While BCM tends to be the cause of death in dogs that die unexpectedly at around age six, subaortic stenosis is responsible for the death of many younger boxers, sometimes as early as two years of age.
Subaortic stenosis is a heart disease that occurs when two of the three flaps that comprise the aortic valve fuse together. This fusion narrows the opening to the aorta, which in turn restricts the blood flow out of the left ventricle into the aorta. Symptoms of this condition include exercise intolerance, fainting, and a systolic murmur (a murmur or slight noise that is heard when the heart contracts). Reduced blood flow can result in sudden death. A definitive diagnosis must be made by a canine cardiologist after detection of a heart murmur. It is known that this is an inherited disease, although the mode of inheritance is not known at this time. However, affected dogs should not be bred for any reason.
Subaortic stenosis can be fairly expensive and should be treated under the supervision of a canine cardiologist.