Background/aims This was a cross-sectional survey to find the prevalence of visual impairment and eye diseases among adults with Down syndrome (DS) in Hong Kong.
Methods 91 DS patients over the age of 30 were recruited through the Hong Kong Down Syndrome Association. Each patient was provided with a complete ophthalmological assessment including best corrected visual acuity, refraction, slit lamp and fundus examination.
Results In our sample, 56.6% had normal vision to mild vision impairment, 33.7% and 7.2% had moderate and severe vision impairment, respectively, and 2.4% were blind. The mean presenting distant LogMAR visual acuity was 0.66 (Snellen equivalent 20/90), and the best corrected LogMAR visual acuity was 0.48 (Snellen equivalent 20/60). Significant refractive errors were found in 86.3% of the eyes, with spherical equivalent corrections ranging from −23.25D to +3.00D. Myopia and astigmatism were prevalent and found in 59.3% and 72.7% of the eyes, respectively. Blepharitis and chalazion were found in 44% of the eyes, while corneal problems were present in 27.5%. There were low incidences of infective keratitis (0.5%), keratoconus (0.5%) and Brushfield spots (1.1%). Cataracts were found in 72.2% of the eyes; 26.1% were congenital and 44.9% were age-related. Fundal abnormalities were present in 49.5% of the eyes.
Conclusions There is a high prevalence of vision impairment among Chinese DS adults. Uncorrected refractive errors, high myopia and cataracts are the main visually debilitating ophthalmological abnormalities. Vision may be improved through the simple use of glasses and early treatment of age-related cataracts.
- Public Health
- Eye (Globe)
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Down syndrome (DS) occurs in 1 in 767 live births in Hong Kong.1 Advances in medical care and social support nowadays have increased the average life expectancy of DS adults to 50 years in developed countries, with some surviving into their 70s.2
With increasing numbers of DS patients reaching adulthood and even old age, adult-onset and degenerative health problems need to be addressed. Sensory impairment, in particular hearing and vision losses, occurs in up to 80% of older DS adults.3
Common vision disorders found in DS patients include cataract, refractive error, squint, nystagmus, amblyopia and blepharitis.4 A North American study in the 1990s has found that cataracts, recurrent keratitis and keratoconus were the main causes for vision impairment in North American adults with DS.5 However, no data are available for the Asian adult DS population. In Hong Kong, vision screening for DS stops at the age of 216 after which patients are expected to seek medical attention by themselves. DS patients with visual impairment often do not act out or appear outwardly anxious7 in contrast to people with disabilities such as epilepsy or autism. Also, many adult DS patients in Hong Kong are cared for by elderly parents who might have their own health problems. As a result, their visual needs might be neglected. Previous studies have shown that vision impairment and blindness among DS patients can remain unidentified in up to 40% of cases.8
Since many ophthalmological problems are treatable (eg, cataract by surgical extraction), sometimes by simple means (eg, refractive error by spectacles), it seems worthwhile to check the vision of DS patients at regular intervals, since they are less likely to complain.
In this study, we aimed to find the prevalence of ocular disease among a group of subjects with DS who were willing to undergo an eye examination and to determine if any of the conditions found could be treated. To the best of our knowledge, this is the first study of its kind in the adult Chinese DS population.
We performed a cross-sectional survey from March to June 2010 with the target population of persons aged 30 and over diagnosed with DS. We collaborated with the Hong Kong Down Syndrome Association (HKDSA) for subject recruitment. Invitation letters were sent to all eligible members of the HKDSA; telephone calls were made and letters of invitation sent to sheltered workshops, residential and day care centres with DS patients. Interested parties were all referred to the HKDSA for recruitment procedures.
The inclusion criteria were any patient aged 30 or older diagnosed with DS and with a legal guardian or representative who was able to give an informed consent. The exclusion criteria were any patient who had a severe motor impairment deemed unfit to travel on public transport, or had an allergy to dilating drugs such as tropicamide or phenylephrine, or was pregnant or lactating.
Written informed consent was given by the legal guardian or representative of all patients.
Demographic data and general medical conditions were collected prospectively via a standardised questionnaire. The level of intellectual disability was determined according to the Wechsler Intelligence Scale for Children for Hong Kong, which is performed for all Hong Kong children at the age of 5–6.
The subjects were provided with a free complete ophthalmic examination at either the Queen Mary Hospital or United Christian Hospital in Hong Kong. Two optometrists measured the visual acuity (VA), and performed refraction and corneal pachymetry while four ophthalmologists performed the ophthalmological examinations. The following data were obtained:
Best corrected VA. Distant best corrected VA was measured monocularly for both eyes. An opaque occluder was used to cover the fellow eye. Different tests were used depending on the intellectual disability of the patient, including Snellen chart, Sheridan Gardner visual acuity chart, Cardiff acuity cards and Snellen grating. Accomodation was not tested in this study.
Refraction. Subjective refraction was done for all cooperative patients. Otherwise, objective refraction was performed.
Strabismus assessment using Hirschberg test and Krimsky test.
Corneal pachymetry. After anaesthetising the cornea with 0.4% Oxybuprocain, the central corneal thickness was measured using a handheld ultrasound pachymeter on patients who were able to cooperate. A minimum of three measurements were made and the average taken as the corneal thickness reading.
Slit lamp examination. Cataracts were classified into congenital cataracts or age-related cataracts according to morphology.
Intraocular pressure. Goldmann tonometry was performed. For less cooperative patients, Tono-Pen (Reichert, New York, USA) was used.
Fundoscopic examination including peripheral retinal examination.
Retinal nerve fibre thickness (RNFL) measurement. For patients who were found to have abnormal cup to disc ratio, a spectral domain optical coherence tomography (OCT) of the optic nerve head using the Spectralis OCT machine (Heidelberg, Germany) was performed to determine the RNFL thickness. This was compared with normal subjects using the Spectralis age-adjusted RNFL thickness normative database to assess for glaucomatous change.
All statistical analyses were performed using SPSS V.17.0 statistics software.
This study was approved by the Institutional Review Board of the Hong Kong West Cluster as well as the Research Ethics Committee of the Kowloon East Cluster (HKWC reference number: UW 10-074; KC/KE reference number: 100045/ER-3).
Demographics and general medical condition
Invitation letters were sent out to 121 eligible members of the HKDSA, of which 53 members agreed to participate (response rate=44%). A total of 39 other institutions for the mentally handicapped were contacted from which another 47 patients were recruited. There were in total 100 eligible subjects, and 91 attended the clinical exam (attendance rate=91%). The mean age was 38 years (SD 6.5 years, range 30–56); 32% were female subjects while 68% were male subjects. All were of Chinese ethnic origin. Most (75.8%) suffered from moderate intellectual impairment. Other baseline characteristics and general medical conditions of the patients are listed in table 1.
Carer and placement
This is summarised in table 2. About a third (34.1%) of the sampled DS patients lived fulltime at home while 53.8% lived fulltime in institutions (shelters). The mean age of the parents of DS patients living in institutions was on average 9 years older than those who lived at home (p<0.05).
Presenting VA and best corrected VA
The mean presenting distant LogMAR VA in the sample was 0.66±0.41 (range: 0.00–2.00), with Snellen equivalent of 20/90 (figure 1). The mean best corrected distant LogMAR VA was 0.48±0.31 (range 0.00–1.52), with Snellen equivalent of 20/60. Using unpaired t test, there was a mean gain of 1.8 lines of vision after correcting refractive error (p<0.05).
Using the new WHO revised International Classification of Diseases (ICD)-10 criteria,9 ,10 presenting VA equal to or better than 20/70 (LogMAR<=0.52) in the best eye is considered normal to mild visual impairment. Moderate vision impairment is defined as VA less than 20/70 (LogMAR>0.52). Severe vision impairment is defined as VA less than 20/200 (LogMAR>1), and blindness is defined as VA less than 20/400 (LogMAR>1.3). In our sample, 47 patients (56.6%) had normal or low vision impairment, 28 (33.7%) had moderate vision impairment, 6 (7.2%) had severe vision impairment and 2 (2.4%) were considered blind.
Overall, 157 eyes (86.3%) had a spherical equivalent refractive error of more than 0.5 dioptres (table 3). The mean spherical equivalent refractive error was −3.42±5.0 dioptres, ranging from −23.25 to +3.00 dioptres. Considering the spherical component only, more than half of the eyes (59.3%) were myopic with a mean spherical refractive error of −5.75±4.6 dioptres, while 30.2% of the eyes were hyperopic. Astigmatism (>=1 dioptre) was highly prevalent and present in 72.7% of the eyes, with a maximum of 6.0 dioptres. The mean astigmatic refractive error was 2.06±1.1 dioptres. Of the 117 eyes with astigmatism, 34.8% had an oblique axis.
Only 23.1% of the patients with visually significant refractive error (SE more than±1.00 dioptres) habitually wore glasses (table 3). For those who lived at home, 45% had been fitted with glasses compared with only 24% for those who lived in an institution (p=0.11).
Multiple ophthalmological abnormalities were found. Table 3 provides detailed findings and their summary statistics. Three eyes (two patients) had an abnormal cup to disc ratio of over 0.6; subsequent OCT of the optic nerve head showed normal RNFL thickness in all three eyes.
High prevalence of vision impairment and uncorrected refractive errors
According to the new ICD-10 criteria, 43% of the DS adults in our sample suffered from moderate visual impairment or worse. Haire and coworkers in 1991 reported that in a sample of 61 mentally handicapped patients where 19 patients had DS, 17 patients (28% of the total sample) had visual impairment with visual acuities of less than 20/60.11 Van Schrojenstein Lantman-de Valk et al in 1994 and 1997 assessed 243 DS individuals and reported a visual impairment prevalence of 36.2%–59.1% in the 30–59-year age group.12 Van Allen et al in 1999 observed in 38 middle-aged to elderly DS patients that 21% suffered from blindness.5 One reason for the higher prevalence of visual impairment in our sample when compared with previous studies could be due to the difference between the ICD-9 and ICD-10 classification in which refractive errors were corrected in the old classification but not in the new classification. If we used ICD-9 criteria for classification, then 23.2% of our patients suffered moderate visual impairment or worse. This reflects that a large proportion of visual impairment in the sample was contributed by uncorrected refractive errors, now considered a major cause of vision impairment worldwide.9 This could be improved cost effectively through the simple use of glasses.9 Our study showed there was a mean gain of 1.8 lines of vision after correcting for refractive errors.
Even after correcting for refractive error, the mean best corrected visual acuity in DS patients in our sample (Snellen VA 20/60) was still significantly lower than that of the general adult population in Hong Kong, where only 4.54% had vision less than 20/60.13 Amblyopia, cataract and myopic fundal degeneration may be other causes accounting for poor VA.
Cataract, astigmatism and high myopia
Cataracts were very common and present in 72% of the eyes. Although a significant proportion of cataracts were attributed to the developmental blue dot cataract which are visually innocuous,14 congenital cataracts which can affect vision, such as posterior polar cataract, were also prevalent (11%), and together with ametropia may have contributed to the development of amblyopia leading to poor vision. Age-related cataracts were also prevalent (45%). This corresponds with the prevalence of 35%–50% reported from other countries.5 ,15 This figure is higher than that of the general population, where the reported prevalence of cataract was 35% for those 40 years or older among Chinese in Singapore (The Tanjong Pagar Survey).16 Susceptibility to oxidative stress17 and the expression and accumulation of amyloid precursor protein and amyloid-β peptides 18 ,19 in the lens have been proposed as mechanisms for the pathogenesis of early onset cataract in DS patients. As cataract surgery is becoming safer and more efficient, screening and early treatment for visually significant cataracts may be beneficial in this population.
Astigmatism was highly prevalent in the sample (72.7%) and the axis was frequently oblique (34.8%). This finding was consistent with previous studies with Asian children.20 Earlier studies suggested the prevalence of astigmatism increased with age.21 As the corneas of DS patients are found to be structurally different and are significantly steeper than the normal population,22 mechanical pressure exerted on the cornea by upslanting palpebral fissures as well as thinner corneas have been implicated as possible causative factors for oblique astigmatism in DS patients.23 This, together with frequent eye rubbing, may predispose DS patients to keratoconus.
In contrast to the paediatric population where hyperopia is more common,24 the majority (59.3%) of adult DS patients suffered from myopia up to −21.50 dioptres (mean −5.8 dioptres). This is around triple the figure reported from Western countries such as Canada (14%), Denmark (21%) and the UK (20%).25 Genetic or environmental factors might play a role, as myopia is highly prevalent in the general population in Hong Kong, where 70% has myopia by 17 years of age.26 Myopic degeneration of the retina is found in half of the sample patients. Besides contributing to low vision, a myopic fundus can predispose the subject to an array of serious but treatable ocular conditions such as myopic choroidal neovascularisation, macula hole, foveoschisis and retinal detachment. This suggests a need for regular ophthalmological screening in DS patients such that prompt treatment could be initiated if such conditions are found.
Other ophthalmological abnormalities
Strabismus (mainly esotropia) was common (40%), consistent with other reports (28% up to 50%).15 ,27 Blepharitis and chalazion were also common, present in up to 44% of the individuals. Brushfield spots, in contrast to Caucasians, were rarely found (1.1%). This is consistent with studies done with the Asian paediatric population.24 ,28 Interestingly, although the corneal thickness in DS patients (519 microns) is much lower than the general population (556 microns),29 there was a low incidence of keratoconus (0.5%) in our sample, compared with 4%–16% in the US or UK.5 ,15 An explanation may be that the cornea in the Chinese population is structurally more resilient against keratoconus. Alternatively, subclinical keratoconus may exist in our sample that was not detected during the clinical examination. Corneal topography may be helpful to diagnose such cases. There was also a low incidence (0.5%) of infective keratitis, in contrast to 21% in the Caucasian population, where keratoconus and keratitis were reported as the main visually debilitating eye diseases.5
In our study, there were two patients who had an isolated optic disc pit (without subretinal fluid). This is a rarely reported finding in DS patients which has recently been observed in a cross-sectional study of paediatric mosaic DS patients.30
DS cared for by elderly parents
DS patients in the sample were looked after by elderly parents with a mean age of 66 years. Older parents may face medical problems of their own and rely on an institution to look after their DS child, as evidenced by the mean parental age of institutionalised patients being older than those living at home by 9 years. Less overt visual needs may be overlooked in such a setting, and our finding that patients living in an institution are less likely to be fitted with glasses than at home (24% vs 45%) seemed to echo this point.
Currently, little information is available regarding the adult DS population in Hong Kong. As even the number of adult DS individuals residing in the territory is unknown, it was difficult to assemble a comprehensive sampling frame to select a fully representative sample of the adult DS population. The male to female ratio of 2.1:1 in our sample could indicate an over-representation of male DS patients. However, the male to female ratio in DS live-born is known to be skewed towards male predominance (1.03–1.62).31 ,32 Whether this ratio changes as the patient ages is not well established.
Second, the sample may be biased to a certain extent towards those with vision problems, as the patients and/or guardians were aware during recruitment that the visit was for an eye exam. However, we have discovered that most of the visually debilitating ocular conditions were treatable, and without the free ophthalmic screening exam these vision problems would have remained unidentified. Last, only distant VA was tested in the current study. With a mean age of 38, many patients could be suffering from presbyopia, and near VA may be more directly related to their quality of life. Testing for near vision in future studies may be useful to determine if DS adults could benefit from wearing bifocal glasses.
There is a high prevalence of vision impairment among Chinese DS adults. Uncorrected refractive errors, high myopia and cataracts are the main visually debilitating ophthalmological abnormalities. Vision may be improved through simple use of glasses and early treatment of age-related cataract. The high proportion of patients with visual impairment living in the community cared for by elderly parents suggests the need for special ophthalmic services that provide regular screening for adult DS individuals.
The corresponding author would like to thank optometrists Ms Gloria Leung of from the Eye Institute, the University of Hong Kong, and Ms Carol Chan from the United Christian Hospital for their help in data acquisition. She would also like to thank Mr LM Ho from the Department of Community Medicine, the University of Hong Kong, for his kind assistance with data analysis. She would like to thank the Down Syndrome Association, in particular Ms Eva Sun and Ms Anna Lee, for their tremendous help in the arranging of the logistics for the programme. The assistance of Ms Monica Lee, Ms Ada Yeung from the Department of Ophthalmology, United Christian Hospital, and Ms Lee Man Yee from the Department of Ophthalmology, Queen Mary Hospital, is also gratefully acknowledged.
This content of this paper was partly presented in the Hong Kong Ophthalmological Symposium on 5 November–12 December 2010 at the Hong Kong Convention and Exhibition Centre, Wanchai, Hong Kong.
Contributors The six authors are justifiably credited with authorship, according to the authorship criteria. In detail: AF—conception and design, acquisition of data, analysis and interpretation of data, drafting of the manuscript, final approval given; JS and AN—acquisition of data, analysis and interpretation of data, final approval given; KL—conception and design, acquisition of data, critical revision of manuscript, final approval given; SM—critical revision of manuscript, final approval given; DW—conception, design, interpretation of data, critical revision of the manuscript, final approval given.
Funding This research is supported by the Timothy Liu memorial fund from the Hong Kong ophthalmological society to cover the transport costs and mailing fees, and The Eye Institute, The University of Hong Kong, to cover the examination fees required by the Hospital Authority for the clinical visit.
Competing interests None.
Ethics approval Institutional Review Board of the Hong Kong West Cluster and the Research Ethics Committee of the Kowloon East Cluster (HKWC reference number: UW 10-074; KC/KE reference number: 100045/ER-3).
Provenance and peer review Not commissioned; externally peer reviewed.