Local Anesthesia-Solution to Pain : An Overview

6Dental Surgeon, Bhubaneswar, Odisha Abstract Local anesthetics have been used clinically for more than a century, but new insights into their mechanisms of action and their interaction with biological systems continue to surprise researchers and clinicians alike. Local anesthetics must traverse several tissue barriers to reach their site of action on neuronal membranes. In particular, the perineurium is a major rate-limiting step. Previously it was assumed that patients are rarely allergic to local anesthetic agents, but variation in individual patient’s response to local anesthetics is larger than previously assumed adjuncts available to block sensory nerver are there, but these typically also prolong motor block.

LOCAL ANESTHESIA -SOLUTION TO PAIN : AN OVERVIEW of this review is to highlight key aspects of LA pharmacology and toxicology and delineate current research.

Definition
It is defined as a loss of sensation in a circumscribed area of the body caused by depression of excitation in nerve endings or an inhibition of the conduction process in peripheral nerves(Stanley F Malamed,1980). (4) History (5,6) The 1st chemical local anesthetic came with discovery of cocaine in 1860 by Albert Nieman, but its anesthetic property was not realized until in 1862 when Schraff noted its local effect on tongue. William Halsted Steward carried out the 1st recorded inferior dental nerve block using cocaine in 1884.Modern chemical LA agents came of age when Alfren Einhorn achieved esterification of the base alcohol with benzoic acid to synthesize procaine in [1904][1905]. Lofgren succeeded in synthesizing Lidocaine from a series of aniline derivatives in 1943. Bupivacaine became the longest acting amide LA in 1980's to be followed by Rupivacaine in mid 1990.
On the basis of myelinated and non myelinated, diameter and velocity, nerve fibres have been classified into different categories Table 1 (7)

Classification of LA (4)
Based on structure (1) Esters: They possess an ester linkage between the benzene ring and the intermediate chain. (3) Quinolone: Centbucridine. Pharmacokinetics (4,9) Uptake: All LA produce vasodilatation of vascular bed into which they are deposited except cocaine which is a potent vasoconstrictor.A significant effect of it is vasodilatation, increased absorption of LA into blood leading to decreased duration and potential for toxicity.The rate at which LA is absorbed into the blood and reach their peak level vary acc. to the route of administration. Distribution: Distributed to all tissues especially to highly perfused organs(brain, liver, kidney).But skeletal muscle (not among the highly perfused organ) which form the largest mass of tissue in body contains the greatest percentage of LA.

Metabolism
Ester LA : Hydrolysed in plasma by psuedocholinestrase.Faster the hydrolysis , lesser is toxic potential. Chloroprocaine (least toxic) &Tetracaine (most toxic). Allergic reactions in response to ester drugs are due to PABA (major metabolite of ester LA).
Amide LA : Liver is the primary site.Patient with lower than usual hepatic blood flow (hypotension, CHF, cirrhosis) are unable to biotransform amide LA at a normal rate leading to increase chance to toxicity. These conditions represent a relative contraindication to amide LA.

Excretion:
Mainly excreted by kidney.% of a given dose of LA will be excreted unchanged in urine.Esters appear in small concentration.Amides are present in urine as parent compound .
Characterstics of an ideal local anaesthetic (10) • Adminstration of agent is non irritating • Little or no allergenicity • A rapid onset & adequate duration • Completely reversible.
• Anesthesia is selective to nociception pathway
• Incision and drainage of abcesses.
• Cavity preparation especially in deep painful cavities.
• Periodontal surgery and gingival surgery. • Sore spots as a result of denture get relieved • Treatment of trismus and trigeminal neuralgia • In patients who shows gagging especially during placement of film.
• For anesthesia of oral cavity and jaw bones for routine surgical procedures like treatment of fractures etc.
• Allergy to local anaesthetic solution.
• Mentally retarded and unco-operative children or very young children.
• Anatomic anomalies. Peripheral Nerve Paresthesia: Articaine is associated with fivefold higher incidence of paresthesia compared with lidocaine. as it can cause damage to inferior nerve or lingual nerve.
Allergic Reaction: The amide local anesthetics appear to have an extremely little immunogenic and therefore low rate of allergic reactions.
Reaction to Anesthetic Formulations contaning a Sulfite Antioxidant: Allergic reactions like urticaria, bronchospasm & anaphylaxis. The use of local anesthetic without vasoconstrictors is a possible alternative with these patients.

Caused by Vasoconstrictor Drug
For prolong action of local anesthetic solution and to reduce its toxicity, vasoconstrictors have been added but its addition lead to contraindication of local anesthetic solution in various patients like in cardiac patients especially those suffering from refractory dysrhythmias, angina pectoris, postmyocardial infarction (6months) and uncontrolled hypertension. Other contraindications to vasoconstrictors are endocrine disorders such as hyperthyroidism, hyperfunction of the medullary adrenal (pheochromocytoma) and uncontrolled diabetes mellitus.

Symptoms: Palpitation, Tachycardia, Headache, Apprehension
Treatment: Brief duration reaction, so stop drug administration and reassure the patient.

Caused by local reactions
Infections caused by contaminated solutions are rare because of high standard of asepsis practiced by manufacturers.
Prevention: Use LA cartridges only once. Store cartridges as aseptically as possible. Before inserting needle into the cartridge, rubber diaphragm should be wiped with sterile disposable alcohol sponge. • Stop the dental procedure.

Caused by Needle Insertion
• Lower the chair back and elevate the legs of the patient.
• If patient is conscious, instruct him to take deep breath.
• Check patients BP, pulse rate and color.
• Ensure adequate oxygenation and CVS stability.

(B) Muscle Trismus
Common and mainly occurs after inferior alveolar nerve block.

Treatment
Mild: Slight exercises coupled with application of moist warm compresses for 15-20 min. /h, Mild analgesics, Physiotherapy consist of opening closing and side to side movement for 5-10 min. after every 3-4 hrs.
Severe: Add centrally acting muscle relaxant

(C) Pain or Hyperesthesia
Most commonly occur due to carelessness of dentist.
Prevention: Use Sharp needle, No multiple traumas, Needle insertion should be Atraumatic and slow, LA should be forced into the tissue slowly.

(D) Broken Needle
Most annoying and depressing complication of anesthesia.
Prevention : Do not force needle against resistance, Do not change the direction of the needle while embedded in tissue, Do not use needle of too fine a gauge, Do not use resterilized needle & Inform the patient before inserting the needle

(E) Hematoma
It is associated with posterior superior alveolar nerve block and infraorbital nerve block. Occurs because of improper technique.

Treatment
Immediate: Direct pressure to the bleeding site for at least 2 minutes.
Subsequent: Do not apply heat to the area for 6 to 8 hours after the incident. Application of ice to the region immediately and reassure the patient.

Techniques of Local Anesthesia
1. Topical Anesthesia (4,19,20) : For obtaning the anesthesia of mucosa prior to injection, a suitable agent is applied to an area of either the skin or mucous membrane which it penetrates to anesthetize superficial nerve endings.  Preanesthetic evaluation (26) Preanesthetic evaluation should be done before administering any anesthetic drug. It is done to secure pertinent information to evaluate and not to diagnose or treat the patient for any medical problem. It is done to determine the following: • Patients general and psychological condition.
• Need for medical consultation.
• History of any previous unpleasant esthetic experience.
• Specific drug sensitivity of the patient.
• The need for premedication or intraoperative sedation.
• The time to be allotted for procedure.
• The technique or method to be used.
• Choice of an anesthetic solution.
• The need and quantity of vasoconstrictor.
On first visit patients pulse rate and blood pressure should be taken. Brief medical history should be taken. It involves: • CVS Status • Any respiratory difficulties, nervous system disorder, metabolic deficiencies, endocrine imbalance, hematological pathologies & iatrogenic conditions • Presence of allergy, patient's size and age, emotional or psychological problems.
• Medications the patient may be taking CVS status: Conditions concerned to dentists are Congenital heart disease, Acquired heart disease, Rheumatic heart disease, Atherosclerotic heart disease, Hypertension, CHF, Valvular heart disease Arrhythmia(conduction system defect).

Precautions
• Consultation with patient physician taken when indicated.
• Procedure should be planned to fit the individual patient condition.
• If patient is anxious, he should be moderately premedicated or sedated during appointment.
• He should be given short appointment to prevent undue tiring.
• Least possible amount of anesthesia should be used.
• Vasoconstrictors, although not contraindicated, should be kept at a minimum dose or eliminated if necessary.
• Patient may be given oxygen by nasal cannula during procedure.
• Prophylaxis with appropriate antibiotics should be given if indicated Respiratory System: Bronchitis, Bronchiectasis, Emphysema, Asthma Precautions • Treatment should be given in afternoon.
• Preoperative medications such as adhesives, hypnotics and narcotics should be used with extreme caution as they interfere with cough reflex and depress ventilation.

CURRENT MEDICAL RESEARCH AND OPINION
Jaan A et al.
• Choice of local anesthetic or vasoconstrictor is not of utmost importance provided there are no other complicating pathologies.
• Oxygen can be given by nasal cannula if required during dental procedure.
• Evaluate the patient treatment whether diabetes is controlled by diet/hypoglycemic agents/insulin.
• Patient controlling diabetes by diet pose no problem.
• Patient on insulin should be treated between 9.00 am -12pm because as a result of food and insulin intake, it is during these hours that they are best able to tolerate stressful situations.
(2) Hypothyroidism These patients do not metabolize drug as well as the normal individual therefore doses of vasoconstrictors in drug should be kept minimum because of relative CVS conditions.

Local Anesthesia in Pregnancy & Postpartum (27)
Local anesthetics can be safely used when treating pregnant & postpartum patients if careful guidelines are followed . Because teratogenic risks are highest in the first trimester, the 2 nd trimester is usually the period chosen for routine dental care. Lidocaine is least associated with medical complications.

LA Consideration in Endodontics
Effect of Inflammation on LA (2): Inflammation and infection lowers the tissue pH , altering the ability of a LA to provide clinically adequate pain control. There are two methods of obtaining adequate nerve block are: 1. Administer LA away from the area of inflammation: It helps in preventing the spread of infection to uninvolved regions. It also provides adequate pain control because of presence of more normal tissue condition. Regional nerve block anesthesia is the major factor in pain control for pulpally involved teeth.
2. Deposit a larger volume into the region : It will provide a greater no. of uncharged base molecule to diffuse through the nerve sheath to give satisfactory nerve block. Some patients respond unfavorably to instrumentation of their root canal, even when canals are debrided thoroughly. LOCAL ANESTHESIA -SOLUTION TO PAIN : AN OVERVIEW study has favored its use as its efficacy in pain control has been described as comparable to local anaesthesia while at the same time avoiding the possible side effects associated with commonly used local anaesthetic agents and the inconvenience of post-operative anaesthetic effect. (33) Another study suggested EDA could be indicated for needle-phobic children; however, studies that have tested its effectiveness in children are few. (34)

SUMMARY & CONCLUSION:
The science of LA is an active research field and LA will continue to be one of the mainstays of contemporary perioperative medicine. Anxiety, fear & apprehension should be recognized & managed before administration of a local anesthetic. Vasoconstrictors should be included in all local anesthetics unless specifically contraindicated. Partial resistance to LA may be more frequent than previously thought. LA are toxic on many tissues but clinically apparent nerve damage is very rare and LA-induced toxicity after peripheral nerve block has a good prognosis overall.