The prognosis of a cat with hypertrophic cardiomyopathy (HCM) is highly variable. Some cats may develop only mild hypertrophy and suffer little compromise of heart function, while others progress to more severe disease.
HCM may worsen quickly over a period of months, or it may progress slowly over several years. Its severity may not change for many years and then suddenly worsen. Some cats with HCM die very suddenly, even though they had no clinical signs of heart disease. It is the most common cause for spontaneous death in indoor cats.
A cat with mild to moderate disease may enjoy an essentially normal life for a number of years. However, the prognosis is much more guarded once the cat has more severe disease.
The risk of developing congestive heart failure is proportional to disease severity, which is often classified by measuring wall thicknesses and left atrial size. Although congestive heart failure can be treated medically, severe heart failure may become difficult to manage over time as the disease progresses.
The prognosis for a cat with heart failure, unfortunately, is guarded to poor. On average, survival for cats with HCM and heart failure is 12-18 months after diagnosis.