Giant perivascular space: a rare cause of acute neurosurgical emergency.

We present a rare case of giant perivascular space in mesencephalo-thalamic region causing hydrocephalus. The patient presented insidiously over 6 months. However, the patient suddenly deteriorated in the hospital with visual symptoms, increasing headache and papilloedema, prompting urgent VP shunt and biopsy. Patient's symptoms resolved completely after decompression and he continues to remain symptom free. This patient is only the second described case of giant perivascular space with sudden deterioration of symptoms. This case report is intended to highlight this rare presentation of this cyst which can potentially suggest a more aggressive underlying lesion and prompt a biopsy which can be risky, given the proximity to perforators and normal structures, which is otherwise not necessary.