In this study, we demonstrated that only 35.7% of the patients under 3 years of age with low vision had isolated ocular pathologies. Remaining 64.3% had CVI alone or CVI and ocular pathologies together. While the most common ocular pathologies were oculocutaneous albinism and Leber congenital amarousis (LCA), the most common causes of CVI were HIE and prematurity. The patients with CVI had lower visual function score than the patients with ocular pathologies. The patients with neurological structural disorders and HIE had worse visual function than the other etiologies of CVI. When the MRI findings were evaluated, the most common finding was widespread white matter involvement. The patients with widespread involvement of grey and white matter had lower visual function score. Presence of seizure negatively affected the visual function.
Factors causing low vision in children vary according to the development levels of the countries. Cerebral visual impairment was reported as the most common cause of visual impairment in children in developed countries. [6–8] Additionally, avoidable causes including preventable and treatable causes are less in these countries. [9, 10] Although preventable and treatable causes are more common in developing countries, the frequency of CVI is also increasing. [11–13] With advances in perinatal care, survival of babies with low gestational age increased. Therefore, CVI has become one of the main causes of low vision in children nowadays. In our study, we found CVI as the main cause of visual impairment in children under 3 years, like the results of developed countries.
Although CVI may occur due to different etiologies including hypoxia, seizure, hypoglycemia, trauma, neurodevelopmental and neurodegenerative disorders, HIE was reported as the most common cause.[14] In the presence of hypoxia and accompanying hypercarbia, vascular autoregulation of brain is impaired, and this results in decreased cerebral perfusion and brain damage. [15] Khetpal et al. evaluated 161 children with CVI, and they found that the most common etiology was perinatal hypoxia (36%). [16] Huo et al. reported the ratio of perinatal hypoxia in patients with CVI as 22.3%. [17] Similar to the literature, HIE was the most common cause of CVI (27.5%) in our study. Another important finding of this study in terms of HIE was that patients with HIE had lower visual scores than other causes of CVI, at the time of admission. Due to the proximity of the visual pathways to the periventricular white matter, this region is of great importance for visual function. Hypoxic ischemic encephalopathy often involves periventricular white matter. The poor visual function in HIE patients can be explained by the damage to the visual pathways.
Prematurity is the other common cause of CVI. The incidence of prematurity in CVI patients was 30% in the study of Khetpal et al. [16] Similar to the reported study, the second most common cause of CVI was prematurity (26.9%), in the present study. Periventricular leukomalacia, reduced volume of periventricular white matter and delayed myelination result in CVI in premature babies. As the periventricular leukomalacia is the most common form of cerebral injury in premature infants, these babies have a high risk of developing visual impairment. Additionally, in this study we demonstrated that patients with CVI alone or CVI and ocular pathology together had worse visual scores than patients with ocular pathologies. This finding suggests that effect of brain structures that are responsible for visual physiology, results in worse visual score of low vision patients.
Different types of cerebral injury may be seen in children with low vision. Depending on the cause, optic nerve, optic tract, optic radiation, occipital cortex, brainstem, basal ganglia, gray and white matter may be affected. In addition, the severity and spread of the involvement may differ between patients depending on the etiopathogenesis. Location and severity of the involvement affect the ocular findings and visual function. Association between different types of brain injury and visual function was evaluated in a few studies. Hoyt et al. found that visual functions of patients with periventricular leukomalacia and striate cortex damage were similar in CVI patients. [15] Cioni et al. showed that patients with damage to periventricular white matter involving optic radiations had worse visual function than visual cortex involvement.[18] Either extensive periventricular white matter damage or striate cortex infarcts was correlated with poor visual function in the study of Eken et al. [19] We also evaluated the association between the location of brain damage and visual function. The most common involvements were widespread white matter involvement (24.5%) and widespread involvement of grey and white matter (15.3%). Patients with widespread involvement of grey and white matter had the worst visual function in our study. Different results of these studies may be related to the method for evaluating visual function. While Eken et al. used Teller acuity tests, Hoyt et al. used a 6 point scoring system. Unlike other studies, we developed a new 15 point scoring system which can evaluate visual function parameters more detailed.
Cerebral visual impairment is rarely seen alone and is often accompanied by another ophthalmic or neurological disorder. Seizures may be the cause of CVI alone or accompany other causes leading to CVI. Huo et al. reported that the most common neurological abnormality in the CVI patients was seizure. [17] Additionally Handa et al. found the coexistence of seizure with CVI as 93%. [20] We evaluated the effect of seizure on visual function, and we found that patients with seizure had worse visual scores. The negative effect of the seizure on visual function can be explained by the damage of the seizure to optic radiations and visual cortex.[17, 21]
In our study the most common causes of ocular pathologies were albinism and LCA. These diseases are hereditary diseases and the prevalence of hereditary diseases are higher in countries where consanguineous marriage is more common. In the present study, hereditary ocular pathologies were the most common causes since consanguineous marriages were more than one third of the patients. The third most common ocular pathology was ROP. Nowadays, with the development of neonatal intensive care conditions, ROP does not take the first place among ocular pathologies in developed countries. [22] Ocular pathologies responsible for visual impairment were like the results of developed countries in our study.
This study evaluated a broad number of low vision children between the age of 6 months and 3 years. Thereby, etiologies in children with low vision could be studied in a large population. Unlike other studies, we developed a new scoring system to evaluate visual function. Thus, we were able to evaluate the visual function even in children with very low vision. Additionally, we could identify the parameters that affect the visual function.
This scoring system will enable us to make a numerical evaluation in following the visual functions of babies and young children. In addition, it will provide an opportunity to objectively evaluate the effectiveness of visual habilitation programs that we prepare and implement individually depending on whether they are ocular or cerebral.