His doctors heard of an experimental treatment at the Kaiser Permanente medical center in Los Angeles. Neurologists there were using a potent immune-suppressing medication to try to protect the nervous system from additional attacks and give it a chance to recover. His neurologist arranged for the patient to be seen there.

Dr. Brandon Beaber, a fellow in a branch of neurology specializing in diseases like M.S., was assigned to the patient. He introduced himself to the couple; he had already reviewed the man’s extensive records and had more questions. Your father died when you were young? he asked. The patient was a teenager when his father died, his wife explained when her husband seemed unable to answer. He had severe dementia and died of what was thought at the time to be arteriosclerosis. His mother died of breast cancer within a year. He had two brothers who both died in their 40s. It felt as if there was almost no family left — except for his children.

A Rare Disorder

“I don’t think you have M.S.,” the doctor said slowly. His symptoms, his M.R.I. and his family history suggested that he had a rare and newly described inherited disease called Cadasil. Beaber had seen a few cases of this unusual disorder. A report published in 1955 about two sisters who developed a severe dementia in early middle age is now thought to be the first described case. At the time, it was viewed as a variation of a type of dementia known as Binswanger’s disease, which can be associated with personality changes. In the earliest stages of the disease, the imaging looks like that seen in M.S. But we now know that patients with Cadasil often have the kind of severe headaches this patient described, as well as strokes and an early and relentlessly progressive dementia associated with often significant personality changes. Symptoms can start when patients are in their 30s. The diagnosis tends to be made a decade later.

The gene behind the disease was identified in 1993 — just a few years before this patient got his diagnosis of M.S. The genetic error affects the muscles in the small vessels in the brain, causing them to thicken, cutting blood flow to the segments of the brain they supply. In short, those who have Cadasil have thousands of tiny strokes that kill off the brain bit by bit. There are no treatments for the cognitive effects and no known cure for the disease.

Confirming a Gene Problem

Beaber sent the man for genetic testing. Cadasil and all its devastation are caused by a single error in a critical single link in a genome containing three billion otherwise normal links. Worse still, if he had it, then his children had a 50-50 chance of having inherited the same bad link. Testing showed he had Cadasil. Next they had to tell their two children that they might have it as well. It was, the man’s wife told me, the hardest thing she has ever done.

Over the next three years, the patient worsened. His world became smaller and smaller. He spoke less and less. He was unable to walk. He also lost his anger. His wife told me that toward the end, he actually seemed content. He spent his last few weeks surrounded by those who loved him, eating ice cream and watching baseball. She was grateful he never knew he was dying.