People do report warnings from dreams or interpretations of dreams.I've,personally,had a few things come true in waking life from dreams and phases,so a 'warning' interpretation is something that I'm open minded to. I'm not saying this is the case,I don't really know.If it was a 'warning' it was a really ambiguous one...If it had any effect on my decision to get an eye exam it would have been subliminal one....
In this instance,I suppose it would entail a general feeling of concern about my vision because this is what I did feel afterwards,albeit it was a concern for my phase vision and not my waking vision.There was/is a real uneasiness about not being able to see,in general,when I desire to.
If there is a constraint on what the subliminal can convey to the egoic-surface mind,say that it must use symbolism and not direct language,then it's even possible that the surface mind could often miss warnings or messages from the deeper psyche.What I'm saying is plausible even in the 'all in the brain' belief about the phase.
Excluding issues over mental vs physical properties and intentionality:The retina is a part of the nervous system and information about it's looming degeneration would be present at it's level.This information could be received by other non linguistic areas,say in the right brain,which could later become incorporated into phase content.(how ever that might happen)This information could be shared with other areas and even register as just a 'gut feeling' at the ego level. An uneasiness about not being able to see when desiring to.
Say,the organism's strategy here is to attempt to draw executive level attention,through dream content, to it's visual system's changes at an early stage of a disease process.
......who knows,I'm just pulling shit outta my ass for fun.It could be though...

"The closer you get to the meaning;the sooner you'll know that you're dreaming" -Dio