### instruction:
If you are a doctor, please answer the medical questions based on the patient's description.

### input:
Hi, I have had a severe case of psoriasis (covering my entire body - apart from my face) for the last 3 months.  I would like to try the puva light but my derm keeps saying that we can start that type of treatment as my skin is so red??? I wouldve thought that the light would have improved it signigicantly??? Thanks :))

### output:

Hello. Thank you for writing to us at Chat Doctor Since you seem to have a generalized involvement with psoriasis; this type of involvement is also known as erythrodermic psoriasis Or unstable psoriasis. This type of psoriasis involvement is associated with thermoregulatory as well as metabolic complications as a result of the lack of normal temperature regulatory functions of the skin as well as loss of proteins, and fluids from the skin. In general unstable forms of psoriasis like erythrodermic, pustular psoriasis is managed conservatively with topical emollients and Oral / Systemic Immunosuppressive like Methotrexate, Cyclosporin etc. Before starting on PVA, patients skin is tested for the tolerable effective dose of PVA, known as MED (minimal erythema dose). The starting dose of PVA is then calculated as 75% of MED. However, if all the skin is already red, there cannot be an accurate determination of MED. Therefore, if inadvertent higher doses of PVA are given in erythrodermic psoriasis without calculating MED, PVA can further irritate the skin and therefore is not the first choice amongst dermatologist in unstable psoriasis. Nevertheless, PVA can be given in low doses along with other systemic treatment like methotrexate, cyclosporin etc. even in erythrodermic psoriasis. Regards