He pinched the bridge of his nose. When he removed his fingers, it seemed as though a flyaway strand of hair had been caught up in his pincer and now dangled irritatingly over his eye. He must have failed to dislodge the follicle, for it still caused his vision to writhe and squirm. He sent his fingers on a sortie, careful to draw his nails in like a cat with its claws when preening itself. And still the prickling sensation continued, as his brain fired barrage after barrage in protest at this trespass on its visual cortex.
It didn't feel like  piece of grit, since there was none of the usual burning sensation that usually accompanied such a thing. He bathed the eye in water and though that appeared to have shifted the interloper, soon enough back came the impression as he was drying his face with a towel. He pressed his face right up close into his mirror and lifted his reptilian lid to scour for the troublesome detached cilium. The problem was his hair was turning grey, so it was harder to isolate in contrast to his skin tone.
Still there was nothing there. He didn't like to do this much, since he didn't like to admit he was growing older, but he even put on his spectacles to try and bear down on the invisible interloper. It only made things worse, like a magnifying glass, though he still couldn't find anything trespassing across the focal plane of his lens.
He removed his specs and scanned them for fluff, dust or hair of their own. He wiped them clean on his shirt, maybe it was the little stains in the glass itself. But no amount of polishing yielded him any better vision. He inspected the goggles and was appalled at just how much gunk and grime was wedged beneath the nose pads. He cleaned it out with a pin and noted its flocculent texture. It was disgusting, especially when he considered that he donned his glasses so rarely and yet here was such an accumulation of his shed skin, pressed and layered beneath the pads' plastic.
But no amount of scouring ameliorated the hairline crack laying across his sight. If anything it had got worse, now taking on the appearance of some of the strands of a spider's web. He even convinced himself he could see part of the bloated body of the spider. Clearly his mind was running away with itself and playing tricks on him. He was so damn tired and that couldn't have been helping his overwrought brain.
As he turned his head in despair, his glass lenses seemed to catch the bathroom light in them right at the side of his vision. That was all he needed to compound his current inconvenience. He switched the light out and two tiny electrical flashes seemed to fizz across the periphery of his perception. Or just below where the glass lenses ended. This was getting ridiculous. Now his addled brain must have been overcompensating, or going into hyper-drive perhaps, as he failed to clear this agitation from its sensory apparatus. He could take a hint, he needed to go to bed and wake up fresh, when hopefully his brain would have settled and the hair itself might have been shaken free.
The next morning he awoke and while not quite a Gregor Samsa moment of imagining himself turned into a cockroach, it did appear as though an insect might have inhabited him and was busy crawling across his retina. He shook his head, partly to clear his watery vision, but also in terror. He slowly unsheathed the eyelid, but still some mote was fluttering and wafting there. It was like viewing bacteria under a microscope. That his eye had become a petri dish prison and insects were slithering over his eyeball. He felt turned utterly inside out. Tears on the inside of his peepers.
He sat up in bed and rubbed his eyes with the heel of his hand. Again two shooting lightning bolts seared past him. He saw them and yet knew that they weren't there, so that it was his brain which had extrapolated or maybe constructed them. So much for his rested and clear grey cells. Perhaps such corsucations were an after-image of the brain's flare of distress. Perhaps he was being afforded an insight into the brain's electrical transmissions themselves?
He got out of bed. Standing on the floor, his eyes were filled with laboriously moving flecks, like the motion of a lava lamp. They formed shadows on his vision and this freaked him out as he wondered if he was losing his vision entirely? Was there a micro-organism living inside his eyes, scuttling over his irises so that they amplified their own size like when a bug crawls over a movie projector? Either it was on the move the whole time, or there were many of them. A veritable army of invaders parasiting his brain perhaps?
Though such notions made him feel queasy, he managed to stumble over to his computer. Squinting through his compromised eye, he managed to discover that he was probably experiencing floaters. Just like the degrading ozone layer that protects the earth from the sun's rays, the protective gelatinous barrier of his own eyes was deteriorating. Thus light was being refracted in strange, debilitating ways his brain was struggling to process. Or else it was projecting shadows where none had previously existed. He flopped back against his chair with relief, that at least he wasn't playing host to some alien invasion. He knew floaters were not themselves threatening blindness or any more serious impairment.
But then he was struck with a wave of despondency. It still represented a decaying, a degeneration of his physical being. Another marker on the migration towards death. And everyday now, he would have reminders of putrefaction's flies hovering right in front of him where they could not be denied. The veil of permanent darkness was drawing in over him.

Perhaps unsurprisingly this story was prompted by me waking up a couple of weeks ago and having these strange sensations appearing in my vision in my right eye. it took me a couple of days to figure out what they were and then I had to go get checked out that there was no retinal damage causing them, which it turned out there wasn't. This was merely a feature of aging and I haven't even reched 50 yet!