Ataxia, optic atrophy, spastic tetraparesis, and cognitive decline are suggestive of progressive neurologic deterioration, likely of metabolic etiology. 

MRI with demyelination sparing U fibers is especially concerning for a leukodystrophy 

Arylsulfatase-A testing would fit the diagnosis of metachromic leukodystrophy 

– disorder can present with infantile-onset, juvenile-onset (in this case), or adult-onset