Special forms of comitant exotropia

Introduction

Exotropia is a manifest divergent squint. A comitant squint is one in which the angle of deviation is the same in all positions of gaze, regardless of which eye is fixing.

This chapter addresses infantile exotropia, cyclic exotropia, micro-exotropia (monofixation), sensory exotropia, and consecutive exotropia. Other than consecutive exotropia, many of these entities are uncommon. A guide for the practising ophthalmologist on the background and the practical management of these, sometimes rare, conditions is provided.

Western and Asian studies differ on the incidence of convergent and divergent squint in their childhood populations. Western studies report a higher proportion of esotropia (convergent squint), whilst in Asia more children are divergent than convergent. However, the subgroup types appear to be similar in both Western and Asian studies, with intermittent divergent squint being by far the commonest form of exotropia. Intermittent divergent squint and primary constant exotropia are covered in Chapter 80 . Incomitant divergent deviations, such as third nerve palsy and Duane syndrome, are covered in Chapters 84 and 85 .

Infantile Exotropia

Infantile exotropia is best defined in an equivalent way to infantile esotropia. Thus, the term should be used to describe a large angle divergent squint presenting in a neurologically normal child before the age of 6 months. However, it is much less common than infantile esotropia, which may explain why the literature is both less extensive and often less rigorous than that for infantile esotropia. Estimates for the frequency of infantile exotropia are given as 1 in 30,000 births.

Publications frequently include children presenting with intermittent exotropia as well as constant exotropia, and it has been reported that some children present initially with an intermittent divergent squint which, within the first year of life, becomes a classic constant infantile exotropia.

The literature includes both children with no underlying disorder and those with squint secondary to underlying systemic or ocular conditions. It is important to separate out healthy children with infantile exotropia, or primary infantile exotropia, from those with ophthalmic or systemic disorders which can be called early onset exotropia or secondary infantile exotropia.

Studies on newborn babies, undertaken to look at the incidence and development of infantile esotropia, have reported that exotropic deviations are common in newborn infants and nearly all will resolve within the first 3–4 months. Studies of newborn babies report 32.7%–60% of them as having a divergent deviation, which can be constant or intermittent. ^. By between 6 weeks to 6 months of age over 97% of the babies studied had straight eyes. It has been reported that normal infants tend to have more intermittent deviations and premature infants more often have a constant divergent deviation. This suggests a normal development pattern with the eyes straightening by going from divergent to a more convergent deviation.

Once transient neonatal deviations have resolved, true infantile exotropia is rare. Infantile esotropia is reported at between 8 and 50 times more common than infantile exotropia.

Early onset exotropia can be seen in children with developmental delay, an ocular abnormality or orbital anomalies – in particular, craniofacial disorders.

Secondary infantile exotropia is significantly associated with developmental delay. Studies have shown that 20%–38% of children presenting with exotropia in the first year of life will have developmental delay associated with neurological problems (including cerebral palsy, hydrocephalus, prematurity, perinatal asphyxia, periventricular hemorrhage, chromosomal or genetic abnormalities).

Nearly 75% of children with a constant deviation of more than 25 prism diopters (PD) in the first year of life have an underlying systemic or ocular abnormality. The systemic disorders include neurological, prematurity, and genetic issues. The ocular disorders identified included albinism, ptosis, oculomotor apraxia, optic atrophy, optic nerve hypoplasia, cataract, retinoschisis, and iridolenticular adhesion.

Abnormalities of the orbit are also associated with early onset exotropia. Apert, Crouzon, and Pfeiffer craniosynostosis syndromes are all significantly associated with V pattern exotropia (see Chapter 26 ).

In normal children without developmental delay or a neurological disorder a larger biorbital angle, defined as the angle between the medial and lateral orbital walls, has been reported in children with infantile exotropia.

Cyclic Exotropia

A cyclic or periodic squint is a rare condition, first described in 1958. In most cases the deviation is convergent but vertical and divergent deviations have been reported. It has been estimated that cyclic esotropia occurs in 1 in every 3,000–5,000 squints, with cyclic exotropia being considerably rarer. Cyclic squints usually occur as an alternate-day deviation with a 48-hour cycle; there is a squint for a 24-hour period followed by 24 hours when the eyes are orthotropic. The deviation has also been reported as periodic, with 3–4-day cycles. Shorter and longer cycles have also been reported.

The etiology of cyclic deviations is unknown. A central processing defect or disruption of the circadian biological clock have been suggested. It can be acquired following ocular trauma, central nervous system lesions, or ocular surgery.

In children, very few cases of cyclic exotropia have been reported. The first were case reports in the Japanese literature. In English language articles there are two case reports involving three children, all girls, with alternate-day exotropia. At presentation the children were 1, 3, and 5 years of age. All the children were healthy with good vision in each eye. All three cases demonstrated 48-hour cycles with 24 hours of constant squint followed by 24 hours of straight eyes. The children were binocular on straight days. Behavioral changes were reported, with some children being unhappy or cross on the squinting days. Two of the children had previously undergone squint surgery, one for esotropia and the other for intermittent exotropia. Two patients required squint surgery, with a good outcome, and one improved spontaneously. There is a report of a child in the Korean literature with a 72-hour cyclic exotropia with 2 days squinting and 1 day straight whose condition resolved after squint surgery. A Japanese boy has been reported with a periodic exotropia of 1 day of squint every 7 days.

In children at least, a cyclic exotropic deviation does not appear to be associated with visual loss, whereas in adults, cyclic deviations are often associated with visual loss, particularly in the deviating eye. It has also developed after surgery for cyclic esotropia. The ophthalmic conditions associated with cyclic strabismus in adults included Graves disease, craniofacial surgery for fronto-orbital fibrous dysplasia, traumatic cataract with aphakia, retinitis pigmentosa, and glaucoma. All these adults had a 48-hour cyclic pattern to the deviation.

A literature review of cyclic strabismus in adults reported 24 cases, including two personal communications to the authors. Of the 24 cases, 6 included a divergent deviation, with 5 of the 6 cases also having a vertical misalignment. Only one case was reported as solely divergent. Five of these 6 cases required surgery, with only one requiring a correction for the divergent deviation which was combined with surgery for the coexistent vertical squint.

Micro-Exotropia

Confusingly, microtropia has had at least 10 different names in the last 50 years including esophoria with retinal slip, esophoria with fixation disparity, strabismus spurius, microtropia unilateralis anamalo-fusionalis, flick (or flicker) eso, micro strabismus, monofixation syndrome, small angle squint, microtropia, and manifest mini strabismus with harmonious abnormal retinal correspondence. These descriptions all essentially refer to the same condition, with the more recent literature usually referring to microtropia.

The condition consists of straight or nearly straight eyes with a deviation of less than 10 PD, usually unequal visual acuity, foveal suppression of the affected eye and abnormal binocular single vision. There is peripheral fusion with stereopsis.

Optimal binocular single vision (BSV) exists with bifoveal fusion. If both foveas are not used simultaneously then there is reduced binocularity. In microtropia there is subnormal BSV with foveal (central) suppression in the non-preferred eye and reduced 3D vision. Motor fusion is present but the range may be reduced. Microtropia can be primary or secondary, occurring after surgical or optical treatment for a larger-angle squint or with ocular pathology. It is commonly associated with anisometropia and amblyopia in the deviating eye. Microtropia can be present with other strabismus, for example, a fully accommodative microtropia.

Microtropia often represents a good outcome after strabismus surgery, as it is a small angle deviation with good cosmetic appearance and often some binocularity. A microtropia is often considered to be a stable position but it is not always a long-term stable alignment and can decompensate, with the patient developing diplopia. The strabismus expert needs to be aware of this diagnostic possibility when a patient presents with new onset diplopia.

Microtropia is an uncommon squint and micro-exotropia even more so. Lang reported it to occur in 2.84% of 33,600 patients, most commonly as a microesotropia. In another report of 26,762 patients he identified only 19 (0.07%) as having micro-exotropia. A similar figure (0.089%) was reported in a study of Chinese preschool children which examined 5,667 children and identified only 5 micro-exotropes. The sensory aspects of micro-exotropia are similar to those of micro-esotropia, which is discussed in detail in Chapter 79 . The ophthalmologist should remember that a 4 PD base-in test may be needed to diagnose a micro-exotropia rather than the standard 4 PD base-out test.