*4.2. Ocular Alignment Di*ff*erences between Slow and Fast Gross Motor Development*

Heterophoria is ocular misalignment in the absence of fusional vergence. Exophoria is a divergent misalignment, while esophoria is a convergent misalignment [54]. The measurement of heterophoria is an important clinical test since it indicates the demand of the fusional vergence system [55]. It has been fully evaluated in adults and older children, but is difficult to measure in toddlers younger than five years because of their limited cooperation [38,54,56,57]. We found authors who established that many toddlers under five years old were orthophoric [38,56]. However, our results are in line with more recent studies showing that the phoria of children without visual impairment between two and seven years old was mainly exophoric, with small differences in that age range [54,55,57].

Our study found that all of toddlers had clear exophoria, which significantly increased their slow motor development. Overall, 72.5% in the slow gross motor development group had a right eye kappa angle that was more positive (exophoric) than 61.8% in the fast development group (P = 0.01). The left eye kappa angle had significant differences (P = 0.09). Correspondingly, the right eye Hirschberg reflex was significantly more nasal (exophoric) (P = 0.02) and considerable (P = 0.08) in the slow motor development toddlers. Furthermore, the slower gross motor development toddlers had an NPC of 2.46 ± 4.07 cm, significantly higher than in the fast-gross motor development group, where the NPC was 1.00 ± 2.02 cm (P = 0.01).

The findings of Jeon et al. [58] were very revealing. They reported that the prevalence of exotropia was higher in toddlers with several motor impairments. Motor function was studied using gross motor function. In this case, our patients were typical developing, and we did not have cases of strabismus but had cases of exophoria. In this sense, the slow motor development group had greater exophoria than the fast motor development group. In addition, we found a decrease in the ability to merge with an NPC increase in the slow gross motor development group. These results agree with Jeon et al. [58]; however, they found the same prevalence in exotropia and esotropia.

On the one hand, exophoria is characterized by a divergent deviation in the line of sight when the visual axes are at rest. Exotropia is defined as a more serious, manifest, and fixed situation of ocular exodeviation [59]. On the other hand, Barbosa et al. [60,61] categorized infants by the degree of motor performance in three motor levels: cerebral palsy, motor delay, and typical development. Therefore, our results could be very useful for anticipating below average cases of motor development in which a higher prevalence of exophoria is demonstrated.
