The purpose of the study is to evaluate the effect of ALN-APP on measures of CAA disease progression and to characterize the safety, tolerability, and pharmacodynamics (PD) of ALN-APP in adult patients with sporadic CAA (sCAA) and Dutch-type CAA (D-CAA). The study will be conducted over 2 periods: a 24-month double-blind treatment period and an optional 18-month open-label extension (OLE) period. The estimated duration of study participation, inclusive of screening, treatment, and additional safety follow-up, is up to 50 months.