The condition known as heart valve thickening is actually heart muscle thickening. Heart muscle thickens due to an increase in heart muscle mass or the accumulation of different tissues within the heart muscle. The biggest cause of this is hypertension. Symptoms of heart muscle thickening include arrhythmia, palpitations, swelling in the legs in later stages, and easy fatigue. Hypertension patients and those with heart valve problems are among the highest risk individuals for this condition.