Excessive thirst, increased urinary frequency and blurry vision can all be early signs of undetected diabetes mellitus.
What you already know. Diabetes mellitus is the body’s inability to decrease the sugar in the blood. This “serum glucose” rises after every meal as our food is broken down into its basic components. In the normal situation, a rise in blood sugar causes insulin to be secreted into the blood stream. Insulin takes blood sugar out of the blood and delivers it to our tissues. As a result, serum glucose levels are kept low and maintained at a steady state. If insufficient insulin or no insulin is produced, then the sugar remains in the blood stream and the sugar level rises.
High levels of sugar increase the “osmolarity” of the blood. Osmolarity is a difficult term to understand, but it reflects the ability of a substance to attract water. The higher the osmolarity of a liquid, the more water it will attract and retain. In this case, as the serum glucose increases, it will literally draw water out of our tissues. We feel thirsty because our tissues are actually dehydrated, hence the increased thirst. The increased water in our blood then causes more urination.
What causes the blurry vision? Again, it is the high sugar and osmolarity, but with a slight twist. The high sugar leaks into the eye and then gets absorbed by the natural lens. The sugar, or glucose, is then changed to sorbitol, another form of sugar. The sorbitol does not leave the lens very readily and is basically trapped. Sorbitol, like glucose, also adds to osmolarity. So, the sorbitol attracts water and causes the lens to swell. This causes the vision to blur. Even with correction of sugar, it may take weeks for the lens to return to its more normal state.
Usually, these are the first, early signs of diabetes. The vision changes are usually not due to diabetic retinopathy at this time as this takes years to develop.
In the end, all three symptoms are based on the same mechanism. Once the abnormality is identified and sugar returns to normal using either diet, oral agents or insulin, water redistributes normally, our tissues rehydrate and the eye returns to normal. Vision, too, should return nicely.
Randall V. Wong, M.D.
Ophthalmologist, Retina Specialist