To present a rare case of herpes simplex virus (HSV) encephalitis and IgLON5 encephalitis in a patient initially presenting with ischemic stroke.

HSV encephalitis has been associated with autoimmune encephalitis, most commonly NMDAR encephalitis. However, co-occurrence of HSV with IgLON5 encephalitis is exceedingly rare. This case describes the clinical course of a patient presenting with both conditions and his response to treatment.

A 62-year-old man with a medical history of diabetes and hypertension presented with acute left-sided hemiparesis. Brain MRI revealed a subacute right thalamic ischemic stroke. During his hospitalization, he developed a fever greater than 102°F, progressive confusion, diabetic ketoacidosis, and seizures, with EEG showing right temporal epileptiform discharges. He was started on broad-spectrum antibiotics, but his mental status rapidly deteriorated, and he experienced worsening seizures, ultimately requiring intubation.

A lumbar puncture revealed HSV encephalitis, and the patient was treated with 21 days of acyclovir. Despite this, his clinical status worsened, and he remained in a vegetative state, necessitating the placement of a tracheostomy tube. A repeat lumbar puncture was performed, revealing positive results for IgLON5 antibodies. Serum testing confirmed an IgLON5 antibody titer of >1:2560.

The patient underwent seven sessions of plasmapheresis (PLEX), followed by an additional four sessions of intravenous immunoglobulin (IVIG). Post-treatment, his serum IgLON5 titer remained at 1:2560. Despite aggressive management, the patient showed minimal clinical improvement and remained paraplegic, mute, and only minimally responsive to verbal and noxious stimuli.

IgLON5 disease is a rare autoimmune encephalitis, occasionally seen in conjunction with HSV encephalitis. In cases of refractory HSV encephalitis, it is essential to investigate for concurrent autoimmune encephalitides, such as IgLON5. This case highlights the poor prognosis associated with concurrent HSV and IgLON5 encephalitis, underscoring the need for further research on treatment strategies and outcomes for these patients.