The DRSC has just updated the DIOPT-DIST tool to allow for more 'forgiving' searches in the search field, "Or disease full text search."
Before, a search with "Parkinson" might pull up different results than a search with "Parkinson's" or "Parkinsonism" would deliver. Now, all these terms pull up the same list, and even a misspelling like "Parksinson" will get you where you mean to be.
The back end does not have an extensive set of disease synonyms, so the DRSC still suggests trying different synonyms of diseases (or OMIM IDs). But hopefully the fuzzy search will make things a whole lot easier.