GAD65-Positive Limbic Encephalitis and Large Temporal Arachnoid Cyst: A Diagnostic Conundrum

Background/Purpose: Arachnoid cysts of the middle cranial fossa have been associated with seizure activity as well as behavioral and psychiatric symptoms. However, in case of an atypical presentation the diagnostic challenge of identifying the arachnoid cyst as the causative pathology or as a mere bystander can arise.

Methods: We present a case study of two concomitant intracranial pathologies and highlight the diagnostic challenges and therapeutic considerations.

Results: A 15-year-old boy presented with a one week history of dizziness, diffuse dysesthesia of the body and loss of the sense of reality. Cranial MRI revealed a large right temporal arachnoid cyst with midline shift and significant compression of the right cerebral hemisphere, formally classified as Galassi type III. Electroencephalography (EEG) did not demonstrate epileptiform activity. A microsurgical fenestration of the arachnoid cyst into the basal cisterns was performed without complications. After an initial improvement following surgery, the patient presented with up to 16 daily episodes of temporal lobe seizures. EEG now showed interictal and ictal epileptic activity. Treatment with levetiracetam was commenced. Cranial MRI now revealed a decrease in arachnoid cyst size and an improvement in right hemispheric compression. However, fluid attenuated inversion recovery (FLAIR) sequences now demonstrated thickening and increased signal intensity of the right hippocampus. In retrospect, subtle limbic signal alteration could be detected also on preoperative MRI, although findings were masked by severe right hemispheric compression due to the untreated arachnoid cyst. A working diagnosis of limbic encephalitis was made and confirmed by a pathological glutamate decarboxylase (GAD) autoantibody serum titer of 1:40,960. Despite the protracted diagnosis of limbic encephalitis, immunotherapy was initiated due to significant persistent memory deficits und persistent GAD 65 autoantibodies.

Conclusion: This is a case of an atypical clinical presentation initially attributed to a Galassi type III arachnoid cyst of the right middle cranial fossa. As symptoms persisted despite successful fenestration, further diagnostic workup revealed a concomitant diagnosis of limbic encephalitis. In retrospect, the patient’s symptoms appear to be related to autoimmune disease rather than to the large arachnoid cyst. This diagnostic conundrum highlights the complexity of differentiating causal and incidental findings in patients presenting with atypical clinical features.