My daughter recently backed into a sea urchin and was stung on her buttock. The wound was debrided by a surgeon, and she was placed on penicillin. Soon afterwards, she developed a stiff neck and local tenderness over the back of her neck. Her buttock is now OK, but there is questionable swelling over the back of her neck. The immediate effects of a sea urchin sting are usually a burning sensation, followed by swelling, redness and an aching pain.
More severe complications can include infection, numbness or even paralysis.
If a person is allergic to the venom, they may even have a life-threatening anaphylactic reaction, much like what someone who is allergic to bee stings experiences. Usually, the symptoms of an urchin's sting will subside over a few days with appropriate treatment, which your daughter received. However, it's not uncommon to have some achiness or discomfort for some weeks, or even months, after the event. It's not particularly clear what causes these long-term effects, but it's likely a neurotoxic effect of the urchin's venom. In any case, I have been unable to find any case reports of permanent neurological damage from an urchin sting, and her symptoms should resolve with time.