Approach to Diagnosing and Managing Painful Leg Bullae

History and Physical Examination

The American Academy of Dermatology suggests that distinguishing between autoimmune bullous pemphigoid and life-threatening necrotizing fasciitis is critical, as these require fundamentally different and time-sensitive treatments 1
A history of pruritus preceding blisters by weeks to months is highly suggestive of bullous pemphigoid, according to the British Association of Dermatologists 1, 3
The presence of tense versus flaccid bullae can help differentiate between bullous pemphigoid and infection, with tense bullae on erythematous or normal skin suggesting bullous pemphigoid 2, 1, 3
A distribution pattern of symmetric bullae on flexural surfaces of limbs, inner thighs, and abdomen favors bullous pemphigoid, as noted by the British Association of Dermatologists 2, 3
Pain disproportionate to physical findings is a critical red flag for necrotizing fasciitis, according to the American College of Emergency Physicians 1
Systemic toxicity, including fever, tachycardia, and hypotension, indicates an infectious cause requiring immediate hospitalization, as stated by the Infectious Diseases Society of America 1

Bullous pemphigoid is an autoimmune blistering disease affecting elderly patients, characterized by tense bullae, pruritus, and minimal mucosal involvement, according to the British Association of Dermatologists 2, 1, 3
Necrotizing fasciitis requires immediate hospitalization, parenteral antibiotics, and emergent surgical consultation, as emphasized by the American College of Surgeons 1
Pemphigus vulgaris is characterized by flaccid bullae with a positive Nikolsky sign and prominent mucosal involvement, as noted by the American Academy of Dermatology 3
Stevens-Johnson syndrome/toxic epidermal necrolysis is characterized by a positive Nikolsky sign with flaccid bullae, extensive mucosal involvement, and constitutional symptoms, including fever, according to the American College of Emergency Physicians 3

Direct immunofluorescence (DIF) from perilesional skin is the single most important and essential test for bullous pemphigoid diagnosis, showing linear IgG and/or C3 deposits along the dermoepidermal junction, as stated by the American Academy of Dermatology 2, 1, 3, 4
Histopathology from an early intact bulla can show subepidermal cleft with eosinophilic infiltrate, according to the British Association of Dermatologists 2, 3
Serum ELISA for anti-BP180 and anti-BP230 antibodies can help diagnose bullous pemphigoid, with anti-BP180 >27 U/mL indicating increased relapse risk, as noted by the American Academy of Dermatology 1, 3

Delaying hospitalization if necrotizing fasciitis is suspected can lead to rapid deterioration, as emphasized by the American College of Emergency Physicians 1
Assuming infection based on culture alone can lead to misdiagnosis, and a fungal stain should be obtained to distinguish true infection from colonization, according to the Infectious Diseases Society of America 1
Using oral prednisone >0.75 mg/kg/day in bullous pemphigoid can lead to significant mortality in elderly patients, as noted by the American Geriatrics Society 1
Relying on histopathology alone can lead to misdiagnosis, and DIF from perilesional skin should always be obtained when bullous pemphigoid is suspected, as stated by the American Academy of Dermatology 2, 4