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Some brain functions decline at a linear rate throughout adulthood. Others remain relatively stable until very late in the life cycle. This study characterized the effects of aging on the regional cerebral distribution of hexamethylpropylene amine oxime (HMPAO) in healthy human volunteers. The sample consisted of 26 men and 18 women with a mean age of 41.6+/-14.9 years (range: 19-73). Their past medical histories, physical examinations, and laboratory screening tests were normal. Single-photon emission tomography (SPET) scans of the brain were performed with a standardized acquisition and processing protocol on a triple-headed camera equipped with fan beam collimators. A 3-D restorative filter and a correction for uniform attenuation were applied before the images were reinterpolated in planes parallel to the line connecting the frontal and occipital poles. Mean counts per pixel were measured in multiple regions of interest (ROIs) within each hemisphere by custom fitting a set of templates to the images. The mean activity in each ROI was compared with the mean activity per pixel in the whole brain. Regression analyses were used to relate the activity ratios to age with both linear and nonlinear models. The relative concentration of radioactivity decreased significantly with age in most, but not all, gray matter structures. It increased in the white matter regions. The nonlinear model of aging fit the data significantly better than a straight line did. Most of the changes with age occurred during young adulthood. No further changes were detectable after the onset of middle age. The median breakpoint age at which the rate of change became negligible was 36.6 years. Aging significantly affects the relative uptake of HMPAO in healthy humans. It decreases in many gray matter regions and increases in most white matter regions. However, the changes do not appear to be linear. Most seem to occur during young adulthood before people reach their late thirties. The distribution then appears to remain relatively stable throughout middle age.

Objective. —To characterize the natural history of viremia with human immunodeficiency virus type 1 (HIV-1) and its association with disease progression from infection to acquired immunodeficiency syndrome (AIDS). Design. —Prospective cohort study. Annual specimens were tested for quantitative virion-associated HIV-1 RNA, p24 antigen, and CD4 + lymphocyte levels. Participants. —A total of 42 homosexual men who seroconverted to HIV-1 between 1982 and 1985. Main Outcome Measures. —Trends over time in serum HIV-1 RNA level, correlations between serum HIV-1 RNA and other markers, and prediction of AIDS using these markers. Results. —HIV-1 RNA levels were stable overtime, increasing by 10-fold or more in only six (14%) of the 42 subjects during 3 to 11 years of follow-up. Mean HIV-1 RNA levels were 10 3.8 copies/mL if AIDS occurred in less than 4 years, 10 3.07 copies/mL if AIDS developed within 4 through 9 years, and 10 2.27 copies/mL if AIDS did not develop within 6 through 11 years. In both univariate and multivariate models, initial and subsequent HIV-1 RNA levels, p24 antigenemia, and percentage of CD4 + lymphocytes were independently predictive of AIDS. Conclusions. —The stability of virion-associated HIV-1 RNA levels suggests that an equilibrium between HIV-1 replication rate and efficacy of immunologic response is established shortly after infection and persists throughout the asymptomatic period of the disease. Thus, defective immunologic control of HIV-1 infection may be as important as the viral replication rate for determining AIDS-free survival. Because individual steady-state levels of viremia were established soon after infection, HIV-1 RNA levels may be useful markers for predicting clinical outcome. ( JAMA . 1995;274:554-558)