Henoch-Schönlein purpura: the childhood rash with the sore joints, and the urine checks that matter most
Last updated September 3, 2026.
Henoch-Schönlein purpura, HSP, now called IgA vasculitis, is the commonest small-vessel vasculitis of childhood: an immune misfire, usually after a cold or throat infection, that inflames the small blood vessels of the skin, joints, gut, and kidneys. The picture is distinctive: a raised, blotchy purple rash over the legs and buttocks, aching joints, tummy pain, and sometimes blood in the urine. It looks alarming and, in the large majority of children, it passes fully over weeks. The treatment is mostly support: rest, simple painkillers, and watching, with steroids for the severe tummy pain or the rarer complications. The one system that decides the follow-up is the kidneys: a minority develop kidney inflammation, usually mild, occasionally significant, and it can be silent, which is why the urine checks and blood-pressure measurements run on a schedule for weeks to months after the rash has faded, and why keeping those appointments matters more than anything about the rash. Recurrences of the rash and the aches happen in about a third, usually milder, and they do not change the good overall prognosis. Most children look back on HSP as a strange, sore month of childhood, nothing more.
What does it look like?
The rash first: raised, purple-red, non-blanching spots and blotches over the legs, buttocks, and sometimes arms, appearing in crops. Then the extras: aching, sometimes swollen ankles and knees; tummy pain that can be crampy and severe, occasionally with blood in the stool; and, in the kidney-affecting minority, tea-colored urine or protein and blood on the dipstick. The child is otherwise often well between the symptoms, which is part of the strangeness.
Why does it happen?
The immune system deposits a particular antibody, IgA, into the walls of small blood vessels, usually a week or two after an ordinary cold or throat infection, and the vessels inflame and leak. Why one child's immune system does this and another's does not is unknown; it is not caused by anything the child ate, caught beyond the usual cold, or that the parents did, and it is not contagious itself, though the triggering cold was.
How is it treated?
- Support is the treatment for most. Rest, simple painkillers for the joints and tummy, fluids, and time: the large majority resolve fully over about four weeks without any specific medicine.
- Steroids for the severe forms. Bad tummy pain, significant kidney inflammation, and the rarer complications earn steroids or specialist care, and the threshold for them is the team's call, watched closely.
- The urine checks are the part that matters most. Kidney involvement can be silent, so urine dipsticks and blood-pressure checks run on a schedule for weeks to months after the rash fades, and keeping every one of those appointments is the single most important job in this condition.
- Recurrences are common and usually milder. About a third of children get a second crop of the rash and aches in the following months; it settles the same way and does not change the overall good outlook.
When does it need urgent review?
Severe tummy pain, blood in the stool or vomit, tea-colored urine, a very swollen painful scrotum in boys, or a child becoming listless and unwell are each a same-day assessment. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
What a Pymander AI doctor consult looks like
Illustrative example, not a real member's messages.
Common questions
What is the worst case, plainly, and how likely is it?
Plainly: the worst case is the kidney form, and here are the real numbers. About a third of children with HSP show some kidney involvement on testing; the large majority of that is mild and passes without treatment; significant kidney inflammation needing steroids or more is uncommon; and lasting kidney damage is rare. The reason the team keeps saying the kidneys are the thing to watch is not dread, it is that this condition's serious version announces itself on a dipstick, silently, weeks to months early, which makes it uniquely catchable. The worst case is real, uncommon, and, in a child whose urine appointments are kept, almost always caught while still easy to treat.
Why is everyone so calm when the rash looks horrific?
Because the rash is the least dangerous part of the condition, for all its drama. It is the surface signature of small blood vessels leaking under the skin, it looks like the emergency and is actually the reassurance: it fades over weeks, leaves no scars, and a second, milder crop appears in about a third of children before the condition is done, which changes nothing. The parts the professionals actually watch, the gut and the kidneys, are the parts that are quiet or invisible, which is why their attention seems oddly absent from the thing you can see. Their calm is the calm of people watching the right horizon, and it is worth joining.
What are the urine appointments actually for?
They are the entire safety net, and they are the single most important job in this condition. Kidney involvement in HSP is often silent: no symptoms, a child bouncing around, and the only evidence is blood or protein on a dipstick or a rising blood pressure, weeks to months after the rash has faded. Caught on that schedule, the kidney form is treated early, when it is easiest to turn around; caught late, it is a different conversation. That is why the list of appointments is long, why it outlasts the rash, and why every one of them is worth keeping even when he looks perfectly well, especially then.
What should make me take him back the same day?
The gut rules and the general rules, and they are worth pinning to the fridge. Severe tummy pain, especially crampy and worsening. Blood in the stool or the vomit. Tea-colored urine, or urine that nearly stops. A very swollen, painful scrotum. And him becoming listless and unwell rather than just sore and annoyed. Any of those is a same-day assessment, because the gut is the other place this condition occasionally bites hard, and the rare complications there are time-sensitive. The ordinary aches, the rash crops, and the bad-tempered days of a sore six-year-old are the condition being itself and do not need the trip.
Will it come back? And will he grow out of it?
About a third of children get a recurrence, a second crop of the rash and the aches in the following months, and it is almost always milder, settles the same way, and changes nothing about the outcome, though a recurrence is worth mentioning to the team because the kidney watching may restart with it. On growing out of it: HSP is a one-chapter condition for the large majority, a strange, sore month or two of childhood, closed fully, with no adult echo. The follow-up exists to make sure of the kidney side, and when the schedule ends clean, the file closes with it. He will remember it as the month his legs looked dramatic and everyone made a fuss.
Did the cold he had cause this? Could his sister get it?
The cold triggered it, but did not cause it in the way you mean, and his sister is at no special risk. The sequence is: an ordinary cold or throat infection, then, in a small minority of children, the immune system misfires afterward and deposits an antibody called IgA into the small blood vessels. The cold was contagious; the vasculitis is not. Why his immune system misfired and his sister's did not is unknown, it is not anything he caught beyond the cold itself, not anything he ate, and not anything you did or missed. If his sister ever developed the same rash after a cold, it would deserve the same look, but the odds are the ordinary ones, unchanged by his chapter.