gms | German Medical Science

Objective: Arachnoid cysts of the middle cranial fossa have been associated with seizure activity as well as behavioural 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 years 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 acute anxiety, disorientation, short term memory loss as well as gustatory and olfactory sensations. EEG now showed scattered sharp waves, although it was not considered diagnostic for 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 significant 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) auto-antibody serum titre of 1:40,000. Given the protracted diagnosis of limbic encephalitis and the spontaneous improvement subsequently observed in our patient, immunotherapy was withheld.

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.