Divergence is one of the vergence movements of the eyes and corresponds to a disjunctive movement that allows fixation of objects at a distance. The opposite eye movement is convergence, which is used to fixate objects in close proximity. It, too, is a vergence movement. Divergence disorders may refer to neurological damage.
What are the vergence movements?
Divergence is one of the vergence movements of the eyes and corresponds to a disjunctive movement that allows fixation of objects at a distance. The human eye is extremely mobile to selectively detect visual stimuli from different directions. Five eye movement patterns constitute the main classes of eye movement. These movement patterns include fixation, following movement, and saccades, as well as vergence movements. These eye movements correspond to a disjugate movement of the eyes. These are opposing eye movements, as they are present in rotational movements about the parallel axis with opposite directions of rotation. The so-called convergence is to be distinguished from the divergence. This is the opposite eye movement in which the movement from parallel position causes both facial lines of the eyes to overlap. The convergence corresponds to a bilateral adduction. In contrast, the divergence takes place from bilateral abduction and brings the facial lines behind the two eyes to overlap. So-called positive vertical divergence always occurs when the line of sight on the right eye shows an upward deviation compared to that on the left eye. Negative vertical divergence is when the left line of sight deviates upward compared to the right. Convergence and divergence are collectively referred to as vergence movements.
Function and task
The vergence movements of the eyes correspond to slow movements that occur in relativity to each other. When the left and right lines of gaze intersect directly in front of the eyes, we speak of convergence. If the point of intersection is behind the eyes, the physician speaks of divergence. Divergence movements of the eyes play a role especially in connection with fixation. In medicine, fixation is the directing of the gaze to a certain object or subject in the environment, which takes place with the help of the highest resolution retinal point. The straight line between this retinal point and the observed object is in turn called the visual field line. Divergence and convergence movements allow the fixed point to move in depth. Furthermore, when an object is moving in depth, these eye movements ensure maintenance of directed fixation. All vergence movements rely on a bilateral motion impulse about the respective axis. While the motion impulse of convergence takes place toward the nose and thus corresponds to an adduction, that of divergence takes place toward the forehead and thus corresponds to a bilateral abduction motion. The movement forms of vergence are irreplaceable in the targeted observation or fixation of objects and subjects in the immediate vicinity and the far distance to the observer. If they do not function sufficiently, double image perceptions may result. Convergence movements belong to a control loop of neurophysiology consisting of a mechanism of close focus in the sense of accommodation and pupil constriction in the sense of miosis. Together with the convergence movements of the eyes, these mechanisms are called the triad of near adjustment. Divergences, on the other hand, belong to the triad of distance adjustment. When the observer fixates and thus observes an object or subject at a distance, his or her visual axes diverge due to the divergence movement. All vergence is also known as disjugate, disjunctive, or opposite eye movements and is further subdivided into accommodative vergence for adaptation and fusional vergence. The nature of the eye movements dictates which control variables the retina senses. The hallmark of all vergence is a change in the angle at the instantaneous axes. A total of six muscles are involved in the movement of the eyeballs. In addition to the musculus rectus lateralis, the musculus rectus medialis and the musculus rectus superior, they include the musculus rectus inferior, the musculus obliquus inferior and the musculus obliquus superior. The latter shifts during vergence of the visual axis and enables voluntary object tracking.Nerves involved in movement are the abducens nerve, the oculomotor nerve, and the trochlear nerve.
Diseases and complaints
Convergence and divergence movements are controlled by different areas in the cerebrum and brainstem that precede the oculomotor nuclei of three oculomotor cranial nerves. At different object distances, the orientation of the visual axes at the two eyes cannot be changed if divergence and convergence are disturbed. Thus, the visual axes at the fixated object no longer intersect. Damage to the rostral midbrain can disrupt the control of vergence movements. Divergence disorders occur in the form of divergence paresis and thus usually do not occur in isolation, but are usually associated with other gaze paresis. A divergence disorder results in a horizontal type of double vision as the most important leading symptom, which occurs especially when looking into the distance. The closer the distant object approaches the patient’s eyes, the less distance there is between the two double images. Divergent paresis can occur in the context of circulatory disorders or hemorrhage. Neurological trauma or disease may also be a primary cause. One of the most common primary causes of impaired divergence is multiple sclerosis. In this autoimmune disease, the patient’s immune system attacks central nervous tissue and causes inflammation in it. In addition to MS and a stroke or central nervous tumors, craniocerebral trauma (concussion) can also cause divergence disorders. Due to these correlations, an ophthalmologic examination is usually not sufficient to clarify a divergence disorder. At a minimum, a neurologic examination should be performed in addition to clarifying the cause.
