FLAIR fornix hyperintensity in VGKC-related autoimmune encephalitis

A 52-year-old man presented with progressive fatigue, drowsiness, weight loss, and memory impairment associated with left-sided temporal lobe seizures. Brain MRI revealed increased bilateral FLAIR signal in the hippocampus, amygdala, mammillary bodies, hypothalamus and fornix (Figs. 1, 2). Blood analysis showed normal CRP, TSH, vitamin B1 and B12 levels. Routine immunological screening, and HIV and syphilis serology were negative. Anti-voltage-gated potassium channel (VGKC) antibodies were positive (463 pmol/l; normal range 0–100 pmol/l), in the absence of antiganglioside antibodies, NMDA/TPO/ GAD antibodies and other paraneoplastic antibodies. CSF analysis showed oligoclonal bands in the absence of other abnormalities. Brain FDG-PET revealed bilateral frontal, parietal and temporal hypometabolism (all equally involved). Whole-body FDG-PET, thoraco-abdominal– pelvic CT and testicular echography were normal. Oral antiepileptic treatment (valproic acid 500 mg bid) led to seizure freedom. Important clinical improvement after corticosteroid treatment (60 mg od for 1 month, followed by tapering during 3 months) was seen, in the presence of persistent MRI abnormalities (although increased signal became less severe) 4 months later. After 1 year of followup, no clinical relapse occurred, and thoraco-abdominal– pelvic CT and whole-body FDG-PET remained normal. A diagnosis of VGKC-related autoimmune, probably nonparaneoplastic, encephalitis was made. Autoimmune-mediated encephalitis can involve several brain areas with notably frequent involvement of limbic system structures (referred to as limbic encephalitis) including the hippocampus, amygdala and the mammillary bodies [1, 2]. MRI (together with FDG-PET) is probably the most adequate imaging technique to reveal abnormalities in these structures. MRI abnormalities due to autoimmune-mediated encephalitis are best seen on FLAIR sequences as an increased signal. To the best of our knowledge, despite radiological involvement of other limbic substructures and associated brain areas (e.g., the hypothalamus), fornix signal changes have never been reported in autoimmune encephalitis. The fornix is an important fiber bundle or projection bundle within the limbic system explaining probably its involvement in our patient. The limbic system includes territories of the diencephalon (the epithalamus, hypothalamus, subthalamic zona incerta), telencephalon (the septal and preoptic regions, hippocampus and some adjacent cortical areas, amygdala, the bed nucleus of the stria terminalis), and mesencephalon. Fornix involvement has been described in multitude other diseases [3]. The fornix is a compact fiber bundle connecting the hippocampus with the hypothalamus and various other structures. Its anterior–posterior orientation makes coronal MRI views particularly interesting. Because of the close proximity of the fornix to structures containing CSF, FLAIR imaging is of special interest to visualize fornix signal abnormalities. Neuropathological findings in autoimmune limbic encephalitis are non-specific, including gliosis, perivascular lymphocyte infiltration and microglial activation. In our patient, it is unclear if signal changes of the fornix were secondary (e.g., due to edema or Wallerian degeneration) D. Renard (&) D. Ratiu Department of Neurology, CHU Nimes, Hopital Caremeau, Place du Pr Debre, 30029 Nimes Cedex 4, France e-mail: dimitrirenard@hotmail.com