First, those symptoms are far from unique to heroin overdose. They are also consistent with, for example, a severe head and neck injury.
Second, I was responding to a poster who suggested that, because Gray was "high on drugs," it is likely that he hurt himself by trying to violently throw himself against the door. That's just not the type of behavior one would expect from someone high on the types of drugs found in his system.
Heroin was found in his system. Even the M.E. said that he may have been having convulsions. There were several notations of him thrashing around in the back of the van. That is why they checked him several times and added restraints.
The M.E. also aid that the injury likely occurred after the third stop, based on the timeline, video and statements of the officers. This was reported by the Baltimore paper that had the autopsy.