As many people already know, diabetes is a disease that can have great consequences for the eyes and vision.

It is commonly understood that if poorly controlled over a long period of time, diabetes can damage the eye and result in loss of vision.  But quite often undiagnosed or poorly controlled diabetes can also cause fluctuations in vision.

So it happens that now and then a patient will present with a complaint of headache, blurry vision, or maybe both.  She reports that her current glasses “just aren’t working anymore”.  Following a thorough exam, it’s found that a significant change in prescription is necessary.

The patient compares the new prescription to her current spectacles and agrees that the change is quite beneficial — “I can see better”!  So, the change is agreed upon and the spectacles are sent off to the lab.

A week or two later, the patient picks up her new glasses and reports that “it does not seem clear”. In fact, she reports that her old glasses “started working” just fine again. There have been no more headaches, and no eyestrain at all. We step into the exam room, re-check the prescription, and the result is again the old prescription. What’s going on?

As blood glucose levels rise and fall, a spectacle (or contact lens) prescription will fluctuate.  This phenomenon occurs in both undiagnosed diabetics, and known diabetics with poor blood sugar control.  A change in diabetic medication can also cause a short term change.  If the fluctuation in blood glucose levels is small, then the variation in the prescription will be minimal.  The patient will often times write these fluctuations off as “just being tired”.  On the other hand, large changes will be very noticeable and will cause the patient to go to the eye doctor believing that their prescription has simply changed.

An elevated blood glucose level causes swelling of the lens inside the eye.  As a result, the patient’s prescription may change.  A nearsighted person may become less so.  
A farsighted person may become more so.  The change is not always predictable, and depends on the particular swelling of the lens.  Once the glucose level is stabilized, so too will the prescription.

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