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Secondary & Consecutive Exotropias

What are secondary and consecutive exotropias? How do we assess patients before surgery?

Video Transcript:

This is a type of divergent squint that has arisen due to poor vision in one eye. There are many different causes of poor vision including problems with the retina or optic nerve. Adults who lose vision in one eye tend to develop a divergent squint rather than a convergent one, whereas the opposite is true of children.

If there is no treatment that is likely to improve the sight in the eye squint surgery or botox injections to the inner muscle of the eye, can be carried out to improve reduce the size of the divergent squint. Unfortunately patients with poor vision are much more likely to suffer a recurrence of their squint following surgery and their eye may drift outwards again. This is why many ophthalmologists prefer to treat this type of squint with repeat botox injections.

Consecutive Exotropias

This refers to a divergent squint that has occurred following surgery for an esotropia. The most common scenario is an adult who had surgery as a child for an infantile or partially accommodative esotropia, whose eyes were straight for many years, but who in later life developed goes on to develop a divergent squint. This is more likely to occur if they have no stereopsis and/or an amblyopic eye. Consecutive exotropias are much more common than consecutive esotropias.

Assessment

Most patients who have developed a consecutive exotropia have no stereopsis and do not experience double vision, because they still have the ability to switch off (suppress) the image from their divergent eye. However, when the eye is returned to a straight position some patients are no longer able to suppress the image from this eye and they develop double vision. By carrying out orthoptic tests using prisms it is usually possible to predict if the squint can be fully corrected without causing double vision. However, if there is any doubt a botox injection into the outer muscle of the divergent eye can be used to temporarily straighten the eyes. When the eyes are straight we can tell whether double vision is actually going to be a problem for the patient. Fortunately in the vast majority of cases botox treatment shows that the risk of double vision is low and we can proceed with surgery.

How are the eyes straightened?

Surgery

Surgery is normally carried out using an adjustable suture technique to give the surgeon the opportunity to “fine tune” the position of the eyes. In most cases we aim to leave the eye slightly turned inwards as all eyes will have a tendency to drift outwards following surgery.

As the majority of patients with a consecutive exotropia have no stereopsis (and / or poor vision in one eye) they have no mechanism to keep their eyes in this good position and it is not uncommon for the eye to wander back out again after surgery. It is difficult to predict how long the eyes will stay well aligned for, but 10 years would normally be deemed to be a good result. If the eye was to diverge again, further surgery may be possible, but often this would need to be carried out on the opposite eye. It is a rather strange to think that surgery on the “good” eye can straighten the divergent or convergent eye, but because of the way the eye muscles are “wired” together this is in fact what happens.

Botulinum toxin

In patients who do not want to undergo further surgery or where further surgery is not possible (usually because the muscles of both eyes have been moved as much as they can be) repeat botox injections, carried out every 3-6 months, can help keep the eyes in a straight position.