Internuclear Ophthalmoplegia (INO)

See also:  saccades, and INO cases

Timothy C. Hain, MD• Page last modified: August 31, 2019

The most common source of asymmetrical saccadic velocity is normal abduction with slowed adduction. This occurs mainly in internuclear ophthalmoplegia or "INO" . INO is due to a lesion in the median longitudinal fasciculus or "MLF", which connects the paramedian pontine reticular formation and the oculomotor nucleus. The MLF is located immediately adjacent to the cerebral aqueduct.

Location of MLF (7) on diagram, from U. Michigan

INO is most often found in patients with multiple sclerosis or cerebrovascular accidents involving the brainstem (Fischer, 1967; Wall and Wray, 1983). The images below show an INO due to a vascular event.

ino saggital ino AXIAL
MRI of patient with bleed into MLF Saggital image of same patient


The hallmark of INO is slowing of adducting saccades, accompanied by an overshoot of the abducting eye. The condition can be unilateral or bilateral. A reduction of adducting velocity into the abnormal range, accompanied by normal abducting velocity, for medium size saccades (about 20 deg), should cause one to consider INO. In this case, one should also examine the position traces of each eye. The combination of an overshoot of the abducting eye, and significant slowing of the adducting eye occurring simultaneously, confirms INO.

The recording below shows an INO due to MS.
Internuclear ophthalmoplegia (INO) recorded in right eye (recording method: VNG, Micromedical Technologies). Note how the rightward going saccades are brisk, while the leftward going saccades (adducing) are slow.

: Video of Internuclear ophthalmoplegia due to multiple sclerosis (same patient)

Examination technique:

It can be challenging to diagnose subtle INO's, because it requires one to observe both eyes at the same time. It is also difficult to use the video frenzel goggles here, because there is a limited field of view, and one has the same problems faced by direct examination.

The technique that the author of this page uses, is to watch the abducting eye a few times,to learn the timing. In other words, tell the patient to look back and forth between finger-nose, or finger-finger. For example, have the right eye go between far right and your nose several times. When the right eye goes out -- it is abducting. When it goes to the nose -- it is adducting. Look for slowing of adduction. Also check the other eye -- if there is obvious slowing on adduction, then stop.

Otherwise, after the timing is learned, switch attention to the adducting eye, and look for that eye still moving, after the abducting eye has finished. In other words, there is some use of the examiners ability to predict timing.

Several other case examples can be found here