convergence

The designation convergence goes back to the Latin word "convergere" and means "to lean towards one another", "to lean towards one another". The position of the eyes with which the lines of sight intersect directly in front of the eyes is referred to as convergence.

What is the convergence?

Young adults and children are able to see clearly despite farsightedness (hyperopia) by compensating for their ametropia.The technical jargon describes this balance as accommodation. The ciliary muscles of the eyes become tense, which increases the refractive power of the lens.

People with no visual impairment do not need to adjust their visual acuity to see clearly in the vicinity. In order to take the appropriate starting position for close vision, both eyes move inwards at the same time. This process is called convergence in technical terms.

Both processes together are referred to as close focus or near fixation. Through this natural process, humans are able to look at objects up close without seeing double images.

Function & task

The triggering 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 fix parallel objects in the vicinity and do not deviate from one another. The so-called strabismus occurs only when the reflex constriction of the pupils is impaired. Both eyes then have limited inward movements. Depending on the severity of the convergence disorder, there is a different strength of squint. Doctors speak of a convergence excess.

Without convergence and convergence reaction, humans would not be able to see in three dimensions. Three-dimensional vision requires that both eyeballs align to the same point in order to create a three-dimensional image via the central nervous system (CNS).

The convergence reaction is a neurophysiological process. This control loop also includes the constriction of the pupils (miosis) and accommodation. The accommodation is an adjustment of the eyes to guarantee an undisturbed near vision. The complex of pupil constriction, convergence reaction and close focus is called the close focus triad.

The convergence reaction occurs via the third cranial nerve. The technical language refers to this as the oculomotor nerve. Together with the sixth cranial nerve (abducens nerve) and the fourth cranial nerve (trochlear nerve), this is responsible for making eye movements. A contraction of the outer eye muscles is triggered via the motor core of the third cranial nerve. With the help of these eye muscles, the eyeballs are able to move inward. This process is known as the convergence movement. By contraction of the eye ring muscles (Musculus sphincter pupillae) a temporary constriction of the pupil occurs. At the same time, the external muscles of the eye contract to fixate on nearby objects.

By turning the eyes inward, the convergence reaction enables the two lines of the face to overlap and avoids double vision. Without this process, viewing objects up close would not be possible.

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Illnesses & ailments

If the convergence reaction is restricted, there is an under- or over-function. The degree of a convergence disorder present is determined by means of the AC / A quotient in strabology. It is an indicator of the pathological condition of binocular vision (binocular vision).

The doctor determines to what extent the patient is able to use the motor and sensory abilities of both eyes together. The convergence of both eyes is two to three degrees per diopter. The degree of the convergence disturbance can be determined by the gradient and heterophoria method.

The squint is triggered by an excessive convergence reaction called the convergence excess. If a person looks into the distance, his eyes move forward in parallel. When looking close, the eyes move inward and point slightly downward. If the gaze is directed back into the distance, there is a divergence. The outer eye muscles (ciliary muscles) are responsible for undisturbed vergences.

In the presence of a weak convergence, the eyes are unable to adapt to distances because the muscles are too weak and can no longer perform adequate contraction. Humans can then no longer clearly see objects in the vicinity.

The brain activates the visual center in order to reduce this convergence disorder by trying to optimize the quality of the perceived images through retouching and empirical values. However, this process is exhausting and clear vision is only temporarily possible. In the long run, the visual acuity decreases, the optical deficiency can no longer be compensated. Permanent poor eyesight develops, which must be corrected. The impulse of one eye is then switched off while the other takes over near vision.

This creates different types of squint. Presbyopia sets in between the ages of 40 and 50. A far-sighted person quickly notices these changes, as they often need to adjust their near vision.

A partially accommodative squint is when glasses do not completely eliminate this ametropia, but only reduce the squint angle. A spasmodic convergence is present in the case of a spasm that is accompanied by a constriction of the pupil and an increased focus. Insufficiency is mostly caused by a disturbance in the change of the corner of the eye. The cause can be a neurogenic or sensorimotor lesion. This visual disturbance can be partially corrected by using prism glasses or visual exercises. Eye surgery is also possible.

The endocrine orbitopathy has a weak convergence. The term “endocrine” indicates a thyroid disorder that causes this autoimmune disorder. The protrusion of the eyeballs (exophthalmos) with an enlarged eyelid is characteristic. It is triggered by changes in the tissue behind the eyeballs. These changes in size and structure affect connective, muscle and adipose tissue. The eyes swell due to the infiltrated tissue, while the flexibility of the muscles is limited. The eye movement is painful and the turning of the gaze is restricted.