8.6:
Local Anesthetics: Adverse Effects
Local anesthetics, or LAs, often diffuse into the systemic circulation and affect the organs that depend on the functioning of sodium channels.
The central nervous system is typically most affected, followed by the cardiovascular system.
The extent of the adverse effects depends on the LA concentration in the bloodstream.
At low systemic concentrations, LAs induce dizziness, and auditory and visual disturbances. They also numb the tongue and leave a metallic taste.
As the serum concentration of the LA peaks, the untreated initial symptoms progress to cause tremors, convulsions, respiratory arrest, vascular collapse, and even death.
Additionally, some individuals are allergic to LAs and suffer from allergic dermatitis or asthma. Most allergic reactions are caused by the metabolism of ester-linked LAs, which yields the allergen para-aminobenzoic acid.
An allergy from amide-linked LAs is rare but can occur due to the presence of preservatives like methylparaben.
8.6:
Local Anesthetics: Adverse Effects
While local anesthetics are generally safe and well-tolerated, they can occasionally cause adverse effects that vary in severity. Local anesthetics can induce toxicity at two distinct levels. They can either produce local effects through direct contact with the neural elements or be absorbed into the bloodstream from the injection site, leading to systemic effects.
Once absorbed into the systemic circulation, local anesthetics can affect the organs that depend on the functioning of sodium channels. The central nervous system is the most affected, followed by the cardiovascular system. Low systemic concentrations of local anesthetics can induce symptoms such as dizziness, auditory and visual disturbances. Higher systemic concentrations of local anesthetics can lead to tremors, seizures, and in severe cases, central nervous system depression or coma. Some individuals may develop allergic reactions to local anesthetics, leading to symptoms such as rash, itching, hives, or even anaphylaxis in severe cases. In rare cases, local anesthetics can cause tissue necrosis if they inadvertently infiltrate blood vessels or are administered in excessive amounts.
The mechanism of neurotoxic effects of local anesthetics has been extensively studied using cell culture, ex vivo, and in vivo models. The potential factors contributing to neurotoxicity are excessive or prolonged sodium channel blockade, disruption of the cytoskeleton of neurons, neuronal membrane damage, disruption of axonal transport, and apoptosis. Furthermore, local anesthetics can cause transient neurological symptoms, like dysesthesia or transient pain syndrome, which can occur when administered spinally or epidurally. These symptoms can cause mild to severe pain, sometimes surpassing the pain induced by surgical procedures.