Diabetes causes damage to small and large blood vessels. This in turn causes damage to heart, eyes kidneys and feet.
Very high
sugar 300 to 500 will cause this kind of damage. It has been observed that good blood sugar control
hba1c of 6.5 can prevent such damage.
Additionally very high blood sugars can cause osmolar symptoms like blurred vision, frequent urination etc. The chance of infections increase. Also a rare serious condition of high sugar is coma (HHNS)