Internuclear ophthalmoplegia (INO) is a disorder of conjugate lateral gaze in which the affected eye shows impairment of adduction

Adduction

Adduction is a movement which brings a part of the anatomy closer to the middle sagittal plane of the body. It is opposed to abduction.-Upper limb:* of arm at shoulder ** Subscapularis** Teres major** Pectoralis major** Infraspinatus...

. When an attempt is made to gaze contralaterally (relative to the affected eye), the affected eye adducts minimally, if at all. The contralateral eye abducts

Abduction (kinesiology)

Abduction, in functional anatomy, is a movement which draws a limb away from the median plane of the body. It is thus opposed to adduction.-Upper limb:* of arm at shoulder ** Supraspinatus** Deltoid* of hand at wrist...

, however with nystagmus. Additionally, the divergence of the eyes leads to horizontal diplopia

Diplopia

Diplopia, commonly known as double vision, is the simultaneous perception of two images of a single object that may be displaced horizontally, vertically, or diagonally in relation to each other...

. That is, if the right eye is affected the patient will "see double" when looking to the left, seeing two images side-by-side. Convergence

Convergence (eye)

In ophthalmology, convergence is the simultaneous inward movement of both eyes toward each other, usually in an effort to maintain single binocular vision when viewing an object. This action is mediated by the medial rectus muscle, which is innervated by Cranial nerve III...

 is generally preserved.

Causes

The disorder is caused by injury or dysfunction in the medial longitudinal fasciculus

Medial longitudinal fasciculus

The medial longitudinal fasciculus is a pair of crossed fiber tracts , one on each side of the brainstem. These bundles of axons are situated near the midline of the brainstem and are composed of both ascending and descending fibers that arise from a number of sources and terminate in different...

 (MLF), a heavily-myelinated tract that allows conjugate eye movement by connecting the paramedian pontine reticular formation (PPRF) -abducens nucleus

Abducens nucleus

The abducens nucleus is the originating nucleus from which the abducens nerve emerges - a cranial nerve nucleus. This nucleus is located beneath the fourth ventricle in the caudal portion of the pons, medial to the sulcus limitans....

 complex of contralateral side to the oculomotor nucleus

Oculomotor nucleus

The fibers of the oculomotor nerve arise from a nucleus in the midbrain, which lies in the gray substance of the floor of the cerebral aqueduct and extends in front of the aqueduct for a short distance into the floor of the third ventricle...

 of the ipsilateral side.

In young patients with bilateral INO, multiple sclerosis

Multiple sclerosis

Multiple sclerosis is an inflammatory disease in which the fatty myelin sheaths around the axons of the brain and spinal cord are damaged, leading to demyelination and scarring as well as a broad spectrum of signs and symptoms...

 is often the cause. In older patients with one-sided lesions a stroke is a distinct possibility. Other causes are possible.

Variants

A rostral lesion within the midbrain may affect the convergence center thus causing bilateral divergence of the eyes which is known as the WEBINO syndrome ( Wall Eyed Bilateral INO) as each eye looks at the opposite "wall".

If the lesion affects the PPRF (or the abducens nucleus) and the MLF

Medial longitudinal fasciculus

The medial longitudinal fasciculus is a pair of crossed fiber tracts , one on each side of the brainstem. These bundles of axons are situated near the midline of the brainstem and are composed of both ascending and descending fibers that arise from a number of sources and terminate in different...

 on the same side (the MLF having crossed from the opposite side)then the "one and a half syndrome

One and a half syndrome

The one and a half syndrome is a rare ophthalmoparetic syndrome characterized by "a conjugate horizontal gaze palsy in one direction and an internuclear ophthalmoplegia in the other". The most common manifestation of this unusual syndrome is limitation of horizontal eye movement to adduction of...

" occurs which, simply put, involves paralysis of all conjugate horizontal eye movements other than abduction of the eye on the opposite side to the lesion.

External links

• Animation at mrcophth.com