14 Accommodative esotropia

Background

Definition

Accommodative esotropia is a type of convergent strabismus that is thought to arise because of inappropriate convergence due to increased accommodative effort.1 This can occur either because excessive accommodation is required to overcome a hypermetropic refractive error, or because of inappropriate gearing between accommodation and convergence (High AC/A ratio). The diagnosis is made on the basis of the response to the correction of significant hypermetropic refractive errors or the prescription of additional plus lenses for near fixation.

Three forms are recognised:

1Refractive – esotropia eliminated by correction of hypermetropic refractive error

2Non-refractive – esotropia present at near only despite correction of any hypermetropic refractive error but eliminated by plus lenses at near

3Mixed – esotropia at distance eliminated by hypermetropic correction but requires plus lenses to eliminate esotropia at near fixation.

When the deviation responds completely to correction with lenses it is termed fully accommodative. Where the deviation reduces in size but is not completely eliminated by refractive correction, it is termed partially accommodative.

Incidence/prevalence

Although accommodative esotropia is occasionally seen in infancy, the condition most commonly presents in the third or fourth year of life when a convergent strabismus is seen, particularly for near fixation. There is frequently a family history of a similar deviation. Graham estimated the prevalence per thousand of fully and partially accommodative strabismus at 6·7 and 15·3 respectively.2

Aetiology

The cause of fully accommodative refractive esotropia is a hypermetropic refractive error, which is usually bilateral and reasonably symmetrical. Not all significant bilateral,

hypermetropic refractive errors lead to an esotropia; in some cases bilateral amblyopia occurs if accommodation is not used to overcome the error. Very asymmetrical or unilateral refractive errors usually lead to unilateral amblyopia.

Non-refractive accommodative esotropia is caused by a high ratio of accommodative convergence: accommodation (AC/A). Little is known about the constitutional factors associated with the range of AC/A ratio or about whether it changes over time.

Prognosis

Fully accommodative esotropia, by definition, responds completely to refractive correction or the use of plus lenses at near sight. Normal binocular single vision is demonstrable when this is in place. In some cases, it is possible to reduce the strength of the lenses over time until the child is able to control the strabismus without refractive correction.

Partially accommodative esotropia does not respond fully to refractive correction or plus lenses at near. Plus lenses at near are not indicated unless there is a functional benefit from their use (which is not the case in partially accommodative esotropia). Surgery is required if it is desired to reduce the angle of strabismus further. Even following surgical correction in addition to refractive correction, perfect ocular alignment and good responses to tests of stereoscopic vision are not usually achieved.

The issues

Given the relationship between hypermetropic refractive errors and accommodative esotropia, the possibility exists that early detection and prophylactic treatment of refractive error might reduce the incidence of this type of strabismus. Similarly, early treatment of fully accommodative esotropia might prevent the deterioration of this type of strabismus into a partially accommodative esotropia with a worse prognosis for binocular single vision. Surgical treatment of fully accommodative esotropia has been proposed as an alternative to refractive correction, and the possibility now exists for surgical correction of refractive error as an alternative to spectacle or contact lens correction. Finally, where surgical treatment is undertaken for partially accommodative esotropia, preoperative adaptation with

prisms has been proposed as a useful manoeuvre to demonstrate the full extent of the strabismus and avoid surgical undercorrection.

Question

Does early correction of hypermetropic refractive errors lead to a reduction in the incidence of accommodative esotropia?

The evidence

This question has been examined by Ingram et al. in a series of papers. In the first,3 306 children aged one year with bilateral spherical hypermetropia of +2·00 DS dioptre sphere or more, or with anisometropia of +1·00 DS or cylinder were randomly allocated to spectacle correction or no treatment and followed until three and a half years of age. Treated children were divided into two groups – compliers and non-compliers with spectacle wear. It is not clear how this distinction was made. Although the incidence of squint at three and a half years was less in the treated group (19/129 compared to 28/136 in non-treated group) this was not statistically significant (P = 0·28) Compliers and non-compliers in the treatment group were not analysed separately.

In a second paper,4 372 children aged six months with +4D of hypermetropia in any meridian were randomised to spectacle correction or no treatment. There was no difference in the incidence of strabismus in the 285 children followed to three and a half years. Further analysis of these children5 showed that strabismus was found more commonly in those children whose refractive error remained at or above +3·50 DS. Those children who consistently wore spectacle correction were more likely to remain significantly hypermetropic.

Comment

Despite these findings, it is widely held that early correction of refractive error is important in preventing strabismus6 and this belief is fundamental to the use of techniques such as photoscreening in preliterate children.7 This view is supported by the work of Atkinson et al.8 This study had two phases of recruitment. In the first, every infant living in the city of Cambridge was offered an examination at the age of six to eight months. Cycloplegic photorefraction was undertaken on the 3166 infants (74% of those offered appointments) who attended. In the second phase, all children born in a two year period were offered screening at around eight months of age. Appointments were sent to 5923 and 5091 attended. Non-cycloplegic videorefraction was performed. A control group was selected

randomly from the same clinic as children meeting the refractive criteria for follow up.

Children who were hypermetropic in infancy but who did not comply with spectacle wear were 13 times more likely to develop strabismus than controls, compared to a

four-fold increase in risk in children who were compliant with spectacle wear compared to controls (P <0·05).

The results of Atkinson et al. and Ingram et al. are contradictory with regard to the benefit of prophylactic spectacle wear on the incidence of strabismus. Atkinson et al. speculate that this is due to differences in the protocol for spectacle prescription and in the degree of compliance. Atkinson et al. do not specify their method of randomisation and seem to have excluded children who were not compliant with spectacle wear (as judged by questioning whether the child wore spectacles 50% of the time) from the analysis although this is not explicitly stated. Both studies agree that the presence of a high refractive error in infancy is predictive of the later development of strabismus and amblyopia.

Question

Is surgical treatment an alternative to refractive correction in the treatment of accommodative esotropia?

The evidence

No trials were found comparing the two approaches.

Comment

Surgical treatment of fully accommodative strabismus has been advocated in the European literature, particularly by Gobin,9,10 but this approach has not found favour in the English-speaking world because of concern about the possibility of late overcorrection and diplopia following surgery without refractive correction.

Question

Does uncorrected fully accommodative esotropia become partially accommodative?

The evidence

No trials were found that address this issue.

Comment

There is a widespread clinical belief that neglected fully accommodative esotropia develops a non-accommodative component with a poorer sensory outcome after treatment.1

Question

Is preoperative prism adaptation necessary to determine the amount of surgery required to correct partially accommodative esotropia?

The evidence

The surgical treatment of acquired esotropia frequently results in undercorrection of the deviation. The Prism Adaptation Study11 (PAS) was a randomised multi-centre clinical trial of patients with acquired esotropia who had a residual angle of 12 prism dioptres or greater with refractive correction. The PAS considered whether the preoperative use of prisms helped to determine more accurately the target angle for surgical correction, so reducing the incidence of undercorrections.

Of the 3574 patients screened, 333 were randomised. One of the exclusion criteria was the presence of a strabismus at near greater than 10 D larger than at distance. This would exclude many patients with partially accommodative esotropia.

One hundred and ninety-nine patients were randomised to preoperative treatment with prisms and 134 underwent surgery for the amount of strabismus present at recruitment. The amount of prism required to neutralise the strabismus was placed on spectacles and the patient was re-examined at weekly intervals with additional prism correction being applied until there was evidence of a fusion response (in which case the patient was designated a prism responder) or the deviation stabilised or was overcorrected without evidence of fusion (prism non-responder). Prism nonresponders also underwent surgery for the amount of strabismus measured at recruitment. Prism responders were then randomised either to surgery for the entry angle or surgery based on the prism-adapted angle. Surgery was standardised and checked photographically. The primary end-point was the distance deviation measured by a masked observer six months after surgery. A successful outcome was defined as 8 D or less of esotropia or exotropia.

The overall success rate among patients who underwent prism adaptation was 83% compared to 72% among those patients who did not undergo prism adaptation (P = 0·04). Those patients who showed a prism response had a motor alignment success of 89% compared to 79% for prism responders operated on for their entry angle (P = 0·23). The observed benefits were concentrated mainly in the patients who built up to larger angles in the prism adaptation process and underwent surgery for their adapted angles.

Comment

Although prism adaptation is widely used for accommodative esotropia, the exclusion of patients with a

near angle more than 10 D than the distance angle means that most cases of accommodative esotropia were excluded from the analysis and so the benefits of prism adaptation for this class of patients remains uncertain.

Question

Is surgical treatment of refractive error an alternative to conventional refractive correction in accommodative esotropia?

The evidence

Refractive surgery has been used to correct the hypermetropic refractive error of some patients with accommodative esotropia,12 but there are no randomised trials of the outcome of this approach.

Implications for practice

There is a lack of agreement about the benefits of prophylactic refractive correction with regard to the subsequent development of accommodative esotropia. The amount of refractive error that requires prophylactic correction in this age group (to benefit both esotropia and amblyopia) will continue to be based on eminent opinion and professional consensus. There is little evidence to support surgical treatment for fully accommodative esotropia. While there is evidence that there is a marginal benefit of prism adaptation on a subclass of esotropia, the benefits of prism adaptation for accommodative esotropia are not proven. Refractive surgery, particularly for hypermetropia, remains an experimental procedure and its use in children with accommodative esotropia is ethically dubious.

Implications for research

Long-term, well-designed studies are required to provide guidance about prophylactic correction of refractive error in the paediatric age group. Such studies would help guide the use of photoscreening and other screening techniques.

References

1.Lambert SR. Accommodative esotropia. Ophthalmol Clin North Am 2001;14:425–31.

2.Graham PA. Epidemiology of strabismus. Br J Ophthalmol 1974;58: 224–31.

3.Ingram R, Walker C, Wilson J, Arnold P, Lucas J, Dally S. A first attempt to prevent amblyopia and squint by spectacle correction of abnormal refractions from the age of 1 year. Br J Ophthalmol 1985;69:851–3.

4.Ingram R, Arnold P, Dally S, Lucas J. Results of a randomised trial of treating abnormal hypermetropia from the age of 6 months. Br J Ophthalmol 1990;74:158–9.

5.Ingram R, Arnold P, Dally S, Lucas J. Emmetropisation, squint and reduced visual acuity after treatment. Br J Ophthalmol 1991;75: 414–16.

6.Kvarnstrom G, Jakobsson P, Lennerstrand G. Visual screening of Swedish children: an ophthalmological evaluation. Acta Ophthalmol Scand 2001;79:240–4.

7.Simons K. Photoscreening. Ophthalmology 2000;107:1619–20.

8.Atkinson J, Braddick O, Robier B et al. Two infant vision screening programmes: prediction and prevention of strabismus and amblyopia from photoand videorefractive screening. Eye 1996;10:189–98.

9.Gobin M. Strabismes accommodatifs, peut-on les operer? [Should accommodative strabismus be operated on?] J Francais d’Ophthalmol 1992;15:483–91.

10.Gobin MH. Surgery for fully accommodative esotropia. Binocular Vis Q 2001;16:80–2.

11.Group PARS. Efficacy of prism adaptation in the surgical management of acquired esotropia. Arch Ophthalmol 1990;108:1248–56.

12.Stidham DB, Borissova O, Borrisov V, Prager TC. Effect of hyperopic laser in situ keratomileusis on ocular alignment and stereopsis in patients with accommodative esotropia. Ophthalmology 2002;109: 1148–53.