We recently saw an adorable 2-year-old with a tan-brown patch on the lower back that had been present since birth. The lesion wasn’t painful, but the family noted that it occasionally became itchy. It had never turned red or swollen with touch, and no one had ever seen it change—until their visit to us.
During the exam, the lesion appeared smooth and slightly raised with a uniform tan-orange hue. As part of our evaluation, we gently rubbed the area—and within moments, it swelled and reddened, revealing a classic Darier sign, a telltale feature of a condition called solitary mastocytoma.
We were able to capture this response in clinic, and the before-and-after images (shared with parental consent) clearly demonstrate the diagnostic Darier sign.
What is Solitary Mastocytoma?
Solitary mastocytoma is a benign skin condition that typically appears in infancy or early childhood. It results from a localized accumulation of mast cells, which are immune cells involved in allergic and inflammatory responses. This is the most common form of cutaneous mastocytosis seen in children.
Most solitary mastocytomas are harmless and tend to fade over time, often resolving by age 2 to 5 without intervention.
Typical Features
Lesions often appear as:
• A single tan, orange, or brown patch or bump
• Present from birth or early infancy
• Smooth surface; may be flat or slightly raised
• Occasionally itchy, especially with friction or heat
• Located most commonly on the trunk or limbs
Our patient’s lesion fit this description perfectly. Although the family had never observed it change, our in-office test revealed the diagnostic Darier sign, confirming the diagnosis.
What is the Darier sign?
The Darier sign is a skin reaction in which rubbing or scratching the lesion causes it to turn red, swollen, or hive-like. It occurs due to histamine release from the mast cells concentrated in the lesion. This sign is a hallmark of mast cell–related skin conditions and often confirms the diagnosis without needing a biopsy.
When Should You Be Concerned?
While solitary mastocytomas are typically benign, dermatologists remain alert to signs that might suggest more widespread or systemic involvement:
• Multiple lesions (usually more than five)
• Blistering, ulceration, or rapid growth
• Systemic symptoms such as flushing, vomiting, diarrhea, abdominal pain, or wheezing
• History of fainting, anaphylaxis, or food-triggered reactions
• Physical findings such as liver or spleen enlargement
• Elevated serum tryptase on lab work
These signs may prompt additional testing to rule out systemic mastocytosis, a rare condition where mast cells affect internal organs as well as the skin.
Management and Parental Guidance
For isolated lesions with no systemic symptoms, treatment is usually unnecessary. However, families can take some simple precautions:
• Avoid rubbing or scratching the lesion
• Be cautious with heat, pressure, or tight clothing over the area
• Use topical corticosteroids or antihistamines if the lesion becomes itchy
• Inform healthcare providers before any surgical or dental procedures, in case preventive medications are needed
Certain medications (such as NSAIDs, opioids, or radiologic contrast agents) can trigger mast cell activation and may need to be used with care in select patients.
In Summary
Solitary mastocytoma is a self-limited, benign skin condition commonly seen in young children and often present since birth. While it can easily be mistaken for eczema or a birthmark, the Darier sign offers a quick and reliable diagnostic clue. In our 2-year-old patient, this sign had never been observed before—but once we gently elicited it in the clinic, it confirmed the diagnosis and gave the family clarity and peace of mind.
Images from the visit show the lesion before and after gentle rubbing, highlighting the diagnostic Darier sign. These photographs are shared with explicit parental consent and may not be used, copied, or shared without written permission.
Disclaimer: The photos shown are actual images of our patient, shared with explicit written consent. These images are for educational use only and may not be copied, distributed, or reproduced without permission from our office.
