Complex sentence: First some MRI-abnormal areas with hidden damage appear in the brain and spine (NAWM, NAGM, DAWM), followed later by leaks in the blood--brain barrier where immune cells infiltrate causing the known demyelination and axon destruction.
Split into simpler sentences: First some MRI-abnormal areas with hidden damage appear in the brain and spine (NAWM, NAGM, DAWM). Second, there are leaks in the blood--brain barrier where immune cells infiltrate causing the known demyelination and axon destruction.