This paragraph was near the end of the essay. Says it all of the emotional losses of this disease.
"People who love each other — husbands and wives, parents and children, friends — cross borders throughout their relationships. If they are lucky, the borders are permeable and either person can cross back. For dementia sufferers the borders are not permeable. The crossings are one-way only. So each week I stand at another border waving goodbye to someone I love, and she doesn’t know I am waving goodbye. She just smiles back her enormous smile from the foreign country where she now lives — and says thank you."