What is the most common cause of toxic reactions to local anesthetics?

• A. Inadvertent intramuscular injection
• B. Overuse of anesthetic without withdrawal
• C. Allergic reaction to the preservative
• D. Inadvertent intravascular injection

Answer: (D)

Systemic toxicity from local anesthetics most often occurs when the drug is injected into a blood vessel, causing a rapid spike in blood concentration. When an intravascular route is unintentionally used, the local anesthetic reaches the brain and heart quickly, so the first signs are CNS effects (numbness around the mouth, dizziness, tinnitus, blurred vision, agitation or seizures) followed by potential cardiovascular collapse if the dose is large. This rapid rise explains why intravascular injection is the most common cause of toxic reactions. In contrast, intramuscular injection tends to absorb more slowly, so it doesn’t produce such a sudden high plasma level. Allergic reactions to preservatives are uncommon causes of systemic toxicity, and toxicity from overuse without withdrawal isn’t the typical mechanism; the key issue is getting a high, rapid systemic concentration, usually from an inadvertent intravascular entry. To reduce risk, use careful aspiration before injection, inject in small increments, and monitor for early CNS symptoms.