Anatomically, a foramen is a hole or passage, usually through a bone. The foramen magnum, or "great hole," is the hole in the bottom of the skull through which the spinal cord passes in order to connect to the brain by merging with the brain's lowermost portion, the medulla oblongata. Passing through the occipital bone, it is the largest opening in the base of the skull, though there are several others. Several other nerves, blood vessels and tendons also pass through the hole.
When evaluating skeletons, the angle of the foramen magnum tells scientists whether the head of the animal in question was carried in a horizontal position, as in most four-legged creatures, or in a more vertical position, as in bipedal animals. Anthropologists also have used this measurement of the position of the hole to determine the ability of human ancestors to walk upright. In modern humans, the passage is located more toward the back of the head than in the great apes, so that less heavy musculature in the neck is required to support the skull.
Occasionally, congenital issues lead to a condition called foramen magnum stenosis, in which the opening is not large enough for the spinal cord to easily pass through. This most often happens in conjunction with achondroplasia, a condition in which bones and cartilage do not form properly, causing a form of dwarfism. In all children who have this condition, the opening is smaller than normal. This often does not present a danger to the patient's overall health, but in about 5% of children with achondroplasia who suffer from an abnormally small passage, the reduced size causes compression to the spinal column, which can be dangerous and even fatal.
In cases of achondroplasia that include spinal cord compression, surgery is necessary to prevent long-term disability or death. Considerations of this surgery can be complex because of the special needs of children born with the condition that leads to the malformation of the foramen. The best results usually can be expected from surgeons who have had experience not only with the stenosis, but also with achondroplasia.
Children with achondroplasia have a much higher mortality rate than the general population. A large percentage of these deaths are believed to be related to spinal cord compression. Because of the danger of sudden death, it is important that these children be treated as soon as possible to correct the problem.