The convergence reaction is the reflexive constriction of the pupils during convergence, on the one hand, and the inward movement of both eyes during fixation of near objects, on the other. Impairments of convergence can cause strabismus, among other conditions.
What is the convergence response?
Convergence is a specific type of opposite eye movement. Without convergence response, objects could not be viewed up close. Convergence is a certain kind of counter-intuitive eye movements. Without convergence reaction, objects could not be viewed up close. The convergence response is part of a neurophysiological process. Accommodation and constriction of the pupil (miosis) are also part of this control loop. The complex of convergence response, accommodation, and miosis is also referred to as the near triad.
Function and task
The convergence response is mediated by the third cranial nerve. This is called the oculomotor nerve in medical terminology. Together with the trochlear nerve and the abducens nerve, it is responsible for eye movements. The convergence reaction can be divided into two reaction steps. Via the motor nucleus of the nervus oculomotorius, the nucleus nervi oculomotorii, a contraction of the musculi recti mediales is triggered. The musculi recti mediales are muscles of the external eye musculature. They provide an inward rotation of the eyeballs. This movement is also called convergence movement. A miosis is also induced via the parasympathetic part of the ocolumotor nerve, more precisely via the nucleus oculomotorius accessorius. A miosis is a temporary constriction of the pupil. This is triggered by a contraction of the sphincter pupillae muscle. Parallel to the convergence reaction, a contraction of the ciliary muscles is also induced via the parasympathetic part of the third cranial nerve. The ciliary muscles are located on the outer side of the corpus ciliare and are responsible for near accommodation. In the convergence reaction, the inward rotation of the eyes allows the two facial lines to intersect. Without this reaction, an object could not be viewed up close without creating a double image. Convergence is what makes three-dimensional vision possible in the first place. For this vision it is necessary that both eyeballs are directed to the same point. Only in this way can a three-dimensional image be generated from the perceived image in the central nervous system (CNS).
Diseases and complaints
Impairments of the convergence response can lead to hyper- or hypofunction. The type of convergence impairment is assessed by the AC/A quotient. The AC/A quotient reflects the ratio of accommodative convergence to accommodation provided. The ratio averages two to three degrees of convergence movement per diopter of accommodation achieved. The AC/A ratio can be determined by the heterophoria method and by the gradient method. Strabismus caused by an excessive convergence reaction is also called convergence excess. In this case, the near squint angle is very large and the distance squint angle is very small or completely absent. As a rule, the eyeballs point inward in strabismus. However, outward strabismus also belongs to convergence excesses. Here, the near squint angle is less pronounced than the distance squint angle. In total, three forms of convergence excesses can be distinguished. In the case of non-accommodative convergence excess, the strabismus is purely motor-related. There is usually no influence by accommodative components. The non-accommodative convergence excess can be corrected by glasses. Strabismus surgery may be necessary. The hyperkinetic accommodation excess is caused by accommodation. In this case, the accommodation width is normal, but the convergence power is too high. Thus, the AC/A quotient is also increased. The therapy is done by special spectacle lenses. In hypoaccommodative convergence excess, the near squint angle is greatly increased and the accommodation width is accordingly significantly reduced. Due to the reduced accommodation, the body tries to see sharply at near with an exaggerated convergence movement. The AC/A quotient is also increased in this case. Hypoaccommodative convergence excess is treated with bifocals. Strabismus surgery should not be performed under any circumstances. A convergence spasm is an excessive spasmodic convergence movement.It is associated with strong accommodation and pupillary constriction. In convergence insufficiency, the AC/A ratio is decreased. This is often due to a disturbance in the change of the angle of vergence. The causes of convergence insufficiency are manifold. Sensory-motor disturbances or neurogenic lesions can be the cause. Therapy is carried out with prism glasses, other special glasses or visual exercises. Strabismus surgery can also be used. The best results are usually achieved with a combination of several measures. Endocrine orbitopathy is also characterized by convergence weakness. This is also referred to as Möbius sign. Endocrine orbitopathy is a disease of the orbit (eye socket). The disease belongs to the autoimmune diseases and usually occurs in the context of thyroid dysfunction. Characteristic of endocrine orbitopathy is the protrusion of the eyeballs. This phenomenon is also called exophthalmos. Associated with this protrusion of the eyes is also a widening of the palpebral fissure. Endocrine orbitopathy is triggered by tissue changes behind the eyeball. Muscle, fat and connective tissues are affected by these structural and size changes. Exophthalmos, together with enlargement of the thyroid gland and palpitations, forms the so-called Merseburg triad. This triad of symptoms is a classic sign of Graves’ disease. Due to the swelling and infiltration behind the eye, the extensibility of the eye muscles is restricted. This results in pain when turning the gaze and restricted movement of the eyeballs. Möbius sign is a typical symptom of endocrine orbitopathy. Other clinical signs include Graefe’s sign or Stellwag’s sign.
