What is the fine-grained topic of the following text: Before the procedure the three could read only the largest group of letter on a special eye test chart, as they had retinitis pigmentosa, an inherited condition which incidentally affects up to 25,000 people in Britain, that slowly constricts the vision, but 18 days after being injected with the stem cells , their sight had improved the point where they could read three letter sizes smaller. Only return the topic.
Improvement in vision due to stem cell treatment in people with retinitis pigmentosa.