Syneretic response of aging normal human lens to pressure

The study was designed to observe whether a reversible syneretic response to pressure is operative in normal human lenses and whether such a response demonstrates a uniform age dependence.

Seven sections (from the anterior outer cortex to the posterior outer cortex) of 10 human lenses were imaged at 2 atmospheres (atm) pressure and the T(1) (spin-lattice) and T(2) (spin-spin) relaxation data on each section were collected. The pressure was then released and NMR relaxographic data were collected under 1 atm.

Both T(1) and T(2) relaxation times were at their minimum in the nuclear region and at their maximum at the two outer cortexes. With increasing pressure, T(2) relaxation times decreased. The pressure-dependent change in T(2) relaxation times decreased with age. Changes in T(1) relaxation times showed no consistent pressure or age dependence. The population index of T(2) relaxation, M(2), had a maximum in the nucleus and a minimum in the two cortexes. The population index of T(1) relaxation, M(1,) was minimal in the nucleus and maximal at the two cortexes. M(2) increased with increasing pressure, whereas M(1) did not show consistent pressure dependence. The percentage of change in M(2) (DeltaM(2)) showed a statistically significant increase with increasing age, whereas the %DeltaM(1) showed no significant age-dependent trend.

The positional dependence of relaxation times and the population indexes indicated that spin-spin relaxation represents the behavior of the bound water and the spin-lattice relaxation that of total water. As pressure increases, the strength of hydrogen bonding as well as the amount of bound water increases. The pressure-induced change in the total water is minimal. Thus, the free water-to-bound water ratio decreases with increasing pressure, demonstrating a significant syneretic response. The extent of reversible syneretic response decreases with age and is actually reversed in older lenses. The implication is that the ability of the human lens to respond reversibly to pressure decreases with the decrease in accommodation, and, when the ability is lost altogether, an increase in free water, a possible source of cataract formation, may ensue.