exodeviations

An exodeviation is a divergent strabismus that can be latent or manifest.

etiology

The exact etiology of most exodeviation is unknown, proposed causes include:

Anatomical and mechanical factors within the orbit.

Innervational abnormalities such as excessive tonic divergence.

Pseudoexotropia

An appearance of exodeviation when in fact the eyes are properly aligned.

Wide interpupillary distance. Positive angle kappa without other

ocular abnormalities. Positive angle kappa together with

ocular abnormalities such as temporal dragging of the macula in ROP.

exophoria

Is an exodeviation controlled by fusion. An exophoria is detected when binocular

vision is interrupted, as during an alternate cover test.

Exophoria is usually asymptomatic. Prolonged, detailed visual work or

reading may bring about asthenopia. Treatment is usually not necessary

unless an axophoria progresses to an INT XT.

Intermittent exotropia

With the possible exception of exophoria at near, the most common type of exodeviation is intermittent exotropia.

Clinical characteristics

The onset is an early, before age 5. The deviation often becomes manifest during

times of visual inattention, fatigue, or stress. Parents report that the exotropia occurs late in

the day with fatigue or during illness, daydreaming, or drowsiness upon awakening.

Exposure to bright light often causes a reflex closure of 1 eye.

During the early stages, the deviation is usually larger for distance than for near.

Later, the near and distance exodeviations tend to be more equal in magnitude.

Intermittent exotropias can be associated with small hypertropias, A or V patterns, and oblique muscle dysfunction.

In many patients, untreated intermittent exotropia progresses toward constant exotropia.

Children younger than 10 years of age may initially have diplopia but often develop the cortical adaptations of suppression and ARC.

However, NRC and good binocular function remain when the eyes are straight.

Amblyopia is uncommon unless the exotropia progresses to constant at an early age.

Clinical evaluation

History of the age of onset of the strabismus.

The clinician records how often and under what circumstances the deviation is manifest.

A qualitative measurement of the control of the deviation, is an important component of the evaluation.

Good control: exotropia manifests only after cover testing, and the patient resumes fusion rapidly without blinking or refixating.

Fair control: exotropia manifests after fusion is disrupted by cover testing, and the patient resumes fusion only after blinking or refixating.

Poor control: exotropia manifests spontaneously and may remain manifest for an extended time.

The deviation at near fixation is often less than the deviation at distance fixation.

The difference may be due to either a high AC/A ratio or to tenacious proximal fusion.

Tenacious proximal fusion is a proximal vergence aftereffect that occurs in some patients with intermittent XT.

This aftereffect is due to a slow-to-dissipate fusion mechanism that prevents intermittent exotropia from manifesting at near fixation.

For patients with significantly more exodeviation in the distance than at near, 1 hour of monocular occlusion, eliminates the effects of tenacious proximal fusion and may help to distinguish between a truly high AC/A and a pseudo high AC/A.

A patient with a pseudo high AC/A ratio would have equal distance and near measurement after occlusion.

Classification

Basic type. Divergence excess type. Convergence insufficiency type.

Basic type

Is present when the exodeviation is approximately the same at distance and near fixation.

Divergence excess

Consist of an exodeviation that is greater at distance fixation than at near.

It can be divided into 2 subtypes. 1-true divergence excess. 2-simulated divergence excess.

True divergence excess

Refers to those deviations that remain greater at distance than at near even after a period of monocular occlusion.

Some of these patients prove to have a high Ac/A ratio when tested at near with +3 D lenses.

Simulated divergence excess

Refers to a deviation that is initially greater at distance than at near but that becomes about the same after 1 eye is occluded for 1 hour ( to remove the effect of tenacious proximal fusion).

The convergence insufficiency

Is present when the exodeviation is greater at near than at distance.

This type excludes isolated convergence insufficiency.

Sensory testing usually reveals excellent stereopsis and NRC when the exodeviation is latent.

And suppression with ARC when the deviation is manifest.

However, if the deviation manifests rarely, diplopia may persist during those manifestations.

Treatment

Some ophthalmologists prefer to delay surgery in young children.

Other ophthalmologists worry that delaying surgery could allow for development of permanent suppression and loss of long-term stability following surgical correction.

Nonsurgical management

Corrective lenses for any significant refractive error.

Mild to moderate hyperopia are not routinly corrected.

Hyperopia of more than 4.00 D and astigmatism of more than 1.50 D is better to be corrected.

Correction of even mild myopia may help to control the deviation.

Additional minus lens power to stimulate accomodative convergence may help to control the deviation.

Part-time patching of the dominant eye 4-6 hours per day, can be an effective treatment for small-to moderate-sized deviations. persumably due to disruption of suppression.

Active orthoptic treatments

Antisuppression therapy, diplopia awareness and fusional convergence training, can be used alone or in combination with patching, minus lenses, and surgery.

This type of treatment can be effective for deviations of 20 P or less, but is not recommended for poorly controlled deviations.

Surgical treatment

Many patients with intermittent exotropia ultimately require surgery,

The best sensory outcomes are probably achieved with motor alignment before age 7 or before 5 years of strabismus duration, or while the deviation is still intermittent.

Many surgeons use manifestation of the deviation more than 50% of the time as criterion for surgery.

Symmetric recession of both LR is the most common surgical procedure.

R&R procedure is an acceptable alternative and may be preferred in basic type INT XT.

Unilateral LR recession may be performed in small-angle exodeviations.

Management of surgical overcorrection

A temporary overcorrection of up to 10-15 P is desirable after bilateral LR recession.

Persistent overcorrection may require treatment with base-in prisms or alternate patching to prevent amblyopia or diplopia.

Corrective lenses or miotics should be considered if hyperopia is significant.

Bifocals can be used for a high AC/A ratio.

Unless in a case of slipped or lost muscle, a delay of several months is recommended before reoperation because spontaneous improvement is common.

Bilateral MR muscle recession, R&R of the fellow eye.

Botulinum toxin injection into 1 MR muscle may be effective.

Management of surgical undercorrection

Mild to moderate residual XT is only observed alone if fusional control is good.

Aggressive base-in prism management with a gradual weaning of the prism dosage.

Postoperative patching and orthoptic treatment can be useful.

Botulinum toxin injection into LRM.

Indication for reoperation

If the nonsurgical management failed. The type of surgery is related to the type

of previous surgery. The surgical dose-response curve

appears to be similar to that for the initial surgery.

Constant exotropia

Congenital exotropia. Sensory exotropia. Consecutive exotropia.

Congenital exotropia

Presents before age 6 months. Large angle constant deviation. Most of these patients have associated

neurologic problems or craniofacial disorders.

Early surgery can lead to gross SBV. DVD and IO overaction may be seen in

these patients.

Sensory exotropia

Any condition that reduces visual acuity in 1 eye can cause sensory exotropia.

Anisometropia, corneal and lens opacities, optic atrophy or hypoplasia, macular lesions, and amblyopia.

Both sensory ET and XT are common in children, but exotropia predominates in older children and adults.

Loss of fusional abilities, known as central fusional disruption, or horror fusionis, can lead to constant and permanent diplopia when adult-onset sensory exotropia has been present for several years prior to visual rehabilitation and realignment.

In these patients, intractable diplopia may persist, even with well aligned eye.

Consecutive exotropia

An exotropia that follows previous surgery for esotropia.

Treatment of consecutive exotropia depends on many factors, including the size of the deviation, the type and amount of previous surgery, the presence of duction limitation, lateral incomitance, and the level of visual acuity in each eye.

Exotropic Duane retraction syndrome

Duane syndrome can present with exotropia, usually accompanied by a face turn away from the affected eye.

Adduction is most often markedly deficient.

Other signs include eyelid narrowing, glob retraction, and upshoot and downshoot.

Dissociated horizontal deviation DVD may contain vertical, horizontal, and

torsional components. DHD is when the dissociated abduction is

predominate. It may be confused with a constant or

intermittent exotropia. DVD and latent nystagmus often coexist with

DHD. Treatment consists of unilateral or bilateral LR

recess in addition to any necessary oblique or vertical muscle surgery.

Convergence insufficiency

Characteristics are: Asthenopia, blurred near vision, reading

problems. Poor near fusional convergence

amplitude Remote near point of convergence.

The patient, typically an older child or adult

May have an exophoria at near. Rarely accomodative spasm may occur

in an effort to overcome the convergence insufficiency.

Treatment

Orthoptic exercises. Base-out prisms can be used to

stimulate fusional convergence. Stereogram, pencil pushups and other

near point exercises are often used. If these exercises fail, base-in prism

reading glasses may be needed.

Unilateral or bilateral MR resection may be used in rare cases but may be associated with risk of diplopia in distance viewing.

Patients with combined convergence and accomodative insufficiency may benefit from plus lenses and base-in prisms for reading.

Convergence paralysis

Is usually secondary to an intracranial lesion. Is characterized by normal adduction and

accomodation with exotropia and diplopia on attempted near fixation only.

It differs from convergence insufficiency in its relatively acute onset and the patient,s inability to overcome any base-out prism.

It usually results from a lesion in the corpora quadrigemina or the nucleus of the cranial nerve 3 and may be associated with Parinaud syndrome.

Treatment

Is limited to providing base-in prism at near to alleviate diplopia.

When accomodation is also weakened, plus lenses also may be needed.

If SBV can not be restored at near by any way, occlusion of one eye is indicated during reading.

Eye muscle surgery is contraindicated.