Perfusion ¹²³IMP-SPECT shows reversible abnormalities in GABA_B receptor antibody associated encephalitis with normal MRI

A new subtype of autoimmune encephalitis associated with antibodies against GABA_B receptor was recently identified. Although immune-mediated functional abnormalities are suggested for the pathogenesis, functional brain imaging such as perfusion SPECT has not been documented. A 62-year-old woman with anti-GABA_B receptor associated encephalitis underwent ¹²³I-IMP SPECT in the beginning and after methylprednisolone pulse therapy. Three-dimensional stereotactic surface projection analysis was used to evaluate the cortical distribution of perfusion abnormality. The patient presented with clinical features of subacute limbic encephalitis. Antibodies to GABA_B receptor were identified in her serum and cerebrospinal fluid (CSF), but no tumor was detected. Despite normal magnetic resonance imaging (MRI) findings, the first SPECT revealed hypoperfusion of the frontal, parietal and medial temporal lobes, as well as thalamus, and cerebellum. These areas are known to contain high levels of GABA_B receptors. In contrast, the SPECT revealed hyperperfusion in the motor strip and left temporal lobe, which are areas related to some of the patient's symptoms, including seizures, orolingual dyskinesia, and Wernicke aphasia. After treatment with pulses of methylprednisolone, the neuropsychiatric symptoms resolved and the SPECT findings showed substantial improvement in most of these regions. In conclusion, the findings suggest that immunotherapy improved the cortical dysfunction mediated by GABA_B receptor antibodies.