Both psoriasis and eczema are chronic skin diseases that cause red, scaly skin rashes. Though they are quite similar, the range of symptoms for each is usually different enough for your doctor to tell them apart without doing a skin biopsy or other diagnostic testing.
Psoriasis lesions are typically thick, red, and scaly (dry). Although eczema lesions may be similar if they are chronic, they also can appear as moist and oozing areas. Both tend to affect the hands, feet, and nape of the neck. When the problem is in these areas, it can be difficult for even a doctor to distinguish one condition from another. But, there are some telltales: Psoriasis likes to involve the back of the elbows and front of the knees (extensor surfaces), while eczema favors the inside of the arms and the back of the knees (flexor surfaces). Both rashes appear frequently on the scalp, while chronic eczema is found on the ankles more often than psoriasis.
Hands and feet are the most difficult areas to tell psoriasis and eczema apart. For one thing, fungus may be along for the ride as well. Even a biopsy of hand and foot rashes may show confusing overlap, causing pathologists to "hedge" a bit and give a non-specific reading or report. One small clue to psoriasis of the hands is that nails will show pitting. Nail changes also occur in eczema and fungus cases, but pitting -- especially in a nail where the cuticle is not involved with any rashes -- is fairly uncommon. Fortunately, both psoriasis and eczema of the hands and feet will respond to topical steroid creams, so treatment is usually not hindered by this diagnostic confusion.
Some treatments work for both psoriasis and eczema, though others are quite specific. It's important to see a dermatologist so that you rash can be assessed and you can get the most effective treatment.