This is part of NPR's series Inside Alzheimer's, about Greg O'Brien's experience of living with the illness. This time we hear from Greg's wife, Mary Catherine.

Greg and Mary Catherine O'Brien will celebrate their 38th wedding anniversary next month. She knows him better than anyone — his moods and sense of humor, his devotion to their three children and his love of Cape Cod.

When Greg was diagnosed with early-onset Alzheimer's five years ago, Mary Catherine had already started to notice little differences in his behavior, she says. Now, as his symptoms continue to worsen, she takes a moment to reflect on how the disease is changing their marriage, sometimes in unexpected ways.