Strabismus (squint): convergent, divergent, in babies and adults, when to see a doctor

Strabismus, or squint, is a misalignment of the eyes: the two eyes are not fixing on the same point at the same time. In infants, brief deviations are common in the first weeks; an eye that turns all the time, or is still turning after three or four months, should be examined, because the risk in a child is amblyopia, the “lazy” eye. Glasses, patching and orthoptics treat many cases; surgery, when it is needed, aims to realign the eyes. In an adult, a sudden squint with double vision means seeing an ophthalmologist the same day or, failing that, going to the eye emergency department.

UNDERSTANDING

What is strabismus? A simple definition

Each eye is moved by six small muscles. When they work in a coordinated way, both eyes aim at the same point and the brain fuses the two images into a single one, in three dimensions. In strabismus, one eye fixes on the object being looked at while the other drifts away from it, constantly or intermittently, either visibly or so subtly that the deviation is only picked up during an examination. A “slight squint” is not without consequences: in a child, a small angle is enough for the turned eye to be neglected.

Strabismus is common. A 2019 meta-analysis covering 56 studies and 229,396 people (Hashemi) estimates its worldwide prevalence at 1.93%, with divergent strabismus (1.23%) more common than convergent strabismus (0.77%), each form having been estimated separately, with wide variations between regions of the world.

Convergent, divergent, vertical strabismus: the different forms

IN CHILDREN

My baby has a squint: is it normal?

In the first weeks of life, the two eyes have not yet learnt to work together. In a British study in which orthoptists observed their own babies (Horwood, 2003), these misalignments occur during the first two months, reflect the normal development of convergence and rarely become a true strabismus; but before two months, an early-onset convergent strabismus that is just beginning cannot be told apart from them. In usual practice, an intermittent squint before three or four months is therefore no cause for concern, and a squint that persists after that age should be examined. An eye that turns all the time, at any age, should be examined by an ophthalmologist, according to the recommendations of the American Academy of Ophthalmology (Sprunger, 2023).

Many babies appear to squint without having a real strabismus: a fold of skin at the inner corner of the eyes, the epicanthus, hides part of the white of the eye, which then seems to turn towards the nose. This false squint becomes less noticeable as the face grows, but only an examination can settle the question.

The risk in children: amblyopia, or lazy eye

An adult who develops a squint sees double. A young child, on the other hand, does not see double: their brain suppresses the image from the turned eye. If this suppression keeps happening during the years when vision is developing, the turned eye stops developing its sharpness of vision. This is amblyopia, the “lazy eye”: the eye itself is normal, but the brain has learnt not to use it. The earlier it is detected, the more easily it is treated; after childhood, recovery becomes much more difficult.

Screening relies on a simple examination: the visual acuity of each eye as soon as the child can name pictures; the cover test, which reveals the deviation when one eye and then the other is covered; and measurement of the eyes’ prescription under cycloplegia, after drops that put focusing (accommodation) at rest, because hidden long-sightedness (hypermetropia) is the cause of many convergent squints.

Glasses, patching, orthoptics, surgery: how strabismus is treated in children

Glasses: the first treatment for convergent strabismus

A long-sighted child has to accommodate, that is, make a focusing effort, to see clearly. This effort makes the eyes converge; when it is too great, one eye turns towards the nose. This is accommodative strabismus (accommodative esotropia), and its treatment is full correction of the long-sightedness (hypermetropia). In an Irish series of 103 children followed for an average of 4.5 years (Mulvihill, 2000), 39.8% of accommodative squints disappeared entirely with glasses; the proportion of amblyopic children fell from 61.2% to 15.5%, and 89.3% had three-dimensional vision. No child was able to give up their glasses while keeping the eyes aligned: glasses for strabismus are worn all the time, for years.

Patching: treating the lazy eye

Once amblyopia has set in, the good eye is covered to force the brain to use the other one. A 2014 Cochrane review (Taylor) concludes that occlusion, together with the necessary optical correction, is more effective than glasses alone in amblyopia linked to strabismus. The patch does not need to be worn all day: in a randomised trial of 189 children under 7 years of age with moderate amblyopia (Repka, 2003), two hours of patching a day, combined with at least one hour of near activities, improved acuity as much as six hours, by 2.4 lines on average at four months. The patch corrects the vision of the amblyopic eye, not the alignment of the eyes.

Orthoptics

The orthoptist measures the angle of the squint, the vision of each eye and three-dimensional vision, and follows amblyopia treatment over time. For intermittent divergent strabismus, the 2021 Cochrane review (Pang) finds that part-time occlusion improves alignment at near and at distance at six months compared with simple observation, with no difference in three-dimensional vision, in children aged 1 to 10 years.

Strabismus surgery: what it is for, at what age, how it is covered

When glasses and amblyopia treatment are not enough to align the eyes, the surgeon moves back a muscle that is too strong or shortens a muscle that is too weak, under general anaesthesia in children. The operation aims to align the eyes and can help binocular vision when it is carried out early; it does not remove the need for glasses or for amblyopia treatment.

On the question of the age at which to operate on early-onset convergent strabismus, the European ELISSS study (Simonsz, 2005), which was controlled but not randomised, recruited 231 children to be operated on between 6 and 24 months and 301 to be operated on between 32 and 60 months, assessed at the age of six. At that age, 13.5% of the first group versus 3.9% of the second recognised the coarsest test of three-dimensional vision; the residual angle was the same, and about 35% of the children in each group were not within the target alignment range. In other words, surgery realigns the eyes in the majority of cases, a second operation is sometimes needed, and fine three-dimensional vision remains rare when the squint appeared in the first months of life. When it is indicated, the operation is covered by the Assurance Maladie, the French national health insurance, for children and adults alike.

IN ADULTS

Strabismus in adults: two very different situations

A sudden squint with double vision: an emergency

In an adult, an eye that suddenly starts to turn causes double vision, because the brain no longer suppresses the image from the turned eye. The cause is most often a palsy of one of the three nerves that control the eye muscles, which after the age of 50 is frequently linked to a poor blood supply to the nerve, made more likely by diabetes or high blood pressure. But not always: in a prospective study of 109 patients aged 50 and over with an isolated ocular motor nerve palsy (Tamhankar, 2013), another cause was found in 16.5% of them, including tumours, a brainstem infarction and giant cell arteritis; the authors recommend brain MRI from the initial assessment onwards. A sudden convergent squint without a palsy can also reveal a neurological cause: a 2023 review (Nouraeinejad) describes several, particularly when there are headaches, nystagmus or balance problems.

Faced with a squint that comes on suddenly, with or without double vision, you should therefore see an ophthalmologist the same day or, failing that, go to the eye emergency department. If this comes with a violent headache, a drooping eyelid with a dilated pupil, weakness on one side of the body or difficulty speaking, call 15, the SAMU emergency medical service in France (see sudden double vision due to ocular motor nerve palsy).

A long-standing squint: correction is still possible

Many adults have lived with a squint since childhood, either never operated on or returning years after a first operation. The idea that it would be “too late” is wrong. The American Academy of Ophthalmology report on strabismus surgery in adults (Mills, 2004) notes, in large series, successful realignment rates of 68% to 85%, with functional benefits for many patients, and puts figures on the risks: new double vision in 1% to 14% of patients, unplanned reoperation in up to 21%. In a series of 424 adults operated on for a long-standing squint (Kushner, 2002), 9% had temporary double vision, which disappeared within six weeks, and 0.8% persistent double vision; a prism test before the operation helps to identify patients at risk. Fine three-dimensional vision, however, is generally not regained when the squint dates from early childhood. We devote a separate article to this: correcting a squint in adulthood.

When to see a doctor, and whom to see

• A baby with one eye that turns all the time, or who is still squinting after three or four months: an eye examination, without delay.
• A child who closes one eye in the sun, tilts their head or squints when tired: an intermittent squint is possible, even a subtle one.
• A squint that comes on suddenly, at any age: see an ophthalmologist the same day or, failing that, go to the eye emergency department.
• A long-standing squint that bothers you: a consultation makes it possible to measure the deviation and discuss a correction.

Dr Tourabaly sees children from the age of 3 at the Cachan practice for strabismus and amblyopia screening, measuring the eyes’ prescription under cycloplegia and prescribing glasses and patching, working with orthoptists, who carry out the orthoptic assessment and the eye exercises. Children under 3 are referred to a paediatric ophthalmology consultation. When an operation on the eye muscles is indicated, in children as in adults, Dr Tourabaly does not perform it himself: he refers the patient to a surgeon specialising in strabismus (strabology) and follows the vision before and after the operation.

Frequently asked questions

Consultations in Cachan, Paris 13 and at the Sorbonne Saint-Michel practice (Paris 5th) · Tel. +33 1 45 47 08 11

Scientific sources

• Hashemi H, et al. Global and regional prevalence of strabismus: a comprehensive systematic review and meta-analysis. Strabismus. 2019. PMID 31012389
• Sprunger DT, et al. Esotropia and Exotropia Preferred Practice Pattern. Ophthalmology. 2023. PMID 36526451
• Horwood A. Neonatal ocular misalignments reflect vergence development but rarely become esotropia. Br J Ophthalmol. 2003. PMID 12928285
• Mulvihill A, et al. Outcome in refractive accommodative esotropia. Br J Ophthalmol. 2000. PMID 10873987
• Taylor K, Elliott S. Interventions for strabismic amblyopia. Cochrane Database Syst Rev. 2014. PMID 25051925
• Repka MX, et al. A randomized trial of patching regimens for treatment of moderate amblyopia in children. Arch Ophthalmol. 2003. PMID 12742836
• Pang Y, et al. Interventions for intermittent exotropia. Cochrane Database Syst Rev. 2021. PMID 34516656
• Simonsz HJ, et al. Final report of the early vs. late infantile strabismus surgery study (ELISSS), a controlled, prospective, multicenter study. Strabismus. 2005. PMID 16361188
• Tamhankar MA, et al. Isolated third, fourth, and sixth cranial nerve palsies from presumed microvascular versus other causes: a prospective study. Ophthalmology. 2013. PMID 23747163
• Nouraeinejad A. Neurological pathologies in acute acquired comitant esotropia. Graefes Arch Clin Exp Ophthalmol. 2023. PMID 37145335
• Mills MD, et al. Strabismus surgery for adults: a report by the American Academy of Ophthalmology. Ophthalmology. 2004. PMID 15177982
• Kushner BJ. Intractable diplopia after strabismus surgery in adults. Arch Ophthalmol. 2002. PMID 12427063

This article is intended for information purposes and does not replace a medical consultation. Indications, expected benefits and risks depend on each individual situation. For any question concerning your vision, consult an ophthalmologist.