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Editor,—The development of spontaneous consecutive exotropia is not uncommon in patients with esotropia.1They characteristically have an early onset partially accommodative esotropia associated with poor binocular function including weak motor fusion, which is thought to prevent stable eye alignment. We report the findings in five patients with partially accommodative esotropia who developed spontaneous consecutive exotropia despite having adequate motor fusion.
We reviewed the case records of five children with partially accommodative esotropia who attended the orthoptic clinic within the department of ophthalmology and were noted to have developed spontaneous consecutive exotropia despite having motor fusion. Each child had undergone cycloplegic refractions, with full hypermetropic correction prescribed. A full orthoptic and ophthalmic examination had also been performed. None had any significant medical history of note.
The findings are summarised in Table 1. The mean age of presentation of the esotropia was 2½ years (range 1½–3¾), with the mean size of the esodeviation being 33 prism dioptres for near (range 15–50) without spectacle correction. Accurate data were not consistently available for the distance deviation at this time owing to the young age of the children. The mean refractive error (spherical equivalent) was +6.6 dioptres for the right eye (range +4.00 to +8.75) and +6.8 dioptres for the left eye (range +4.75 to +10.75). Patients 1 and 2 had anisometropia of 2.50 DS and 2 DS respectively. The remaining patients had no significant anisometropia. At initial presentation all the children had a degree of amblyopia, which improved with appropriate occlusion therapy and spectacle wear. At subsequent visits abnormal binocular single vision was established with the Bagolini lorgnette, although none demonstrated stereopsis, with no response being elicited for either the Frisby or Titmus (housefly) stereotests. The presence of motor fusion was confirmed by the ability to overcome a 20 dioptre base out prism. This is a routinely used test for preschool visual screening,2-4 with the failure to overcome the prism being interpreted as evidence of poor or absent motor fusion. At the clinic visit immediately before the onset of the exodeviation all had visual acuities of 6/12 or better, with the mean size of the esodeviation with spectacle correction being 6 prism dioptres for near (range 0–16) and 5 prism dioptres for distance (range 0–16). The mean size of the subsequent exodeviation with spectacle correction was 8 prism dioptres for near (range 2–16) and 15 prism dioptres for distance (range 4–20). The mean interval between initial diagnosis and appearance of exotropia was 2 years (range 1¼–3) with the mean age at onset of the exodeviation being 4½ years (range 3–6). Cycloplegic refraction was rechecked following the onset of the exotropia and found to be unchanged. Visual acuities were also unchanged.
The consecutive exotropia was managed in all patients by decreasing the strength of the hypermetropic prescription by a mean of 1.00 DS (range 0.50–1.50). This resulted in three children developing a microexotropia and two children developing a microesotropia during the follow up period (mean 14 months, range 3 months to 3 years).
This study demonstrates that spontaneous consecutive exotropia can develop in certain patients with accommodative esotropia despite the presence of motor fusion. Moore5 reported four patients with evidence of peripheral fusional amplitudes who developed consecutive exotropia, although the actual extent of their fusional reserve was not recorded. It has been suggested that full correction of a significant hypermetropic refractive error in children with early onset esotropia predisposes to the development of spontaneous exotropia in some cases.67 The degree of hypermetropia in our cases is similar to that of patients reported in earlier studies,5-7 and it may well be that full correction of this amount of hypermetropia is the cause of the exodeviation. Just before the onset of the exodeviation all of our patients had acuities of 6/12 or better in the worse eye, with one subject seeing 6/6 with each eye. This is in keeping with the findings of Beneishet al7 and Moore,5who concluded that amblyopia was not an important factor in the development of spontaneous consecutive exotropia. The time interval between initial diagnosis and development of exotropia in the present study is similar to that of Beneish et al7 although our patients were slightly younger (4½ as opposed to 51/3). However, the children are much younger than those described by Moore5who reported an average age of onset of 9.
Why should the eyes of patients with motor fusion diverge? Although these patients had “risk factors” for developing consecutive exotropia and could be expected to show a reduction in the angle of esotropia with the passage of time, the presence of satisfactory motor fusion, as judged by the 20 dioptre base out prism test, should have prevented them diverging. Ciner and Herzberg8 suggested that spontaneous consecutive exotropia is the result of poor accommodative function, while van Lammeren et al9 proposed that it is related to anomalies of the vergence system. In addition Burian6 found that a low AC:A ratio was a risk factor for the development of a consecutive exotropia but unfortunately this ratio was not formally recorded in our subjects.
Our patients were managed by reducing the strength of the hypermetropic prescription, which was effective in all cases. This might suggest that they were all overcorrected to begin with. However, repeated refraction by optometrists experienced in the assessment of paediatric patients confirmed that their initial refractions were accurate. Interestingly, previous reports document a variable response to reducing the hypermetropic prescription in the treatment of spontaneous consecutive exotropia. For example Swan10 reported that it was of little benefit, with all six of his patients subsequently requiring surgery, while Beneish et al7found that eight out of 22 patients improved significantly following a reduction in their hypermetropic prescription. It is conceivable that such a response is only observed in patients with underlying motor fusion.
When motor fusion is present there may a tendency not to think about the possibility of a consecutive exodeviation developing in children with esotropia but as this study illustrates this is not necessarily the case.