LAs are combined, e.g.:
LA solutions for injection are sometimes mixed with vasoconstrictors to increase the duration of local anesthesia by constricting the blood vessels, thereby safely concentrating the anesthetic agent for an extended duration, as well as reducing hemorrhage. Because the vasoconstrictor temporarily reduces the rate at which the systemic circulation removes the local anesthetic from the area of the injection, the maximum doses of LAs when combined with a vasoconstrictor is higher compared to the same LA without any vasoconstrictor. Occasionally, cocaine is administered for this purpose. Examples include:
One combination product of this type is used topically for surface anaesthesia, TAC.
Using LA with vasoconstrictor is safe in regions supplied by end arteries.
The commonly held belief that LA with vasoconstrictor can cause necrosis in extremities such as the nose, ears, fingers, and toes, is invalidated, since no case of necrosis has been reported since the introduction of commercial lidocaine with epinephrine in 1948.

Ester group

Most naturally occurring local anesthetics with the exceptions of menthol, eugenol and cocaine are neurotoxins, and have the suffix -toxin in their names. Cocaine binds the intracellular side of the channels while saxitoxin, neosaxitoxin & tetrodotoxin bind to the extracellular side of sodium channels.

History

In Peru, the ancient Incas are believed to have used the leaves of the coca plant as a local anaesthetic in addition to its stimulant properties. It was also used for slave payment and is thought to play a role in the subsequent destruction of Incas culture when Spaniards realized the effects of chewing the coca leaves and took advantage of it. Cocaine was 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 anesthetics stovaine in 1903 and procaine in 1904. Since then, several synthetic local anesthetic drugs have been developed and put into clinical use, notably lidocaine in 1943, bupivacaine in 1957, and prilocaine in 1959.
The invention of clinical use of local anaesthesia is credited to the Vienna School which included Sigmund Freud, Carl Koller and Leopold Konigstein. They introduced local anaesthesia, using cocaine, through ‘self-experimation’ on their oral mucosa before introducing it to animal or human experimentation. The Vienna school first started using cocaine as local anaesthesia in ophthalmology and it was later incorporated into ophthalmologic practice. Dr. Halsted and Dr. Hall, in the United States in 1885 described an intraoral anaesthetic technique of blocking the inferior alveolar nerve and the antero-superior dental nerve using 4% cocaine.