Microvascular cranial nerve palsy (MCNP) is a neurological condition involving the small blood vessels that affects the muscles that move the eyes. With MCNP, there is a blockage of blood flow to the nerves between the brain stem and muscles within the eye socket, as well as other nerves that affect visual function.
With MCNP, you may not be able to move your eyes in one or more directions, depending on the muscles affected, and you may experience double vision. You may also have a droopy eyelid. Double vision and droopy eyelids can also be symptoms of a number of life-threatening conditions, so if you experience either symptom you should see your ophthalmologist (Eye M.D.) as soon as possible.
MCNP is one of the most common causes of acute double vision among older people. It occurs more often in people with diabetes and high blood pressure. MCNP is sometimes called "diabetic" palsy of the eye. Occasionally MCNP can occur in younger people with migraine. This condition almost always resolves on its own without leaving any double vision.
Six muscles move your eyes. Four of these muscles attach to the front part of your eye (just behind the iris, or the colored portion of the eye). Two muscles that attach behind the four forward muscles are responsible for some of the up-and-down (vertical) movement and most of the twisting movement of each eye. These six muscles receive their signals from three cranial nerves that begin in the brain stem.
A nerve cannot function properly when its blood flow is blocked. If the sixth cranial nerve (also called the abducens nerve) is affected, your eye will not be able to move to the outside and you will be aware of horizontal double vision (seeing side-by-side images). If the fourth cranial nerve (also called the trochlear nerve) is affected, you will be aware of vertical double vision (one image on top of another). You may be able to eliminate or decrease the double vision by tilting your head towards the opposite shoulder.
The third cranial nerve (also called the oculomotor nerve) supplies four of the six eye muscles. These are some of the muscles that control the eyelid and the size of the pupil. When the third cranial nerve is affected, your eye may be limited in its up-and-down motion and may turn away from your nose. The eyelid may droop. You usually experience combined vertical and side-by-side double vision.
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