Developmental coordination disorder, or DCD, is uncommon, and was recently discovered. I was the first in my county, possibly even state, to be diagnosed with it. It is commonly mistaken for arthritis. It is a mutation. While in utero, the muscles are developed a lot. When these muscles aren't developed correctly, it results in DCD. This slows (or in some cases, even stops) certain social checkpoints, such as tying shoes or walking. While in most cases it does not result in a child being confined to a wheelchair, they may find they trip constantly, tire quickly, and have overall trouble balancing. Most times, this can cause distancing from peers, because they have trouble playing catch or riding a bike. I was unable to ride a bike properly until after age 8. Handwriting with children of DCD is often poor, as a result of tired, aching hand muscles that simply are not comfortable in the typical writing hand position.
I just scratched the surface, but I hope that you'll cover DCD soon!