In a patient with cardiomyopathy and associated symptoms, what is the most likely diagnosis if chest x-ray findings show no significant increase in heart size?

In a patient with cardiomyopathy and associated symptoms, the absence of significant heart enlargement on a chest x-ray suggests that the heart failure may not yet be advanced. In cases of heart failure, particularly with preserved ejection fraction, one can see cardiomyopathy symptoms such as shortness of breath or fatigue without notable cardiomegaly. The condition can be due to various types of cardiomyopathies, including hypertrophic or restrictive types, which may result in heart failure symptoms while maintaining a normal-sized heart on imaging studies.

In contrast, subacute bacterial endocarditis would typically present with systemic findings related to infection or embolization, and it is usually associated with other physical findings that would not support a diagnosis in this context. A pulmonary embolus might produce chest pain or acute shortness of breath but would likely present with other acute changes on imaging, such as signs of right heart strain or characteristics consistent with infarcts in the lung fields. Pneumonia might produce respiratory symptoms as well but would generally show abnormalities on an x-ray, such as infiltrates, instead of involving the heart directly.

Thus, the presence of cardiomyopathy along with the normal heart size on a chest x-ray supports the diagnosis of heart failure without significant heart enlargement