Ms. Sanders declined to explain what led to the diagnosis, what symptoms the senator had or what indicated that the disease had progressed.  “We on staff who work here with him every day haven’t found any sort of effect that’s interfered with his ability to do his job and do it well,” she said.  She said that Mr. Domenici had regular examinations by a doctor at Johns Hopkins, and that a checkup in April detected no signs of progression. At that point, he was planning to run for re-election, but changed his mind after the examination in September.  Dr. Bruce Miller, a neurologist at the University of California, San Francisco, and a leading expert on frontotemporal dementia, called it “a very tough disease,” and said it could progress rapidly, twice as fast as Alzheimer’s.  “In a study we did, the average person, from the time of diagnosis, lived only about three and a half years,” Dr. Miller said.  He said many patients died from problems like those in Parkinson’s disease that impaired swallowing.  Dr. Miller is not involved in Mr. Domenici’s care and said he did not know the details of his case.  But asked if it was reasonable for someone with frontotemporal dementia to continue serving in the Senate until 2009, Dr. Miller said, “It would be worrisome, I would say. Not knowing any of the specifics, and given the possibility that this is incredibly mild, it might be O.K.. I would say that generally, most of our patients don’t work.”  Dr. Miller also said he was surprised by the case, because at 75, Mr. Domenici is well beyond the average age at which the disease is usually diagnosed.  “It’s possible the diagnosis is wrong,” he said.  Typically, the disease is diagnosed after patients consult a doctor about speech problems or because relatives have noticed changes in behavior. It is sometimes mistaken, at least initially, for a psychiatric disorder. Mental tests can help identify it, and magnetic resonance imaging scans can detect a characteristic pattern of brain shrinkage. Both tests can monitor the progression of the disease.  Another neurologist who specializes in dementia, Dr. Norman Relkin, of New York Presbyterian Hospital/Weill Cornell Medical Center, said, “In some cases I’ve followed patients for many years and they have been able to maintain a relatively high level of function in positions of responsibility.”  But as the disease progresses, it impairs insight and judgment, something patients themselves cannot recognize, Dr. Relkin said.  “Usually those around them become aware of it,” he said.  Patients who continue working may need other people to let them know if they have done something irrational, irresponsible or out of character, Dr. Relkin said. And they need regular medical visits to monitor the progression of the disease.  Dr. Jason Karlawish, an associate professor of medicine and medical ethics at the University of Pennsylvania, said: “This a case study for something that will be an issue more and more as people don’t necessarily retire at 65. Just because he has a diagnosis doesn’t mean he’s not capable of doing his job.”  The larger question for society, Dr. Karlawish said, will be figuring out how and by whom decisions should be made about other people’s mental capacity to work, vote and make choices for themselves. Often, he said, doctors will have to be involved.  Ms. Sanders said Mr. Domenici’s doctor “feels comfortable with” his continuing to serve in the Senate.  A version of this article appears in print on , on Page A32 of the New York edition with the headline: Senator’s Illness Requires Monitoring, Doctors Say. Order Reprints| Today's Paper|Subscribe
The announcement of Senator Pete V. Domenici’s progressive brain disease has left unanswered questions about how the illness is affecting him.