Rangers officials said that Hamilton saw an optometrist and was diagnosed with ocular keratitis, a drying of the cornea caused by too much caffeine and energy drinks.
Hamilton, who hit his league-leading 43rd homer in the fifth inning of the Rangers' 5-4 win, said he was "fixated" on one spot with his eyes, and it was as if he couldn't stop staring. He said it started bothering him last Saturday in Arlington before things got worse.
"It's me we're talking about here," Hamilton told reporters prior to Monday's game. "Guys, it's me -- it's Josh. It's going to be something weird."
Hamilton, 31, said he plans to back off the caffeine, which included chocolate after games and an energy drink before games -- not to mention what he was consuming all day long.
"I was loading up on caffeine, and I'm out there in the bright lights," Hamilton said. "I can't control my eyes. They are stuck."
Hamilton left in the middle of last Tuesday's loss to the Los Angeles Angels in Anaheim after telling manager Ron Washington he was having trouble seeing.
Hamilton tried to participate in batting practice on Thursday in Anaheim but had difficulty taking fly balls and was sent back to Arlington for more tests. He did not play at all in this past weekend's three-game series in Seattle.
Hamilton said he knew he was ready to return after hitting in the cages and running the bases late Sunday. The All-Star admitted it was difficult to leave the Rangers during an important road trip.
"It was hard," he said. "Obviously, we know what the situation is. ... We want to win as many as we can heading into the last 10 (games). I talked with (general manager) Jon Daniels and (head athletic trainer) Jamie Reed and said, 'Let's figure it out quickly. The same things aren't working.'
"If I had been with the team, I would have been drinking more caffeine."
Texas is 9-7 in games played this season without Hamilton in the lineup.
The five straight games are the most Hamilton has missed consecutively this season. He missed three games in Toronto in late April-early May due to back stiffness and another three games in Houston with an intestinal virus.