Introduction Leptomeningeal carcinomatosis (LMC) refers to diffuse or multifocal seeding of the leptomeninges by neoplastic cells. Patients with LMC can have transient neurological dysfunction due to seizures or plateau waves from paroxysmal elevation of intracranial pressure (ICP). There are very few descriptions of the clinical and EEG correlates of plateau waves in patients with LMC. We present the video-EEG findings in a patient with LMC and episodes of transient anisocoria and unresponsiveness. Methods Case report. Results A 20-year-old man with history of colorectal cancer treated with surgery and chemotherapy four years earlier, presented to an outside hospital with severe headache, diplopia, fever and neck rigidity. Brain MRI showed mild meningeal enhancement. Serum and CSF evaluation was normal except for positive Lyme antibodies. He was treated with intravenous ceftriaxone for Lyme meningitis, but showed no improvement. On transfer to our hospital, spine MRI revealed multiple enhancing lesions. Lumbar puncture showed opening pressure > 55 cm H20 and atypical tumor cells positive for AE1/AE3 and CK20, consistent with LMC from a colorectal adenocarcinoma with raised ICP. He had multiple episodes of impaired consciousness and tremulous arm movements with pupillary anisocoria (right pupil dilated and nonreactive, left pupil normal size and reactive). The episodes lasted for 5–10 min followed by spontaneous recovery. Video-EEG monitoring was performed. Background EEG revealed continuous, diffuse, moderate amplitude, polymorphic theta/delta activity. One recorded event was characterized by pupillary anisocoria, unresponsiveness, left sided tremulous movements and repetitive mouth movements. EEG changes preceded the clinical event by a few seconds and consisted of diffuse, high amplitude, rhythmic delta activity. This lasted for 7 min and gradually returned to baseline with clinical improvement. There were no epileptiform discharges. EKG showed bradycardia but no asystole or arrhythmias during the event. A ventriculoperitoneal shunt was placed. The patient was given intraventricular chemotherapy with methotrexate and gemcitabine through an Ommaya reservoir. He had improvement of headache and episodes of transient neurological dysfunction. Subsequent EEGs showed only diffuse polymorphic slowing. Conclusion In patients with LMC, clinical distinction between epileptic seizures and plateau waves resulting from raised ICP as the etiology of transient neurological dysfunction can be difficult. Video-EEG monitoring is helpful not only to rule out seizures but also to identify the electrographic correlates of plateau waves, consisting of diffuse, high amplitude delta activity. It can also be useful in monitoring the response to treatment.