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ARACHNOID CYSTS CAN occur wherever there is arachnoid tissue, although nearly half of all arachnoid cysts arise in the temporal fossa–Sylvian fissure. In general, clinical manifestations of arachnoid cyst are often mild. Several articles have suggested that there is an etiological relationship between arachnoid cyst and psychiatric symptoms.1

A 24-year-old man first presented with compulsive checking at the age of 21 years. In addition, he had experienced delusions of persecution and auditory hallucinations. He had no neurological abnormality and was treated with antipsychotic medication. The antipsychotics, however, did not improve the psychotic symptoms. At that time, computed tomography showed an arachnoid cyst in his left middle cranial fossa. The basic rhythm on electroencephalogram (EEG) was posterior dominant alpha wave. The psychotic symptoms were considered to be caused by the arachnoid cyst because of the antipsychotics' low effectiveness. Magnetic resonance imaging (MRI) showed a large anteromedial left temporal fossa arachnoid cyst (80 mm in diameter) expanding into the left Sylvian fissure. Event-related potentials (ERP), such as P300, and mismatch negativity (MMN) were low in amplitude. Some reports have suggested that surgical intervention for arachnoid cyst improved psychotic symptoms.2 Therefore, endoscopic fenestration of the cyst was performed. The delusions and hallucinations improved postoperatively, and the patient was discharged. The delusions of persecution, however, later increased. MRI using time–spatial labeling inversion pulse showed inadequate flow of cerebrospinal fluid through the fenestra. Hence, another craniotomy was performed. Fenestra closure caused by arachnoid regeneration was found during this procedure. After partial removal of the cyst wall, the delusions remitted partially. The amplitude of P300 and MMN in the frontocentral region increased after each surgery. In contrast, MMN amplitude in the bilateral mastoid region showed no change. There was no difference between the basic rhythm and amplitudes of EEG before and after surgery. This patient provided written, informed consent.

In some reports, left temporal arachnoid cysts were found to be associated with schizophrenia-like symptoms.2 Recent studies have demonstrated volume loss in the left superior temporal gyrus in schizophrenia.3 The temporal lobe is regarded as an important region for P300 and MMN, the amplitudes of which are reduced in schizophrenia and the other psychoses. This is one of the reproducible abnormal biological findings in schizophrenia. Therefore, small P300 and MMN amplitude, schizophrenia-like psychotic symptoms, and left temporal lobe volume loss are closely related.3 In the present case, the compressed and/or defective development of the temporal lobe due to the arachnoid cyst probably caused the low P300 and MMN amplitudes. If low ERP were of organic origin, the amplitudes would improve.

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