Complications in strabismus surgery

Introduction

Strabismus surgery is generally considered safe. It is, after all, a superficial, peribulbar procedure, involving surgery to the conjunctiva, Tenon’s capsule, extraocular muscle tendons, and anterior sclera. It thus avoids the inherent risks of both intraocular surgery and retrobulbar orbital surgery. Nevertheless, complications can occasionally occur and their recognition is important from the perspectives of both surgical training and patient consent.

Definitions

At first sight, one would not think it difficult to define what counts as a complication of strabismus surgery. A starting definition may be: any departure from the normal peroperative or postoperative course, which leads to an adverse outcome for the patient. There are some immediate problems with this, however, some of them specific to strabismus. What is the adverse outcome we are talking about? Reduced visual acuity is a certain complication. But what about double vision? Leaving a binocular patient with constant double vision after surgery is definitely a complication, but what about a small amount of double vision on extreme lateral gaze (something not at all uncommon), which causes the patient no problems in daily life? What about the case where a patient’s pre-existing diplopia is improved in 80% of the field of vision, but worsened in 20%?

Then there is the problem of angular outcomes. Many surgeons consider the goal of surgery is to get the angle to some arbitrary bracket around orthophoria, e.g. within 10 prism diopters of straight. They call this a success, and call outcomes outside of the bracket a failure. But is that “failure” also a complication? Or is it just part of the inherent inaccuracy of strabismus surgery? What if the angle is technically a failure but the patient is delighted? It has been shown that there is very little correlation between patient satisfaction and angular outcome in adult strabismus surgery. One may try to circumvent this issue by considering instead rates of reoperation – the frequency of cases whose angular, or other, outcome was poor enough that they required further surgery. However, that brings up a new problem of timing. At what time point is the need for further surgery considered a complication. Going back to theater the next day for a consecutive exotropia caused by a slipped muscle is clearly a complication. Going back a year later for a consecutive exotropia caused by a stretched scar is an event most people would not want counted as a complication of the first surgery – but in reality, is it not?

Furthermore, there is the question of when does normal postoperative healing turn into a complication? All strabismus surgery results in some scarring. Often it is hardly visible, but occasionally it can be unsightly or irritating to the patient. How much redness or elevation in a scar counts as a complication. Again, we have the problem of where the cut-off lies.

These are just some of the issues of defining what a complication of strabismus surgery is. Ultimately, it will be arbitrary what we include as a complication and what we exclude. What we can do, at least, is to group complications according to severity.

Classification and Grading of Complications

Serious complications of strabismus surgery can be categorized as either sight-threatening (e.g. globe perforation, orbital infection, scleritis) or ocular-motility related (e.g. slipped muscle, lost muscle). Both of these categories are clearly important, as is whether any further unintended intervention after the primary surgery was required. The first author has published a classification system which combines these three elements, which we have simplified further in Table 89.1 . By shifting the discussion away from definitions of complication to generic outcomes, many of the issues discussed above have been circumvented. Six months was chosen as the time point for inclusion. In this paper, a severe complication was defined as one potentially associated with reduced acuity or intractable double vision. In reality, though, many such complications (e.g. globe perforation), while having the potential to cause problems to the patient, very rarely actually do. What matters most then, for audit, reporting and consent, is a significant complication, which would be defined as: a complication that leads to a poor visual outcome (acuity or double vision) at 6 months. Severe complication rate must in some way reflect surgical skill; significant complication rate reflects also how much the surgeon got away with it!

Incidence

The incidence of complications in strabismus surgery has been studied in two large studies, which though very different in their methodologies, led to remarkably similar findings. The first (the BOSU study) was a nationwide, 2-year prospective survey of severe complications of strabismus surgery carried out by the first author through the British Ophthalmic Surveillance Unit. The second (the Moorfields study) was a 5-year prospective audit by the second author of 4000 consecutive strabismus operations in a single large teaching hospital. The incidences of complications found by these two studies are shown in Table 89.2 . The key message from these two incidence studies is that for strabismus surgery, there is about a 1 in 400 risk of severe complications (including globe perforation, cellulitis, lost/snapped/slipped muscle); but less than a 1 in 2000 risk of a significant complication (meaning loss of visual acuity or unexpected double vision in the primary position).

Severe Complications