Briana Nathaniel, 14, lies listless in her hospital bed, exhausted from days of nearly unbearable pain. Her voice is small, barely a whisper, and even lifting her hand seems to take enormous effort.

Half an hour later, she’s transformed. A virtual reality visor covers half her face, but it can’t hide her grin. She’s sitting up, her hands waving around as she calls out, “Hi, dolphin! Hi, whale!”

Nothing, Briana says, can completely remove the pain she experiences from sickle cell disease. But a virtual reality game that transports her to an underwater adventure helps more than almost anything else.

“It calms you down a lot because you’re not thinking about the pain and stressing about when it’s going to stop,” Briana said. “It’s like you’re just a scuba diver, and you’re swimming with the dolphins and the whales.”

Briana is part of a study at UCSF Benioff Children’s Hospital Oakland testing the use of virtual reality technology to distract kids with sickle cell disease from the at-times excruciating pain that is a hallmark of their condition. Twenty-five children are enrolled in the study so far, and five more will join over the next few months.

Sickle cell disease is an inherited disorder that causes blood cells to take on a distinctive sickle shape. Blood cells, which typically look like plump doughnuts, are soft and pliable, making them easy to squeeze through narrow vessels. The rigid, sickled cells can interfere with blood flow, causing sudden, extreme pain.

“One of the best descriptions I heard was it feels like you have little shards of glass moving through your veins, and every time your heart beats those shards move another inch along,” said Dr. Anne Marsh, director of the pediatric sickle cell program at the hospital. “Thinking about that, it makes my hair stand on end.”

Episodes of pain can come out of nowhere, and often can be resolved only with a trip to the hospital and intravenous pain medication like morphine. Even then, it may be days before the pain fades enough for a patient to go home.

Marsh said nurses and doctors at the Oakland hospital already use distraction techniques — usually movies, sometimes video games — to help patients get through a pain crisis. Even so, she was shocked when she saw how some kids reacted to the virtual reality therapy.

“It’s amazing. You walk into the room, and they’re like a different kid for these 30 minutes,” she said.

Similar painkilling techniques are being studied at a handful of other sites around the country — in particular, in helping burn patients — and many experts in virtual reality say there is a lot of untapped potential for the therapy in health care settings.

Pain, and how to treat it, is an urgent topic of research as the United States struggles with a crisis of painkiller addiction. Drug overdose is a leading cause of death in the U.S., killing about 47,000 people a year, and opioids account for more than a third of those cases.

Among the alternatives to prescription drug treatments are many nonmedical therapies, like meditation or acupuncture. With virtual reality, the basic idea is that it distracts patients from their pain. The mind, doctors say, can only focus on so much stimulation at once. Give the brain something else to focus on and the pain a person experiences will diminish.

The more pain someone is in, the more distraction they need. So while watching a movie or reading a book might work for someone who has mild postoperative discomfort or is recovering from a broken leg, it’s not going to be stimulating enough for someone with severe pain, like that caused by sickle cell disease.

“The more immersive — the more we block out the rest of the world — and the more engaged you are in the VR, the more we distract you from the pain,” said Walter Greenleaf, director of the Mind Division at Stanford, who was an early pioneer of using virtual reality technology as medicine. “If you’re going through chemotherapy or dialysis, a little bit of distraction may work. Whereas if you’re in a burn clinic and having wounds debrided, then you need a lot of distraction.”

In the virtual reality technology now being used in pain research, patients typically are immersed in a gamelike simulation, where they’re encouraged to look around and interact with the environment.

The study at Children’s Hospital Oakland was designed by an independent developer, Simon Robertson, with a background in video games and animation. His thought was to create a soothing environment that would also be fun and exciting for children and teens.

His first virtual reality setup involved a laptop and other hardware that was clunky and hard to drag around the hospital. The one he uses for the study is simple — with an adjustable visor that slips over patients’ eyes, a remote control, noise-canceling headphones and a cell phone onto which Robertson has uploaded his game.

Patients are “submerged” into an underwater world, where they drift forward as though they were in a submarine-type vehicle. Soft music plays in the background, over the rush of deep ocean water and the distant squeaks and moans of aquatic animals. Fish slowly come into focus from the front and sides, then a giant sea turtle and a pod of playful dolphins. A giant whale for a moment nearly fills the scene.

Users hold a remote control in their laps and are encouraged to push a button that shoots a ball into the water. When the ball hits a sea creature, it changes color. Patients who want a bit of competition can collect points for hitting the fish, but they can also just sit back and take in the sights. The game lasts for 15 minutes, and patients often play it back to back.

Robertson wanted patients to feel stimulated and engaged — but not to the point where it became stressful.

“I wanted to transport patients outside the hospital. I wanted it to feel magical, but also grounded in reality,” he said. “Outer space or a roller coaster might be too intense.”

Also: no sharks.

“Often I’m dealing with patients who are experiencing a 9 out of 10 pain. And they’re on a lot of drugs,” Robertson said. “I want to make sure I’m not adding any stress on top of that.”

The game had to be accessible for patients who can’t move their heads or aren’t able to hold a remote control, while the equipment had to be easily sterilized, portable and easy to use.

When Robertson approached the hospital with his idea, doctors were immediately interested, he said. They all agreed that children with sickle cell disease would be the best audience.

Their pain can be overwhelming, even when hospitalized, and there are too few treatments for them. They probably will always need some kind of opioid to cope with their worst pain episodes, but often not even the strongest drugs can fully alleviate their discomfort.

Studying these children could not only reduce their pain, but inform how the therapy might work for other patients, doctors hope. Robertson, who has started a company in Oakland called kindVR, plans to collaborate with Children’s Hospital Oakland on several more studies, involving adults with sickle cell disease and children with cancer.

When Briana took off the visor at the end of her game, she looked — as Marsh had noted with other patients — like a new person. She sat up straighter in bed, grinning, her eyes lit up. She spoke, her voice now strong and impassioned, about the future of virtual reality and how much it could help patients like her.

“I’m still in pain,” Briana later admitted, quietly, while her mom sat nearby, getting a demonstration of the technology. She, too, was laughing, periodically reaching out to “touch” dolphins or calling out to them.

“But at least I’m calm now,” Briana said. “It’s not so bad.”

Erin Allday is a San Francisco Chronicle staff writer. Email: eallday@sfchronicle.com Twitter: @ErinAllday