(See Figures 10.13.1 and 10.13.2.).
Critical
Weakness or paralysis of adduction, with horizontal jerk nystagmus of the abducting eye.
NOTE: |
INO is localized to the side with the weak adduction. |
Other
May be unilateral or bilateral (WEBINO: wall-eyed, bilateral INO). Upbeat nystagmus on upgaze may occur when INO is unilateral or bilateral. The involved eye can sometimes turn in when attempting to read (intact convergence). A skew deviation (relatively comitant vertical deviation not caused by neuromuscular junction disease or intraorbital pathology) may be present; brainstem and posterior fossa pathology should be ruled out. With skew deviation, the three-step test cannot isolate a specific muscle. See 10.7, ISOLATED FOURTH CRANIAL NERVE PALSY. In addition, presence of other neurologic signs, including gaze-evoked nystagmus, gaze palsy, dysarthria, ataxia, and hemiplegia, favors skew deviation rather than fourth cranial nerve palsy.
NOTE: |
Ocular motility can appear to be full, but a muscular weakness can be detected by observing slower saccadic eye movement in the involved eye compared with the contralateral eye. The adducting saccade is assessed by having the patient fix on the examiners finger held laterally and then asking the patient to make a rapid eye movement from lateral to primary gaze. If an INO is present, the involved eye will show a slower adducting saccade than the uninvolved eye. The contralateral eye may be tested in a similar fashion. |
Treatment/Follow Up