What Causes Painful Calves?

Question: I have a four year old son who has been having problems with legs, especially his calves for 9 months. He stopped walking one night because of pain. My husband and I took him to his pediatrician who tested him for the flu and nothing else. We were told he has acute myositis as a result of flu. We went back three days later because my son couldn't flex his feet and was walking on his toes. Two months later my son still hadn't regained total mobility. We were eventually referred to rheumatologist due to slightly elevated ANA tested and very high SED. She diagnosed my son with hypermobilty of knees and ankles and said that he had an injury. We have since seen rheumatologist four times and we keep getting told it's a recurrent injury. I recently started taking my son to a sports medicine doctor. He ordered X-ray to confirm flat feet. The sports medicine doctor is worried but my sons muscle development. He's a large very muscular kid. His height is 3'9" and weight 51 lbs. He has what I am told is extremely developed muscles. He is suggesting we see a neuromuscular specialist. We redid lab work today and we are still waiting on results. So far all we know is his SED was 11 on a scale of 1-10. The problems he keeps having is extreme pain in and around calves muscles, always both legs. Once the pain was right below his knees. A rash under his arms is also present with leg pains. Pains come and go and last 3-4 days. He can only toe walk during an episode and he can go weeks without symptoms. He has been prescribed naproxen. My son is a mirror image twin. His twin has no leg pain, isn't hypermobile, and is bigger and more muscular. When my son isn't having an episode, he's an extremely active happy child.

1. Pain is present at rest
2. He did 6 weeks of PT to regain a normal walk. He sometimes has to be reminded to walk flat footed and his walk to slower. He also had trouble walking on staits alternating right foot, left foor pattern. PT has helped some with that but he os much slower and more careful walking on stairs now.
3. Calves are extremely hard when he's having pain
4. Cpk was normal. I am not sure if our test were done
Attached is picture of his rash. Would you suggest we see another rheumatologist as well as a neoromuscular specilaist?