This one-year descriptive study was conducted on completely blind and visually-impaired patients who presented at a rehabilitation clinic to receive visual aids. All patients provided written informed consent. Patients were examined by an expert optometrist. At the end of the examination, vision aids were prescribed based on visual acuity and patients' requirements. The vision aids consisted of typical far glasses, telescopic glasses (power 2× and 4×), max TV (power 2×) and electronic devices for far and near vision, hand-held magnifiers (with or without illumination), closed-circuit television, and electronic devices for near-vision.

Refractive error was measured using a retinoscope. Best-corrected visual acuity (BCVA) and uncorrected visual acuity (UCVA) were assessed using the Snellen chart. Patients were divided into three groups according to their BCVA following the World Health Organization classification method: 1) mild to moderate low vision (20 / 60 < BCVA ≤ 20 / 200), 2) severe low vision (20 / 200 < BCVA ≤ 20 / 400), and 3) blind (BCVA < 20 / 400) [5].

All physicians who referred the patients were asked to describe the main cause of low vision based on their clinical and paraclinical evaluations. For any referred patient without an exact diagnosis, further evaluation and diagnostic procedures were recommended and patients were referred to an expert ophthalmologist.

The International Classification of Diseases (ICD-10) was used to classify the patients according to the primary cause of low vision and blindness for both eyes [6]. Based on this classification, any disease is categorized by a specific code which itself is a subheading of a general heading that classifies the disorders based on the anatomic site of involvement.

To describe data, we used mean ± SD and frequency (percent). All statistical analyses were performed using SPSS ver. 15 (SPSS Inc., Chicago, IL, USA).

During the study period, 423 consecutively referred patients including 275 men (65%) and 148 women (35%) were enrolled. All were Caucasian. The age ranged between 3 to 92 years old and the mean age was 43.6 ± 25.2 years. Of those, 68 patients (16%) were illiterate and 305 patients (72%) reported having under high-school diploma degree and 50 patients (12%) reported having a high-school diploma or higher educational degrees.

All patients had some degree of visual impairments. BCVA less than 20 / 200 was seen in 71.2% of the patients. Table 1 displays the number of patients in each level of the visual impairment. Diagnosis was confirmed in 418 out of 423 patients. The most common cause (74.5%) of visual impairment in all ages was retinal and choroidal diseases (Table 2). In the retinal and choroidal group, macula and posterior pole degeneration were seen in 92 patients (29.6%) and diabetic retinopathy, retinitis pigmentosa, Stargart disease and macular dystrophy were seen in 58 (18.7%), 54 (17.4%), 35 (11.2%), and 30 (9.6%) patients, respectively (Table 3).

The main causes of low vision were categorized based on age (Table 2). It was noted that retinal and choroidal diseases were the leading cause of visual impairment across all age groups. We also examined the most common disorders in the retinal subgroup by age (Table 3) and we found that the distribution of the retinal disorders varied by age. The most common retinal disorder in the patients less than 30 years of age was macular dystrophy, while in the age group of 31 to 50 and above 50 years of age, it was diabetic retinopathy and degeneration of the macula and posterior pole, respectively.

This study demonstrated that the leading causes (74.5%) of visual impairment, based on the major headings of ICD-10 classification, in patients referred to a visual rehabilitation clinic in Iran were retinal and choroidal diseases. Optic nerve and optic tract diseases, vitreous and globe disorders, congenital cataract and glaucoma were less common causes. Although this pattern of prevalence was observed across all age subgroups, the main causes responsible for visual impairment, according to the subheadings of the classification, varied among them. In patients less than 30 years of age, retinitis pigmentosa and Stargardt disease were the most prevalent. In adults between 31 to 50 years of age, diabetic retinopathy and retinitis pigmentosa were the most prevalence and in those above 50 years of age, degeneration of the macula and posterior pole were the most common causes.

In a retrospective study conducted with 4,711 patients who presented to a rehabilitation clinic in Tubingen (Germany), age-related macular degeneration was the most important cause (40%) of visual impairment, followed by tapetoretinal dystrophy, optic nerve atrophy and diabetic retinopathy [7]. In another retrospective study with 573 patients in a Malaysian rehabilitation clinic, the most common cause of visual impairment varied by age; congenital disease was most prevalent in individuals less than 30 years of age, those between 30 and 60 years of age reported the most retinitis pigmentosa, and those greater than 60 years of age reported the age-related macular degeneration [8]. In a study conducted with 362 students in three schools for the blind in Tehran (Iran), the rate of severe low vision was 8.9%. The most common cause of low vision in that study was retinal diseases (51%). Cataract, optic nerve atrophy, corneal and anterior segment disease, glaucoma, anophthalmia and globe malformations were other common causes of low vision in schools in Tehran [9]. In the current study, 41.8% of our patients were above 50 years of age, which might explain the high prevalence of diabetic retinopathy and age-related macular degeneration. This finding is in line with the global data on visual impairment in the year 2002 [1].

The extant literature thus far suggests a preponderance of males with visual impairments. Nguyen et al. [7] found that 58.9% of the patients were male. In the current study, the majority (65%) of the enrolled populations were male as well. If we divide the cases into two groups (under 30 and above 30 years old) the male preponderance is still greater in the older group; 75.8% in patients above 30 years old versus 54.2% in adults under 30. The two most common diseases among patients older than 30 years old in our study were diabetic retinopathy and age-related macular degeneration, which have not been reported to be more frequent in men. In this sample, it appears that men more than women seek services from the rehabilitation clinic. Additionally in the current study, we noticed that a significant number of people with elementary or below a bachelor's level of education (88%) presented to the visual rehabilitation clinic, suggesting that highly educated people are not the only clients seeking to improve and increase their visual ability.

In the current study, retinal and choroidal diseases were the most common causes of visual impairment. There are three explanations possible: 1) retinal and choroidal diseases are less treatable and reversible compared with other ophthalmologic problems, 2) retinal and choroidal diseases are more frequent in our society, and 3) patients with these types of ophthalmic problems may be referred more often to rehabilitation clinics by ophthalmologists. Cataract problems are the main cause of visual impairment in most population-based studies [1]. Other causes in decreasing order of frequency include refractive errors, trachoma, glaucoma, macular degeneration and retinal diseases [6]. Since our study was conducted with referred patients to a visual rehabilitation clinic, the two common diseases (i.e., cataract and refractive errors), were not seen as major causes of vision loss in our population. These findings confirm the first hypothesis because patients with cataract and refractive errors are usually managed properly in our society, especially in the capital city of Tehran.

The assumption of a higher prevalence of retinal diseases among our society (the second hypothesis) is supported by data from an epidemiologic population-based study performed in Tehran. It reported that the most common cause of visual impairment after two treatable diseases (refractory errors [33.6%] and cataract [25.4%]) was macular degeneration (20%). In this report, the prevalence of retinal vascular diseases was 4% [10]. Additionally, retinal diseases were also found to be the most common etiology (51%) of visual impairment among youth from schools for the blind in Tehran [9]. It is possible that not only the acquired but also the congenital forms of retinal disease are highly prevalent in our population. This finding converges with similar studies conducted in Brazil [11], Germany [7], India [12], Thailand [13], and the Netherlands [14] that show that the most common cause of low vision is retinal diseases. Conversely, in studies performed in Ethiopia [15] and Uganda [16], the anterior segment of diseases had a more important role in vision loss, which might reflect a high prevalence of infectious diseases and vitamin A deficiencies in these societies. Further, in a study with 3,210 children in Brazil, the most common cause of visual impairment in children with multiple disabilities were optic nerve atrophy (37.7%) and cortical blindness (19.7%). The authors emphasized the physical examination of other parts of the body in a child with optic nerve atrophy [11]. In the current study, optic nerve diseases had the second highest prevalence after the choroidal and retinal diseases.

This study did not involve all-blind people because they are not usually referred to rehabilitation centers. It should be cautioned that because this current study only examined those who presented at a rehabilitation clinic, the generalizability of these results is limited. However, this study identified the irreversible and untreatable causes of low vision in our society and could be helpful in planning for screening programs. Moreover, the results of these types of studies can be used in creating and informing guidelines of practice for rehabilitation clinics. Further studies especially in other cities with larger populations are warranted.