Originally Posted by Karlin
It definitely can be associated with bowel movements and urination. This is a common symptom for humans with SM too -- because when you strain, it increases the pain (hence sneezing can be really painful for people with Chiari malformation and/or SM). Scratching is a response to pain and discomfort in the syrinx area usually, so the pressure from straining would cause that area to become more uncomfortable. Loss of bowel and bladder control is also a human symptom of SM.
The air scratching, and scratching to one side (the side the syrinx presses on most) are also possible signs -- air scratching where the foot never makes contact with the skin, in particular.