What is Childhood Exotropia ?
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:
- Intermittent exotropia
Constant exotropia:
Constant exotropia may be of following types:-
- Congenital exotropia
- Decompensated intermittent exotropia
- Sensory deprivation exotropia
- Consecutive exotropia
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:
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:
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:
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).
Childhood Exotropia Symptoms
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.
Childhood Exotropia Causes
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.
Childhood Exotropia Diagnosis
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.
Intermittent exotropia:
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.
Congenital Exotropia:
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.
Consecutive exotropia:
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.
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