When we refer to an actor’s performance as breathtaking, we’re usually engaging in hyperbole. Rarely if ever do we mean it actually affected our ability to breathe.

But during “Still Alice,” watching the vital, sharply intelligent woman played by Julianne Moore slowly cede her mental faculties — and, most painfully, her identity — to Alzheimer’s disease, I found myself frequently needing to gulp in big breaths of air, merely to steel myself for the next scene.

Of course this is partly due to the nature of the material. There’s no way to tell a story about Alzheimer’s that isn’t ultimately devastating, and writer-directors Richard Glatzer and Wash Westmoreland clearly have no intention of sugar-coating the cruelty of this disease.

But as its title suggests, “Still Alice,” based on the novel by neuroscientist Lisa Genova, is about one woman, and thankfully we have Moore, one of our most sensitive and nuanced actresses, in the role.

We first meet Alice, a linguistics professor at Columbia, as she’s celebrating her 50th birthday with family. Chic and accomplished, she’s managed to work and travel and raise three adult kids in a beautiful home. She and her husband, John (Alec Baldwin), also an academic, live in a lovely brownstone with a great kitchen.

But one day, giving a lecture, Alice suddenly stops, mid-sentence. She can’t remember a key word. She recovers nimbly with a joke, but we shudder. Back home, taking her usual jog around campus, she gets lost. The camera blurs, along with the connections in her brain. We shudder again. We know what’s coming.

The meeting with the neurologist, for example, where her memory lapses become clear. Her first warning to her husband that something’s wrong — he brusquely dismisses her worst fears, as many would to protect themselves. The diagnosis of early onset Alzheimer’s. The devastating news that her children might carry the gene.

Time passes — too quickly, of course. Alice tries to keep working, but that proves unrealistic. Also unrealistic: that loved ones, in such circumstances, would behave like saints. Of course they don’t. Alice’s younger daughter Lydia (an excellent Kristen Stewart), going through a self-indulgent phase, struggles to make room for her mother’s affliction. And John, as subtly portrayed by Baldwin, has trouble balancing his devotion to his wife with fears for the future — and his own career goals.

But though these two relationships are key, the movie distinguishes itself from others about Alzheimer’s by being, essentially, about Alice’s relationship with Alice. It’s about her fight to retain what she can of herself — to remain the person that she knows.