Literature covers only hypertrophy of the heart.
Atrophy not mentioned.
Theoretically atrophy of the heart would require some factors
- mutations (which can destroy the gene to build up the organ),
- poor nourishment,
- poor circulation,
- loss of hormonal support,
- loss of nerve supply to the target organ,
- excessive amount of apoptosis of cells, and
- disuse or lack of exercise or disease intrinsic to the tissue itself.
Atrophy can happen locally in some part of the heart only, like in some parts of myocardial infarction.
However, the surrounding healthy tissue then undergoes hypertrophy so the total heart undergoes hypertrophy.
Some discussion about Cardiac atrophy here.