“I was dumbfounded,” Ms. Sherman-Lewis said. “It’s O.K. for other people’s little grandchildren to be running around sometimes. What am I supposed to do, keep him in a crate in the car?”

Like so many caregivers, she has discovered that along with the abandoned career, the hands-on tasks, the medical scheduling, the insurance tussles and the disrupted sleep, she faces another trial: social isolation.

“It’s hurtful,” she said. “You need friends more than ever.”

But where are they? Betsey Brairton, 48, cares for her mother, Sue, in rural Olean, N.Y. The elder Ms. Brairton, 79, suffers from spinal stenosis, arthritis and lingering damage from a stroke, so she has limited mobility. “We hardly go anywhere, and nobody comes here,” said her daughter. When she does leave for an hour or two, she’s afraid to put down her cellphone.

Though a couple of friends occasionally invite her out for dinner, “I can’t commit to anything, in case my mom is having a bad day,” Ms. Brairton said. She has begun to worry that when she does spend time with others, her narrowing life leaves her with nothing interesting to say.

Those who work with caregivers know this phenomenon well, especially when the cared-for person has dementia, a particularly arduous responsibility.