As shown in the attached pictures, my boxwood are dying slowly, branch by branch. Hopefully you might be able to identify the cause - as well as any treatment available. We have several dozen of them here on the Eastern Shore, Chestertown area.
Based on your photos the dieback may be due to volutella, a fungal disease and/or vole damage. You will have to look for this.
Voles - Look for holes about the size of a silver dollar around the base of the plants. Voles are a type of meadow mouse that feeds on the roots of trees, shrubs, and herbaceous plants and causes dieback. Control involves trapping with mouse traps baited with apple slices or peanut butter. See our publication on voles extension.umd.edu/publications/PDFs/FS654.pdf
Volutella is a fungal disease that causes branch dieback. All you can do is prune out all dead wood. The plants should be thinned to improve good air circulation and light penetration. Shake out and remove all old leaves that remain in the branch crotches and from around the base of the plants. See our website for more information http://extension.umd.edu/learn/volutella-blight-boxwood
and our publication http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG52_IPM_Boxwood.pdf mh