Actually, this website I just found says 359.8 is the correct code:
Neuromuscular Conditions
334.8 Ataxia telangiectasia (other spinocerebellar diseases)
356.1 Charcot Marie Tooth Disease (peroneal muscular atrophy)
359.1 Duchenne / Becker Muscular dystrophy (hereditary progressive muscular dystrophy)
359.1 Facioscapulohumeral (FSHMD) (hereditary progressive muscular dystrophy)
359.1 Limb Girdle MD (hereditary progressive muscular dystrophy)
334.0 Fredreich Ataxia
333.4 Huntington disease
359.8 Mitochondrial myopathy (other myopathies)
359.2 Myotonic dystrophy (myotonic disorders)
359.1 OPMD (Hereditary progressive muscular dystrophy)
335.0 Spinomuscular atrophy, type I (Werdnig-Hoffmann disease)
335.11 Spinomuscular atrophy, type II (Kugelberg-Welander disease)
335.19 Spinomuscular atrophy, type III (other spinal muscular atrophy)
334.2 SCA (primary cerebellar degeneration)
http://www.kumc.edu/gec/prof/gcicd9.html
I'd probably go with their advice...