Childhood exotropia (outward deviation) is a horizontal exodeviation characterised by visual axis forming a divergent angle. It usually begins as exophoria. Exophoria is a condition in which eyes are straight without deviation when both eyes are open. However, eye under cover deviates on cover-uncover test or alternate cover test. It may progress to exotropia which may be latent (exophoria) or manifest (Exotropia). Fluctuation between phoria and tropia is common in exotropia.
Classification of childhood exotropia:
It is broadly classified as:
Constant exotropia may be of following types:-
According to distance-near relationships, exodeviations may be further subdivided as (Duane classification):-
Convergence insufficiency exotropia: Due to convergence insufficiency, exotropia is worse for near vision.
Divergence excess: Due to divergence excess, exotropia is worse for distance vision.
Basic exotropia: Exotropia is equal for both near and distance vision.
Intermittent exotropia frequently begins around the age of two years. A child with intermittent exotropia does not develop diplopia (double vision) due to bitemporal suppression, unlike acquired manifest exotropia in adults. With progressive suppression, constant exotropia may develop. Development of amblyopia (functional suppression of retina) is very rare. Manifest exotropia may be precipitated by factors such as fatigue, light glare, ill-health or visual distraction.
Congenital exotropia is rare and present at birth and may be associated with neurological abnormalities like cerebral palsy, midline defects or craniofacial syndromes. Infantile exotropia manifests during the first year of life.
Decompensated intermittent exotropia:
Manifest intermittent exotropia may increase with time and become constant exotropia.
Sensory deprivation exotropia:
Sensory deprivation exotropia is due to disruption of binocular reflexes by acquired conditions like opaque media due to a disease or cataract. It begins in children over five years of age or in adults.
Consecutive exotropia may develop following surgical overcorrection of esotropia (inward deviation of eyes), especially in an eye which is amblyopic. Occasionally, a deeply amblyopic convergent eye may become divergent (acquire resting position of eye).
Secondary exotropia results from a primary sensory deficit (sensory deprivation exotropia) or occurs as a result of treatment for esotropia (consecutive exotropia).
Patient with Childhood exotropia may present with:-
- Patient may experience eyestrain following prolonged near work.
- Running together of words or missing of the word being read, due to divergence of eyes.
- Some patients may be aware of divergence and are able to control it voluntarily.
- Voluntary control of exodeviation may lead to accommodative convergence which makes letter appear small in size.
- Some patients have panoramic view i.e. increase in temporal visual field.
- A child may close one eye (eye which diverges) in bright light.
Heredity appears to have role in exodeviation.
The cause of exodeviation is thought to be multi-factorial. However, successive generations in a family tend to have exotropia earlier and of greater severity.
For diagnosis of childhood exotropia:-
A complete eye examination is conducted including record of ocular motility.
Ocular deviation for gaze at near (33 centimeters), distance (6 meters) and far distance (beyond 6 meters) is recorded.
Assessment of the control of deviation is noted which helps in monitoring progression of intermittent exotropia. This deviation may be noted by the parents or is detected on eye examination.
The degree of deviation may be different in primary (straight gaze) and lateral (side) gaze positions. This is important to record from surgical point of view, to avoid post operative diplopia in lateral gaze.
Patients with intermittent exotropia rarely have any complaints due to well developed suppression mechanism. Patient may have symptoms like eyestrain, headache, blurring of vision or difficulty in prolonged reading. However, these symptoms are quickly controlled by development of sensory adaptation. Not all intermittent exotropias are progressive. The deviation may remain stable for many years. The patient should be followed over time to know whether exotropia is stable or deteriorating.
It is characterised by:
- Fairly large and constant angle of deviation.
- Since infant uses left eye in left gaze and right eye in right gaze (uncrossed homonymous fixation), development of amblyopia is uncommon. In some, if one eye is preferred for vision, then other eye may develop amblyopia.
- Infant has normal refraction.
- Adduction is not restricted.
- No lid involvement or pupillary abnormalities distinguishing it from oculomotor nerve palsy ( third cranial nerve).
Decompensated intermittent exotropia:
In some patients, exophoria progresses to intermittent exotropia that eventually may lead to constant exotropia. Deviations usually occur first for distance and later appear for near fixation. However, there are exceptions. The deviation remains constant or rarely may decrease.
Sensory deprivation exotropia:
An eye with poor vision (may be due to opaque media), or a blind eye drifts outwards into exodeviation. This usually occurs in children 2 to 4 years of age and in adults.
Development of consecutive exotropia, after correction of esotropia, may take many years. Usual factors for surgical overcorrection of esotropia are excessive amount of surgery, amblyopia, high hypermetropia, and poor preoperative evaluation of patient.
Exotropia should be distinguished from conditions like oculomotor nerve palsy or pseudoexotropia. In pseudoexotropia, visual axis of both eyes is straight, but the eyes appear divergent.
Nonsurgical methods of correction are adopted in children having good control of deviation as well as in those where risks of overcorrection are undesirable.
- Correction of refractive errors: A trial of spectacle correction is given in all cases of refractive errors. Myopics, in particular, may control the deviation with glasses and may recover from intermittent exotropia.
- Prescription of overcorrecting concave (minus) glasses: Overcorrecting concave lenses stimulate accommodative convergence and may improve quality of fusion. It may decrease the exotropia.
- Part time occlusion: Part time occlusion of non deviating normal eye may treat suppression and amblyopia in deviating eye and thereby may correct exotropia and revert it to exophoria. Alternate occlusion of eyes may be done in cases showing equal preference. Part time occlusion has been found useful in very young children.
- Use of prisms: Base-in prisms enforce bifoveal stimulation and help in relieving eyestrain. Prisms may be used to improve fusional control.
- Orthoptic treatment: Fusional convergence exercises may be employed in patients with symptoms of convergence insufficiency type of intermittent exotropia. Active anti-suppression and diplopia awareness techniques may be used in cases of suppression. Patient is encouraged to improve control on exotropia voluntarily.
- Botulinum toxin chemodenervation: It may be difficult to achieve long term alignment in cases of secondary exotropias. Botulinum toxin injection in extraocular muscles may be used in patients with constant exotropia who are at risk of postoperative diplopia and also who have undergone multiple operations. Preoperatively, if after botulinum injection, a patient develops intractable diplopia, then he may be advised against surgical correction.
In sensory deprivation exotropia, management involves elimination or reversal of treatable causes like removal of cataract. Prisms and botulinum toxin denervation do not play a significant role in the management.
Surgical management of exotropia:
Surgical management may be indicated in patients with:
- Poor control of intermittent exotropia: When manifest intermittent exotropia is present atleast half of the time during the day.
- Progressive deterioration of control of intermittent exotropia: Patients with increase in size of deviation, loss of control and a progressive inability to re-fuse images after manifest deviation may be considered for surgery.
- Severe eyestrain.
- Troublesome diplopia.
- Surgery may also be indicated to restore binocularity and also for cosmetic reasons. Surgical results are better for intermittent exotropia as compared to constant exotropia.
Surgical management options include lateral rectus muscle recession, lateral rectus muscle recession with ipsilateral medial rectus muscle resection and bilateral medial rectus muscle resection.
Surgery with preoperative conservative orthoptic or occlusion therapy gives the highest success rate.
Management of the disease should be under medical supervision.