Chewing fingers | ADHD Information

My 9 year old son with ADHD and Sensory Integration Dysfunction has begun t chew on his fingers. Just randomly there does not seem to be a trigger but he does it when he is playing, or watching tv or really anything. Anyone else have this issue?  Ditto for my 10 yr old son with adhd and sensory integration dysfunction.  Just started semi-biting his nails, a brand new habit.Will does this also, especially when playing video games.   I can't say too much, I seem to do it too though...