The term convergence comes from the Latin word “convergere” and means “to incline toward each other,” “to lean toward.” Convergence is the position of the eyes at which the lines of sight intersect immediately in front of the eyes.
What is convergence?
Convergence is the position of the eyes with which the lines of sight intersect immediately in front of the eyes. Young adults and children are able to see sharply despite farsightedness (hyperopia) by compensating for their refractive error. The technical term for this compensation is accommodation. The ciliary muscles of the eyes are tightened, which increases the refractive power of the lens. People without visual impairment do not need to adjust their visual acuity to see sharply at close range. To assume the appropriate starting position for near vision, both eyes move inward at the same time. This process is known as convergence. Both processes together are referred to as near focus or near fixation. Through this natural process, humans are able to view objects up close without seeing double images.
Function and task
The initiation of an arbitrary convergence movement is often referred to as strabismus, but this is not correct because the facial lines of the left and right eyes fixate parallel near objects and do not diverge. Only when the reflexive constriction of the pupils is impaired, the so-called strabismus is present. Both eyes then show restricted inward movements. Depending on the severity of the convergence disorder, the degree of strabismus varies. Physicians speak of a convergence excess. Without convergence and convergence reaction, people would not be able to see in three dimensions. Three-dimensional vision requires that both eyeballs align to the same point to create a three-dimensional image via the central nervous system (CNS). The convergence response is a neurophysiological process. This control loop also includes pupil constriction (miosis) and accommodation. Accommodation is an adjustment of the eyes to ensure near vision without interference. The complex of pupil constriction, convergence response, and near adjustment is called the near adjustment triad. The convergence response occurs via the third cranial nerve. The technical term for this is the oculomotor nerve. Together with the sixth cranial nerve (nervus abducens) and the fourth cranial nerve (nervus trochlearis), this nerve is responsible for the execution of eye movements. The motor nucleus of the third cranial nerve triggers contraction of the external eye muscles. With the help of these eye muscles, the eyeballs are able to move inward. This process is called convergence movement. Contraction of the eye ring muscles (Musculus sphincter pupillae) causes temporary pupil constriction. At the same time, the external eye muscles contract to fixate nearby objects. The convergence reaction, by inward rotation of the eyes, allows the two facial lines to overlap and avoids double images. Without this process, viewing objects at close range would not be possible.
Diseases and disorders
If the convergence response is impaired, an under- or overactivity is present. The degree of convergence dysfunction present is determined by means of the AC/A quotient in strabismus (strabology). It is an indicator of the pathological condition of binocular vision. The physician determines the extent to which the patient is able to use the motor and sensory abilities of both eyes together. The convergence of both eyes ranges from two to three degrees per diopter. The degree of convergence disorder can be determined by the gradient and heterophoria method. Strabismus is caused by an exaggerated convergence reaction called convergence excess. When a person looks into the distance, his eyes move forward in parallel. When looking close up, the eyes move inward and direct themselves slightly downward. If the gaze is directed back into the distance, there is a divergence. The outer eye muscles (ciliary muscles) are responsible for vergence without interference. In the presence of convergence weakness, the eyes are unable to adjust to distances because the muscles are too weak and can no longer contract sufficiently.The person can then no longer see objects in the vicinity clearly. The brain activates the visual center to reduce this convergence disorder by trying to optimize the quality of the perceived images through retouching and experience. However, this process is exhausting and clear vision is only possible temporarily. In the long run, visual acuity decreases and the optical defect can no longer be compensated. A permanent visual impairment sets in, which must be corrected. The impulse of one eye is then switched off, while the other takes over near vision. In this way, different types of strabismus develop. Between the ages of 40 and 50, presbyopia sets in. A farsighted person notices these changes quickly, as he or she is often dependent on adjusting his or her near vision. Partially accommodative strabismus is when glasses do not completely eliminate this refractive error, but only reduce the squint angle. A spasmodic convergence is present when there is a spasm associated with pupillary constriction and increased near vision. Insufficiency is most often caused by a disturbance in the change of the angle of the eye. The cause may be a neurogenic or sensorimotor lesion. This visual disorder can be partially corrected by prism glasses or visual exercises. Eye surgery is equally possible. In endocrine orbitophaty, there is a weakness of convergence. The term “endocrine” refers to a thyroid disease that triggers this autoimmune disorder. Characteristic is the protrusion of the eyeballs (exophthalmos) with a widened palpebral fissure. This is triggered by tissue changes behind the eyeballs. These size and structural changes affect the connective, muscular and fatty tissues. The eyes swell due to the infiltrated tissue, while the extensibility of the muscles is limited. Eye movement is painful and gaze use is limited.
