Background and importance: Both asymptomatic unruptured intracranial aneurysms (AUIA) and idiopathic normal pressure hydrocephalus (NPH) have been found in the elderly population to increase in incidence with advancing age. In NPH, external lumbar drainage (ELD) is a gold standard and widely accepted low-risk procedure for supplemental testing of CSF responsiveness and suitability for surgical intervention. Herein, we reported the first case of aneurysmal subarachnoid haemorrhage (SAH) as a cause of neurological decline following ELD for the testing of patients with NPH. Clinical Presentation: A 65-year-old gentleman with probable NPH was admitted for supplemental testing using ELD. He had demonstrated CSF responsiveness as evidenced by improving Tinetti score. Unfortunately, on the third day of post-insertion, his GCS deteriorated from 15 to 3 due to rupture of a previously undiagnosed anterior communicating artery (Acom) aneurysm. He underwent left frontal external ventricular drain in an emergency setting and his GCS improved to E1VtM4. After frank discussion with his family, we proceeded with craniotomy and clipping of the aneurysm. However, his neurological recovery remained poor post-operatively and he had eventually undergone tracheostomy and ventriculo-peritoneal shunt. Conclusion: As SAH is also a known trigger of secondary NPH, there may be a critical role for screening of AUIA prior to ELD for NPH. The characterization of this risk would then allow for a full discussion of surgical or endovascular treatment options for AUIA, as well as the risk-benefit ratio of proceeding with supplemental testing and intervention for NPH.