Mononeuropathy and diplopia
People with diabetes occasionally have damage to a single nerve (mononeuropathy). It's not clear why this happens, but there's generally thought to be some relationship with lack of glucose control. There doesn't seem to be an association between diabetic mononeuropathy and other forms of diabetic neuropathy.
Mononeuropathies are not always diabetic in origin. For this reason patients with diabetes and a mononeuropathy should have other causes of neuropathy considered, including alcoholism, familial, nutritional, and drug-induced. In the absence of other medical disorders known to cause neuropathy, extensive investigation is not likely necessary.
How mononeuropathy causes diplopia
Among the nerves that might be affected by a mononeuropathy are the cranial nerves (nerves supplying the head). There are a dozen cranial nerves, but some of them may have specific problems that occurs in people with diabetes: usually the third or sixth cranial nerves. The third nerve is also called the occulomotor nerve; the sixth, the abducent. Another name for damage to the nerve is "nerve palsy" (hence the names "third nerve palsy" and "sixth nerve palsy").
The third, fourth, and sixth cranial nerves control the extra-ocular muscles (EOM) of the eyes, allowing the eyeball to rotate in different directions. So, if one of the these nerves is damaged on one side, the eye on that same side can't fully move, causing double vision (diplopia, from the Greek, diplous = double + ope = sight) when the patient looks in certain directions. This double vision is extremely annoying, but easily temporarily treated, by putting a patch over the eye on the side where the nerve is affected.
In addition to the double vision, damage to the third nerve may result in ptosis (dropping eyelid) and a dilated pupil. These findings are variable in diabetic nerve palsies, and if present should prompt the physician to do further evaluation (such as a neurologic consultation and/or an MRI) to look for other causes of the damage.
Although diabetic EOM palsies are extremely frustrating, they resolve gradually over a period of months without any specific treatment.