NEDHSIL is the alternative name for MEF2C-associated syndrome. It describes the core clinical features without naming the gene. You might see it in older papers or clinical notes.
Why the different names: Medical genetics sometimes names syndromes descriptively (by what they look like) before the gene is discovered. It's like calling "Type 1 Diabetes" "Juvenile Onset Diabetes". The descriptive name came first, and the genetic cause was identified later. NEDHSIL = MEF2C deficiency. They're the same condition, just named differently.