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Abstract
A summary of the events occurring in galactose cataract is presented in Fig. 10. The primary factor initiating this type of cataract is the high concentration of galactose in the aqueous humor. The abnormal level of galactose in the lens triggers the enzyme aldose reductase to convert galactose to dulcitol. Since the lens membranes are relatively impermeable to sugar alcohols, dulcitol once formed begins to accumulate, creating a hypertonic condition. To maintain osmotic equilibrium, water is drawn into the lens fibers. Unless, at this point, the swelling is checked by withdrawing galactose, the viability of the lens steadily declines. Even before any lens changes are grossly visible--the prevacuole stage--the resulting osmotic swelling has deleterious effects on the lens. The increase in hydration markedly affects the amino acid transport mechanism, accounting for the decrease in amino acid content. In the prevacuole stage, osmotic changes probably are also responsible for the appearance of hydrops, which later, with further increase in water, disintegrates to form vacuoles.
In the late vacuolar stage the dulcitol content is maintained at the level observed in the initial stage. This probably does not mean its synthesis has stopped, but since the permeability properties are sufficiently altered the rate of exit is equal to that of its formation. There is only a slight increase in water, but changes in cations reveal a dangerously abnormal condition. The Na content is approaching the level of K, indicating that the lens is having difficulty excluding Na. The significant lowering of ATP may be contributing to the marked changes in cation. In spite of these changes there is no major change in hydration during the period just prior to the appearance of the nuclear opacity.
The nuclear cataract stage is characterized by another and more pronounced increase in lens hydration. The swelling is due to the accompanying increase in electrolytes. Apparently a complete loss in the selective permeability results because dulcitol is no longer retained. Moreover, the electrolytes are so freely diffusible that Donnan swelling develops.