How effective is skelaxin (metaxalone) in helping with the spasmodic symptoms associated with myotonic dystrophy (related to muscular dystrophy)?

Muscle relaxants. It's really just trial and error. It does have the advantage of being one of least likely to be sedating. You may need to trial Baclofen or Zanaflex (tizanidine) if skellaxen does not work.
Not very. Muscle relaxants such as skelaxin (metaxalone) generally have little or no benefit in this situation. There are meds available that can be very effective for the spasticity. Discuss this with the neurologist.