My 5 yr. old son has had two surgeries for his ACM1. We've seen several doctors and have been amazed at how many educated people can look at an MRI and have a completely different opinion. Our son's herniation was 13 mm, but that is extreme and I have heard of 3-4 mm being called ACM1. I would seek a second opinion and would suggest a special MRI to see how the CSF fluid is flowing. It is called a "syni-flow" and it, not the level of herniation, is the best indication of whether or not a decompression is needed.