We pretty much knew that this was the case, but it is good to get official confirmation (as reported in the Washington Post!)
In an interview with The Washington Post, Francisco J. Diaz, the medical examiner, said the autopsy found no evidence the 42-year-old officer suffered an allergic reaction to chemical irritants, which Diaz said would have caused Sicknick’s throat to quickly seize. Diaz also said there was no evidence of internal or external injuries. . .
Diaz said Sicknick suffered two strokes at the base of the brain stem caused by a clot in an artery that supplies blood to that area of the body. Diaz said he could not comment on whether Sicknick had a preexisting medical condition, citing privacy laws.