**3. Results**

This study included 116 toddlers, 53 males (45.68%) and 63 females (54.31%). The mean ages of the toddlers were 29.57 ± 3.45 (24.16 to 36.90) months. Population flow chart diagram is shown in Figure 1. In the gender comparison, no statistically significant differences were found in any variable except for the fine motor quotient (FMQ) where the males toddlers obtained 100.56 ± 16.64 and the females toddlers obtained 108.09 ± 12.21 (t = 2.79, *p* < 0.01). Motor development variables included the dominant hand and foot, stationary percentile, locomotion percentile, object manipulation percentile, grasping percentile, visual motor integration percentile, gross motor percentile (GMP), fine motor percentile (FMP), total motor percentile (TMP), gross motor quotient (GMQ), fine motor quotient (FMQ), and total motor quotient (TMQ). They are presented in Table 1. Visual development variables included the Cardiff visual acuity (VA) test for right, left, and both eyes; broken wheels VA test for right eye, left eye, and both eyes; retinoscopy refraction in the mean spherical equivalent; kappa angle for right and left eyes; Hirschberg reflex for right and left eyes; near point of convergence; base-out and base-in test; Lang stereopsis test; Bruckner test; fixation, accuracy, and head tests for tracking movements; and reflection and head tests for saccades movements. These are also presented in Table 1.

**Figure 1.** Population flow chart diagram.

Visual development differences between the fast and slow GMQ development groups (GMQ > 100 and GMQ ≤100, respectively) are presented in Table 2. The GMP demonstrated identical results for the GMQ groups. We found no significant differences in visual development between the fast and slow FMP, TMP, FMQ, and TMQ development groups. In the right eye kappa angle, only 17.5% of the toddlers had a centered reflex in the slow gross motor development group (GMQ ≤ 100), while 36.8% of the toddlers had a centered reflex in the fast motor development group (GMQ < 100) (χ<sup>2</sup> = 8.28, P = 0.01). Similar results were found in the right eye Hirschberg test: only 17.95% of the toddlers had a centered corneal reflex in the slow motor development group, whereas 39.5% of the toddlers had the same type of corneal reflex in the fast motor development group (χ<sup>2</sup> = 7.64, P = 0.02). The same situation was found in the kappa angle and Hirschberg test for the left eye, although the results were not statistically significant (χ<sup>2</sup> = 4.74, P = 0.09, χ<sup>2</sup> = 4.88, and P = 0.08, respectively).

The Krimsky and Bruckner tests obtained similar findings. In the slow motor development group, 87.5% and 88.8% of the toddlers had a normal test while 12.5% and 11.3% had a deviated test, respectively. However, in the fast motor development group, all of the toddlers had normal Krimsky (Figure 2) and Bruckner tests and none had any deviations in their eyes (χ<sup>2</sup> = 4.92 and P = 0.02 for the Krimsky test and χ<sup>2</sup> = 4.39 and P = 0.03 for the Bruckner test). The last statistically significant visual development variable was near point of convergence (Figure 3C). The slow motor development group had 2.46 ± 4.07 cm, whereas better results were reported in the fast motor development group, 1.00

± 2.02 (t =2.56, P = 0.01). For visual acuity (Figure 3A), retinoscopy refraction (Figure 3B), base-out and base-in test, stereopsis test, fixation test, accuracy and head position for tracking movements, and reflection and head position for saccades movements, there were non-statistically significant differences between slow and fast motor development including gross, fine, and total percentiles and quotients.

**Figure 2.** Population pyramid for kappa angle (expressed in percentage and count of toddlers in square).

**Figure 3.** Slow and fast motor development characteristics for main optometry variables. (**A**)—Box and plot graph for both eyes' visual acuity (Cardiff test, expressed in LogMAR). (**B**)—Box and plot graph for mean retinoscopy refraction (expressed in diopters). (**C**)—Near point of convergence (expressed in cm).



Values were presented with mean ± SD (standard deviation) and (Range) in quantitative variables or expressed with frequency and percentage in qualitative variable.



VA: Visual Acuity; NPC: Near point of convergence; RE: Right eye; LE: Left eye. Quantitative value with mean ± SD and qualitative with frequency (percentage).

#### **4. Discussion**

This study evaluated the visual function and motor development in typical developing toddlers. The objective was to determine the possible presence of visual dysfunction and/or motor disorders and analyze possible differences between slow and fast motor development for visual system variables. Visual acuity, alignment of visual axes, stereopsis, and ocular motor skills were included in the present study. The percentiles and quotients of gross, fine, and total motor development in a group of 116 toddlers aged from two to three years were also reported. Two main GMQ groups were established, a slow motor development group (GMQ < 100) and fast motor development group (GMQ > 100). We found statistically significant differences between slow and fast motor development for certain visual system variables. Toddlers with slow gross motor development had a greater tendency of exophoria and a further near point of convergence (NPC) than toddlers with fast gross motor development. These findings agree with previous studies that demonstrated the link among the visual and motor systems in babies, toddlers, and children [4,13–17,19,21,24–26].

Similar to our outcomes, Thompson et al. [26] studied a two-year-old large toddlers group. They fully evaluated vision state (visual acuity, stereopsis, visual axes alignment, ocular motility, and auto-refraction) and its possible relationship with motor development. They measured using the Bayley Scale of Infant and Toddler Development 3rd edition (BSID-III), which has an excellent correlation with the PDMS-II [41,42]. Their outcomes revealed that global movement perception and binocular vision were associated with motor function at an early development stage. This study included toddlers born with risk factors for neurological development. Thus, their early development rates differ from ours since our sample was based on toddlers with neurotypical development. To the best of our knowledge, our study is the first that analyzed typical developing toddlers two- to three-year-old both in motor and visual function.

To date, the prevalence of visual, accommodative, and non-strabismic binocular dysfunction has increased in the pediatric population [43–45]. Early diagnosis and proper management can improve vision-related life in this population, thus guaranteeing its correct evolutionary development in all areas. Different symptoms and signs can be used to diagnose visual function [46,47].

#### *4.1. Visual Acuity Di*ff*erences between Slow and Fast Gross Motor Development*

Amblyopia refers to the unilateral or bilateral reduction of the best corrected visual acuity that is not directly attributed to a structural alteration of the eye or visual pathways [48]. It constitutes one of the main causes of visual impairment in children, with prevalence values ranging from 1% to 4% [49]. Amblyopia is related to the presence of strabismus, refractive errors, astigmatism, and anisometropia. Determination of visual acuity is generally the first clinical step to identify the presence of amblyopia [50]. When we classified the subjects according to their GMQ values, we found no differences in visual acuity values between the two groups. Visual acuity was identical in the RE and LE (0.18 ± 0.10 LogMAR) measured with the Cardiff test, and there were no statistically significant differences between both eyes (OD = 0.36 ± 0.05 LogMAR, OI = 0.37 ± 0.04 LogMAR) when evaluated with broken wheels. Similar findings were obtained in binocular visual acuity values and did not differ between the two groups. Many studies link visual acuity deficits with motor delays [14–17,21,51,52]. The presence of refractive errors, mainly hyperopia, is common in children with amblyopia, which is normally associated with mild delays in many aspects of development [53]. In our study, the sample consisted of toddlers without visual impairment whose objective refraction did not exceed +1.50 diopters in either of the two eyes, which could be why we did not find differences between the slow and fast motor development groups.
