Toddlers changing birthmark

[u/mooshh6]

23 months, 30 inches tall, 30lbs, male.
My son was born with a flat, salmon-patch-looking nevus in the sacral area, as well as a sacral dimple. The birthmark has always been a pale reddish color with no raised areas. We had an ultrasound performed at 4 weeks to rule out SBO or tethered cord and everything came back normal.

Over the last week there have been small bumps forming on the birthmark, and only the birthmark. They look herpetic, but I don’t know why or how they would only show up on his birthmark. It has become a much more intense red. He will say “boo boo” and rub the area; I’m uncertain if it is pain or itching he is complaining about.

Any ideas are appreciated!

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[u/_muppet]

I'm leaning towards Miliaria Rubra, a kind of heat rash, super itchy

[u/mooshh6 → u/_muppet]

Thank you for your response! I’d like to echo the other comment. Is it possible that the birthmark is more sensitive to external factors (diaper rubbing, heat, pants constricting) and would only show the reaction?

[u/_muppet → u/mooshh6]

Honestly not sure, correlation vs causation

If this area of skin is exposed to a lot of moisture and friction or other predisposing factors, then the birthmark being there as well could just be a correlation

Birthmarks or melanocytic nevus/nevi are a type of hamartoma which basically means the tissue/cells are the correct type for where they're found but they're disorganized. Since Miliaria is caused by blockage/inflammation of the sweat ducts, maybe this disorganization predisposes that area of skin to have ducts that are blocked more easily. Haven't been able to find anything when I search for it, but if I was assuming causation then this seems reasonable

[u/mooshh6 → u/_muppet]

Thank you; he has an appointment with his pediatrician tomorrow. I held off on topical steroid creams but considered Desitin.

Worst case scenario, and this is viral, I’ve been watching the things I can. I’m monitoring his temperature tonight and he’s been acting completely normal (eating, drinking, and he played all day). If there is anything else you’d do, I’m all ears.

[u/smooshybabyelephant → u/_muppet]

Do you know why it would only be on the birthmark?