Pruritic bullae in an older adult with diabetes and CKD
A 77-year-old woman presents with 6 weeks of an intensely pruritic eruption. She describes water blisters on the abdomen, thighs, and forearms that rupture and leave crusts. Mucous membranes are spared. Examination shows urticarial plaques with several tense 1-2 cm bullae and many eroded crusted lesions on the trunk and proximal limbs, involving about 15% of body surface area, with no Nikolsky sign. Medications: insulin glargine, amlodipine, atorvastatin, alendronate, and occasional ibuprofen. Comorbidities: type 2 diabetes (HbA1c 8.1%), osteoporotic vertebral fracture, and chronic kidney disease stage 3b with eGFR 38 mL/min/1.73 m2. She lives alone and drives herself to appointments. An urgent care physician started prednisone 40 mg daily 5 days ago, with minimal improvement and a home glucose reading of 320 mg/dL.
1. Give your differential and the tests that will establish the diagnosis.
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Bullous pemphigoid is most likely: an older adult with pruritic urticarial plaques evolving into tense bullae, no mucosal involvement, and a negative Nikolsky sign. Alternatives: epidermolysis bullosa acquisita (mucosal involvement, trauma-prone sites, healing with milia and scarring, but can mimic BP); pemphigus vulgaris (flaccid blisters, mucosal erosions, positive Nikolsky); a drug-induced bullous eruption (dipeptidyl peptidase-4 inhibitors such as sitagliptin or linagliptin, PD-1/PD-L1 inhibitors, furosemide - ask specifically about recently stopped drugs and supplements, since her current list is low risk); bullous arthropod reaction; and linear IgA disease.
Tests: two biopsies - lesional or perilesional skin in formalin for H&E, showing a subepidermal split with eosinophils, and perilesional normal-appearing skin in saline for direct immunofluorescence, which should show linear IgG and C3 along the basement membrane zone. Serum BP180 (NC16A) and BP230 ELISA, with BP180 correlating with disease activity, and indirect immunofluorescence on salt-split skin, which localizes to the roof in BP and the floor in EBA. Add CBC, liver and renal panels, glucose, and albumin. Malignancy screening is not routine; pursue it only for red flags such as weight loss, anemia, or hematuria.
2. How do you treat her, given the diabetes, CKD, and the prednisone already started?
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The prednisone was started without a diagnosis and is already causing hyperglycemia, but it should not be stopped abruptly because rebound is likely. The plan is to confirm the diagnosis this week (biopsies, ELISAs) and simultaneously build a steroid-sparing regimen.
Reduce prednisone toward roughly 0.5 mg/kg/day (about 20-30 mg) with a structured taper over 8-12 weeks, not an immediate stop. Start doxycycline 100 mg twice daily plus nicotinamide 500 mg three times daily: renally safe, no laboratory monitoring burden, and evidence-supported as a steroid-sparing option in moderate disease. Add clobetasol 0.05% ointment to active truncal and limb lesions, sparing the face and intertriginous areas, plus intralesional corticosteroid to small localized plaques - topical therapy is a genuine component of BP treatment, not an afterthought. Coordinate with her primary physician to intensify diabetes therapy during the taper and to monitor glucose; continue calcium, vitamin D, and alendronate for bone protection.
If disease is refractory or prednisone cannot be reduced below about 7.5 mg/day, dupilumab is an attractive steroid-sparing option with randomized evidence in BP and no renal dose adjustment; rituximab is reserved for refractory disease. Methotrexate should be avoided at eGFR 38, and azathioprine requires TPMT/NUDT15 testing and hepatotoxicity vigilance. She lives alone, so plan infusion or injection support and driving safety during sedation or antihistamine use.
3. At 4 weeks, which findings would make you doubt the diagnosis or escalate therapy?
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Doubts about the diagnosis: negative direct immunofluorescence; disease concentrated on mucosa or head and neck rather than trunk and limbs (think epidermolysis bullosa acquisita or pemphigus); exclusively targetoid lesions or a tight temporal link to a new drug (bullous drug eruption); milia and scarring at healed sites (EBA); or continuing new blisters despite adequate therapy, which warrants re-biopsy and reconsideration of linear IgA disease, mucous membrane pemphigoid, or bullous lupus.
Escalation triggers: three or more new blisters per day, or persistent urticarial lesions at 4 weeks on doxycycline and nicotinamide; or inability to taper prednisone below 7.5 mg/day without relapse. In either case move to dupilumab or rituximab rather than simply increasing prednisone again.
Also reconsider if new systemic symptoms appear - weight loss, night sweats, lymphadenopathy, hematuria - since a lymphoproliferative or internal malignancy can present with BP-like disease, though routine search is not indicated. Monitor for secondary infection of eroded skin, venous thromboembolism in severe BP, and steroid complications: hyperglycemia, hypertension, delirium, and falls in a patient who lives alone and drives.