Squint & Pediatric Ophthalmology
A newborn's eyes are anatomically complete and functionally almost useless. Acuity at birth is poor, binocular vision does not yet exist, and the cortical machinery that will eventually deliver stereopsis has not been assembled.
That machinery is built by use, over the first years of life, and only during that period. Everything in paediatric ophthalmology follows from this single fact, and it is why conditions that merely inconvenience an adult can permanently blind a child.
1. Built, Not Born
The visual cortex at birth contains the raw connections but not their organisation. Ocular dominance columns, binocular cells and the fine tuning of receptive fields all develop in response to clear, matched images from the two eyes.
Development is competitive. Inputs from the two eyes contest the same cortical territory, and the eye supplying the better image wins.
The organising tool is therefore that anything degrading the image during this window causes permanent cortical loss, and there are only three ways to degrade it.
| Mechanism | Cause |
|---|---|
| The image is blurred | Uncorrected refractive error, particularly if unequal between the eyes |
| The image is blocked | Deprivation, as in congenital cataract or ptosis |
| The images do not match | Strabismus, so the two eyes send conflicting images |
The result of all three is amblyopia, meaning reduced vision in a structurally normal eye because the cortex never learned to use it.
2. The Child Does Not See Double
An adult who develops a squint has diplopia, sometimes disablingly so. A child with the same misalignment almost never complains of it, and this difference is the most important concept in the chapter.
The immature cortex adapts by suppressing the input from one eye, which eliminates the double image at the cost of that eye's development. The adult cortex cannot do this, having lost the plasticity, so the adult is left with diplopia instead.
The consequence is that a child with a squint is losing vision silently. There is no symptom to bring them in, which is why screening exists and why a parent's observation that the eyes look crossed must never be dismissed.
Anomalous retinal correspondence is a further adaptation, in which the cortex re-maps the deviating eye's fovea to correspond with a peripheral point in the fixing eye, restoring a crude form of binocularity around the deviation.
Constant unilateral squint is the dangerous pattern, because the same eye is always suppressed. Alternating squint, in which the child fixes with either eye in turn, causes little or no amblyopia because both eyes get used.
3. The Muscles and How to Assess Them
Six muscles move each eye, and their actions are not intuitive because four of them do not lie in the visual axis.
| Muscle | Primary action | Nerve |
|---|---|---|
| Medial rectus | Adduction | Third |
| Lateral rectus | Abduction | Sixth |
| Superior rectus | Elevation, with intorsion and adduction | Third |
| Inferior rectus | Depression, with extorsion and adduction | Third |
| Superior oblique | Intorsion, with depression and abduction | Fourth |
| Inferior oblique | Extorsion, with elevation and abduction | Third |
The obliques are the counterintuitive pair. The superior oblique depresses and the inferior oblique elevates, which is the reverse of what the names suggest, and both act through a posterior insertion behind the equator.
The vertical recti act most purely in abduction and the obliques in adduction, which is why elevation is tested in the abducted position for the superior rectus and in the adducted position for the inferior oblique.
Testing vision in a preverbal child relies on behaviour rather than charts. A normal infant should fix and follow by about three months. Objection to occlusion of one eye but not the other indicates that the uncovered eye sees worse, and preferential looking tests using cards of varying grating give a quantitative estimate.
Assessing alignment
The corneal light reflex, or Hirschberg test, is the first assessment and requires only a torch. The reflex should sit symmetrically in both pupils, and roughly one millimetre of decentration corresponds to about fifteen prism dioptres of deviation.
The cover test detects a manifest squint. Covering the fixing eye forces the deviating eye to take up fixation, and the movement it makes reveals the direction of deviation. Inward movement means the eye was out, and outward movement means it was in.
The cover-uncover test detects a latent squint, or phoria, in which fusion normally holds the eyes straight and the deviation appears only when fusion is interrupted by the cover.
Prism cover testing quantifies the angle, using prisms to neutralise the movement.
Pseudosquint
Pseudo-esotropia is extremely common in Indian and East Asian children and is the commonest reason a child is brought to an eye clinic for a squint.
Prominent epicanthic folds and a broad flat nasal bridge cover the medial sclera, so less white is visible nasally than temporally and the eyes appear crossed.
A symmetrical corneal light reflex with no movement on cover testing settles it immediately, and the appearance resolves as the facial bones grow. The examination costs a minute and prevents years of unnecessary anxiety.
4. Concomitant and Incomitant
A concomitant squint has the same angle of deviation in all directions of gaze. These are the childhood squints, arising from disordered development of binocular control rather than from any muscle or nerve defect.
An incomitant squint has an angle that varies with the direction of gaze. This means a paralytic cause, as in a cranial nerve palsy, or a restrictive one, as in thyroid eye disease or an orbital floor fracture with entrapment.
An incomitant squint of new onset in a child requires neurological evaluation, because the childhood squints are concomitant and a nerve palsy indicates something else.
The Parks three-step test identifies which cyclovertical muscle is paretic in a vertical incomitant deviation, and it works by successive elimination.
The first step asks which eye is higher in primary position, which halves the eight possible muscles to four. The second asks whether the deviation increases on right or left gaze, halving it again to two. The third asks whether it increases on head tilt to the right or left, leaving one.
The commonest answer is a fourth nerve palsy affecting the superior oblique, which is also the commonest cyclovertical palsy, and it is why patients adopt a head tilt away from the affected side.
The congenital restrictive syndromes
Two congenital incomitant squints are examined because they mimic nerve palsies and must not be operated on as though they were.
Duane retraction syndrome results from absence of the abducens nucleus, with the lateral rectus innervated instead by a branch of the third nerve. Attempted adduction therefore fires both horizontal recti simultaneously, and the co-contraction retracts the globe and narrows the palpebral fissure.
Limitation of abduction makes it resemble a sixth nerve palsy, and the globe retraction with fissure narrowing on adduction is what distinguishes it. It is present from birth and non-progressive, so it needs no neuroimaging.
Brown syndrome is a restriction of the superior oblique tendon at the trochlea, so the eye cannot elevate in adduction. The pattern mimics inferior oblique underaction, and forced duction testing distinguishes restriction from weakness.
5. Esotropia
Convergent squint is the commonest childhood squint in most series, and the important distinction is whether accommodation is driving it.
Infantile esotropia presents before six months with a large constant angle, minimal refractive error, and frequently cross-fixation, in which the child uses the adducted eye to look at the opposite field. It requires surgery, because glasses will not correct it.
Accommodative esotropia is the one that glasses cure. A hypermetropic child must accommodate to see clearly, and accommodation is linked to convergence through the near reflex, so sustained accommodative effort drives the eyes inward.
Full cycloplegic refraction is therefore mandatory in any child with esotropia, because a child's accommodation is powerful enough to conceal several dioptres of hypermetropia from a standard refraction.
Correcting the hypermetropia removes the accommodative drive and straightens the eyes, which is one of the few situations in medicine where spectacles are a curative treatment.
A high accommodation-to-convergence ratio produces esotropia that is greater at near than at distance, and bifocals may be required. Partially accommodative esotropia improves but does not fully correct with glasses, and the residual angle is treated surgically.
6. Exotropia
Divergent squint is commoner in some Asian populations and behaves differently.
Intermittent exotropia is the usual form, appearing when the child is tired, unwell, daydreaming or looking into the distance, and controlled by fusion the rest of the time.
Parents often report that the eye drifts outward in bright sunlight, and children characteristically close one eye outdoors.
Because it is intermittent, fusion is being exercised and amblyopia is uncommon, which is the opposite of the constant unilateral esotropia situation.
Sensory exotropia occurs when an eye with poor vision drifts out, and the squint is the consequence rather than the cause. In a child, that poor vision must be explained before the squint is treated.
7. Treating Amblyopia
Treatment proceeds in a fixed order, and skipping the first step is the commonest error.
Refractive correction comes first. A period of spectacle wear alone, called refractive adaptation, improves a substantial proportion of amblyopic eyes without any further intervention, and any occlusion started before this is being applied to an uncorrected eye.
Occlusion of the better eye is the classical second step, forcing the amblyopic eye to work and allowing it to compete for cortical territory.
Atropine penalisation is an alternative, blurring the better eye pharmacologically. It is useful where a patch is refused or removed, and it cannot be taken off by the child.
The dosing evidence is worth knowing because it contradicts older teaching. Two hours of patching daily is as effective as six hours for moderate amblyopia, and weekend atropine is as effective as daily atropine, with atropine overall comparable to patching. Less burdensome regimens improve adherence without sacrificing outcome.
The window matters. Treatment is most effective in the first few years, remains worthwhile to around seven or eight, and produces some benefit in older children, so age alone should not be used to refuse treatment.
Surgery aligns the eyes but does not treat amblyopia, and operating on an amblyopic eye without first treating the amblyopia wastes the opportunity.
8. The Watering Eye in Infancy
Congenital nasolacrimal duct obstruction is the common cause, arising from a persistent membrane at the lower end of the duct, and it presents with watering and mucoid discharge from a few weeks of age.
More than ninety percent resolve spontaneously within the first year, so initial management is conservative, with lacrimal sac massage to generate hydrostatic pressure against the membrane and antibiotic drops only for infection.
Probing is deferred until after the first year, when spontaneous resolution has become unlikely.
The diagnosis that must not be missed is congenital glaucoma, which also presents with watering. The distinguishing features are photophobia and blepharospasm alongside the epiphora, and corneal enlargement or clouding, and it requires urgent surgical treatment.
Blocked ducts do not cause photophobia. That single symptom is what separates a reassuring diagnosis from a blinding one.
9. Refractive Error and the Myopia Epidemic
Uncorrected refractive error is the leading cause of visual impairment in children worldwide, and it is entirely correctable with a pair of spectacles.
This is what school eye screening under the national blindness programme addresses, with teachers performing initial vision screening and free spectacles supplied to children found to need them. The intervention is cheap and the effect on schooling is substantial.
Myopia prevalence is rising rapidly, particularly in urban Asian populations, driven by prolonged near work and reduced time outdoors.
Time outdoors protects against the onset of myopia, and the effect appears to relate to light intensity rather than to distance viewing, which is why the recommendation is time outdoors rather than less reading.
Low-concentration atropine slows progression once myopia has begun. The LAMP study compared 0.05, 0.025 and 0.01 percent atropine against placebo in children and found a concentration-dependent effect.
At one year, 0.05 percent atropine reduced progression of spherical equivalent by 67 percent and axial elongation by 51 percent, against 27 and 12 percent respectively for the 0.01 percent concentration. The 0.05 percent concentration remained the most effective over three and five years with minimal rebound.
Axial length is the more important outcome, because the pathological complications of myopia, meaning retinal detachment, myopic maculopathy and glaucoma, follow from the eye being physically too long rather than from the refractive number.
10. Other Paediatric Presentations
Congenital ptosis is usually due to dysgenesis of the levator muscle, and the key question is whether the lid covers the visual axis, since deprivation amblyopia is the risk that determines urgency.
Marcus Gunn jaw-winking is a congenital synkinesis in which the ptotic lid elevates on jaw movement, from aberrant innervation between the trigeminal and oculomotor systems.
Ophthalmia neonatorum, congenital cataract, retinoblastoma and retinopathy of prematurity are each developed in their own chapters, and together with squint they constitute the conditions in which a delay of weeks changes the outcome permanently.
A child who does not fix and follow by three months, or whose parents report a white pupil or a squint at any age, requires examination rather than reassurance.
The common thread across all of them is that the child cannot report the problem, the eye usually looks normal to a parent, and the window in which intervention works is measured in weeks to months rather than years.
This is why paediatric ophthalmology is organised around screening at fixed ages rather than around symptoms, and why the newborn red reflex, the three-month fixation check and school vision screening exist as separate scheduled events rather than as responses to complaint.
11. Worked Examples
Example 1. A 2-year-old is brought with eyes that appear crossed. The corneal light reflex is central in both eyes and there is no movement on cover testing. What is the diagnosis?
Pseudo-esotropia from prominent epicanthic folds and a broad nasal bridge, which is very common in Indian children. Less sclera is visible nasally than temporally, creating the appearance of convergence. A symmetrical light reflex and an absent cover test movement exclude a true squint, and the appearance resolves as the facial skeleton develops.
Example 2. A 3-year-old has a convergent squint. Cycloplegic refraction shows plus 5 dioptres in each eye. What is the treatment?
Full spectacle correction of the hypermetropia. This is accommodative esotropia, in which the child must accommodate heavily to see clearly and the near reflex drives convergence with it. Removing the accommodative demand removes the convergence and straightens the eyes. Cycloplegia is essential, because a child's accommodation conceals hypermetropia from ordinary refraction, and surgery would be inappropriate for the accommodative component.
Example 3. A 5-year-old has moderate amblyopia. The parents cannot achieve six hours of patching. What can be offered?
Two hours of daily patching, which trial evidence shows is as effective as six hours for moderate amblyopia, or atropine penalisation of the better eye, which is comparable to patching and can be given at weekend dosing with equal effect. The choice should follow what the family can actually sustain, since adherence rather than prescribed hours determines the outcome. Refractive correction must be in place first.
Example 4. A 4-month-old has a watering eye with mucoid discharge, no photophobia, and a clear cornea of normal size. Management?
Conservative. This is congenital nasolacrimal duct obstruction, and more than ninety percent resolve spontaneously within the first year, so lacrimal sac massage is taught to the parents and antibiotic drops used only for infection, with probing deferred beyond twelve months. The absence of photophobia, blepharospasm and corneal enlargement excludes congenital glaucoma, which is the diagnosis that would require urgent surgery.
Summary
The visual system is built by use within a limited window, and development is competitive between the eyes.
Three things degrade the image: blur, deprivation and mismatch, and all three cause amblyopia.
A child suppresses rather than seeing double, so squint in childhood is silent and blinding while in adults it is symptomatic and harmless to vision.
Constant unilateral squint causes amblyopia; alternating squint largely does not.
Hirschberg first, then cover test for tropia and cover-uncover for phoria.
Pseudo-esotropia from epicanthic folds is settled by a symmetrical light reflex.
Concomitant squints are the childhood ones; a new incomitant squint needs neurological evaluation.
The superior oblique depresses and the inferior oblique elevates, and Parks three-step identifies the paretic cyclovertical muscle.
Duane syndrome retracts the globe on adduction, which is what separates it from a sixth nerve palsy.
Cycloplegic refraction is mandatory in esotropia, because glasses cure the accommodative form.
Intermittent exotropia exercises fusion and rarely causes amblyopia.
Correct refraction before occluding, and two hours of patching matches six, with weekend atropine matching daily.
Surgery aligns eyes; it does not treat amblyopia.
Watering with photophobia is congenital glaucoma, not a blocked duct.
Uncorrected refractive error is the leading cause of childhood visual impairment, and outdoor time prevents myopia while 0.05 percent atropine slows it.
