ROCHESTER, MNDr. Erich Stellbrach, a general practitioner at the Mayo Clinic, could barely contain his exhilaration Monday upon discovering that patient Oliver Patterson, 54, has the extremely rare degenerative nerve disease Gertsmann-Straussler-Scheinker syndrome. 'Mr. Patterson, I'm so sorry to tell you this, but you haveyou're not going to believe itspinocerebellar ataxia!' Stellbrach said, waving an x-ray of Patterson's spinal cord. 'It afflicts only one in 2.9 million people!' Stellbrach recommended Patterson begin aggressive treatment to mitigate his impending brain dysfunction and onset of dementia, and made the patient promise to remain in his care. 