Phototherapy is most often initiated in the presence of widespread disease (generally defined as more lesions than can be easily counted). Resistance to topical treatment in a patient with widespread disease is one indication for phototherapy; however, in many patients, topical therapy and phototherapy are combined and are more effective than either therapy alone.
The two main forms of phototherapy are ultraviolet B (UVB) irradiation and psoralen plus ultraviolet A irradiation (PUVA). Proper facilities are required for both UVB irradiation and PUVA photochemotherapy.
UVB phototherapy is extremely effective for treating moderate to severe plaque psoriasis. The major drawback of this therapy is the time commitment required for treatments and the accessibility of the UVB equipment. Patients may dislike the unpleasant odor when coal tar is added.
Home ultraviolet therapy can overcome some of the logistical problems associated with phototherapy. Because of the expense of home units, it is most suitable for patients who require long-term maintenance therapy.
PUVA photochemotherapy, also known simply as PUVA, uses the photosensitizing drug methoxsalen (8-methoxypsoralen) in combination with UVA irradiation to treat patients with extensive disease. UVA irradiation uses light with wavelengths of 320-400 nm. PUVA interferes with DNA synthesis, decreases cellular proliferation, and induces apoptosis of cutaneous lymphocytes, leading to localized immunosuppression.
More than 85% of patients report relief of disease symptoms with 20-30 treatments. Therapy is usually administered two to three times per week in an outpatient setting, with maintenance treatments every 2-4 weeks until remission.
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Cite this: William James. Skill Checkup: A 27-Year-Old Woman With Erythematous Plaques on Her Elbow, Thighs, and Scalp - Medscape - Dec 20, 2021.