So, I've been doing some skulking around and I didn't really see anything like this, but if there is just let me know!
Anyway, cerebellar abiotrophy is found in horses, dogs, humans, and cats. But, CA causes the death of neurons in the cerebellum. This leads to tremors in the head and lack of balance (ataxia). One of the most prominent factors is a wide, stiff stance and some animals are unable to move from specific positions. It isn't always severe, but it can be. Some animals with CA are startled easily and are prone to falling. In any event, CA causes fluctuating degeneration of responses/reflexes. Some of them may fall over while walking or may stumble a lot.
In cats, ataxia shows around 18 months, blindness in 36.

I think this could be nonlethal, but would bring penalties to lions. For example, males cannot be kinged or go out on patrols. Lionesses cannot hunt, but can be broodmothers, and I feel their stats may be reduced as well. Particular stats that'd be affected would likely be speed, agility, stamina, and strength and they could reduce over time.
Lineart may not need to be changed, but I was thinking possibly lines similar to Primal Felis or Adol. Females - wide/stiff stances. Also, I was unable to find many picture references, so if you find one, drop it by. c:

It could add interactions, such as the following:
"Cub passes by you and you offer them a greeting, causing them to nearly lose all their balance... Did you almost give them a heart attack?"
"You spent your day playing with Cub - you can't help but think they always look like they're marching around."
"Lion is trying to attract some attention from the lionesses with squinted eyes, but you decide to let it slide... He's talking to a chameleon..."
"Upon coming back from sparring, Lion is suddenly greeted by one of the cubs. Lion fell over from the shock - is he alright?"
"Lioness is convinced there's something in her eyes, and she's asking you to look. Only problem is, she's talking to a rock."
"You notice Cub chasing a butterfly - it's cute how ungraceful they are, but you wince as they fall over."
"Lioness has cub duty today, but as she stumbles after them, she nearly falls into you."
"You decide to try to spar with Lion. He seems honored and quickly stands on trembling legs - was he walking normally for once, or was that just your mind?"
"Your sub males were out with the kittengarten. When everyone returns, Lion is stuck in a low position... What's he stalking? Or can he just not move?"

I can't see there being any birth notifications, but if anyone thinks of one - tips, criticism, ideas... anything can be helpful. c:

Possible Birth Notification:
Lioness notices one of her cubs seems a little unsteady. Maybe you should keep an eye on them.

Or perhaps instead of a birth notification, one may get a notification when ataxia and blindness come into play.
Lately Lion has been acting like a newborn cub - did they forget how to walk, or is there something going on?

You noticed Lion hasn't been seeing very well. They mentioned that they were afraid that the sun hasn't shone for a few days. Have they gone blind?