This study has presented data from a population of low vision clinic patients. A major advantage of low vision clinic studies when compared with population surveys, blind school studies or blind register studies is that they provide more reliable and usually detailed ophthalmic information about people with low vision [11
]. However, such information may be rather clinic-specific and be strongly influenced by the sources of referral as well as the acceptance and utilisation of low vision services by the population served by the particular low vision clinic. In addition, the sample size of such studies is often limited as was the case in our study. Thus, they are prone to sampling errors and are limited in their extrapolation to the general population [13
]. Notwithstanding, the information obtained from such studies can be very useful for planning low vision services, active care and rehabilitation [6
The age distribution of our patients is different from previous reports from developed countries but is similar to those from other developing countries. Although the incidence of low vision has been reported to increase with age [12
], in our study, a significant proportion (38.9%) of patients was below 30
years and majority (58%) were below 50
years of age, while less than a third were aged 60
years and above. This depiction of a younger population is similar to findings from Malaysia [14
], Korea [6
], Nepal [15
], and India [16
] in which the proportion of patients aged below 50
years were 74%, 69%, 58% and 68% respectively. In these developing countries, the proportion of low vision patients aged 60
years and above ranged between 16% and 26%.
On the other hand, in studies from developed countries, Leat and Rumney [17
] (United Kingdom) found 77% of their patients to be aged 60
years and above; Elliot et al. [11
] (Canada) reported that 66% of patients were 70
years or older; while in Australia, Wolffsohn and Cochrane [12
] observed that 87% of patients were aged 60
years and above. This difference in the pattern of the age distribution may be a reflection of the older general populations in developed countries [11
] and low life expectancy in developing countries [16
]. Indeed, the proportion of the elderly (65
years and above) within the general population of Nigeria is only 3.3%, based on figures from the 2006 census [18
]. Despite this fact, the difference may actually be an indication that the older population in developing countries are less likely to access and utilise low vision services than those in developed countries as a result of lower literacy and a relative lack of interest in reading.
The relatively high male to female ratio in our study is similar to that of other studies conducted in developing countries as follows: Korea- 1.8:1 [6
], Malaysia- 2.2:1 [14
], Nepal- 2.3:1 [15
], and India- 2.6:1 [16
]. It is, however, different from the pattern in developed countries where more females were found to present for low vision services [11
]. This probably demonstrates the reduced access and utilisation of eye care services by females in developing countries [20
In addition, it has been reported that the female predominance observed in studies from developed countries becomes more noticeable with age and may be related to greater longevity in women [12
]. In our study, however, we found that the proportion of females reduced with age, though this trend was not statistically significant. Further research into the gender distribution among low vision clinic patients may shed more light on this observation.
Majority of our patients considered their problems with near and distance vision to be of equal importance. However, elderly patients were more likely to deem near vision as being their major problem; while children had a tendency to judge distance vision as more important. This observation perhaps portrays the additional effect of presbyopia on low vision in the elderly, although it may also signify that the elderly have a greater likelihood of central visual loss from macular disease.
Posterior segment disease accounted for the majority of causes of low vision in this study. This correlates with findings of most low vision clinic studies [6
]. One difference, however, is that some previous reports found age related macular degeneration (ARMD) to be the commonest cause [11
], while retinitis pigmentosa was the commonest in our study. Despite this difference, we found ARMD to be the second commonest cause, occurring in one out of every seven subjects seen in our low vision clinic. Similarly, in the Nigerian National blindness survey, ARMD was the third most common cause of low vision, accounting for 11.0% of subjects with low vision. Besides, there are other reports, specifically from developing countries, in which ARMD was also not the commonest cause [6
]. Possible reasons for lower prevalence of ARMD in developing countries may include nutritional factors, less cigarette smoking, and lower body mass index (BMI) [22
]. Further research is required for a better understanding of the role these factors in developing countries.
Retinitis pigmentosa has been found to be a major cause of low vision in only a couple of previous studies. Khan found it to be the commonest cause in a population of 410 low vision patients in India [16
]. While Mohidin and Yusoff observed it to be the second commonest cause in a Malaysian low vision clinic [14
]. Further research is necessary to investigate this finding in our patients in order to elucidate a possible explanation.
Glaucoma occupied the fourth position as a cause of low vision in our study population (11.4%) in contrast to findings of the Nigerian National Blindness and Visual Impairment Survey in which glaucoma was the most common (26.6%) cause of functional low vision [3
]. The small sample size of our study may account for this difference. It may, however, be suggestive of poor uptake of low vision services by patients with glaucoma. Some glaucoma patients needing low vision care may not have been referred to the low vision clinic and may therefore have been missed. It is thus necessary to educate eye care providers and glaucoma patients about the available options of low vision assessment and low visual aids.
The low frequency of diabetic retinopathy as a cause of low vision in our study is contrary to findings from most of the previous reports from both developing and developed countries in which it usually featured as the second or third commonest cause [6
]. Our finding is, however, in keeping with the low prevalence of diabetic retinopathy as a cause of blindness observed during the Nigerian National Blindness and Visual Impairment Survey [23
Contrary to the previous report by Richard [24
] in which cataract was the most common cause of low vision, cataract was uncommon in our study because most cases of cataract were satisfactorily managed in the main eye clinic by surgery. The few patients with cataracts in our study either had another disease as the primary cause of low vision (e.g. retinitis pigmentosa) or could not be operated because of poor health or other social reasons.
The major causes of low vision within the different age groups as shown in Figure are quite similar to other reports [12
]. As expected, congenital and heritable conditions were more common in children, while age related diseases were predominant in the elderly patients.