Eric Lamaze and Hickstead. © Spruce Meadows
The aorta is the main artery that carries blood from the heart to all other arteries, except the pulmonary artery.
Rarely, a sudden tear or break can occur in the wall of the aorta, usually quite close to where the aorta joins the heart.
Blood in the aorta is under high pressure and pours into the sac surrounding the heart, compressing the heart to the point where it can no longer pump blood.
Depending upon the scale of the tear, death can result from a few seconds to a few minutes later.
Diagnosis is made during a post mortem, the first obvious sign being that the chest cavity is filled with blood.
Further investigation is likely to reveal a rupture in the aorta.
Little is known about why aortic ruptures happen.
Specialists who have conducted microscopic examinations of ruptured aortas have usually found nothing to indicate why the wall failed, unless it was the site of an aneurysm - an area of dilation in a blood vessel where the wall thins and bulges under pressure.
It is possible ruptures result from a congenital defect, or some form of degenerative disease process that weakens the wall.
Some experts wonder whether strongyles during their migratory phase might play a part in a small number of cases, but they have yet to find a "smoking gun".
Another possible cause suggested has been copper deficiencies, with ruptures in a uterine artery in mares linked to such a deficiency.
Copper is needed by one of the enzymes that makes strong bonds between elastin and collagen in blood vessel walls. However, aside from the uterine artery ruptures aforementioned, no direct links have been established.
There is little to indicate gender, age, breed, or workload play a significant part in ruptures.
Not all sudden collapses and deaths in horses can be aattributedto aortic ruptures. In fact, the rate of aortic failure in horses, as far as is known, is well below that of humans.
Hickstead was 15 when he died on November 6. Gribaldi, the sire of the world's top dressage horse Totilas, also died of a ruptured aorta, at the age of 16 in 2010.