Answer to Image of the Month June 2015
Submitted by Mani Makhija and Vibhu Mendiratta
Subcutaneous fat necrosis of newborn
The key features on histopathology of subcutaneous fat necrosis of newborn and the histologically identical post steroid panniculitis are:
1. Necrosis of the fat lobule with crystallisation of lipocytes.
2. Radial or needle shaped crystals in the lipocytes and giant cells (see figure below)
3. Granulomatous inflammation with multinucleated giant cells (see figure below).
Three entities in the lobular panniculus are characterised by needle shaped clefts in lipocytes and they are : sclerema neonatorum, subcutaneous fat necrosis of new born and post-steroid panniculitis. The latter two are identical dermatopathologically and show granulomatous reaction while the former lacks the inflammation and giant cells.
The pathogenesis of these characteristic crystals is fascinating. Unlike in adults, subcutaneous fat in infants has a higher percentage of saturated fats (including palmitic and stearic acids) than unsaturated fatty acids like oleic acid. This higher saturated to unsaturated fatty acid ratio results in a higher melting point and promotes crystallisation under certain conditions like hypothermia. It is the larger type B crystals compared to the micro-sized type A crystals (seen in sclerema neonatorum) which are capable of inciting a granulomatous response in subcutaneous fat necrosis of newborn and post-steroid panniculitis. Hence sclerema neonatorum has the finer needle shaped crystals and they are characteristically devoid of inflammatory infiltrate or giant cells.
Obstretic complications and hypothermic cardiac surgery have been consistently implicated as the aetiology of subcutaneous fat necrosis of the newborn. Fortunately, the condition is self resolving as a rule but sometimes results in lipoatrophy. Also, hypercalcemia from extra renal production of calcitriol by activated macrophages within the granulomatous panniculitis may need monitoring.