The compression developed from unknown advanced arthritic changes in his neck.
Key was never lame or showed any resistance/stiffness while under saddle to give any indication of the condition the was silently coping with; talk about heart.
One day in he looked uncomfortable and stiff in the stall, unable to lower his head fully and was immediately attended by Veterinarians, then taken to a clinic where a surgeon delivered a devastating diagnosis. Nothing could be done, the nerves and spinal cord were affected. The decision was made to free him from his pain. He was only 11, arthritis is cruel in its selection.