Please check your pachysandra for evidence of a sucking insect called euonymus scale.  Look along the stems and leaves for small white oystershell shaped insects that can be scraped off. They can cause leaf yellowing, stunting, and dieback. See our Plant diagnostic Website for more information http://plantdiagnostics.umd.edu/level3.cfm?causeID=643
If you do not find evidence of this insect, please send us another email so we can see else you may be dealing with.
mh