Cutaneous Microcirculation and Nervous Sensitivity in Psoriasis (MicroPso)

Psoriasis is a chronic inflammatory cutaneous disease, affecting 3% of the French population. Among psoriatic patients, 80% feel pain or cutaneous discomfort related to their pathology. Neurogenic inflammation's role in psoriasis has recently been put forward by a study showing that TRPV1 ion channels are necessary to establish psoriasiform inflammation in mice. The investigators hypothesize that there is a link between cutaneous sensory neuropathies and altered cutaneous microcirculation during psoriasis. In order to test this hypothesis, local vasodilators will be delivered to patients by iontophoresis and their skin blood flow in response to these molecules will be followed by laser Doppler recordings. Two recordings will be performed for each patient, both on a psoriasis plaque and on uninvolved skin, in order for the patient to be his own internal control.

Local vasodilators (acetylcholine, sodium nitroprussiate and ppi water as control) will be delivered to patients by iontophoresis on healthy skin. The skin blood flow in response to these molecules will be followed by laser Doppler recordings.

Local vasodilators (acetylcholine, sodium nitroprussiate and ppi water as control) will be delivered to patients by iontophoresis on a psoriasis plaque (on the patient's back). The skin blood flow in response to these molecules will be followed by laser Doppler recordings.

Acetylcholine will be locally delivered to patients by iontophoresis successively on healthy skin and on psoriasis plaque (order determined by randomization).

Sodium Nitroprussiate will be locally delivered to patients by iontophoresis successively on healthy skin and on psoriasis plaque (order determined by randomization).

ppi water will be locally delivered to patients by iontophoresis successively on healthy skin and on psoriasis plaque (order determined by randomization).

Skin blood flow will be recorded by laser Doppler during 2 minutes before iontophoresis, and during 30 minutes following iontophoretic delivery of vasodilators.

Skin blood flow variation on healthy skin and on psoriasis plaque will be measured by Laser Doppler in response to iontophoretic delivery of vasodilator substances (sodium nitroprussiate and acetylcholine) and ppi water. Skin blood flow will be recorded during 2 minutes before iontophoresis, and during 30 minutes following iontophoretic delivery of vasodilators.

Increasing size of Von Frey filaments will be placed onto patients' skin to assess sensory detection threshold.

Increasing size of Von Frey filaments will be placed onto patients' skin to assess potential discomfort sensation threshold.

Inclusion Criteria: Type I psoriasis Psoriasis plaques on the patient's back Exclusion Criteria: BMI > 25 Diabetic patient Arteriovenous disease history Ongoing anti-inflammatory treatment Major cardiovascular history (<3 months) Hypertension Topical treatment on the back skin (<7 days) Systemic treatment (steroids, methotrexate, retinoids, cyclosporine) or phototherapy (<1 month) Pregnant women Subject within exclusion period following a previous or ongoing biomedical study