AFTER Mark Donham’s wife, Chris, fell under the spell of early-onset Alzheimer’s, he doubled down on his marriage vows. He quit his job as a well-paid sales representative in the printing industry and became his wife’s 24-hour caregiver: dressing her, doing laundry and scheduling social visits with friends. Faith, hope and courage became his new mantra.

As Alzheimer’s slowly progressed and Chris became frailer, their lives narrowed. To explain their painful emotional journey to friends and family, Mr. Donham, who lives in Portland, Ore., began making videos and posting them on YouTube.