Bullous pemphigoid is the most common type of bullous skin disease and is clinically characterized by clear-tense bullae, which result in post-bullous cutaneous erosions, altering the skin barrier. The treatment of this pathology consists of the application of high doses of topical corticosteroids (clobetasol propionate) for a prolonged period of at least 6 months. The main objective of this study is to demonstrate a change in bone mineral density at 6 months after initiation of treatment, in subjects with bullous pemphigoid and treated with topical corticosteroid.

Glucocorticoids have direct effects on bone remodeling by suppressing bone formation (inhibition of osteoblastic differentiation, inhibition of mature osteoblasts function and apoptosis of mature osteoblasts) and by increasing bone resorption (decrease in osteoclast apoptosis and stimulation of osteoclastogenesis). They also have indirect bone effects by decreasing the intestinal absorption of calcium and increasing its urinary excretion, and by inhibiting the somatotropic and gonadotropic axis. This pathophysiology results in excessive bone fragility. Bone loss and increased incidence of fractures occur within 6 months after the introduction of oral corticosteroid therapy, with a partially reversible phenomenon within months of discontinuation. The extent of bone loss depends on the dose and duration of glucocorticoid administration.

The systemic transition of topical corticosteroids depends on several parameters such as excipients, anatomical location, cutaneous state, the dose used and the duration of exposure.

Clobetasol propionate, used for long-term use in bullous pemphigoid, is a Class IV dermocorticoid (highly potent). Patients with bullous pemphigoid will benefit from bone densitometry at the initiation of treatment, at 3 months (theoretical end of the treatment of attack) and at 6 months (theoretical end of the treatment).

Patients will also benefit a blood test of serum calcium, phosphoremia, albumin, 25 OH vitamin D and cortisol at 8 to highlight possible correlations between changes in bone mineral density and phosphocalcic parameters and 8 cortisolemia (braking of the hypothalamic-pituitary-adrenal axis).

Patients will also benefit from standard radiographs of the thoracic and lumbar spine at the initiation of treatment and at 6 months. Follow-up is planned over 6 months, with 2 follow-up visits at 3 months and 6 months.

Inclusion Criteria:

• patients presenting a multi-bullous pemphigoid newly diagnosed or relapsed more than 3 months after stopping corticosteroids and treated according to the national protocol for diagnosis and care issued by the reference center for autoimmune bullous diseases of April 2016
• patients having received written and oral information and signed informed consent
• patients covered by national health insurance

Exclusion Criteria:

• Patients under tutorship or curatorship or inability to give informed consent
• Patients receiving an anti-osteoporotic treatment
• Patients requiring an anti-osteoporotic baseline treatment (T-score ⩽ -3DS on at least 1 site or FRAX score above the therapeutic intervention threshold)
• Patients with one or more major risk factors for osteoporosis
• Patients who have received topical corticosteroids in less than 3 months