An ABR test, by evaluating the hearing nerve pathways, can identify cases of sensorineural hearing loss. It is a sleep EEG hearing test which shows the softest sounds your child's ears can detect at various pitches. The ABR compares changes in brain activity to the timing of repetitive sounds to determine whether the particular intensity of sound can be heard.
You can read more about it on the Boston Children's Hospital's webpage, which is where I found this definition.
From what I understand, this will determine exactly what type of hearing loss Mason has (although I thought it was already known...there are only two main types, and only one has to do with the nerve that the tumor was wrapped around), and if he will need/be able to use a hearing aid or not. I'll post more tomorrow after his test, when I know more of what will happen after the test.
I also wanted to note really quick, that Mason went to the hospital by our house for a blood draw on Friday, because we couldn't make it up to Primary's that day. This hospital isn't equipped to access ports, so he had to have the blood drawn from his arm, without numbing cream. We talked about it all morning, and he was ready for it when it happened. He got scared when he saw the needle and it did hurt him pretty badly, but he left saying that he would be okay doing it again on days when he just needed blood drawn, and didn't have to see the doctor. Brave boy! :) I'm not sure if he'll remember saying that the next time it comes around, but we'll see how he feels then.