Mechanisms of disease: cataracts.

Studying the mechanisms that are responsible for the transparency of the lens we see that multiple factors are involved in the maintenance of lens clarity as well as in cataractogenesis. Is there a unifying mechanism? Since the function of the lens is the transmission of electromagnetic radiation in the visible spectrum, fundamental physical laws should apply. Attenuation of a light beam that passes through a medium happens by two major processes: absorption and scattering. In absorption, light energy is dissipated into heat as the result of energy-absorbing electronic transitions. In scattering, radiation is redirected away from the incident pathway, thereby reducing the transmitting power. Depending on the angular dependence of the scattered light, the wave front of the transmitted light is distorted. In the case of cataract, the primary factor is turbidity secondary to scattering. A perfectly uniform medium exhibits no light scattering; thus a continuous medium can scatter light only when it contains spatial fluctuations around the mean density governed by specific equations. These equations simply state that the amplitude of scattering is proportional to the Fourier amplitude of the density fluctuations in the medium [12]. Electron micrograph analysis using Fourier transformation has shows an increase in the fluctuation of spatial density of the opaque fiber [12,26,37,93]. Thus, all transparency mechanisms and all cataractogenic factors can be thought of as opposing effectors of spatial density fluctuation affecting scatter and antithetical producers of light absorbing moieties.