Discuss honestly, the possible course the disease may take. Understand your patient's views clearly. Encourage him to make an oral or written "living will". This way, you will have a clear understanding of which treatment and extent of medical support the patient desires. Convey this information to the primary physician too. Doing all this when the patient is in relatively stable or good health, is always better.
Ultimately, the patient is the one who endures all the physical trauma. Your emotions may cloud your judgement, if you are called upon to make these choices. If your patient is not in a position to convey his own options, your choices will be hard and difficult. It is far better to be armed with clear knowledge of the patient's will and intent, to execute such decisions.
Prepare yourself mentally to the possibility of having to actually execute such wishes. Insisting upon "Do Not Revive" may be the hardest thing you ever have to do. But if your patient has insisted upon this, like any other "will", his wishes should be honored. As also, any eye or organ donations that he may have willed