neurologic disorders
Internuclear Ophthalmoplegia
Introduction
Internuclear ophthalmoplegia is characterized by paresis of ipsilateral eye adduction in horizontal gaze but not in convergence. It can be unilateral or bilateral. (See also Overview of Neuro-ophthalmologic and Cranial Nerve Disorders .) During horizontal gaze, themedial longitudinal fasciculus(MLF) on each side of the brain stem enables abduction of one eye to be coordinated with adduction of the other. The MLF connects the following structures: 6th cranial nerve nucleus (which controls the lateral rectus, responsible for abduction) Adjacent horizontal gaze center (paramedian pontine reticular formation) Contralateral 3rd cranial nerve nucleus (which controls the medial rectus, responsible for adduction) The MLF also connects the vestibular nuclei with the 3rd and 4th cranial nerve nuclei. Internuclear ophthalmoplegia results from a lesion in the MLF: In young people, commonly caused by Multiple Sclerosis (MS) and often bilateral In older people, typically caused by Ischemic Stroke and is unilateral Rarely, the cause is Arnold-Chiari malformation, Late or tertiary syphilis , If a lesion in the MLF blocks signals from the horizontal gaze center to the 3rd cranial nerve, the eye on the affected side cannot adduct (or adducts weakly) past the midline. The affected eye adducts normally in convergence because convergence does not require signals from the horizontal gaze center. This finding distinguishes internuclear ophthalmoplegia from Third Cranial (Oculomotor) Nerve Disorders , which impairs adduction in convergence (3rd cranial nerve palsy also differs because it causes limited vertical eye movement, ptosis, and pupillary abnormalities). During horizontal gaze to the side opposite the affected eye, images are horizontally displaced, causing diplopia; nystagmus often occurs in the abducting eye. Sometimes vertical bilateral nystagmus occurs during attempted upward gaze. Treatment of internuclear ophthalmoplegia is directed at the underlying disorder. 