Visual Impairment Scotland Report
Chapter 5 Aetiology: when and how a child acquires visual impairment
Number of children in each aetiological group
Most children (82.9%) acquired the cause of visual impairment by the end of their first month of life.
Few children (7.5% acquired visual impairment after the first month of life. These children tended to be more visually impaired compared to children who acquired the impairment earlier.
(For 9.6% of children the time of onset was unknown.) Children who acquired visual impairment around the time of birth were more likely to have developed additional disabilities (89%) compared to those who acquired impairment before birth (42% with additional disabilities).
The pattern of additional disabilities across aetiology groups found by VIS is similar to that found in Sweden. This indicates that the new notification system has identified the full spectrum of children with visual impairment.
5.1 Number of children in each aetiological group
The time of acquiring the cause of childhood visual impairment can be divided into three periods:
- Prenatal - before birth including both genetic and intrauterine causes
- Peri/neonatal - at the time of birth and within the first 28 days of life
- Postnatal - during childhood, after the first 28 days of life
The VIS project uses the NORDSYN study group aetiological classification system. The distribution of the 333 children notified to VIS with significant visual impairment is shown in Table 5.1. The Swedish figures13 are shown for comparison in Figure 5.1.
Disorders acquired in the prenatal period occur most frequently (67%). The most common time prenatally to acquire childhood visual impairment is at conception (41.4%): that is to say by genetically acquiring a visually impairing condition. The time of onset of symptoms of visual impairment is often different from the time of acquiring the cause of impairment. For example, although retinitis pigmentosa is a genetic condition, acquired at conception, it usually becomes slowly and progressively symptomatic in later childhood or early adulthood. Other genetic conditions such as Leber’s optic atrophy can become symptomatic over a short period of time in late childhood or early adulthood, although it is a genetic condition acquired at conception.
It is difficult to identify the exact cause and timing of many conditions prenatally acquired therefore many (21.9%) were categorised as ‘prenatal unspecified’. This is similar to studies from Scandinavia. Many conditions such as microphthalmia and coloboma are likely to have multifactorial aetiologies, both environmental and genetic. There is no way to decide categorically which predominates.
The second most common time to acquire visual impairment in childhood is at and around the time of birth (15.9%). Prematurity and oxygen deprivation are the two main causes of acquired visual impairment at this time.
This study like previous ones found that it is uncommon to acquire a visually impairing condition during childhood (7.5%). However, as described earlier, many progressive conditions (such as retinitis pigmentosa and Leber’s optic atrophy), although present at birth, may become worse during childhood.
The ratio of males to females in the general under 16-year-old population in Scotland is 1.0547. There is a tendency for boys to develop visual impairment prenatally and postnatally but for girls to develop visual impairment peri/neonatally.
Children who prenatally acquire a condition that impairs vision are more likely to be less visually impaired and be found in NORDSYN group A than children who acquire visual impairment postnatally. (Section 4.1 describes the NORDSYN visual function groups.)
Children who acquire visual impairment prenatally are significantly less likely to develop visual impairment with additional disabilities compared to children who acquire visual impairment peri/neonatally.
The pattern of additional disabilities across aetiology groups found by VIS is similar to that found in Sweden13. This indicates that the new notification system has identified the full spectrum of children with visual impairment.