Internuclear ophthalmoplegia is the difficulty in performing horizontal eye movements, caused by injury to specific connections between nerve centers in the brain stem (the lowest part of the brain).
In internuclear ophthalmoplegia (กล้ามเนื้อตาอ่อนแรง, which is the term in Thai), the nerve fibers responsible for coordinating the movements of both eyes in the same horizontal plane are damaged (that is, those responsible for the action of looking from one side to the other). These nerve fibers connect groups of neurons (the so-called nerve centers or nuclei) that originate from the third cranial pair (the so-called common ocular motor nerve), the fourth cranial pair (trochlear nerve), and the sixth cranial pair (the so-called nerve external eye motor).
Internuclear ophthalmoplegia is usually a consequence of the following:
- In the elderly: a stroke (usually affects only one eye)
- In younger people: multiple sclerosis (quite often, both eyes are affected)
The least common causes of internuclear ophthalmoplegia are Lyme disease, tumors, traumatic brain injuries, nutritional deficiencies, and certain medications, such as phenothiazines (which are antipsychotics) and tricyclic antidepressants.
Eye movements in the horizontal plane are affected, but there is no impairment of vertical movements. The affected eye does not rotate toward the inside of the face, but it can instead rotate outward. Therefore, when only one eye is affected and the person looks in the opposite direction to the affected eye, the following occurs:
- The affected eye, which to look in the desired direction should turn towards the inner part of the face, is unable to go beyond the midline of its normal course, and therefore remains facing forward.
- As the other eye rotates outward, it often makes involuntary, and repeated pendulum movements called nystagmus. That is, the eye moves rapidly in one direction and then slowly moves in the opposite direction.