This video details the dermatopathological analysis of a skin biopsy from a child presenting with multiple itchy, hyperpigmented papules on their trunk. Microscopic examination revealed a dense dermal infiltrate of oval to spindle-shaped cells with finely granular, bluish cytoplasm, accompanied by eosinophils, notably sparing the epidermis. Initial consideration of Langerhans cell histiocytosis was ruled out due to the exclusive dermal involvement. Immunostaining confirmed the presence of mast cells, with strong diffuse positivity for CD117 (C-kit) and negativity for Langerhans cell markers (CD1a and S100 protein) in the dermal infiltrate. The diagnosis of cutaneous mastocytosis, specifically Urticaria Pigmentosa, is established.
Summarizing a Dermatological Biopsy: Diagnosis of Cutaneous Mastocytosis
0:00:05 Patient Presentation: A child presents with multiple hyperpigmented, very itchy papules on their trunk, prompting a skin biopsy.
0:00:20 Dermal Infiltrate Observed: Microscopic examination reveals that the dermis is abnormal, filled with an infiltrate of oval to spindle-shaped cells mixed with abundant pink collagen.
0:00:34 Epidermal Sparing: A critical observation is that the cellular infiltrate is entirely confined to the dermis and does not involve the epidermis, which helps narrow down the differential diagnosis.
0:00:47 Eosinophil Presence: Scattered bright pink or orange eosinophils are also noted within the dermal infiltrate.
0:01:17 Cytoplasmic Characteristics: Closer inspection reveals that the cells possess a very finely granular, slightly bluish cytoplasm, a key morphological feature suggestive of mast cells.
0:01:42 Mastocytosis Suspected: Based on the cellular morphology and dermal localization, mastocytosis is strongly considered. This condition can cause multiple itchy papules in children.
0:01:54 Confirmation with CD117 (C-kit): Immunostaining for CD117 (C-kit), a marker for mast cells, shows strong, diffuse positivity throughout the dermal infiltrate, supporting the diagnosis.
0:01:59 Caution and Alternative Stains: A word of caution is given that CD117 is not entirely specific and can also stain melanocytes. More specific stains for mast cells include mast cell tryptase or Giemsa.
0:02:49 Ruling Out Langerhans Cell Histiocytosis: Stains for Langerhans cells (CD1a and S100 protein) are performed. The dermal infiltrate is almost entirely negative for these markers, effectively ruling out Langerhans cell histiocytosis, which typically involves the epidermis. Normal Langerhans cells and melanocytes in the epidermis serve as positive internal controls for these stains.
0:04:15 Final Diagnosis: The conclusive diagnosis is cutaneous mastocytosis, specifically the form known as Urticaria Pigmentosa, characterized clinically by itchy, hyperpigmented papules and nodules.