Diabetes Can Leave Us Seeing Double
Understanding what your eyes are seeing
Diabetes is a frequent cause of double vision, what eye doctors call "diplopia." This phenomenon is attributable to diabetic neuropathy affecting the fine nerves controlling the coordinated movements of both eyes. This can result in a "crossed," "wandering," or "lazy" eye that causes diplopia.
For a person with "straight eyes" (i.e. no crossed eye or ‘strabismus'), precise alignment of both eyes is required to maintain clear, single vision (fusion), and any deviation results in double vision (diplopia). The coordinated movements of both eyes are accomplished by six pairs of muscles connected to the outside of the eyeballs. These muscles are controlled by three specific pairs of ‘cranial nerves' (nerves emerging from the base of the brain – there are twelve pairs total, numbered in Roman numerals from I to XII). When chronically high blood glucosedamages the tiny blood vessels supplying oxygen to these cranial nerves, either partial (paresis) or total (paralysis) loss of eye muscle function and double vision occurs (provided the brain does not shut off, or ‘suppress' the affected eye, as happens with infants and children who develop strabismus); loss of any eye muscle function is denoted by the less specific term, palsy.
Symptoms of double vision develop suddenly with cranial nerve palsy due to diabetes. Sometimes, patients are able to eliminate or minimize double vision by tilting or turning their heads such that the incapacitated muscle does not need to be used as much. Closing or covering the affected eye always eliminates such double vision. Diabetes related nerve palsies almost always affect one eye at any given time. Let's briefly consider the three cranial nerves controlling movement of our eyes.
The Third Cranial Nerve (called the ‘oculomotor nerve' or CN III) controls four external eye muscles and two internal eye muscles. It has separate branches allowing opening of the eyelid, turning the eyeball inward toward the nose and upward toward the forehead, and control of muscles allowing the pupil to constrict (get smaller) and the ability to focus from far to near. CN III has a lot of jobs, and cranial neuropathy affecting it has a profound impact upon vision (the lid droops, the pupil is dilated and immoveable, precise focusing is lost, and the affected eye wanders down and out. Vertical and horizontal double vision result provided the eyelid does not block vision, or if the eyelid is held open. Diabetes is a leading cause of IIIrd Nerve palsy, and characteristically does not affect the branch controlling the pupil (80% of cases). Other causes include tumor and aneurysm formation.
The Fourth Cranial Nerve (called the ‘trochlear nerve' or CN IV) controls a single external eye muscle, which is responsible for turning the eye downward and outward, and for torsionally turning the eye inward like the hands on a clock. Diabetes is a common cause of neuropathy affecting CN IV, which results in diplopia with vertical, horizontal, and torsional components; patients typically compensate by tipping the head toward the shoulder that is opposite the affected side. CN IV palsies also are often caused by trauma and stroke.
The Sixth Cranial Nerve (called the ‘abducens nerve' or CN VI) also controls just a single external eye muscle, responsible for turning the eyeball outward toward the ear. The affected eye is unable to turn outward past the midline. Patients typically turn their head toward the affected side in order to minimize horizontal diplopia. Diabetes is a common cause of CN VI palsy, as are stroke, multiple sclerosis and tumor.
Although cranial neuropathies causing double vision are not an infrequent symptom of diabetes, the mere presence of one (or more) of these neuropathies in a patient with diabetes does not exclude other causes. Only thorough evaluation by a knowledgeable, experienced examiner can establish a definitive cause. Fortunately, diplopia related to diabetes typically is caused by dysfunction of only one nerve at a time (and on one side), and resolves without intervention in the majority of cases within 3 to 6 months. Patients with multiple cranial nerve palsies, palsies simultaneously affecting both eyes, or palsies that do not resolve within 6 months should receive neuro-imaging (CT scan or MRI) to rule out other causes. People with diabetes can reduce their chances of developing cranial neuropathy and double vision by maintaining excellent blood glucose, blood pressure, and blood lipid control.
Examples of Double Vision From Various Cranial Nerve Palsies
For more information on diabetic eye disease, consult Dr. Chous' book Diabetic Eye Disease: Lessons From a Diabetic Eye Doctor, Fairwood Press, Seattle, 2003.
Read more about Dr. Chous here.
Visit Dr. Chous' website here.
NOTE: The information is not intended to be a replacement or substitute for consultation with a qualified medical professional or for professional medical advice related to diabetes or another medical condition. Please contact your physician or medical professional with any questions and concerns about your medical condition.
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