@Masa: I don't think that the pic is showing a blind spot. Even if Kakshi's Sharingan eye has deteriorated slightly, he still has his other eye which should be able see normally.

@XanBcoo:

I...don't think you got my point. I don't see what this reply has to do with what I said. Kakashi's question was addressed to Itachi about Itachi's eyesight. Why anyone would pull from that that Kakashi has experience with deteriorating eyesight himself is beyond me.
Its simple... it't not something thats been mentioned for other Sharingan users, so it doesn't apply to them. Therefore if Kakashi still only has the normal Sharingan, he can't possibly know about this side effect.

IMO, the setting of the question and the tone of Kakashi's voice (if I may go so far to say), suggests this is an entirely rhetorical question. He knows Itachi's eyesight has gotten worse, and if my suspicion on normal Sharingan users not having worsening eyesight is correct, he could only know what caused it if he has now obtained the Mangekyou Sharingan, as it only happens to Mangekyou users. It's all just Kakashi's cool way of indirectly telling Itachi he now also has the Mangekyou Sharingan.

If this isn't enough, Itachi's next statement is the final proof: "Kakashi-san, you couldn't have..."

Couldn't have what? Joined MENSA because you're so smart by figuring all this out? He has just finished praising Kakashi all the way through the conversation for his insight, and now he decides to get shocked? I don't buy that.

Kakashi has had plenty of experience with it. It's been used on him before, and he's been able to deduce the effects of MS on the user as a result of Itachi's actions.
No, he doesn't. Being subjected to an attack once is not plenty of experience. Certainly not for deducing intangible weaknesses of an attack.

He also remembers what Kisame (who works alongside Itachi and knows about MS's effects) said.
Kisame told Itachi not to use his eyes too much because it is dangerous for him, not necessarily his eyes alone. Obviously if Itachi drains all his chakra by using the Mangekyou he is going to collapse. I seriously doubt Itachi would have revealed such a severe weakness of one his main attacking options to Kisame. They are in Akatsuki, not a friendly after school club. My money is on Kisame simply telling Itachi not to drain all his chakra at once.