When I got up, Mom was already awake. I could hear her rummaging in the kitchen. Through the glass doors in the living room, sunlight flared so brightly off the hillocks of snow that I had to shield my eyes.

It was my birthday, and I was afraid.

What if my husband had neglected to take Mom shopping for a card? Once Mom found out it was my birthday, she would be devastated that she had forgotten and had nothing to give me. Little matter that she has dementia and can’t remember what we did two hours ago. Birthdays are a big deal to Mom.

Birthdays are not a big deal to me. I hate growing older. I don’t mind if Mom forgets my birthday as long as she still remembers me. That someday she might not recognize me has been my biggest fear ever since Mom got dementia. I can’t imagine anything more devastating than being forgotten by your own mother.

When Mom was diagnosed with vascular dementia seven years ago, I was told she did not have Alzheimer’s disease. I hoped that meant she would never forget her family. But as Mom’s dementia progressed, I realized that I had no idea whether vascular dementia could be as bad as Alzheimer’s. I really didn’t understand the difference.

Recently I decided to find out, and the answers were not very reassuring. Although vascular dementia and Alzheimer’s disease are hardly the same, with time the differences start to blur.

“Theoretically, they are two different beasts,” Dr. Ronald Petersen, director of the Alzheimer’s disease research center at the Mayo Clinic, told me. “In reality, there’s a lot of overlap.”

Said Dr. Claudia Kawas, a geriatric neurologist at the University of California, Irvine: “Many years ago, when I started, we were spending all this time trying to educate people about how Alzheimer’s is different from hardening of the arteries. Now we’re lessening that distinction.”

Vascular dementia is caused by stroke or an accumulation of micro-strokes so small they may not be noticed when they occur. These cause brain tissue to die. Researchers are still not sure what causes Alzheimer’s, though most suspect that amyloid plaques and tangles forming in the brain are culprits.

Traditionally, it was thought that vascular dementia has a more sudden onset and will “plateau” — that is, not get any worse until another stroke hits. If that never occurs, the dementia might never get worse. Alzheimer’s, experts said, always starts gradually and always progresses inexorably.

Nowadays, doctors describe a different set of phenomena. Vascular dementia can progress gradually. Alzheimer’s can plateau for several years. And in the later stages, said Dr. Kawas, Alzheimer’s and vascular dementia look a lot alike. People forget everything in reverse order. First the grandchildren. Then the children. Then the spouse.

According to both researchers, it is very unlikely for someone with dementia to have only vascular dementia or only Alzheimer’s. That was the most disturbing news for me. Older people with dementia almost always have both. “The odds are,” said Dr. Peterson, “your mother has both.”

The sleeping house was quiet except for the sound of Mom’s ministrations to the pots left overnight in the kitchen sink. Outside, dazzling spears of ice hung down from the roof, ticking and dripping as steadily as a metronome. Will Mom live long enough to forget me? Will the day come when she no longer worries about remembering my birthday because she no longer knows that I exist? I think she would rather die first.

In the kitchen, in the glare of the late morning sun, Mom heard me approach.

She hurried to the door, her arms raised for a sudsy cantata, her feet tapping, her face ringing with smiles. “Happy birthday to you,” she sang, swaying to her own chorus. “Happy birthday to you, happy birth-,” she belted out the high note in a trilling soprano, “day, dear Celia. Happy birthday to you.”

We laughed as I hugged her tight, both of us amazed that she had remembered. Moments later, she said, “Oh, there seems to be something here for you on the mantel.” She brought me her card. My husband did, after all, take her shopping. The sentimental birthday-for-a-daughter card was filled with expressions of love and appreciation with many underscorings and “So True” notations in blue ink.

I am made precious by her love. And if one day she does forget she had a daughter named Celia and tells me what a pretty name that is, I will give her a card with many underscorings, and a song for her birthday, and a daughter for all seasons. And she will be made precious by my love.