Local anaesthetics

From Therapeutic guidelines Oral and Dental 2012

Anaesthesia: A state of controlled, temporary loss of sensation. It may involve analgesia, paralysis and amnesia or unconsciousness.
Analgesia: Inability to feel pain

Local anaesthetics in dentistry are used locally or regionally to produce loss of sensation essentially blocking somatic and noniceptive stimulation as well as motor nerve conduction. Their action is mainly blocking of sodium channels along neurons that essentially block the formation of action potentials. The progression of nerve function blockage relates to nerve diameter, myelination and conduction velocity. First, autonomic activity is blocked then nociception, other sensory functions then motor activity. In dental applications, Soft tissue innervation tends to be located on the outside of the nerve trunk and pulpal innervation towards the centre. Therefore soft tissue anaesthesia will precede pulpal anaesthesia and pulpal anaesthesia will wear off before soft tissue anaesthesia.

All popular dental anaesthetics are amino amides (Lidocaine, prilocaine, mepivacaine and articaine) and are metabolised by the liver. Vasoconstrictors can be added which act to prolong the working time and reduce the minimum effective dose of anaesthetics due to local vasoconstriction reducing the washout rate of solution. This also assists in local hemostasis which is useful for surgical procedures.

Adrenaline is the most commonly used vasoconstrictor. It is affected by heat anf light so should be stored in a cool, dark place. Adrenaline is rarely contraindicated due to it being an endogenous hormone however many people report adverse symptoms rather than true allergies. Patients who report an adrenaline allergy should be questioned as to the symptoms they experienced on application. Tachycardia is a physiological response to systemic adrenaline and does not contraindicate use. True allergies are rare and often occur due to a reaction to the adrenaline preservative (Sodium metabisulphate). Non adrenaline containing annaesthetics can be used such as mepivacaine plain or prilocaine with fellypressin. MAOI antidepressants inhibit the metabolism of adrenaline so adrenaline containing solutions are contraindicated.

Fellypressin is a vasoconstrictor resembling the endogenous hormone oxytocin. It has a lack of CNS effects which is useful when adrenaline is contraindicated. Although it is similar in structure to oxytocin (causes contractions in pregnancy) its use in pregnancy is not contraindicated.

Local anaesthetics in dentistry are considered safe because the incidence of adverse effects relative to the number of injections given is very low. Systemic toxicity to LA can arise after intravascular injection, rapid systemic absorption excessive dose administration or impaired drug clearance. This is seen on a continuum as plasma concentration rises.

Minor CNS effects are seen first:
-Circumoral and tongue numbness
-Light headedness
-Excitability
-Nervousness
-Sweating
-Visual and auditory disturbances
-Disorientation
-Vomiting
-Generalised muscle twitching

These precede more severe CNS effects such as:
-Loss of consciousness
-Tonic-clonic seizures
-Coma
-Respiratory arrest and cardiovascular collapse.