1. What is the mechanism of internuclear ophthalmoplegia?
It is due to a lesion in the medial longitudinal fasciculus (MLF) which connects the sixth nerve nuclei to the contralateral medial rectus muscle nucleus. In unilateral case, there is loss of adduction of the contralateral medial rectus on attempted conjugate gaze. There is abducting nystagmus of the ipsilateral eye. The horizontal saccadic, pursuit and vestibuloocular systems are affected but the convergence is usually normal.
2. A patient with internuclear ophthalmoplegia has problem with left adduction, which side is the lesion located?
As explained above, the lesion is contralateral to the eye that has problem with adduction. Therefore the lesion is in the right medial longitudinal fasciculus.
3. Is convergence ever affected in lesion of the medial longitudinal fasciculus?
Convergence is usually not affected in lesion of the medial longitudinal fasciculus (MLF) alone. However, if the lesion extends into the mid-brain the convergence may be involved as in Cogan's anterior internuclear ophthalmoplegia in which the patient has bilateral INO and convergence failure.
4. What is the mechanism of one and a half syndrome?
An extensive lesion in the lower part of the pons can affect the horizontal gaze centre and the adjacent MLF. The lesion causes motility problems with both medial recti and one lateral rectus. On examination, there is a pontine gaze palsy to the ipsilateral side together with an INO on gaze to the contralateral side. In a right-sided lesion, there is failure of the eyes to look to the right, and on left gaze only the left eye will abduct with ataxic nystagmus.
5. What is wall eyed syndrome?
This is the presence of exotropia with bilateral internuclear ophthalmoplegia. It is also called WEBINO which is short for Wall Eyed Bilateral Inter-Nuclear Ophthalmoplegia.