Convergent strabismus

Convergent strabismus (esotropia, manifest convergent strabismus) can be concomitant or paralytic. In concomitant convergent strabismus, differences in the angle of deviation within 5 D are observed in different positions of horizontal gaze. In paralytic convergent strabismus, the angle of deviation in different positions of gaze is different as a result of impaired innervation or restriction.

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Types of convergent strabismus

Accommodative convergent strabismus

• Refractive convergent strabismus
  • fully accommodative
  • partially accommodative
• Non-refractive convergent strabismus
  • with excess of convergence
  • with weakness of accommodation
• Mixed convergent strabismus

Non-accommodative convergent strabismus

• essential infantile
• microtropia
• main
• excess of convergence
• convergence spasm
• divergence insufficiency
• divergence paralysis
• sensory
• secondary
• with an acute onset
• cyclical

Accommodative convergent strabismus

In the act of vision at close range, both processes are involved - accommodation and convergence. Accommodation is the process by which the eye focuses on a nearby object, which is accompanied by a change in the curvature of the lenses. At the same time, the eyes converge to achieve bifoveal fixation of the object. Both processes (accommodation and convergence) are quantitatively related to the distance to the object and are characterized by a relatively constant ratio between them. Changes in the AC/A index are the main cause of some forms of convergent strabismus.

Refractive accommodative convergent strabismus

The AC/A index is unchanged, convergent strabismus is a physiological response to excessive hyperopia. Usually between +4.0 and +7.0 D. In this case, the accommodation tension required to focus even a distant object is accompanied by increased convergence, exceeding the patient's negative fusion reserves. Control is lost, and a manifest form of convergent strabismus occurs. The difference in the angle of strabismus when fixating near and distant objects is small (usually <10 D). Strabismus appears at the age of 2.5 years (from 6 months to 7 years).

1. Complete accommodative convergent strabismus is completely eliminated by optical correction of hyperopia.
2. Partial accommodative convergent strabismus is reduced by optical correction of hypermetropia, but is not eliminated completely.

Non-refractive accommodative convergent strabismus

Caused by a high AC/L index, in which increased accommodation is accompanied by a disproportionately large increase in convergence in the absence of significant hyperopia. There are 2 types:

Excess of convergence. It is characterized by:

High AC/A index due to increased AC (accommodation is normal, convergence is enhanced).

• Normal near point of accommodation.
• Correct position of the eyes when fixating a distant object, convergent strabismus when fixating a close object.

With impaired accommodation (hypoaccommodation). It is characterized by:

• High AC/A index due to decreased A (weak accommodation requires additional efforts, which are accompanied by increased convergence).
• Distance from the nearest point of accommodation.
• When fixating a close object, additional accommodative effort is required, leading to excess convergence.

Mixed accommodative convergent strabismus

Hyperopia and a high AC/A index can be combined, leading to convergent strabismus when fixating a distant object and significantly increasing the deviation angle (>10 D) when fixating a close object. Deviation when fixating a distant object is usually corrected with glasses, convergent strabismus when fixating a close object will persist if it is not corrected with bifocal glasses.

Treatment of accommodative convergent strabismus

Refractive errors should be corrected as described above. For children under 6 years of age, full refractive correction is recommended, as revealed by retinoscopy in cycloplegia. In accommodative refractive convergent strabismus, such correction eliminates the angle when fixating near and far objects. After 8 years, retinoscopy should be performed without cycloplegia (manifest hyperopia) and the maximum tolerated plus correction should be prescribed.

Bifocals are prescribed for accommodative convergent strabismus (high AC/A index). They facilitate accommodation (and, consequently, accommodative convergence), allowing the child to maintain bifoveal fixation and the correct position of the eyes when fixating on a close object. This is achieved with minimal plus correction. The most convenient form of bifocals is when the dividing groove runs along the lower edge of the pupil. The power of the lower lenses should be gradually reduced with age: in early adolescence, it is advisable to switch to monofocal glasses. The final prognosis regarding the termination of spectacle correction is associated with the AC/A index, as well as with the degree of hypermetropia and astigmatism. Glasses may be necessary only when working with close objects.

Miotic treatment may be short-term in children with accommodative convergent strabismus due to a high AC/A index who do not wish to wear glasses. The initial dose is 0.125% ecothiopate iodide or 4% pilocarpine 4 times daily for 6 weeks. If treatment is effective, the strength and frequency are gradually reduced to the minimum effective dose. Formation of iris cysts caused by ecothiopate may be prevented by concomitant administration of 2.5% phenylephrine 2 times daily. The mechanism of miotic treatment is stimulation of "peripheral" accommodation (i.e. stimulation of the ciliary muscle to a greater extent than the action of the third pair of cranial nerves). Less accommodation tension is required, and accommodative convergence is induced to a lesser extent. A possible side effect is blurred vision when fixating a distant object.

Treatment of amblyopia is extremely important and should precede surgical correction.

Surgical correction is indicated after treatment of amblyopia if glasses do not completely eliminate the deviation. The principle of surgical intervention is weakening the internal rectus muscles, i.e. the muscles responsible for convergence.

• Bilateral recession of the intrinsic muscles is performed in patients with symmetrical visual acuity in both eyes, when the deviation when fixating a close object is greater than a distant one.
• If there is no significant difference between the near and far fixation angles and vision is equal in both eyes, some surgeons perform a combined procedure with resection of the medial and lateral rectus muscles, while others prefer bilateral recession of the medial rectus muscles.
• Recession-resection on the amblyopic eye is performed in patients with residual amblyopia.

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Essential infantile convergent strabismus

Essential infantile convergent strabismus is idiopathic, developing during the first 6 months of life in healthy infants in the absence of refractive errors and limitations of eye mobility.

Symptoms

• The angle is usually large (>30 D) and constant.
• In most patients, alternating fixation is in the primary position and crossed fixation of the right eye when looking to the left (Fig. 16.63b), and of the left eye when looking to the right (Fig. 16.63a). This may give the false impression of bilateral abduction insufficiency, as in bilateral sixth cranial nerve palsy. But abduction can usually be demonstrated with the "doll's head" maneuver or by rotating the child. If this is difficult, unilateral occlusion for several hours will unmask the ability of the other eye to abduct.
• Manifest nystagmus is usually horizontal; if manifest, it can be latent or manifest-latent.
• The refractive error corresponds to the child’s age (about +1.5 D).
• Asymmetry of optokinetic nystagmus.
• Hyperfunction of the inferior oblique muscle can be present initially or develop later.
• Dissociated vertical deviation occurs in 80% of patients by 3 years of age.
• Low potential for development of binocular vision.

Differential diagnostics

• Congenital bilateral paralysis of the sixth pair of cranial nerves, which can be excluded on the basis of the previously mentioned methods.
• Sensory convergent strabismus due to organic pathology of the visual organ.
• Nystagmus block syndrome, in which horizontal nystagmus is suppressed by convergence.
• Duane syndrome types I and III.
• Mobius syndrome.
• Fixed strabismus.

The first stages of treatment

Ideally, the correct position of the eyeballs should be achieved surgically by 12 months of age or a little later, by 2 years of age, only after amblyopia or significant refractive errors have been corrected. First, a bilateral recession of the internal rectus muscles is performed. At large angles, the recession can be 6.5 mm or more. Combined hyperfunction of the inferior oblique muscle must also be taken into account. An acceptable result is a residual convergent strabismus of 10 D associated with peripheral fusion (combats diplopia) and central suppression (combats confusion). Such a residual small angle is quite stable even when the child does not have bifoveal fusion.

Follow-up treatment

1. Undercorrection may require re-recession of the medial rectus muscle or resection of one or both lateral rectus muscles.
2. Hyperfunction of the inferior oblique muscle may develop later, most often by the age of 2 years. Therefore, parents should be warned that subsequent surgical treatment may be required despite an initially good outcome. Initially, surgical treatment is unilateral, but often within 6 months, surgery is required on the second eye as well. Procedures aimed at weakening the inferior oblique muscle include myotomy, myectomy, and resection.
3. Dissociated vertical deviation may appear several years after initial surgical correction, especially in children with nystagmus. It is characterized by the following:
   • Upward drifting eye with excyclodeviation under a shutter or with impaired attention.
   • When the flap is removed, the affected eye will move downward without accompanying downward movement of the opposite eye.

Thus, VDD does not obey Hering's law. The deviation is usually bilateral and may be asymmetric. Surgical treatment is indicated for cosmetic reasons. Recession of the superior rectus with or without Faden operation and/or repositioning of the inferior oblique are frequently used interventions for VDD, although complete correction is rarely achieved.

Amblyopia develops over time in 50% of cases.

An accommodative component may be suspected if after surgery the eye position is correct or nearly correct and then reconvergence occurs. Therefore, to correct the emerging accommodative component, it is necessary to reexamine the refraction in all children.

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Basic convergent strabismus

Symptoms

• There are no significant refractive errors.
• The same angle when fixing near and far objects.

Treatment is surgical.

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Excess of convergence

Symptoms

• There are no significant refractive errors.
• Orthophoria or slight esophoria when fixating a distant object.
• Esophoria when fixating a close object with a normal or low AC/A index.
• Normal nearest point of accommodation.

Treatment: bilateral recession of the internal rectus muscles.

Cyclic oculomotor spasm

It is a periodic phenomenon, usually hysterical, but may be of organic origin (trauma or tumor of the posterior cranial fossa).

Symptoms during an attack:

• Esotropia due to supported convergence.
• False myopia due to spasm of accommodation.
• Bilateral miosis.

Treatment with cycloplegic drugs and bifocal glasses.

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Insufficiency of divergence

Affects healthy young people.

Symptoms

• Intermittent or constant esotropia when fixating a distant object.
• Minimal or no deviation when fixating a close object.
• Complete bilateral abduction.
• Reduction of negative fusion reserves.
• Absence of neurological disease.

Treatment: prismatic correction until spontaneous recovery, and in case of failure - bilateral resection of the external rectus muscles.

Divergence paralysis

May manifest at any age. Differential diagnosis with unilateral or bilateral paralysis of the sixth pair of cranial nerves is necessary. Divergence paralysis is characterized by:

• Convergent strabismus, which does not change or may decrease with outward gaze, in contrast to paralysis of the sixth pair of cranial nerves.
• Negative fusion reserves are significantly reduced or absent.
• May be accompanied by neurological disorders such as head trauma, intracranial space-occupying lesions and cerebrovascular accidents.

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Sensory convergent strabismus

Sensory convergent strabismus is caused by a unilateral decrease in visual acuity that interferes with or eliminates fusion, such as in cataracts, optic atrophy or hypoplasia, toxoplasmic retinochoroiditis, or retinoblastoma.

An examination of the fundus under mydriasis is necessary in children with strabismus.

Secondary convergent strabismus

Secondary convergent strabismus follows excessive correction of exodeviation. If the deviation is not very large, surgical correction should be postponed for several months, as spontaneous improvement may occur.

Acute convergent strabismus

Acute convergent strabismus occurs with sudden decompensation of convergent strabismus or microtropia. The patient complains of double vision. It is important to rule out paralysis of the sixth pair of cranial nerves or divergence paralysis.

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Cyclic convergent strabismus

Cyclic convergent strabismus - CEOS is a very rare condition characterized by alternating manifest convergent strabismus and orthophoria lasting for 24 hours. The condition can persist for months or years and eventually lead to permanent convergent strabismus requiring surgical treatment.

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