Dr. John Langdon Hayden Down, a prominent 19th-century British pediatrician specializing in the care and treatment of "idiots and imbeciles," wrote in 1866 (in the first description of the syndrome that now bears his name) that "a very large number of congenital idiots are typical Mongols."

With that description, the brilliant scientist created an ethnic slur that was to last nearly a century, until his modern colleagues chose to substitute "Down's syndrome" for Dr. Down's own appellation of "Mongolian idiot."

It is not only the name that has changed for this particular birth defect, common to virtually all ethnic groups and occurring in this country in approximately one out of 1,000 births. Now, for the approximately half-million members of the Down's population, there may be a new frontier of research and even hope--in a surprising place for a birth defect, the National Institute of Aging. NIA is offering an opportunity for Down's individuals to participate in a program that could have implications for Down's syndrome, of course, but also for Alzheimer's disease, that relentless brain deterioration affecting about 15 percent of the over-65 population.

Scientists know today that Down's syndrome is a genetic disorder almost uniformly characterized by the presence of an extra chromosome--47 instead of the normal 46--in either the sperm or egg of a parent.

Down's individuals, in addition to the facial characteristics that suggested Mongol to Dr. Down, share other characteristics in varying degrees: a tendency to heart and lung defects, a susceptibility to infection, eye and ear problems, speech thickness. Although they are all mentally retarded, the degree of retardation varies widely, and many can learn basic life skills.

Many seem to be natural mimics and often have cheerful, affectionate personalities.

Until the 1940s, half of Down's babies died in infancy. Very few survived into their twenties. Now, since the advent of antibiotics and more recently the exponential breakthroughs in neonatal treatment, including heart surgery, the life expectancy for the Down's individual is about 50 years.

Because they are living longer, scientists have been able to verify one other major common characteristic also common to elderly persons suffering from Alzheimer's disease.

According to Dr. Neal R. Cutler, chief of the Section on Brain Aging and Dementia, Laboraory of Neurosciences, National Institute of Aging, "Somewhere in the third and fourth decade of their lives, all--100 percent--of Down's individuals will develop the neuropathologic changes of Alzheimer's disease."

Not all of the Down's population will have the deterioration in mental functioning exhibited in Alzheimer's, but autopsies invariably show the same twisted and tangled nerve cells seen in the brains of Alzheimer victims.

And some of the Down's individuals will, indeed, begin to lose even those few skills they have been able to master. This similarity in brain changes in the two disorders was first described about 50 years ago, but at the time it had no relevance to other problems and, says pediatrics neurologist Dr. Mark B. Schapiro, "it was just forgotten."

Cutler, who oversees programs involving persons with Alzheimer's disease, is starting a major study of a group of adult Down's individuals with the help of Schapiro.

In preliminary work reported in a current issue of the journal Science, Cutler and his NIA team reported some surprising findings in a group of Down's subjects whose brain metabolism was measured by use of a PET scanner. PET--for Positron Emission Tomography--is an elaborate computerized technique for measuring the metabolic functions of the brain. The researchers found that young Down's individuals (between 19 and 27) had a much higher brain metabolism than normal.

By comparison, says Cutler, Alzheimer's sufferers show "definite reductions" in the way the brain metabolizes glucose.

In one 51-year-old Down's individual, the researchers also found a lower brain metabolism, but no evident deterioration of function. "In fact," they wrote in the Science article, "family members reported that his performance and temperament had, if anything, improved in the last several years . . ."

"We don't know what it means yet," says Cutler, "but we are making a major thrust to follow up on it. We need to understand the relationship between Alzheimer's and Down's."

Some studies have suggested that there may be a higher incidence of Down's in families in which older members have Alzheimer's, which in turn suggests that Alzheimer's is also genetically based. (Other work has suggested environmental factors such as aluminum may be related to Alzheimer incidence.)

According to Cutler, the main question they hope to answer is why some Down's people do not show intellectual deterioration associated with the brain changes. The answer could lead to research on eventual treatments to protect intellect, even if the physiological brain changes cannot be stopped.

For the study Cutler and Schapiro are seeking adults (over 18 but especially those in their thirties and forties) with Down's syndrome. After interviews with individuals and their families, those selected will be admitted for a 7- to 10-day hospital stay, during which they will be assessed for all health risks associated with Down's. They will receive complete physical and neurological examinations, chemical and metabolic analyses along with special attention to cardiovascular, seeing and hearing problems.

"Individuals with Down's," says Schapiro, "need to maximize the capacities they have."

Those interested in either the Down's Program or the ongoing Alzheimer study should write, specifying which study, care of Angela Moore, Laboratory of Neurosciences, National Institute of Aging, Building 10, Room 10N314, National Institutes of Health, Bethesda, Md. 20205 or phone: 301-496-4754.

For a brochure on Alzheimer's Disease, write "The Dementias," the National Institute of Neurological and Communicative Disorders and Stroke, Rm. 8A06, Bldg. 31, National Institutes of Health, Bethesda Md. 20205.

For more information on Down's syndrome write March of Dimes Birth Defects Foundation, 1275 Mamaraneck Ave. White Plains, N.Y. 10605.