Due to a head injury, Sherlock forgets all the little facts he knows that allow him to make deductions. He can't see hairs on someone's sleeve and know if they are human or animal, he can't name perfumes, he can't tell how old bruises are or any of that stuff.

In fact some of his personal memories are scrambled as well, he can generally remember who people are, but he's fuzzy on how he met them and sometimes his exact relationship with them.

The absolute worst part is that he keeps forgetting things he just learned (or relearned). How can he solve any cases like this?