Internuclear ophthalmoplegia (INO) is a medical sign indicative of a particular cause of extraocular muscle weakness (ophthalmoparesis). It can affect either one or both eyes. It is a disorder of conjugate lateral gaze in which the affected eye shows impairment of adduction. The partner eye diverges from the affected eye during abduction, producing horizontal diplopia. That is, if the right eye is affected the patient will "see double" when looking to the left and the images will be side by side. During extreme abduction, compensatory nystagmus can be seen in the partner eye.
The disorder is caused by injury or dysfunction in the medial longitudinal fasciculus (MLF), a heavily-myelinated tract that allows conjugate eye movement by connecting the paramedian pontine reticular formation (PPRF) -abducens nucleus complex of one side to the oculomotor nucleus of the opposite side.
In young patients with bilateral INO, multiple sclerosis is nearly always the cause. In older patients with one-sided lesions a stroke is a distinct possibility.
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 on the same side (the MLF having crossed from the opposite side)then the "one and a half syndrome" 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.