What it is
Glycogen is the body's energy store: when strength is needed, glucose is quickly drawn from it. In this disease one of the enzymes needed to build glycogen properly is missing, so glycogen with a faulty structure builds up in the cells and the body cannot use it. The muscles, nerves and liver are hit hardest.
The disease is inherited: a kitten falls ill only when it receives the faulty gene from both parents, and the parents themselves can look perfectly healthy.
The signs appear in youth, when a kitten should only be growing stronger and bolder. That is why any weakness, trembling or reluctance to jump deserves attention rather than waiting for it to pass. The disease affects the body, not the character: the kitten stays curious and affectionate, only moving becomes harder.
Signs you may notice
A kitten is born weak, or grows like its littermates at first and later begins to tremble, weaken, walk on stiff legs and move less. Sometimes it runs a fever.
When to see a vet
A weak newborn kitten that does not suckle needs a vet immediately; if a young cat weakens, trembles or walks stiffly, book a check-up. When getting a kitten, ask the breeder about the parents' health testing.
This is general information, not veterinary advice. For your own cat's health, tests and treatment, see a vet.