What should i do if emergency room doctor doesn't know about your condition?

Tell him/her!: If you mean a condition you have not shared, then by all means--"spill it, " it is not a guessing game with your health. If you mean you have a rare or complicated condition the er dr is not familiar with--he/she cannot know everything, so can wait while searches internet, or you, as expert in your condition, can educate provider on basics / needs / risks / usual care.