This clinical trial is focused on studying a neurological condition called Cerebral Amyloid Angiopathy (CAA). CAA occurs when amyloid proteins build up in the walls of blood vessels in the brain, which can lead to problems such as bleeding into the brain. The study will evaluate a treatment called Mivelsiran, which is administered as a solution for injection. Mivelsiran is also known by its code name ALN-APP. The purpose of the study is to assess the effect of Mivelsiran on the occurrence of new small brain bleeds, known as Cerebral Microbleeds (CMBs), in patients with CAA.

Participants in the study will receive either Mivelsiran or a placebo, which is a substance with no active medication. The study is designed to be double-blind, meaning neither the participants nor the researchers will know who is receiving the actual treatment or the placebo. This helps ensure that the results are not influenced by expectations. The treatment will be given through a method called intrathecal use, which involves injecting the solution into the space around the spinal cord. The study will monitor participants over a period of time to observe any changes in the incidence of new CMBs and other related outcomes.

The study will also use MRI scans to detect changes in the brain and assess the safety and tolerability of Mivelsiran. MRI, or Magnetic Resonance Imaging, is a technique that uses magnetic fields and radio waves to create detailed images of the organs and tissues in the body. The trial aims to provide valuable information on the potential benefits and risks of using Mivelsiran for treating CAA, contributing to a better understanding of how to manage this condition effectively.