A sleep study is usually the thing to do. We ended up bypassing it, I guess by DS's history and symptoms, and by process of elimination. We had noticed DS's difficulty with breathing at night since infancy. He developed chronic congestion around 6 weeks old, that continued until allergies were eliminated at around 2 1/2 yrs. The congestion lead to frequent, chronic sinusitis, which can be associated with enlarged adenoids. His breathing always sounded relatively normal at night until the moment he fell asleep. Then he always breathed through his mouth, and snored through his nose. Watching him sleep, it was quite obvious that he repeatedly stopped breathing for about 10 seconds, give or take a few seconds depending on whether or not he had a cold. Each time, he began breathing again with a gasp and sometimes jerking his body, usually to roll to his side (side sleeping was better than back). We took him to an ENT who gave him nasal sprays, one with steroid med, one without. We tried it for a year to see that it helped some, but not enough. The ENT said that was the easiest way to rule out one potential cause (the turbinates), and the next most likely cause would be the adenoids. When we went back a few weeks ago he recommended CTscan to be totally sure, and they saw that his adenoids were significantly enlarged, so they had to come out. Now he breathes beautifully, it's only been a week. His behavior has not improved yet, but I suppose that may take some time.
If sleep apnea was actually a factor in producing DS's ASD symptoms, I'm guessing the continued O2 deprivation could have a progressive impact on brain funciton, so I'm glad we didn't wait any longer. I wouldn't want to push anyone to seek treatment that isn't necessary, but I'm a strong believer in what a good, caring doctor might be able to conclude from taking a detailed case history. The only diagnostic exam that was potentially difficult for DS was the CTscan, but that ended up being fine. Everything else was pretty standard office visit other than a more careful look in the nose. And thanks for all those site recommendation, Krissi, I wish I was keeping better track of the good sites I've found so I could pass them along.