A longitudinal study of development in language,cognition,social adaptation,and exploratory behaviour of toddlers with vision impairment

Abstract

Research on children with vision impairment (VI) has primarily focused on comparing their development to that of sighted peers, offering limited insights into their individual developmental trajectories. To enhance early intervention, understanding these individual pathways is crucial. This study examined the developmental trajectories of 24 toddlers aged 8 and 38 months (M = 19.96, SD = 8.06) with VI in four key domains: language, cognition, social adaptation, and exploratory behaviour. Over 2 years, participants were assessed during home visits using the Reynell Zinkin Scales. Individual plots and linear mixed effect models revealed individual developmental trajectories, with most children showing progress in social adaptation and exploratory behaviour before language and cognition. No significant differences in developmental growth were found between children with moderate VI or severe VI. This longitudinal investigation emphasizes diversity within the population of children with VI suggesting that factors beyond VI itself should be considered when addressing developmental differences.

Keywords

Children development longitudinal toddlers visual impairment

In a review study, Veldhorst et al. (2022) found ample examples of delayed development in children with vision impairment (VI), probably because vision has a pivotal role in early childhood development (Dale et al., 2022). Children with VI often experience delays ranging from several months to years in areas such as executive functions (Cavezian et al., 2013), expressive language development (Reynell, 1978), joint attention (Bigelow, 2003), social communication (Greenaway & Dale, 2017), manual skills (Brambring, 2007a), theory of mind (Begeer et al., 2014), and motor skills (Elisa et al., 2002; Levtzion-Korach et al., 2000). The delays can persist into later childhood; school-age children with VI have been shown to encounter difficulties in executive functions (Bathelt et al., 2018), communication (James & Stojanovik, 2007), or adaptive development (Bathelt et al., 2019; Metsiou et al., 2011; Papadopoulos et al., 2011).

While it is well established that children with VI as a group are at risk for developmental delays, there is limited understanding of when these delays begin, how they evolve and the individual variation. Previous studies have examined various developmental domains in children with VI, but it remains unknown whether these group data suffice for designing effective interventions for individual children (Elsman et al., 2019a). To address this gap, the current study adopts an individual differences approach, studying individual developmental trajectories in developmental areas know to be at risk in children with VI and for which objective measures are available: language, cognition, social adaption, and exploration of the environment. By enhancing the knowledge of the development of children with VI, without multiple disabilities, it provides insights for improving early intervention services.

In brief, this is what is known for motor and mobility development, touch, adaptive skills, language, and cognition. Independent exploration of the environment is crucial for mobility, allowing individuals to navigate without external assistance, and enhancing social participation (Altunay et al., 2021). Learning by touch can largely replace learning by vision, though it requires more time to comprehend object form, structure, and function. Despite being less efficient, touch perception is not necessarily less accurate (Reynell, 1978). The severity of the VI affects exploration skills; children with low vision facing fewer problems than blind children (Bigelow, 1996). As children age, vision loss becomes less critical, as cognitive and language abilities aid spatial and object understanding (Reynell, 1978; Withagen et al., 2012). However, research often focus on very specific exploration skills, such as sound exploration (Verver et al., 2019) and haptic exploration within reaching distance (Withagen et al., 2012), leaving a gap in the understanding of general exploratory skills of young children with VI.

Adaptive skills development in children with VI has received limited research attention. It entails social responses to others and self-help skills (Reynell, 1978). Children typically acquire adaptive skills by observing and imitating parents (Bishop, 2004), but this can be challenging for children with VI, due to their inability to visually observe role models. The extent of delays in learning adaptive skills remains uncertain. Whereas Reynell (1978) found no differences between children with moderate and severe VI, Brambring (2007a) observed delays for children with severe VI, albeit varying across different skills. Recent research showed that older children and adolescents having severe VI exhibited greater delays in adaptive behaviour compared to sighted peers (Bathelt et al., 2019). The timing of these delays and individual differences within the group of children with VI remain unknown.

While it is commonly asserted that the language development of young children with VI is impaired or delayed, a more nuanced view is necessary (Vervloed et al., 2005). Language problems primarily occur in word meaning and pragmatics (Tadic et al., 2010). Impaired vision limits exploration opportunities, hindering understanding of referents, that is the objects, people, and situations words refer to. Less semantic knowledge can therefore lead to differences in concept development compared to sighted children (Warren, 1994). For instance, Andersen et al. (1993) found that children with severe VI acquired word labels later than children with moderate VI. Furthermore, Dale and Sonksen (2002) observed delays in language structure among children with severe VI. However, previous research indicated no significant differences in language subdomains such as syntax, grammar, and vocabulary between children with and without VI (Pérez-Pereira & Conti-Ramsden, 1999; Tadic et al., 2010). As children age, language differences mostly disappear, while some semantic and pragmatic issues may persist in children with the most severe vision impairment (Brambring, 2007b; Pérez-Pereira & Conti-Ramsden, 2001).

Cognitive development also appears negatively impacted by VI (Cass et al., 1994; Dale et al., 2017; Sakkalou et al., 2021), though findings are inconclusive (Platje et al., 2018; Sakkalou et al., 2021). Severity of VI seems important, with blind children showing more delays than children with low vision (Cass et al., 1994). However, with age, children improve their understanding of abstract concepts and develop strategies to overcome visual limitations (Reynell, 1978). It is important to realize that a lot of children with VI receive early intervention, with parents supported in applying effective parenting strategies (Platje et al., 2018; Sakkalou et al., 2021; Tadić et al., 2009).

Current knowledge reveals gaps in understanding development of children with VI. While the impact of VI on developmental domains may decrease with age, severe developmental delays or setbacks can still occur (Cass et al., 1994; Dale & Salt, 2008; Dale & Sonksen, 2002), implying possible stasis or regression in the development (Vervloed et al., 2019). However, not all children with VI experience setbacks; this population is highly heterogeneous (Heppe et al., 2024). Most children exhibiting developmental delays in studies by Cass et al. (1994) and Dale and colleagues (Dale & Sonksen, 2002; Dale & Salt, 2008) were either blind or had light perception only, whereas the majority of children with VI retain some residual sight (Ravenscroft, 2019). In addition, the presence of multiple disabilities complicate assessment of development (Dale & Sonksen, 2002).

A recent review showed that children with VI are frequently compared to sighted peers (Veldhorst et al., 2022). While such comparisons provide benchmarks for typical development, it does not offer insights into individual trajectories of children with VI (Ravenscroft, 2019; Warren, 1994). Despite Warren’s (1994) advocacy to focus on variability within the VI population of children rather than group averages, research utilizing an individual differences approach remains rare. Many studies also rely on retrospective designs (Vervloed et al., 2019), risking biased outcomes (Dale & Sonksen, 2002). In contrast, prospective longitudinal studies with frequent measurements and emphasizing individual differences offer most valid results into development over time (Vervloed et al., 2019). However, such studies are scarce (Veldhorst et al., 2022), and cross-sectional designs remaining common. The few longitudinal studies date back to the late 20th century (Fraiberg, 1977; Jan et al., 1977; Norris et al., 1957; Pérez-Pereira & Conti-Ramsden, 1999; Warren, 1984, 1994; Webster & Roe, 1998), reflecting different expectations and demands on children with VI then nowadays.

The current study presents a first step towards increasing understanding of individual developmental trajectories of toddlers with VI in established domains: cognition, language, adaptive skills, and exploratory behaviour. The study adopted a longitudinal, prospective design to capture developmental changes over time. Although primarily explorative and descriptive, this investigation aims to establish a foundation for creating tailored support and interventions reflecting the unique characteristics of individual children with VI (Vervloed et al., 2024), emphasizing the role of the severity of the VI. By accounting for individual variation, the study helps design interventions more precisely attuned to individual needs (Elsman et al., 2019a) or observations to ascertain whether developmental delays self-correct. The study is guided by two research questions:

What are the developmental trajectories of cognition, language, adaptive skills, and exploratory behaviour of toddlers with VI in the first 4 years of life?

To what extent does the severity of the VI affects the developmental trajectories of the toddlers?

Method

Design and procedures

This study is part of the longitudinal study ‘Prospective Longitudinal Cohort study Children with Vision Impairment’ (PLoCC-VI). The study adhered to the ethical principles of the Declaration of Helsinki (World Medical Association, 2022) and obtained approval from the Medical Ethical Committee Eastern Netherlands, Nijmegen, the Netherlands (No. NL74630.091.20). Participants were recruited via regional centres of two Dutch organizations for people with VI. Parents of children meeting inclusion criteria received a flyer from their service provider. Those interested in participating contacted the primary investigator and were provided with a digital information and consent letter. A reminder was send 2 weeks later. Upon participation, parents granted consent for both themselves and their child. Data collection took place through home visits from 2021 until 2024, conducted by the first author or research assistants. Parents were present throughout the home visits. Each participant was visited for a period of 2 years, resulting in five to eight waves in total. The total amount of home visits varied as a result of the child’s age at the time of entry in the study. The end date was fixed for all participants.

Participants

Participants in this study were toddlers with VI. To be eligible for inclusion, children needed a diagnosed VI (either ocular or cerebral) and aged between 6 and 32 months at the start. Parents had to be proficient in either Dutch or English. All children were registered at a regional centre for support for people with VI in the Netherlands. Children with evident multiple disabilities were excluded. The sample consisted of 29 children with VI at the start of the study. However, five children turned out to have multiple disabilities after enrolment, leading to cancellation of home visits an exclusion of the data in consultation with parents. The final sample size was 24, with ages between 8 and 32 months (M = 19.96, SD = 8.06) at the first wave. Based on medical data obtained from the regional centres, children were classified as having moderate low vision (visual acuity between 5/100 and 30/100 or field of vision between 10° and 30°), severe low vision (visual acuity between 5/100 and 30/100 or field of vision between 10° and 30°), or blindness (World Health Organization, 2007). The final group included 18 children with moderate low vision (75%), four with severe low vision (16.70%) and two with blindness (8.30%). Demographic information is presented in Table 1.

Table 1.

Sociodemographic characteristics of the participants (N = 24).

Participant	Sex	Age at onset of the study (months)	Length of gestation (weeks)	Severity VI	Diagnosis^a	Course of the VI	Bi-monolingual home language	Place in the children’s row^b
1	Boy	11	Term	Severe	Coloboma of iris, choroid and retina and optic nerve ODS^c	Uncertain	Monolingual	1(1)
2	Girl	26	Term	Moderate	Achromatopsia	Uncertain	Monolingual	2(2)
3	Girl	14	Term	Moderate	Achromatopsia	Uncertain	Monolingual	1(1)
4	Girl	19	Term	Moderate	Exudative vitreoretinopathy	Progressive	Monolingual	2(2)
5	Girl	8	Term	Moderate	Albinism	Uncertain	Monolingual	2(2)
6	Boy	9	Term	Moderate	Albinism	Uncertain	Monolingual	2(2)
7	Boy	25	Preterm (31 + 3)	Moderate	Congenital nystagmus	Uncertain	Monolingual	1(2)^d
8	Boy	25	Preterm (31 + 3)	Moderate	Congenital nystagmus	Uncertain	Monolingual	1(2)^d
9	Boy	17	Term	Severe	Coloboma of iris and retina coloboma	Uncertain	Monolingual	1(1)
10	Boy	22	Unknown	Moderate	Albinism (suspected)	Uncertain	Monolingual	2(2)
11	Girl	22	Term	Severe	Peters anomaly	Uncertain	Monolingual	1(2)
12	Boy	32	Term	Moderate	Retinal damage due to toxoplasmosis ODS^c	Uncertain	Bilingual	2(2)
13	Girl	24	Term	Moderate	Alternating and intermittent squint with nystagmus and mild refraction errors ODS^c	Uncertain	Monolingual	3(3)
14	Girl	23	Unknown	Moderate	Congenital nystagmus	Uncertain	Monolingual	2(2)
15	Boy	21	Term	Moderate	Axenfield Rieger Syndrome	Uncertain	Bilingual	1(1)
16	Girl	10	Unknown	Moderate	Morning glory disc anomaly OS^c, nystagmus OS^c	Stable	Bilingual	1(1)
17	Girl	9	Term	Blind	Leber Congenital	Stable	Bilingual	1(1)
18	Girl	8	Unknown	Moderate	Congenital microphthalmia OS^c	Stable	Bilingual	1(1)
19	Girl	30	Unknown	Moderate	Alström-syndrome	Progressive	Monolingual	1(1)
20	Boy	27	Preterm (30)	Severe	Persistent foetal vasculature OS^c and symptoms of CVI	Uncertain	Monolingual	1(2)^e
21	Girl	27	Unknown	Moderate	High myopia	Uncertain	Monolingual	4(4)
22	Girl	32	Unknown	Moderate	Binocular single vision, hypermetropia and astigmatism	Uncertain	Monolingual	2(2)
23	Boy	11	Preterm (36 + 4)	Blind	Anopthalmia OD^c, micropthalmia with ablatio retinae OS^c	Stable	Bilingual	4(4)
24	Girl	27	Term	Moderate	Microphthalmia with congenital cataract and palatoschizis	Uncertain	Bilingual	1(2)

Because of the young age of the participants, diagnoses should be interpreted with caution.

Place in the child row. Total amount of children within the family within brackets ().

ODS = both eyes, OD = right eye, OS = left eye.

Participant 7 and participant 8 were twin brothers.

Participant 20 was part of a twin. The twin-sibling had no VI.

Assessments

Children’s developmental outcomes on language, cognition, social adaption, and exploratory behaviour were assessed with the six subscales of the Reynell Zinkin Scales (RZS, Reynell, 1979), utilizing the Dutch revised edition (Vervloed et al., 2000). The RZS are a semi-standardized instrument with norms for blind or partially sighted infants in the UK version (Reynell, 1979) and for children with VI in the Dutch version (Vervloed et al., 2000). The RZS are commonly used in the United Kingdom to study the development of children with VI (Dale et al., 2018; Dale & Sonksen, 2002; Sakkalou et al., 2021). The six subscales cover diverse aspects of development, namely social adaptation (SA); motor and mobility skills in the scale exploration of the environment (EE); cognitive development in the scale sensorimotor understanding (SMU); language understanding in the scale response to sound and verbal comprehension (RSVC); expressive language in the scales vocalization and expressive language structure (ELS); and expressive language, vocabulary, and content (ELVC). Raw scores on the RZS were converted to equivalent developmental age levels following the procedure of the original RZS (Reynell, 1979) and using the reference values from the revised Dutch edition of the RZS (Vervloed et al., 2000). The RZS scores signify a developmental age; this means that for example a participant with a score of 24 on the SMU scale possesses sensorimotor competences akin to a child of 24 months old.

Data analysis

RStudio (version 4.3.1 for Windows) was used for statistical analyses. To address research question 1, individual developmental trajectories across the six RZS were visualized using ggplots. A linear mixed effect model (lmer function) was then applied to analyse the participants’ development across the scales. Despite a small sample size, this approach accounts for the hierarchical structure of the data (multiple observations nested within participants) and group heterogeneity within the group (Bolker et al., 2009; Garson, 2013). Age was standardized (M = 0, SD = 1), and fixed (RZS, age, and their interaction) and random effects (individual differences in intercepts and slopes) were included. Fixed effects are variables that are consistent and predictable across all participants, whereas random effects account for individual differences. For research question 2, a similar model examined whether the developmental trajectories varied by severity of VI, incorporating severity as a fixed effect. For both research questions effects sizes and corresponding 95% confidence intervals were included in the analyses.

Results

Individual developmental trajectories

Figure 1 shows the individual plots for each participant’s development across the six RZS, ordered by their age at wave 1. The plots reveal substantial variation across the different RZS, with SA and EE demonstrating the highest score at wave 1, confirmed by the mixed effect model. SA served as the reference domain (β = 39.32, p < .01), showing the highest score at wave 1. The coefficients for the other scales showed lower scores for ELC, ELS, SMU and RSVC at wave 1 than SA (β = −2.15 to −3.75, p < .01). The difference for EE was not significant (β = −1.57, p = .10). Table 2 presents the fixed effect estimates.

Figure 1.

Individual plots of the developmental trajectories on the six RZS for each participant.

Table 2.

Fixed effects estimates.^a.

	β	SE	df	t-value	p-value	Standardized β	CI lower	CI upper
Main effects
SA (Intercept)	39.32	1.05	22.21	37.48	<.01	0.18	0.02	0.33
EE	-1.57	0.90	19.50	-1.75	0.10	-0.12	-0.24	-0.00
ELC	-3.75	0.89	24.64	-4.21	<.01	-0.32	-0.44	-0.20
ELS	-3.37	0.85	20.23	-3.98	<.01	-0.29	-0.41	-0.17
SMU	-2.15	0.64	49.70	-3.38	<.01	-0.18	-0.30	-0.06
RSVC	-2.72	0.76	28.81	-3.56	<.01	-0.23	-0.35	-0.11
Age	7.27	0.98	31.04	7.41	<.01	0.63	0.54	0.72
Interaction effects
EE × Age	2.16	0.60	276.40	3.63	<.01	0.03	-0.09	0.15
ELC × Age	2.87	0.65	225.61	4.46	<.01	0.28	0.16	0.40
ELS × Age	2.23	0.64	171.68	3.48	<.01	0.13	0.01	0.24
SMU × Age	1.86	0.58	440.74	3.23	<.01	0.14	0.02	0.26
RSVC × Age	1.09	0.61	247.40	1.79	.08	0.12	-0.00	0.24

β, beta; SE, standard error of the mean; df, degrees of freedom; EE, exploration of the environment; SMU, sensorimotor understanding; RSVC, response to sound and verbal comprehension; ELS, vocalization and expressive language structure; ELVC, expressive language, vocabulary and content; CI, 95% confidence intervals.

The plots in Figure 1 show that most children scored above average for their age, achieving maximum scores earlier than the RZS norm group, suggesting a ceiling effect. Children showed the biggest progress in ELC, which was confirmed by the mixed effect model. The interaction between the six scales and age demonstrated that scores changed over time, with EE, ELC, ELS, and SMU increasing significantly more with age than SA (β = 1.86–2.87, p < .01). The increase for RSVC was not significant (β = 1.09, p = .08). Despite lower initial scores on ELC, ELS and SMU compared to SA, children progressed on those scales as they aged.

The random slopes for age and the six RZS (see Table 3) indicated considerable variability in how RZS scores changed with age. Specifically, the variance for the slopes highlighted that the relationship between age and RZS scores was not uniform across participants. Negative correlations suggested that participants with higher intercepts showed slower increases in RZS scores with age than participants with lower intercepts, meaning their developmental progress was more gradual. Positive correlations indicated similar score change patterns, reflecting how different developmental measures tended to increase together with age.

Table 3.

Random effects estimates.^a.

Effect	Variance	SD	Correlation with age
SA (Intercept)	21.673	4.655	-0.27
Age	18.209	4.267
EE	11.676	3.417	0.35
ELC	11.161	3.341	-0.38
ELS	9.323	3.053	0.18
SMU	2.018	1.421	0.35
RSVC	6.213	2.493	-0.31
Residual	20.790	4.560

SD, standard deviation; EE, exploration of the environment; SMU, sensorimotor understanding; RSVC, response to sound and verbal comprehension; ELS, vocalization and expressive language structure; ELVC, expressive language, vocabulary and content.

Examination of the patterns in the plots points towards the possibility to categorize the growth of the children into linear, quadratic, or no growth. However, closer visual analysis reveals this categorization largely reflected the age at which the children entered the study. For example, the youngest children (#1, 5, 6, 16, 17, 23) displayed linear growth, as the RZS allows for improvement. In contrast, many older children (#2, 12, 13, 19, 20, 21, 22) showed little to no growth, due to already high RZS scores. Some children showed slow or minimal growth on ELC (#16, 17, 18), notably all these children were bilingual. A few children showed irregular progression on the RZS (#9, 10, 14, 23, 24), with three having severe VI and/or being bilingual. Despite the individual variation, all children progressed on the RZS by the end of the study.

Differences in developmental trajectories for the severity of the VI

To answer research question 2, we compared the longitudinal development on the RZS for children with moderate VI, severe VI, and blindness. Because only two children were blind, they were added to the group with severe VI, resulting in a dummy variable with moderate VI and severe VI as levels. We used the same mixed effect model as for research question 1, and added the severity of VI as an additional fixed effect. Because of a singularity warning for the full model, we simplified the model by deleting the random slope for developmental domain. No significant interaction terms were found for the scales, age and the severity of the VI (β = −1.14 to 2.00, p > .05). This means that progress on the scores on the RZS did not differ between children with moderate and severe VI (see Table 4).

Table 4.

Fixed effects estimates for the differences between children with moderate and severe VI on the RZS.^a.

	β	SE	df	t-value	p-value	Standardized β	CI upper	CI lower
Main effects
SA (Intercept)	36.88	2.07	29.00	17.85	<.01	0.82	0.43	1.21
EE	-1.52	1.23	705.31	-1.24	.22	-0.21	-0.47	0.05
ELC	-1.33	1.24	705.54	-1.07	.29	-0.65	-0.92	-0.38
ELS	-1.50	1.24	705.54	-1.21	.23	-0.68	-0.95	-0.41
SMU	-3.85	1.23	705.31	-3.13	<.01	0.08	-0.18	0.34
RSVC	-1.57	1.23	705.31	-1.28	.20	-0.36	-0.62	-0.09
Age	8.48	2.02	30.55	4.20	<.01	0.46	0.07	0.85
Severe VI	3.28	2.39	29.29	1.37	.18	-0.32	-0.59	-0.05
Interaction effects
EE × age	1.14	1.28	705.31	0.89	.37	0.01	-0.24	0.26
ELC × age	2.32	1.29	705.40	1.80	.07	0.46	0.21	0.71
ELS × age	0.24	1.29	705.40	0.19	.85	0.38	0.13	0.63
SMU × age	1.49	1.28	705.31	1.17	.24	0.25	0.00	0.50
RSVC × age	0.73	1.28	705.31	0.57	.57	0.24	-0.01	0.49
Interaction effects
EE × severe VI	0.21	1.43	705.31	0.15	.88	0.06	-0.12	0.25
ELC × severe VI	-3.54	1.44	705.48	-2.46	.01	0.24	0.04	0.43
ELS × severe VI	-2.85	1.44	705.48	-1.97	.05	0.28	0.08	0.47
SMU × severe VI	2.43	1.43	705.31	1.70	.09	-0.20	-0.39	-0.01
RSVC × severe VI	-1.67	1.43	705.31	-1.16	.25	0.09	-0.10	0.27
Age × severe VI	-1.24	2.33	30.59	-0.53	.60	0.13	-0.14	0.40
Interaction effects
EE × age × severe VI	-1.14	1.47	705.31	-0.77	.44	0.06	-0.12	0.25
ELC × age × severe VI	1.70	1.47	705.38	1.15	.25	0.24	0.04	0.43
ELS × age × severe VI	2.00	1.47	705.38	1.36	.17	0.28	0.08	0.47
SMU × age × severe VI	-0.02	1.47	705.31	-0.01	.99	-0.20	-0.39	-0.01
RSVC × age × severe VI	1.05	1.47	705.31	0.72	.47	0.09	-0.10	0.27

Discussion

For the current study, a longitudinal design (2 years) was used to examine development in language, cognition, social adaption, and exploration in 24 toddlers with moderate and severe VI. To address the first research question regarding developmental trajectories of cognition, language, adaptive skills, and exploratory behaviour of toddlers with VI, individual participant plots were generated to illustrate the developmental trajectories across the six scales of the RZS. Subsequently, a linear mixed effect model analysed the individual developmental trajectories.

Both plots and the linear mixed effect model demonstrated that development in SA and EE started earlier than in other domains, with younger children displaying lower scores on the scales SMU, ELS, and ELC. However, the model showed that these domains demonstrated stronger progress than SA and EE. Furthermore, the individual plots revealed variation in individual developmental trajectories. For instance, plateaus in development were observed for several children. However, visual inspection suggested that these were attributable to a ceiling effect, as these children attained maximum scores on the scale, rather than a stasis in development. A second variation was that some children showed linear or quadratic growth. This phenomenon could be explained by their age during wave 1. Younger children had more opportunities to demonstrate progression on the RZS than older children. For the second research question, we investigated to what extent the development on the RZS differed for children with moderate VI and severe VI. The results of the linear mixed effect model showed that developmental progress did not differ significantly between children with moderate VI and severe VI.

Language development seemed somewhat delayed compared to the other domains of the RZS. A plausible explanation for the children under study is that their langue development follows motor development. For instance, labelling objects, as part of expressive language content starts in children with typical development, commonly around 18 months of age (Reynell, 1978). As motor skills advance, children expand their environment through exploration, which subsequently stimulates language development (Gonzalez et al., 2019; Oudgenoeg-Paz et al., 2016). Given that the participants had no additional disabilities, this rationale is reasonable. The results also highlighted the heterogeneity in individual responses, represented by each participant’s developmental trajectory, emphasizing the importance of accounting for individual differences when examining the development of children with VI, as addressed by other researchers such as Warren (1994) and Ravenscroft (2019).

The answer to research question 2 showed no differences in development for children with moderate and severe VI. This finding contrasts with previous studies that reported delayed or atypical development for children with severe VI compared to moderate VI (Brambring, 2007b; Cass et al., 1994; Dale & Sonksen, 2002; Hatton et al., 1997). The discrepancies with these earlier outcomes can be attributed to heterogeneity in study samples and incomparability in study designs. It is also crucial to note that children with overt multiple disabilities were excluded from the sample in the present study, which was not consistently the case in previous longitudinal studies. Consequently, regression or stasis in development could have resulted from including children with multiple disabilities or neurodiversity in these study samples (Dale, personal communication). Furthermore, data from previous studies were predominantly collected before 2000. Since then, changes have occurred in the prevalence of pathologies, neonatal care (Boonstra et al., 2012), rehabilitation practices (Reichman et al., 2008), and available knowledge on VI. For instance, in the Netherlands, organizations supporting individuals with VI provide psycho-education, and information is more easily available because it is disseminated on websites such as eduVIP (eduVIP, n.d.). In this respect, the ratification of the United Nations (2007) convention on the rights of people with disabilities also helped. In short, more knowledge is presently available and overall the development of children with VI has improved. However, empirically testing this remains challenging.

Overall, the longitudinal study provided a comprehensive insight in individual developmental trajectories that appeared after drawing individual plots of children’s development and calculating effects with linear mixed effect models. All toddlers with VI progressed in all developmental scales. Moreover, toddlers with VI demonstrated high scores on all scales regardless the severity of the VI.

Limitations

This study is the first longitudinal investigation into the development of children with VI in decades (Veldhorst et al., 2022), but it had limitations. To begin with, longitudinal studies are difficult to conduct due to the need for a representative and sufficiently large sample size and safeguard attrition of participants. In this study, the small population of children with VI without additional disabilities in the Netherlands (Boonstra et al., 2012) posed a challenge. Participants were recruited through regional centres of two Dutch institutes supporting individuals with VI, using convenience sampling, which limits generalizability (Emerson, 2021). All participants were registered with a centre, so potentially unregistered children could be missed.

Another limitation relates to prematurity. Brambring (2007b) urged researchers to correct chronological ages for prematurity. However, this information was not recorded for all children, so adjustments could not be made. Despite this, children scored high or at ceiling level on most scales of the RZS, reducing false positive results due to not correcting for prematurity. Including children under the age of 12 months was also challenging, as procedures to diagnose a child with VI and exclude additional disabilities took time. The project’s start coincided with COVID-19 measures, making several parents reluctant to participate. This resulted in an average age of the children of 19 months at the first wave of the study, whereas we opted for including young infants to make use of the full range of 0 to 48 months for which the RZS was designed (Vervloed et al., 1998).

Moreover, because of the sample’s heterogeneity and nested design, linear mixed effect models seemed appropriate. They are known for their robustness and ability to handle unbalanced data (Field et al., 2012). With our relatively small sample size, a singularity warning indicated potential overfitting or redundancy in the random effects structure of the model. Sensitivity analyses using simpler models confirmed the robustness of our proposed model, but results should still be interpreted cautiously. In addition, effect sizes and 95% confidence intervals were included in the analyses to enhance the interpretation of our findings and understanding of the practical significance of the results. Future research with larger samples could improve estimate reliability.

Finally, compensating factors for VI, such as environmental differences, high-quality early intervention, and parenting strategies, were not included for methodological and statistical reasons, although it is known that they can affect children’s development (Veldhorst et al., 2022). Much like previous studies of Brambring (2007b), Cass et al. (1994), Dale and Sonksen (2002), and Hatton et al. (1997), these factors were not accounted for.

Implications for practice and research

This 2-year longitudinal study on the development of toddlers with VI offers valuable insights for further study and practical applications. The current study used the RZS (Reynell, 1979) which enabled comparison with previous research (Dale & Sonksen, 2002; Sakkalou et al., 2021). The children scored rather high on the RZS. This could be due to sampling bias, but a ceiling effect is also a plausible explanation (see also: Rose et al., 2022) and the Dutch norms (Vervloed et al., 1998) are possibly outdated. This calls for calculating new norms potentially extending the age range beyond 48 months to assess for a longer time.

The study found that within the diverse group of toddlers with VI most of them exhibited satisfactory developmental progress and no delays. Currently, there is no reason to believe the severity of the VI is the main factor for individual variation. Various other factors should be considered in predicting development, rather than solely focusing on the VI as predictor (Ravenscroft, 2019; Warren, 1994). Similarly, cognition and motor development do not fully explain the developmental trajectories in children with intellectual or motor disabilities (Dhondt et al., 2023). Thus, individual variation is not a result of – the severity of – an impairment only, but is affected by other factors as well.

Footnotes

Acknowledgements

This research was conducted in collaboration with Royal Visio and Bartiméus. The authors wish to thank the research assistants for assisting with the data collection and the participants for their time and effort.

Author contributions

C.V.: Conceptualization, methodology, formal analysis, investigation, writing – original draft, writing – review & editing, project administration. S.K.: Conceptualization, methodology, writing – review & editing, supervision, funding acquisition. M.P.J.V.: Conceptualization, methodology, writing – review & editing, supervision, funding acquisition. B.S.: Conceptualization, methodology, writing – review & editing, supervision.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by Stichting ter Verbetering van het Lot der Blinden. [Foundation for the Improvement of the Lot of the Blind].

Ethical approval and informed consent statements

The study adhered to the ethical principles of the Declaration of Helsinki (World Medical Association, 2022) and obtained approval from the Medical Ethical Committee Eastern Netherlands, Nijmegen, the Netherlands (No. NL74630.091.20) on 4 February 2021. Written informed consent was obtained from a legally authorized representative for participation in the study and anonymized patient information to be published in this article.

ORCID iD

Carlijn Veldhorst

Data availability statement

Data are available on request through the Open Science Framework ().

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