Immunodermatology, Connective Tissue Disease and Systemic Disease
Autoimmune blistering disease
Pemphigus vulgaris: flaccid blisters, mucosal involvement, positive Nikolsky sign, intercellular IgG on immunofluorescence, anti-desmoglein 3. Bullous pemphigoid: tense blisters in older patients, anti-BP180/BP230, linear IgG and C3 at the basement membrane, tissue eosinophils. Dermatitis herpetiformis: intensely itchy grouped vesicles on elbows and knees, granular IgA in dermal papillae, gluten-sensitive enteropathy.
Connective tissue disease
Lupus: acute malar rash, subacute annular photosensitive lesions with anti-Ro, discoid scarring lesions. Check ANA, anti-dsDNA and complement. Dermatomyositis: heliotrope rash, Gottron papules, shawl sign, mechanic's hands; anti-TIF1-gamma and anti-NXP2 raise malignancy risk, so age-appropriate cancer screening is mandatory. Systemic sclerosis: sclerodactyly, Raynaud phenomenon, telangiectasia, calcinosis and abnormal nailfold capillaroscopy.
Cutaneous clues to systemic disease
Acanthosis nigricans (insulin resistance, malignancy), pyoderma gangrenosum (IBD, myeloma), erythema nodosum (streptococcus, sarcoidosis, IBD, drugs), Sweet syndrome, necrobiosis lipoidica (diabetes), and palpable purpura (check urinalysis, renal function, blood pressure).
Traps
- Calling every photosensitive rash "lupus".
- Forgetting malignancy screening in new adult dermatomyositis.
- Treating a low-titre ANA as diagnostic.
- Missing renal involvement in IgA vasculitis - always check urine.
Rule of thumb
Skin change plus systemic features (fever, weight loss, myalgia, arthralgia): investigate for connective tissue disease or paraneoplastic process rather than reaching for topical steroid.