The purpose of this study is to examine the changes in amyloid myocardial burden in a subset of the population participating in the ION682884-CS2 (NCT04136171) study, up to 150 participants, after treatment with eplontersen or placebo based on scintigraphy scans performed at Week 140 using the Perugini grade score method.

Participants who were randomized in Study ION-682884-CS2 (NCT04136171) and have had a baseline scintigraphy scan will be offered the opportunity to participate in this sub-study which involves optional scintigraphy scans.

Inclusion Criteria:

• Participants must have been properly randomized into the ION-682884-CS2 study (NCT04136171) and should have had a baseline scintigraphy scan (99mTc-DPD, 99mTc- PYP, and 99mTc-HMDP) within 12 months prior to screening for ION-682884-CS2 with planar and SPECT or SPECT/CT images that can be read by the central reader.

Exclusion criteria:

• Must not meet any of the exclusion criteria of the study protocol ION-682884-CS2 (NCT04136171).
• Must not have weight or body girth that exceeds the limits of the equipment specifications.
• Should not have any previously reported hypersensitivity reaction to Technetium-99m.

Participants who were randomized in Study ION-682884-CS2 (NCT04136171) and who signed informed consent for the Scintigraphy sub-study will participate in this sub-study.