Aging of the human crystalline lens and presbyopia

The human lens undergoes profound optical and physical changes with increasing age. The impact of these changes is evident in the extraordinary loss of the physiological function of accommodation roughly midway through the human life span and in the nearly inevitable development of cataract in the elderly. Various lines of experimental evidence show that age-related changes in the lens occur from birth and include increased mass, increased thickness, increased anterior and posterior surface curvatures, increased hardness, increased light scattering from the zones of discontinuity, possible changes in refractive index distribution, loss of ability to undergo accommodative changes, changes in spherical aberration, increase in the shortest attainable focal length, and decreased ability of the capsule to mold the lens. Under this barrage of insults due to aging, it is no wonder that presbyopia and, ultimately, replacement of the lens with an IOL are the norm. The reasons for the occurrence of these physiological changes are uncertain. The changes may be a cause or a consequence of presbyopia. Certainly, increased hardness, inability to undergo accommodative changes, and alterations in shape due to the capsule, together with an increasing shortest attainable focal length of the lens, all relate directly, either causally or consequentially, to presbyopia. Realistic surgical modalities aimed at restoring accommodation must consider these factors. Although some increased efficacy of the lens may be achievable serendipitously by unusual surgical interventions that either are in use or are being developed, clearly, if accommodation is to be restored, the crystalline lens must be replaced with a prosthesis that is more suited to the function of accommodation than is the aged human lens.