27 March 2017

Seeing eye to eye: treatment options for strabismus

Clinical Ophthalmology

Case study: A 13-year-old boy presents for a second opinion, with a three-year history of an esotropia, (eye turned in) which started after a viral infection when he was aged 10. Although his visual acuity was normal in both eyes, his mother was concerned as her son had become depressed about his condition and that he had self-harmed. 

Initially, he had been deemed unsuitable for surgery by his original ophthalmologist. This time, the boy was treated with a single dose of Botulinum toxin injected directly into the left medial rectus muscle, which was administered in the office under topical anaesthesia. 

The procedure was successful and ocular alignment has been maintained long after the Botulinum toxin’s effects have worn off.  The impact on his life was significant, and, four years later, he has achieved highly at school and has regained self-confidence.

What is strabismus?

Strabismus describes the condition where a patient’s eyes do not simultaneously fix on a target of interest, or where there is misalignment of the eyes. It is often referred to as a squint or being “cross-eyed”.

Strabismus affects 2.5% to 4% of the population and can be very obvious or subtle, intermittent or constant, may turn in (esotropia) or turn out (exotropia), be vertically misaligned or a combination of these. Visual impairment related to the condtion occurs in 6.4% of Australian preschool children, with 2.7% affected in both eyes.

Strabismus may present in childhood or adulthood, though approximately half of those adults have had it since childhood. Risk factors in children for esotropia and exotropia include maternal smoking during pregnancy and gestational age less than 33 weeks.

Significant refractive error is also predictive of esotropia and exotropia in infants and children. A family history of strabismus is also a risk factor. Any other disease that causes vision loss may also cause strabismus, though in most children with strabismus, the cause is unknown and in more than half of these cases, the problem is present at or shortly after birth, which is called congenital strabismus. More rarely, strabismus can be associated with other disorders (See table page 33).

As strabismus may be an early sign of an underlying systemic pathology or of an underlying neurological pathology such as a posterior fossa tumour, strabismus of an acute onset in childhood requires urgent referral to a paediatric ophthalmologist.

Risk factors in children for esotropia and exotropia include maternal smoking during pregnancy and gestational age less than 33 weeks.

In patients with strabismus, the six muscles of the eye do not work together in a coordinated fashion. Most of the time, the problem has to do with muscle control, and not with muscle strength. As a result, one eye looks at one object, while the other eye turns in a different direction to focus on another object.

When this occurs, two different images are sent to the brain, i.e. one from each eye. This confuses the brain. In children, the brain may learn to ignore (suppress) the image from the weaker eye.

If the strabismus is not treated, the eye that the brain ignores will never see well. This loss of vision is called amblyopia, sometimes known as “lazy eye.” Sometimes amblyopia precedes and causes strabismus. If amblyopia is not treated by about age 11, it is likely to be permanent.

Why treat it?

In childhood, the aim of treatment is to ensure:

• that the child develops normal vision;

• that the child develops depth perception if the child has this potential – the earlier these children are referred to a paediatric ophthalmologist the better the chance;

• to help maintain ocular alignment through life in order to optimise quality of life.

Humans assess the facial symmetry of a person within 150 milliseconds of meeting them and the fundamentals of attractiveness are based on symmetry. It is possible that at a basic genetic level, we are pre-programmed to avoid those with “faulty genes” in play, from an evolutionary perspective.

This may help to explain why patients with strabismus are often socially isolated. Most adults with strabismus report adverse effects of their eye misalignment on their quality of life, including subjective visual performance, well-being, and self-image.

Many patients are so embarrassed by their appearance that they report difficulty with eye contact. The psychological status of patients with strabismus has been shown to be between normal individuals and those with depression and anxiety.

Beyond the impaired self-image of the adult strabismic patient is the poor view that others have of an adult with eye misalignment. Adults with strabismus are viewed as less intelligent, less attractive, and less able to perform in leadership positions, while over 90% of dating agencies found adults with strabismus are less likely to successfully find a partner.  In one study, it was found that esotropic patients were less likely to be offered a job than otherwise equal orthotropic patients.

From a patient perspective, the effect of the strabismus on their quality of life score is similar to patients who have bilateral macular degeneration, or a patient who has had a stroke and most adults with strabismus would willingly trade some of their life expectancy to be free from the condition.

For these patients, isolation and discrimination starts in childhood, with children as young as five being excluded from birthday parties or being the last to be picked for sporting events. Hence strabismus is associated with an increased incidence of psychological disorders, including depression.

This has led to the consensus by ophthalmologists that treatment for this group should not be viewed as “cosmetic”, with the associated tone of strabismus being a superficial problem, where in reality it is a significant disease process.

Treatment options

Before treatment for the strabismus can be considered, it is important to assess the patient and perform appropriate investigations to determine, if possible, the underlying cause.  Symptomatic relief is commenced in patients with diplopia, particularly if it occurs when the patient is looking straight ahead or when reading.

This may initially involve occluding an eye to alleviate the double vision, or using a temporary prism such as a stick-on Fresnel prism. Once underlying pathology has been excluded, further treatment for any persistent diplopia may be considered.

The usual options include continuing with the temporary treatments, prisms being ground into glasses where the temporary Fresnel prisms alleviated the patient’s symptoms, Botulinum toxin A intramuscular injections or surgical correction.

Botulinum toxin A

Botulinum toxin A for the treatment of strabismus was first described in a case series published in 1981, which was the first time it was used as a medical treatment. When indicated, the toxin is injected into the rectus muscle under local anaesthetic with electromyographic guidance.

It is two to three days before any effect is noticed and it can take about two weeks to reach full effect. Complications are uncommon and when they occur are usually temporary. They include under or overcorrection of the strabismus, diplopia, ptosis, haematoma or infection.

A very rare complication is loss of vision, which may be permanent. Botulinum toxin A is a temporary treatment that on average lasts four months, and can be used to treat small to large angle squints.

In some patients, Botulinum toxin A is given once to reduce the angle of deviation towards congruence, to a point where the patient is able to regain control of their alignment and they don’t require any further treatment, as in the case above.

In others, it may be given repeatedly to maintaining ocular alignment.

Botulinum toxin treatment is often the best therapeutic option in patients who are unfit or unable to undergo an operation, or who have a strabismus despite having undergone several previous strabismus surgeries. Some patients choose this path temporarily until they can timetable surgery around their work and family commitments.

Surgery

With the accuracy and precision that has been achieved in cataract and refractive surgery, patients often believe the aim of strabismus surgery is to align them precisely. However this is unachievable, as the position of the eyes is not purely related to the length of the muscles or the position of their insertions.

The aim of surgery or Botulinum toxin treatment is to reduce the angle of deviation. In patients who do not achieve complete steropsis, or fusion (alignment), by reducing the angle of deviation between the eyes and getting them into their “fusional range”, alignment may still be achieved due to further neural compensation mechanisms. Even in patients without fusion potential, reducing the angle of deviation makes the strabismus less obvious.

In patients who may be suitable for surgery, Botulinum toxin A is often used to mimic the effects of surgery  to assess the possible outcome, or to gradually prepare for surgery those who have developed suppression due to their condition.

This is because, when strabismus occurs in childhood, the brain usually suppresses part of the image, for example suppression of the temporal part of the image in a patient with childhood exotropia. Surgery may take the patient out of their suppression zone, thus inducing troublesome postoperative diplopia, so Botulinum toxin is a useful step in many patients where surgery is being planned.

Strabismus surgery is a good treatment option for many patients, particularly those with no or limited previous surgery.  Studies suggest it has a better health value than cataract surgery.

In adults, an adjustable suture is often used, with the majority of the surgery performed with the patient under general anaesthetic. The final muscle position is then determined and the suture tied off after the patient has been woken up.

The recovery period for strabismus surgery is usually three to six months. At this point, further surgery may sometimes be considered if further improvement is deemed possible.

Strabismus surgery in adults can be more challenging than similar surgery in children, both in preoperative planning and intraoperative technique. Adult strabismus surgery often requires reoperation of previously manipulated muscles, necessitating meticulous scar removal and changes to the surgical approach.

Patients who have had repeated previous surgery have increased risk of chronic redness postoperatively.

There is increased associated scarring of the conjunctiva with each surgery and for this reason in some cases Botulinum toxin treatment is a better option.

Summary

The management of strabismus is challenging, but can be successful in most patients.

Options for treatment include non-invasive orthoptic measures, while both Botulinum toxin therapy and surgery have an important role to play.

The restoration of normal ocular alignment is extremely valuable to the patient. It is not “cosmetic” surgery and the aim is to restore normal ocular alignment and binocularity while improving the patient’s quality of life.

Dr Shanel Sharma is a consultant ophthalmologist in Sydney and conjoint lecturer at UNSW and University of Sydney

References on request