Cases of nonconvulsive status epilepticus (NCSE) during antidepressant treatment are commonly reported in older adults; however, there are few reports of NCSE in patients who are younger than 60 years old. Herein, we report a man in his 40 s who was receiving antidepressant therapy and developed symptoms due to NCSE. The patient had been treated for depression for 10 years. He was admitted to our department on day X-48. His depressive symptoms improved after he was diagnosed with autistic spectrum disorder, received psychoeducation, and his medication was changed to duloxetine 40 mg and mirtazapine 45 mg. He was discharged on day X-27. Around day X-10, his spouse noticed an increase in the patient's use of a harsh tone of voice. From around day X-5, he experienced fluctuating consciousness and frontal lobe dysfunction. On day X-3, he was urgently transported to our hospital via ambulance following his first episode of a tonic-clonic seizure. The patient experienced repeated seizures and we initiated levetiracetam. On day X, he was transferred to our department for post-seizure follow-up observation. Long-term electroencephalography (EEG) revealed periodic discharges predominantly in the frontal region lasting for 15 minutes and occurring every 30 minutes. After receiving an intravenous fosphenytoin injection, EEG abnormalities and clinical symptoms improved, leading to a diagnosis of NCSE. Around day X+5, manic symptoms, such as talkativeness, irritability, and excessive spending, emerged. After adjusting his medication, including discontinuation of antidepressants, his manic symptoms disappeared by day X+18, and he was discharged on day X+26. Three years post-discharge, the patient remained off antidepressants and had no recurrence of depression, mania, tonic-clonic seizure, or NCSE. The seizures were likely due to the antidepressants affecting the epilepsy-prone parts of the autistic brain, which lowered the seizure threshold. It is crucial to consider NCSE as a differential diagnosis of behavioral abnormalities during treatment with threshold-altering drugs, even in patients younger than 60 years old. The transient manic symptoms after NCSE cessation may be attributed to the side effects of drugs, the first manic episode of bipolar disorder, or transit syndrome. This case suggests the possibility of psychiatric symptoms even after NCSE treatment, highlighting the value of careful observation and treatment by psychiatrists.
 Authors' abstract