Major histopathological changes in the dermis and hypodermis — expanded notes
Dermal architecture
Papillary dermis contains loose collagen, fine elastic fibres and superficial capillaries. Reticular dermis contains dense collagen bundles, elastic fibres, adnexal structures, vessels and nerves. Fibroblasts maintain extracellular matrix; changes in collagen, elastin, mucin or vessels create recognisable reaction patterns.
Inflammatory patterns
Perivascular: inflammatory cells surround superficial or deep vessels.
Interstitial: inflammatory cells lie between collagen bundles.
Nodular/diffuse: larger aggregates or sheets of inflammatory cells.
Granulomatous: epithelioid histiocytes with or without giant cells; consider infection, foreign material, sarcoidosis and granuloma annulare.
Vasculitic: vessel wall inflammation and damage; look for leukocytoclasia, fibrinoid necrosis, endothelial swelling and red-cell extravasation.
Connective tissue alterations
Sclerosis is increased dense collagen and may be localised (morphea) or part of systemic sclerosis. Solar elastosis is abnormal elastic material in sun-damaged dermis. Mucin accumulation occurs in lupus and dermatomyositis. Amyloid is extracellular protein deposition and may be confirmed by Congo red staining with apple-green birefringence.
Panniculitis
Panniculitis is inflammation of subcutaneous fat. Histology is classified as septal or lobular, and whether vasculitis is present. Erythema nodosum is the classic septal panniculitis without vasculitis. Lobular panniculitis occurs in pancreatic disease, lupus panniculitis, infection and alpha-1 antitrypsin deficiency. Biopsy must include adequate subcutaneous fat.
Vascular and deposition changes
Haemosiderin-laden macrophages may be seen in chronic venous hypertension. Calcium deposits may occur in damaged tissue or systemic mineral imbalance. Thrombi, vascular proliferation and ectasia are interpreted with clinical distribution and systemic findings.
Biopsy technique
Choose a fresh representative lesion. A superficial punch may miss panniculitis. For vasculitis, sample a fresh purpuric lesion; direct immunofluorescence may require a separate early lesion. For suspected blistering disease, take lesional skin for routine histology and perilesional skin for direct immunofluorescence.