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In the practice of ophthalmology, the movement of an eye is known as a duction. Duction occurs naturally, and most people have a range of six different eye movements along the eye's axis of rotation. When these eye movements are not possible, it might be a sign of neurological damage or a physical incapacity.
Eye movement is dependent upon the eye's current position on its axis of rotation and the coordination of the six extraocular muscles. The six movements of the eye are abduction, adduction, supraduction, infraduction, incycloduction and excycloduction. The eyes might naturally perform any of these movements, and they usually do so in coordination with each other, thus creating binocular vision. When the eyes function independently of each other, it creates a vergence, which can result in vision problems caused by duction. These vision problems are commonly known as having a lazy eye, double vision or crossed eyes.
When duction is noted during an eye examination, medical terminology is used that might be confusing to the patient. For the sake of clarity, "abduction" means that the pupil of the eye moves toward the temple, commonly known as having a lazy eye, and "adduction" means that the pupil gravitates toward the nose in a duction commonly known as having crossed eyes. In supraduction, the pupil of the eye is elevated, and infraduction causes the pupil to look down. "Incycloduction" and "excycloduction" refer to movements of the eye toward the inside or outside of the body. Incycloduction commonly results in diplopia, also known as double vision.
Uncoordinated duction between the eyes or the inability to move the eye in any one of the six ductions can be a symptom of a serious issue. In these cases, the ophthalmologist might choose to perform a forced duction test as part of the eye examination. The purpose of the forced duction test is to determine whether the problem is a neurological disorder or whether the lack of movement is caused by a physical issue.
The forced duction test is performed by attempting to move the eyeball in the direction of restricted movement. The ophthalmologist accomplishes this movement by anesthetizing the conjunctiva, holding it with forceps and simulating the natural movement of the extraocular muscles. If movement is induced in this way, it can be assumed that the problem is a result of neurological damage and not a physical inability to move the eye.