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 the left abducens (CN VI) nucleus, which controls the left lateral rectus muscle and causes the left eye to abduct (gaze to the left).
- Additionally, the left CN VI nucleus innervates 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 CN VI nucleus 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.
- Flaherty AW, Rost NS. 2007. Eyes and vision. In: Massachusetts General Hospital Handbook of Neurology. Lippincott Williams & Wilkins.
- Frohman EM, Frohman TC, Zee DS, McColl R, Galetta S. 2005. The neuro-opthalmology of multiple sclerosis. The Lancet Neurology; 4:111.
- Ropper AH, Brown RH. 2005. Disorders of ocular movement and pupillary function. In: Adams and Victor’s Principles of Neurology. McGraw-Hill.
- Wilkinson I, Lennox G. 2005. Cranial nerve disorders. In: Essential Neurology. Blackwell.
clear and concise, loves it!
Can you do one for lateral gaze palsy?
Sure! I’ll put it on the to-do list!
The image is not entirely correct. It is the left VI nucleus that sends the signal to the right III nucleus and not the PPRF. Source: Neuranatomy, Blumenfeld
Thanks for the pick-up, s.z! Image and text corrected tonight.
Thank you! excellent explanation
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great. but just to clarify, a lesion to the right MLF will affect the right eye adduction and cause left eye nystagmus. Hence this is called, RIGHT internuclear opthalmoplegia? we name it according to the side of the MLF damage?
Yes, that would be the case. And, for optimal clarity, it helps to say (as in this case) “INO of the RIGHT eye” as that will leave no ambiguity about the side which side the eye and MLF are affected.
Hello, thank you for this. I understand that INO is from damage to the MLF causing bad connection from CN3 to CN6. Does CN4/superior oblique ever come into play here? From what I read it also has a nucleus in MLF, but I can’t find if there is any issues of eye depression/adduction in INO.