Eric, These are hardy in my zone but since they die back and do not emerge until late May I carry them over inside, just barely moist and as warm as possible (above 60 degrees), but mine do have crispy edges on oldest leaves. Also new leaves during winter are progressively smaller. No fertilizer. I don't worry about them growing in winter, just surviving, and when put outside in early April and flooded regularly with water they take off. No more crispy leaf edges, which I think may be due to the cooler temps and less water in winter.

Do you have yours in warm greenhouse? If so, my experience may not be helpful. Also, the brown spots on interior of leaves may be a different problem. I think I'd cut off those leaves just in case it is some type disease which may spread to other plants.

Also, you might want to check out International Aroid Society...they probably have info on diseases, etc. josh

I over-winter a dozen or so different EE's every year.
It looks like a humidity problem to me.
It's not a big deal but it will weaken them.

I use trays w/ pebbles then set the pots on those.
Keep water in this all the time.
Water the plants only enough to the keep the soil from totally drying out.

Try to get them all the light you can.
Remember they grow in full sun.
You can't give them too much light.

Also check for Spider Mites and Scale.
The underside of the leaves can hid them very easily.
They are very prone to these critters in Winter.
Safer Soap will take care of them easily.