Internuclear opthalmoplegia (INO) is an impairment in lateral conjugate gaze (both eyes looking toward one side), caused by a lesion in the medial longitudinal fasciculus (MLF), and associated with multiple sclerosis.
Lateral conjugate gaze requires coordination of adduction (medial rectus muscle, CN III) in one eye and abduction (lateral rectus muscle, CN VI) in the other eye. These movements are coordinated by the paramedian pontine reticular formation (PPRF), also known as the pontine gaze centre. The pathway is as follows:
To look to the left, the right frontal eye field (FEF) sends a signal to the left PPRF. The left PPRF innervates 1) the left abducens (CN VI) nucleus, which controls the left lateral rectus muscle and causes the left eye to abduct (gaze to the left); and, 2) the right oculomotor (CN III) nucleus, which controls the right medial rectus muscle and causes the right eye to adduct (gaze to the left). The MLF is the tract connecting the PPRF to the contralateral CN III nucleus.
In INO, there is damage to the MLF, giving a deficit in adduction of the corresponding eye during conjugate lateral gaze, but convergence (eye crossing) is classically preserved because that is controlled by a different pathway. In very mild cases of INO, the only deficit is a slowed velocity of the affected eye. For naming, a right INO (as in the sketch) involves damage to the right MLF, which means that the right eye can’t adduct to look to the left, but can abduct to look to the right.
INO may also be associated with gaze abnormalities such as nystagmus, skew deviation, and even abduction or convergence deficits.
The causes of INO include: multiple sclerosis, pontine glioma, and stroke.
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