Hi
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There are three main possibilities: contact dermatitis, lymphedema and deep vein thrombosis. If the left hand comes in contact with something that the right hand does not, then this could be the cause of dermatitis. Duplex ultrasounds and color dopplars can help diagnose a deep vein thrombosis or a blocked lymph channel. It is important to examine the left axilla (armpit) and breast for lumps since there is redness and swelling in left arm.
Another possibility is infection of the skin called cellulitis. This also causes redness and swelling and this is painful to touch. Treatment is by antibiotics and anti-inflammatory drugs.
I think you should discuss these possibilities with your doctor. It is difficult to comment beyond this without examining. A comprehensive investigation is required keeping all the points in mind. Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!
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