Tissue injury from tricyclic antidepressants used as local anesthetics

Neurotoxicity has been reported with tricyclic antidepressants (TCAs) used as local anesthetics. We examined the hypothesis that TCAs cause tissue injury, particularly myotoxicity, as occurs with many local anesthetics. Animals were given sciatic nerve injections with 0-80 mM doxepin, amitriptyline, or bupivacaine (1.5 mL for histological studies, 0.3 mL for neurobehavioral studies). Four days after injection, the TCAs caused ischemic tissue injury. Subcutaneous tissue showed expansion and hardening, with hemorrhage and adhesion to overlying skin. Muscle was diffusely pale. Histopathology showed coagulative necrosis of muscle and surrounding soft tissues, with thrombus formation in vasculature near affected areas. These findings were much reduced with bupivacaine. TCA-injected and bupivacaine-injected animals also developed characteristic local anesthetic myotoxicity. Amitriptyline proved less potent than bupivacaine as a local anesthetic: the concentrations required to provide 100 min of nerve block were 20 mM and 3 mM, respectively. Some animals receiving large concentrations of amitriptyline developed spontaneous recrudescence of nerve blockade or had irreversible nerve blockade, both of which may reflect nerve injury. Neither finding occurred in animals injected with bupivacaine. TCAs do not appear to offer any advantages over conventional local anesthetics and do appear to risk substantially increased toxicity.