It is particularly irritating when a coworker eats with mouth open. But as for the extra distress you are going through when your coworker smacks her food, I can only fail to imagine that considering your health clause. However, how you deal with a loud chewer is determined by your level of relationship with the loud eating coworker and as well as how other of your colleagues perceive the condition: if it’s equally disturbing them or not.
Your first approach when a coworker eats with mouth open is to confront her politely. NB: if it’s an issue she can easily fix, then the effort is worthwhile, otherwise it would be rude to confront her. While confronting her, consider presenting yourself at fault for not being able to deal with a loud chewer. You have already spelled the health condition making that impossible; maintaining it will give you an edge. Don’t imply that she is to be blamed. Listen to her response and thank her for being receptive.
If the loud eating coworker seems offended or implies that she can’t do anything to help you focus better at work, be sure to apologize and withdraw your request.
If the challenge persists, you can respond each time the coworker eats with mouth open to distract you by putting on a quality noise-canceling headphone, though this can only be an option if your office permits headsets.
With the first two options not viable, it’s time to privately inform your office HR or management that a coworker eats with mouth open to distract you from work. Don’t hesitate to disclose how others feel when the coworker chews loudly. Your leaders would have to look into how the coworker smacks her food to disturb others. It should lead to a new eating culture that will promote general productivity.