Myoclonus in patient on fluoxetine after receiving fentanyl and low-dose methylene blue during sentinel lymph node biopsy

Serotonin released in the nerve synapses is cleared through reuptake into presynaptic neurons and metabolism with monoamine oxidase (MAO). Therapy with selective serotonin reuptake inhibitors (SSRIs) or MAO inhibitors increases serotonin concentration in the synaptic cleft and may result in serotonin syndrome (SS). Our patient undergoing sentinel lymph node biopsy was on fluoxetine (SSRI) and intraoperatively developed SS after receiving fentanyl (200 μg) and methylene blue (MAO inhibitor), 7 mg subcutaneously into the scalp. Initial presentation was several episodes of generalized muscle activity, which was later diagnosed as lower extremity myoclonus consistent with SS. Upon awakening, the patient showed no evidence of encephalopathy, and the clonus was less intense. The patient was discharge home the next day. Our case suggests the possibility that even a small dose of methylene blue, when administered simultaneously with other serotoninergic medications, may be associated with serotonin toxicity.