[First episode psychosis associated with increased volume of an arachnoid cyst in the posterior fossa].

We describe the case of a 64-year-old woman who experienced a first psychotic episode following an increase in volume of a known arachnoid cyst in the posterior fossa. This was associated with anxiety and panic attacks as well as mild cognitive deficits. A cystoperitoneal shunt was placed three years earlier to treat headaches, poor balance and memory problems associated with the cyst. Increasing the flow rate through the shunt, followed by the initiation of an SSRI, resulted in remission of the psychotic and affective symptoms. This case illustrates the potential association between structural infratentorial lesions, such as arachnoid cysts, and neuropsychiatric symptoms, particularly psychosis. Given that decompression of the cyst was associated with a reduction in symptoms, this suggests a link between the onset of psychotic symptoms and cerebellar dysfunction.