Hi and thank you so much for this query.

I am Dr. Ditah and feel honored having this opportunity to address your health question on this forum today.

I am sorry to hear about this history of capillary malformation with associated hypertrophy probably in your son (hope am correct from the age which shows 7!)

I wish to ask if this large bulge was first noticed after trauma? This is because you also describe bruising too. Supposing this was noticed after some trauma(witnessed) or later with bruises described which points to trauma as most likely origin, I think it is simply bleeding (hematoma) into the skin or the muscles. If it is not increasing in size, not too big(greater than 5cm), I will suggest that you apply ice packs, give some pain killers and observe over a couple of days. This is usually sufficient to get the problem resolved. If it is however very big, increasing in size or too painful, then you may want to visit the ER for a more detailed evaluation. If it should also fail to regress after the above treatment, visiting the hospital should be considered.

If this swelling is not related to trauma, it could be a spontaneous rupture of a blood vessel in the skin. This is even more likely as one blood vessel malformation predisposes to others. In this case, I will suggest that you make plans and take your son to the hospital as soon as possible for proper evaluation. We may need to do a doppler ultrasound(one that evaluates blood vessels and blood flow) to identify what the possible problem is and give the adequate treatment. Also, it may signal a blood clotting disorder if this becomes recurrent or happens with minimal trauma.

I hope this answers your query and provide guidance and information on which you can make informed decisions. I wish you and your soon good health and best possible outcome from this swelling under the arm.

If you think this answer is not conclusive enough, please ask further questions and maybe consider uploading a picture of the swelling too.

Dr. Ditah, MD.

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