The moment Sasuke saw his beloved older brother again, he couldn't even breathe.
It felt like an eternity since he had discovered the truth about Itachi, and even longer since he killed his own brother with his hands. Sasuke hadn't slept since then. His restless mind wouldn't let him surrender into unconsciousness. No, that wasn't entirely true. Sasuke slept a few hours every couple nights or so. But his dreams were stained with the eerie reminder that he killed Itachi, who had given up everything for him. He killed his brother over and over, again and again in his dreams. He would wake every morning covered in sweat, tears pouring from his onyx eyes. His mind thought of nothing but his brother and how the village had wronged him for so long. He grew bitter and wild as time passed. He knew Madara was only fueling his rage, but couldn't help but play right into his hands. Sasuke hated feeling used, but hated the village more. Sasuke felt his mind slipping every so often. He went into fits of rage that he couldn't remember hours late. He was quick to snap at anyone and everyone. What was worse though was every time he looked at his palms, he saw Itachi's blood which had left his fingers stained a deep shade of red. He knew Itachi's blood wasn't really there, but he couldn't not see it. Every time he looked at his slender fingers, he saw red.
So the moment when he saw his brother jump gracefully from tree branch to tree branch above him, he froze. He was almost positive he had to be dreaming. The man that passed him couldn't possibly be his brother. It didn't stop Sasuke's heart from fluttering though. The man continued past Sasuke, turning back to look Sasuke right in the eye.
And then Sasuke knew.
This was no hallucination, this was really Itachi.