CONVERGENCE

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CONVERGENCE

q  DEFINITION

•         It is a simultaneous, synchronous, disjugate movement in which both  eyes rotates inward so that line of sight intersect in front of eye to maintain bifoveal singe vision at any point of object regrads. 

•         SYMMETRIC CONVERGENCE:

•         If the fixation object is situated in the median plane of head , equal angles are formed between each visual axis and a line erected perpendicular at mid-point of a line connecting the centre of rotation of two eyes

•         ASYMMETRIC CONVERGENCE:

•         If the fixation object is right or to the left of the median plain, the two visual axes do not form an equal angle with mid-line

•         NEAR POINT OF CONVERGENCE:[NPC]

•         It is the closest point at which an object can be seen singly during bifoveal fixation.

•         It is usually less than 8cm and always closer than NPA .

•         FAR POINT OF CONVERGENCE:

•         It refers to relative position of eyes when they are completely at rest, usually at infinity .

•         At rest eyes are slight divergence so far point of convergence may be negative i.e is behind the eyes.

•         RANGE OF CONVERGENCE:

•         It is the distance between between the far point of convergence and near point of convergence

•         The part of range of convergence between the eyes and infinity is called as POSITIVE CONVERGENCE.

•         The part and infinity i.e is behind the eyes is called as NEGATIVE CONVERGENCE or DIVERGENCE.

•         AMPLITUDE OF CONVERGENCE:

•         It refers to difference in convergence power exerted to maintain eyes in a position of rest and position of maximum convergence

•         TYPES OF CONVERGENCE :

q  VOLUNTARY CONVERGENCE:

•         It is that amount of convergence of visual axis that can be produced at will.

•         It is not a part of normal convergence movement as it occur in everyday life.

•         Thus, it is separate phenomenon from the activities and that not every one is capable of doing or learning this.

q  REFLEX CONVERGENCE:

•         Its is the converegence of visual axes which is not under complete voluntary control.

•         It has 4 following components:-

1)Tonic Convergence:

  The amount of Convergence brought about by tenus of extraocular muscles in an alert conscious individual.

  

 2)Proximal Convergence:

•           It is the amount of Convergence produce by the awarness of nearness of object.

3)Accommodative Convergence:

•           It is the Convergence induced by the stimulus to accommodation.

•           The stimulus accommodate is initiated by such factor as blurred image and chromatic aberration, but it is the own accommodative effort made to overcome the blurred vision which determines convergence response

4)Fusional Convergence:

•         The amount of Convergence exerted over and above tonic proximate and accommodative, the keep the friction point on corresponding retinal points.  OR

•         The negative movements of eyes so that image is formed on fovea simultaneously and is used as one.

a) Positive Relative Fusional Convergence:

•         The deficit of Convergence in ocular alignment is adjusted by positive relative fusional Convergence.

b) Negative Relative Fusional Convergence:

•         The excess of Convergence in ocular alignment is adjusted by inhibition of negative relative fusional Convergence

  

Anomalies of Convergence

1.   Convergence Insufficiency

2.   Convergence Paralysis

3.   Convergence Spasm

 

1.   Convergence Insufficiency: It is the inability to obtain &/ or maintain adequate binocular convergence for any length of time without under effort.

Etiology:-

Primary:-

·         It is due to wide interpupillary distance & delayed or inadequate functional development.

·         General debility, Psychological instability, over work & worry may be the precipitating factors.

Refractive Error:-

·         It may be associated with uncorrected high hypermetropia & myopia.

·         High hypermetropia ( more than 5D ) usually makes no accommodation & thus there is deficient accommodative convergence.

·         Myopes may not need accommodation thus lack accommodative convergence.

Muscular Imbalance:-

Extraocular muscular imbalances in the form of exophoria, intermittent exophoria, if neglected for a long time may be associated with convergence insufficiency.

Diagnosis

Diagnosis of convergence insufficiency is confirmed by:-

Near point of Convergence: Convergence insufficiency is said to be exist if NPC is more than 10cm from the base line.

Synoptophore : When measured on synoptophore the convergence is said to exist if there is difficulty in attaining 30° of convergence.

Prisms: Prism convergence is low but prism divergence is normal.

Treatment

Optical Treatment:-

Proper refraction should be done

Myopes are give full correction & hypermetropes are under corrected to stimulate their accommodation which will simultaneously stimulate convergence

Orthoptic Treatment:-

Main aim of orthoptic exercise is to improve the binocular convergence & increase the amplitude of fusional convergence.

Prismotherapy:-

When all the exhaustive exercise fails then prismotherapy may be tried to relieve symptoms

Base-in prism reading glasses or bifocal with prism in the lower segment are useful as relieving prism.

Surgical treatment:-

As a last resort, medial rectus resection can be performed in one or both eyes.

 

2.   Convergence Paralysis: It refers to a total lack of ability to overcome any amount of base-out prism.

Etiology:-

Convergence paralysis occurs secondary to some organic disease of the barin. The organic brain lesions reported to be associated with this condition as follows.,

·         Head injury

·         Encephalitis

·         Disceminated sclerosis

·         Tabes dorsalis

·         Narcolepsy

·         Tumours

Diagnosis:-

·         Evidence of intracranial disease.

·         History of sudden onset of crossed horizontal diplopia at near fixation.

Treatment

·         Base-in prisms are prescribed at near to alleviate the diplopia at near.

·         Plus lenses with base-in prisms may be required in patient having weakness of accommodation

·         Occlusion of one eye at near may be indicated in comfortable single binocular vision

·         Eye muscle surgery is contraindicated in this condition.

 

3.   Convergence Spasm: It is usually associated with spasm of accommodation

Etiology:-

Functional cause:-

It occurs in patient with hysteria or neurosis

Organic cause:-

·         Rarely convergence spasm may be secondary to some underlying organic lesion.

·         It has been reported to occur after head trauma, encephalitis, tabes, pituitary adenomas & posterior fossa neurofibroma.

Treatment

·         Prolonged atropinization with plus lenses in lower segment of bifocals for near work may be required to break the cycle.

·         Alternate monocular occlusion mat be considered as an alternative to atropinization.