Galatians 4:13-15 ESV
You know it was because of a bodily ailment that I preached the gospel to you at first, and though my condition was a trial to you, you did not scorn or despise me, but received me as an angel of God, as Christ Jesus. What then has become of your blessedness? For I testify to you that, if possible, you would have gouged out your eyes and given them to me.
There is often speculation about what Paul's "thorn in the flesh" might have been. These verses seem to indicate it had to do with his eyes. The "bodily ailment" was severe enough to be a trial for the Galatians. In the last verse, Paul says they would have offered their own eyes to him to help him.
Considering Paul seeing a great light that blinded him on the way to Damascus, it is not that far of a stretch to think his sight was weakened or damaged.
He will speak of writing big letters by his own hand elsewhere, another clue of the ailment of sight.
His natural sight has diminished, but his spiritual eyes now see clearly.