Transactions 29th European Strabismological Association Meeting – de Faber (ed) © 2005 European Strabismological Association, ISBN 04 1537 211 9

When should the amount of surgery be adjusted during conventional muscle surgery?

A. Roth

Medical School, University of Geneva, Switzerland

C. Speeg-Schatz

Eye Department, Medical School, University of Strasbourg, France

ABSTRACT: In which cases of concomitant strabismus is intra-operative adjustment of the amount of surgery justified? Patients and method: The study involved 80 consecutive cases of infantile esotropia and 44 of intermittent exotropia (124 cases). The angle to be corrected was measured in the awake state. Intra-operative data were recorded during surgery. Results: Results are presented for 86 patients having follow-up of at least two and one years respectively. In 42 patients, surgery was carried out with no allowance for intra-operative data. In 44 patients it was adjusted, or not, according to these latter. Discussion: Our results suggest that the variance of the results is reduced by intraoperative adjustment if the angle under anaesthesia and the extensibility differential in an antagonistic muscle pair indicate a significant imbalance between the passive muscle forces. Conclusion: Drawn from our results, guide lines for intra-operative adjustment are proposed.

The present study is the continuation of a previous one on the parameters to calculate the amount of conventional eye muscle surgery (ESA, 2003 [3]). We concluded that intra-operative data are useful additional parameters that, when taken into account, significantly reduce the variance of results. The aim of the new study is to demonstrate which cases of concomitant strabismus require intra-operative adjustment and by how much.

The squint angle is variable in concomitant strabismus. The horizontal angle A, at a given moment i, can be given according to the following general descriptive formula [4], which sums up the implied factors:

(1)

Dv excess or insufficiency of tonic vergence; Dan anatomized part of the deviation, resulting from lasting Dv; Σi sum of the variability factors; Mc compensatory movements according to Bielschowsky, reducing the angle; Mh and Mac convergence excess independent and dependent of accommodation, increasing the angle. Dv and Dan represent the components of the basic angle, Mc, Mh and Mac the factors responsible for the variability.

The aim of conventional surgery is to correct the basic angle of deviation [1, 2]. This angle, however, differs according to the type of concomitant strabismus. This can be illustrated with the formula (1) as shown by the two opposite types of strabismus set out in this study, infantile esotropia and normosensorial intermittent exotropia with normal AC/A ratio.

1METHOD AND PATIENTS

In infantile esotropia compensatory movements are insignificant: the basic angle corresponds to the minimum angle and any variability results only from convergence excess, whether

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accommodation-dependent or not. Thus the formula (1) can be simplified to:

(2)

The series of 80 children who underwent surgery for infantile esotropia from 1994 to 1998, with ages ranging from 11 to 100 months (median 44 months) and studied a year ago [3] are reconsidered here. Bilateral medial rectus posterior fixation sutures combined with uni or bilateral recession were carried out in all cases. The global amount of recession was calculated on the basis of the minimum angle in the waking state. 51 of the 80 children have been followed for over two to seven years.

In normosensorial intermittent exotropia with normal AC/A ratio the compensatory movements according to Bielschowsky are powerful enough to compensate intermittently for the deviation. The amount of convergence excess, however, is insignificant. The basic angle corresponds to the maximum angle or, more precisely, to the prism power allowing optimal compensation of the deviation at distance. The variability results only from compensatory movements. The formula (1) can therefore be simplified to:

(3)

A series of 44 children who, during the same period, underwent a first surgery for normosensorial intermittent exotropia, with ages ranging from 3 to 12 years (median 5 years) is compared with the previous series. Intra-operative data were recorded routinely in the same way as for infantile esotropia. Combined unilateral surgery, i.e. recession of the lateral rectus and plication of the medial rectus, was carried out in all cases. The global amount of surgery was calculated on the basis of the optimal prismatic correction of the deviation. 35 of the 44 children have been followed over one to nine years.

Of the total 86 patients from both series, in 44 cases (28 infantile esotropia and 16 intermittent exotropia) the amount of surgery was carried out making allowance for the imbalance between passive muscle forces as assessed by the position of the eye under anaesthesia and the extensibility differential in the lateral and medial antagonistic muscle pair; adjustment was made whenever these data indicated a significant imbalance between the passive muscle forces.

2RESULTS

2.1Intra-operative data

In the previous study on infantile esotropia we noted a significant correlation between the angle under anaesthesia and the sum of the extensibility differential of the two eyes. Here, we consider the position under anaesthesia and the extensibility differential of each eye separately. The correlation is similar to the previous one with r2 0.32 (2 outlayers – 4 eyes – being deleted) [3].

In the series with normosensorial intermittent exotropia, data were recorded on the operated eye only. The correlation between the position under anaesthesia and the extensibility differential was r2 0.23 (2 outlayers being deleted).

The plot of the two series on a same graph shows that their distributions are in continuity and both regression lines (polynomial quadratic order) in extension. The regression of both series considered together is r2 0.45 (Fig. 1). This correlation demonstrates that the principle of intraoperative adjustment is valid for all types of concomitant strabismus.

2.2From application to postoperative results

We consider stable microtropia (between –4° and 4°) in infantile esotropia and a controlled exophoria or esophoria in intermittent exotropia to be a good result (it being well known that 50% of intermittent exotropia require a second surgery one or more years later).

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LR MR extensibility differential vs eye position under anaesthesia

Eye position under anaesthesia

(filled circles : infantile esotropia, open circles : intermittent exotropia)

Figure 1. Correlation between the differential of extensibility between lateral and medial rectus (LR – MR) and the position under anaesthesia of each eye (polynomial 2nd order regression)

(a)infantile estropia (filled circles), N 156 eyes (4 out-layers being deleted): r2 0.32

(b)normosensorial intermitent exotropia (open circles), N 42 eyes (2 outlayers being deleted): r2 0.23

(c)all data: r2 0.45.

Table I. Comparison of the results obtained in infantile esotropia depending on whether the adjustment or absence of adjustment was appropriate or not to the (im)balance of the passive muscle forces as shown by the intra-operative data: (a) in infantile esotropia; (b) in intermittent exotropia.

The results varied according to whether or not the adjustment, or absence of adjustment, was appropriate to the intra-operative data. Both series showed proportionally better results when adjustment or no adjustment was appropriate compared to when it was not (Table Ia and Ib): 86% versus 70% in infantile esotropia and 63% versus 26% in intermittent exotropia.

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Table II. Guidelines for intra-operative adjustment of the amount of surgery in conventional surgery. MED: Muscle extensibility differential (lateral rectus extensibility minus medial rectus extensibility).

3DISCUSSION

Conventional muscle surgery pertains to parametric calculation for which the angle measured in the awake state represents the parameter, the amount of surgery the variable and the result the probability of effect [2]. The angle under anaesthesia and the differential of extensibility in an antagonistic muscle pair represent valuable additional, partially independent, parameters, provided their assessment was precise [3]. Our results show that the variance is reduced by an intraoperative adjustment in the cases in which intra-operative data indicate a significant imbalance between the passive forces. The number of cases of our series, however, is too small and the confidence interval remains too large to be certain that the differences between the two subgroups, with or without an appropriate adjustment, are significant or not. This cannot, however, be rejected out of hand. The fact that a similar difference was found in both series is an additional argument in favor of the significance of our results.

Guidelines for the amount of adjustment to be carried out according to intra-operative data are drawn from our results and summed up in Table II.

4CONCLUSION

Our present study provides further supportive evidence to intra-operative adjustment in conventional surgery, this combined or not with posterior fixation sutures. An adjustment is justified whenever a significant imbalance between the passive muscle forces is assessed.

REFERENCES

1.KAUFMANN H. 2004. – Strabismus. Thieme, Stuttgart, 3rd ed.

2.ROTH A. 2004. – Which angle for which surgical strategy in comitant strabismus. The Pratt-Johnson annual Lecture. Amer. Orthop. J., 53: 75–87.

3.ROTH A. 2004 – Parameters to calculate the amount of conventional eye muscle surgery. In: de FABER J.- T. (ed.): Trans. 28th ESA Meeting 2003, Taylor & Francis, London, pp.179–183.

4.ROTH A., SPEEG-SCHATZ C. 2001. – Eye muscle surgery. Swets & Zeitlinger, publ., Lisse, NL.

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