Module 38

Question 1

How can local anesthetics block pain impulses without causing paralysis?

Answer 1

• The selectivity of local anesthetics is based on the fact that they have much more of an effect on small diameter nerves than on large diameter nerves.
  The nerves that carry pain impulses are much smaller than motor neurons, and therefore at relatively low concentrations local anesthetics block pain impulses without causing paralysis.

Question 2

What is the incidence of true allergies to local anesthetics? What is the evidence for the theory stated in many textbooks that if a patient has a history of a reaction to an ester anesthetic, an amide anesthetic is safe but avoid other esters. While if a patient has a history of a reaction to an amide, an ester should be used? What structural feature is probably responsible for most true allergic reactions to local anesthetics? What other components of a local anesthetic can be responsible for adverse reactions to local anesthetics?

Answer 2

• True allergies to local anesthetics are rare. Many of the reactions are probably vasovagal reactions caused by the stress of a dental or other procedure. It is also possible for a patient to have a reaction to preservatives in the formulation; specifically, bisulfite or para-aminobenzoic acid.
  
  • In addition, many formulations contain epinephrine, which causes local vasoconstriction. This has the advantage of decreasing bleeding and keeps the agent localized to the site of injection longer. If the anesthetic is mistakenly injected into a blood vessel the epinephrine can cause significant symptoms.
  • However, true allergic reactions do occur, but the differentiation based on whether it is an ester or amide is illogical. By coincidence, most of that ester local anesthetics are primary aromatic amines, which form reactive metabolites and are more likely to cause an immune response. This separation into ester and amide local anesthetics was proposed many years ago and stuck, even though studies to test the hypothesis have not been performed. And since true allergic reactions are rare, if a clinician switches to a different anesthetic, no matter whether it is an ester or amide, it is unlikely that the patient will have a significant adverse reaction. This has produced a confirmation bias.