Down’s syndrome is a trisomy caused by an extra copy of chromosome 21¹. The extrachromosomal material is transmitted either by non-disjunction, unbalanced translocation, or mosaicism.

It is the most common chromosomal anomaly seen in live births². As an ophthalmologist, it is important to screen for ocular abnormalities since the incidence of ophthalmic disorders is between 46-100%³. One study found 97 % of children with Down’s syndrome with at least one ocular abnormality⁴. This proves the necessity to carefully screen these children for any disorders and their prompt treatment. It is recommended that all children diagnosed with Down’s syndrome must undergo evaluation by a Pediatric ophthalmologist before the age of 6 months and then annually thereafter if no abnormalities are found at the first visit⁵

Although there are many ophthalmological anomalies seen in Down’s syndrome, we will be discussing only the ten most clinically significant ones below.

1. Refractive Error and Visual acuity:

The most common refractive error found in patients with Down’s syndrome is hypermetropia, followed by myopia and astigmatism⁵. Hypermetropia is seen in 55-80%, myopia in around 18-59% of patients⁵. Astigmatism has been noted to be present in around 20-30%⁵. It is interesting to note that refractive errors may not always be present at infancy but may develop later on in life⁶. The reason for a greater prevalence of hypermetropia is supposedly failure of emmetropization, which generally occurs with growing age in normal children^6,7. Various theories have been proposed for this such as poor accommodation which is typically seen in patients with Down’s syndrome. Other theories for poorer development of visual acuity that have been proposed are structurally different visual cortex of patients with Down’s syndrome⁸.

2. Strabismus:

The most common type of strabismus is Esotropia as reported by most studies in literature^9,10. The prevalence of exotropia is far less common, being only 8-20%^9-10. Vertical deviations, though seen are uncommon¹⁰. Studies have shown no association between the presence of hypermetropia and the occurrence of strabismus in patients with Down’s syndrome⁹.

3. Nystagmus:

Nystagmus is usually congenital. It is seen in approximately 5-30 % of all patients with Down’s syndrome¹¹. The nature of this nystagmus is either horizontal jerky or pendular¹². Nystagmus is often associated with refractive errors^10-12. This is one of the reasons why the threshold for prescribing glasses in children with Down’s syndrome is much lower than for regular cases.

4. Nasolacrimal outflow obstruction :

It is extremely essential to look for nasolacrimal duct obstruction in a child with Down’s syndrome since the incidence of nasolacrimal outflow obstruction is as much as 17-36%¹³. The various nasolacrimal anomalies seen are punctal agenesis, canalicular atresia and stenosis, and anteriorly placed turbinates, dacrocutaneous fistula. It is considered more challenging to treat the nasolacrimal outflow obstruction in Down’s syndrome than in the general population primarily because it is a consequence of complex developmental anomalies rather than just failure of the opening of the Hasner’s valve¹³.

As compared to the normal population, syringing and probing may not suffice and may call for aggressive procedures such as stent placements and balloon dacryoplasties as the primary procedure of choice¹⁴.

5. Squamous blepharitis:

A very severe form of squamous blepharitis is seen in patients with Down’s syndrome. A deficient immune response has been held responsible by some as reason for causing severe blepharitis whereas some attribute it to abnormal skin structure in patients with Down’s syndrome¹⁵. It requires aggressive treatment as it can lead to refractive errors and keratoconus¹⁶.

6. Keratoconus:

Keratoconus is most commonly seen around the onset of puberty in patients with Down’s syndrome. Contributing factors include steeper posterior corneal curvature at birth, higher-order aberrations, and eye rubbing consequential to squamous blepharitis^17,18. A genetic theory pointed out by Wang et al states that the corneal collagen fibres are affected in Down's syndrome due to the variations seen in Chromosome 21 and this, in turn, makes the cornea unstable¹⁹.

7. Lenticular opacities:

Congenital or early neonatal cataracts are found to be around 8% in incidence, which also includes visually insignificant opacities²⁰. The incidence of visually significant opacities was found to be only 1% in the neonatal age group by Haargaard B et al²¹. Since this rate is still higher than what is seen in the normal populations, it is imperative to screen for early cataracts in these cases²². A visually significant cataract is more commonly seen in the adolescent and presenile age groups.

The common morphologies of cataracts seen are cerulean, nuclear, zonular and cortical amongst others²¹. IOL implantation is preferable in these cases as it is difficult to manage the patient post-operatively with a contact lens²².

It must be ensured that a multidisciplinary approach is chosen before planning for cataract surgery if need arises. It is important to involve the anesthetist, cardiologist for an uneventful surgery and recovery.

8. Retinal anomalies:

Scott et al²³ found the central macular thickness in Down’s syndrome to be higher than the normal population, suggesting abnormal macular development. However, they did not find any correlation between the thickness of macula and vision in these patients. Studies have also shown the persistence of inner retinal layers at fovea in patients of Down’s syndrome²⁴. All these studies suggest that the retina is structurally abnormal in Down’s syndrome, although more detailed studies are needed for the same.

9. Optic nerve head abnormalities:

The optic nerve head tends to be smaller in patients with Down’s syndrome although the size does not correlate with vision²⁵. It has been found however that smaller disc size makes them susceptible to optic nerve drusen formation²⁶. It has also been noted that the neurosensory abnormalities often seen on VEP in these patients probably do not occur at the level of the ONH but are due to an abnormality in the visual pathway²⁵. Many other insignificant anomalies are seen such as an increased number of vessels crossing the disc, but this is not clinically insignificant.

10. Hypoaccomodation:

Accommodation deficits in children with Down’s syndrome are not associated with poor visual acuity²⁷. Hence, it is extremely essential to incorporate dynamic retinoscopy in the examination of children with Down’s syndrome. If accommodation is found defective, prompt treatment with bifocals is a must²⁸.

In conclusion, Down’s syndrome presents with many ophthalmological abnormalities in addition to systemic anomalies, which require early detection and treatment by a Pediatric ophthalmologist. This is essential to achieve better outcomes visually and also promote the overall psychological well-being of a child with Down’s syndrome.

References:

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