Common types of strabismus (2024)

Of the dozens of different types of strabismus, three types are seen much more commonly in the paediatric population. They are congenital esotropia, accommodative esotropia and intermittent exotropia. However, a large number of children are referred for evaluation and treatment of early onset esotropia when, in fact, they have pseudostrabismus related to prominent epicanthal folds.

PSEUDOESOTROPIA

Transient misalignment of a baby’s eyes is very common up to the age of four months (Figure 1). The eyes may be intermittently esodeviated or exodeviated, but by three months of age, the eyes should be straight. Any strabismus that is apparent after that time is a source of concern (1). Epicanthal folds are present because of the underdevelopment of the nasal bridge, and these folds cover the inner portion of the eye, obscuring the underlying white sclera. The appearance of strabismus is particularly noticeable in a child’s side gaze as the eye travels underneath the epicanthal fold. A simple cover test of each eye separately will elicit no movement in the uncovered eye, confirming a diagnosis of pseudoesotropia related to epicanthal folds.

CONGENITAL ESOTROPIA

Congenital esotropia is much less common than pseudoesotropia, and it presents as a very large angle of esodeviation in a young child (2). The angle of turn is sufficiently large that a diagnosis of congenital esotropia can be made from across a waiting room. Children with this condition may present with or develop an amblyopia, in which case occlusion therapy is indicated. However, the children may chose to cross fixate, using the right eye to look in gaze left and the left eye to look in gaze right. There is no accommodative component to this type strabismus, although it may develop later. Glasses are of no assistance in the early treatment of these children (for accommodative esotropia, see below).

The treatment for congenital esotropia is surgical, consisting of weakening and strengthening the appropriate extraocular muscles. Frequently, more than one surgery is necessary. While children with this condition can have their eyes straightened cosmetically, the children have an inherent inability to fuse, and they never develop complete binocularity. Surgery is customarily performed around one year of age (3), and these children must be followed through their amblyogenic years (up to age eight) to ensure that any preference for one eye is treated satisfactorily with occlusion therapy. The children may develop an accommodative component to their strabismus at a later date, and glasses may ultimately be required.

ACCOMMODATIVE ESOTROPIA

The onset of accommodative esotropia occurs between the ages of 18 months and four years, and may develop acutely, simulating a sixth nerve palsy. It is not at all uncommon for children with this condition to present at the emergency department of a paediatric hospital. These are the children who are referred to the ophthalmologist with a diagnosis of esotropia, and they return wearing a rather substantial pair of spectacles.

Accommodative esotropia (4) develops because of an abnormal relationship between the converging muscles (medial recti) and the focusing muscles (ciliary body) attached to the lens inside the eye. Normally, there is a linear relationship between these two groups of muscles in that a certain amount of convergence is accompanied by a certain amount of accommodation. However, this relationship is abnormal in this group of children, and the children’s eyes over converge in response to a demand to focus clearly. This condition may begin initially as an intermittent deviation where the eyes may be satisfactorily aligned when the child’s gaze is not fixing on an object, only to become markedly esotropic when focusing mechanisms are used.

Children with accommodative esotropia tend to be quite hyperopic (far sighted), and treatment is directed at correcting this far sightedness with glasses. The glasses focus for the children, and because they have no further need to accommodate, the overconvergence does not occur and the eyes are straight (Figure 2). The glasses work instantaneously, and while the eyes are straight when looking through the spectacles, they are markedly convergent when the glasses are removed. Many children achieve perfect binocularity and alignment with the use of hyperopic corrective glasses, but some children’s eyes only straighten part of their turn (partially accommodative esotropia). These children may require surgery for the nonaccommodative component of their strabismus. If their eyes are not perfectly aligned, they run a high risk of developing amblyopia, and must be followed closely through their amblyogenic years to ensure that this condition is detected and treated.

Some children’s eyes have a larger angle of turn at near rather than at distance. These children are treated with a bifocal that incorporates a stronger hyperopic lens in the bottom portion of the glasses where children look to do most activities at a near distance.

In the course of explaining the pathophysiology of accommodative esotropia to parents, their first question is “How long will my child need the glasses?” This can be difficult to predict, and the author always gives parents a “definite maybe” as to whether their child will be able to stop using glasses at a later date. The reason for this is that children tend to become less hyperopic as they go through their adolescent growth spurt, and as the body grows taller, the eye grows longer, decreasing the hyperopia. If this occurs, the strength of the glasses can be decreased gradually and the glasses can be removed eventually. However, many of these children are extremely hyperopic and no amount of growth will change this. These children will require glasses (or contact lenses) throughout their lifetime. Very often, a strong family history is associated with accommodative esotropia. Children whose parents wear thick glasses should have their eyes examined immediately if a strabismus develops or by the age of three-and-one-half years.

INTERMITTENT EXOTROPIA

Intermittent exotropia develops between the ages of 10 months and four years, and can be rather difficult to detect (5). Parents are unable to describe accurately the abnormality that they see in a child’s eye, and will often say that the eye looks ‘funny’ or ‘glassy’. The reason for this is that the ocular deviation presents initially only from a distance, when the child is sick, or when he or she is day dreaming. Parents will notice the misalignment, ask the child to look at them, and of course from near distances the eyes are perfectly straight. Because the eyes are only intermittently deviated, the incidence of amblyopia in this condition is quite low, and the children tend to develop good binocularity and stereoscopic vision.

A classic symptom of intermittent exotropia is the closure of one eye in bright sunlight (6). Until adequate suppression develops, children have double vision when an eye starts to wander. Because this happens most frequently outdoors, the children will begin to close one eye. Detection of this symptom alone is an indication for a referral to an ophthalmologist.

A cover test may be performed on a distant fixation target, perhaps taking the child to an office window to fixate on an interesting object.

Surgery for intermittent exotropia is generally performed around the age of four years when adequate measurements of the angle of deviation can be obtained. Surgery consists of weakening and strengthening appropriate extraocular muscles, and, generally, glasses and exercises are of no particular value.

CONCLUSIONS

Cover testing will detect any type of strabismus, and should be performed routinely both at far and near distances. A single cover test means covering one eye (eg, placing one hand on the child’s head and using the thumb as an occluder), and watching for movement in the opposite eye. If the eyes are satisfactorily aligned, there will be no movement whatsoever in one eye when the other is covered. Alternate cover test involves moving the occluder from one eye to the other, and watching the eye that has just been uncovered; there should be no movement while this manoeuvre is undertaken. Children who are amblyopic in one eye will lustily resist any attempts to cover their good eye, indicating an inequality of vision and prompting a referral to an ophthalmologist.

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