Triphasic waves in leptomeningeal metastases and cycling alternating pattern of encephalopathy during sleep

Abstract Background Triphasic waves (TWs), now termed generalized periodic discharges with triphasic morphology, may be mistaken for nonconvulsive status epilepticus (NCSE), particularly in critically ill patients. Changes in vigilance over time can help distinguish encephalopathic patterns from ictal activity. Case Presentation A 69‐year‐old woman with breast cancer and leptomeningeal metastases developed progressive mutism and impaired consciousness. Cerebrospinal fluid analysis showed atypical cells and markedly elevated protein, while brain MRI demonstrated bilateral sulcal hyperintensities, most prominent in the posterior regions. An EEG performed 1 week after bevacizumab and paclitaxel were discontinued showed only mild background slowing without epileptiform discharges. A repeat EEG 1 week later showed nearly continuous TWs. The discharges disappeared during spontaneous sleep and recurred periodically, consistent with a cyclic alternating pattern of encephalopathy during sleep (CAPE). In the absence of significant metabolic and toxic derangements, these features favored an encephalopathic rather than ictal pattern. Given the advanced disease and poor prognosis, care remained supportive and palliative. Conclusion This case highlights the importance of interpreting TWs within their clinical context and in relation to the state of vigilance. Their disappearance at sleep onset, with periodic recurrence during sleep producing a CAPE, supports an encephalopathic rather than ictal interpretation, helping to avoid misdiagnosis of NCSE and unnecessary administration of antiseizure medications.