Convergence (eye)

As convergence ( Latin convergere "to incline" "to each other tend") is a certain basic type of oppositely directed eye movement ( vergence called) from which in the parallel position out the two lines of sight of the eyes are brought before them to overlap. This is triggered by a bilateral movement impulse around a vertical axis inwards, i.e. towards the nose ( adduction ). This form of movement is indispensable for viewing objects in the vicinity without triggering a double image perception. The point at which an object in the vicinity can just barely be seen binocularly is called the convergence near point .

The convergence movement is part of a neurophysiological control loop, which also includes the mechanism of the close-up adjustment of the optical system ( accommodation ) and the narrowing of the pupil ( miosis ). The entire complex is also called the close-up triad .

The arbitrary triggering of a pronounced convergence movement is often incorrectly referred to or perceived as strabismus , but has nothing to do with it, since normally the lines of sight of the right and left eyes jointly fix an object in the vicinity and do not deviate from each other.

Disorders of Convergence

Classification according to ICD-10
H51.1	Convergence weakness and convergence excess
ICD-10 online (WHO version 2019)

Depending on the type and localization, impairment of convergence can consist of over- and underfunction. The various forms of hyperfunction represent the vast majority of disorders.

For a reliable assessment of the type of convergence disorder, it is essential to consider a special factor, the AC / A quotient . This is the ratio of accommodative convergence to accommodation provided . On average, this amounts to about 2-3 degrees of convergence movement per diopter of accommodation. The AC / A quotient is determined in two different ways: the heterophoria method and the gradient method .

Convergence excess

A squint that is triggered by an excessive convergence reaction is usually referred to as an excess of convergence. This is usually a very large convergent close squint angle with a very small or completely absent remote squint angle. However, there are also forms of outward squint in which the close squint angle is significantly less pronounced than the deviation in the distance. Here, too, one speaks of a convergence excuse type.

A distinction is made between different forms of the convergence excess:

non-accommodative convergence excess
hyperkinetic or norm-accommodative convergence excess
hypo-accommodative convergence excess

Non-accommodative convergence excess

The non- accommodative convergence excess is a purely motor-related squint with a large near angle that is not significantly influenced by accommodative components.

Depending on the situation, treatment is carried out by correcting existing ametropia with glasses and, if necessary, a strabismus operation . The so-called thread operation according to Cüppers is generally used here , an operation method that influences the rolling distances of the muscles concerned and primarily reduces the convergent close squint angle.

Hyperkinetic (norm-accommodative) convergence excess

The hyperkinetic (norm-accommodative) excess of convergence is a squint that is triggered by appropriate accommodation. The range of accommodation is normal here, but the resulting convergence performance is higher than the accommodation used would require. The AC / A quotient is increased.

Therapy is carried out by prescribing suitable bifocal lenses with an additional near part of +2.00 to +3.00 dioptres in order to reduce the accommodation and the close squint angle. The bifocal part can often be weakened from the age of 13-15 and finally removed. If this measure is not sufficient, an additional squint operation is necessary (thread surgery according to Cüppers, see above).

Hypoaccommodative convergence excess

The hypoaccommodative convergence excess is a large close squint angle that is associated with a significantly reduced range of accommodation. Here the patient tries to mobilize all available reserves of his reduced accommodation in order to see clearly in the vicinity, which is acknowledged with a correspondingly excessive convergence movement of the cross-eyed eye. The AC / A quotient is increased.

The treatment of choice for this rare form of excess convergence is in any case the prescription of suitable bifocal glasses. A squint operation is absolutely contraindicated here.

The unambiguous determination and differential diagnostic clarification of the type of a convergence excess is indispensable for the selection of suitable therapy methods and a successful treatment, whether its complexity and also occurring mixed forms.

Convergence spasm

This is used to describe a convulsive excessive convergence movement, which is associated as a close-up spasm with excessive accommodation and miosis.

Convergence failure

A subfunction of the convergence is primarily a disruption of the distance-dependent change in the vergence angle, which can also assume the extent of paralysis. The AC / A quotient is reduced. It must be clarified whether the movement deficit is an isolated motor problem or whether accommodation and / or convergence miosis are also affected. Convergence insufficiencies often occur in exophoria , especially in the vicinity, which is why the close squint angle is usually greater than the deviation in the distance.

In principle, the nearer point of convergence (the point that can still be easily seen binocularly at a minimal distance from the eye) is displaced further into the distance, the more pronounced the convergence weakness is.

The causes of a weak convergence can be diverse and range from sensory-motor disorders of the fusion to neurogenic lesions. Therapeutic approaches can be found in the prescription of suitable glasses, possibly prism glasses , orthopedic exercise treatments or even squint operations . It is not uncommon for successful treatment to include all aspects of the treatment options mentioned.

Möbius sign

The Möbius sign is a symptomatic convergence weakness in the clinical picture of endocrine orbitopathy .

literature

Herbert Kaufmann: Strabismus . With the collaboration of W. de Decker et al., Stuttgart: Enke, 1986, ISBN 3-432-95391-7