What causes brain swelling? My 2 1/2 year old grandson Alex took 18 sleeping?

pills. He is in a coma having seizures. The doctors say his brain is swelling. Can an overdose of sleeping pills make your brain swell? He lives in California with his mom and 3 yr. old sister Ana. My granddaughter was put in foster care. I am having trouble believing the mother's story of how this happened. Alex is @ CHOC which is a children's hospital. I am presently living in Iowa. That is 1,800 miles away from my grandkids. It has been difficult to find out any info. This happened last Friday afternoon and she didn't call my son until 11 p.m. Saturday night. She informed us that he was doing better. It wasn't until yesterday when my son spoke with a doctor that we realized that it is doubtful that he will pull through. He is a tough little boy just like his dad. But swelling of the brain!? I always ociated brain swelling with head injury. I've tried researching this on the web but I would appreciate some first hand knowledge. Thanks.