Crosslinking of near responses in healthy young subjects.

Abstract

With the excessive smartphone use, an increasing number of young people are complaining of ‘double vision (smartphone esotropia)’ or ‘near vision difficulties (smartphone presbyopia)’ (Lee et al. 2016; Long et al. 2017). Resolution of these problems is therefore desirable; however, the mechanisms of these symptoms remain unclear. When viewing nearby objects, ‘near response’, consisting of convergence, accommodation and pupil miosis are activated and these responses are known to be crosslinked in the cortical centre. Accommodation and convergence have a feedback mechanism that interprets these as error information (blurred retinal image and binocular parallax), as well as independent programme mechanisms. However, the control system of the pupil is activated following convergence and accommodation, with pupil miosis occurring at the latest time, indicating the ambiguity in how pupil miosis is related to convergence and accommodation. We hypothesized that an unbalanced crosslink of the near responses is one of the factors causing the onset of esotropia and presbyopia due to smartphone usage. Therefore, we investigated the possible factors affecting the onset of these problems in healthy young subjects. We examined 22 eyes of 22 healthy young subjects (age, 20–23 years). Study protocol was approved by our Institutional Ethics Committee. This study followed the tenets of the Declaration of Helsinki for research involving human subjects, and written informed consent was obtained. Subjects looked at a printed asterisk shaped target (30 min of arc) at 1, 0.5, 0.3, 0.25, 0.2, 0.16, 0.14, 0.12 and 0.11 m and were instructed not to blink during the 10-second recording. HandyRef-K (NIDEK, Gamagori, Japan) was used to measure the spherical equivalent refraction (accommodative response) (D). Iriscorder Dual C-10641 (Hamamatsu Photonics, Hamamatsu, Japan) was used to measure the pupil size (mm). The difference (Δ) of values between the 1 m and other distance were calculated (unreliable data was excluded). These data at each distance of the target were compared using one-way anova, with the Scheffe test for post hoc comparison. A p-value of <0.0001 was considered statistically significant. We discovered that the accommodative response was significantly induced with the visual target at 0.3 m, whereas pupil size was significantly reduced with the visual target at 0.14 m (Fig. 1). Even though the accommodative stimulus was similar, the accommodative response was dissociated with pupil miosis, demonstrating that the crosslink of the near responses becomes unbalanced in healthy young subjects. These findings were thought to occur because young subjects have sufficient accommodative function and can clearly view nearby objects without significant pupil changes. Accommodation lag is known to be dependent on the pupil size corresponding to the depth of focus. Another positive aspect of our results is that preventing the onset of esotropia or presbyopia related to excessive smartphone use, viewing objects 0.14 m away, where the pupil changes were not significantly activated, is considered desirable. Tablet devices, currently, along with the smartphone, most widely used to view mainstream media, are operated at about 20 cm; and virtual reality and augmented reality viewing devices, which are expected to become widespread in the future, will likely project a video at a distance of significantly less than 20 cm from the eyes (Bababekova et al. 2011; Turnbull & Phillips 2017). A virtual reality head-mounted display creates dissociation between convergence and accommodative demands, a shift towards esophoria and an extended near point of convergence (Hara et al. 1996). Consequently, viewing at 0.14 m away may prevent the onset of smartphone esotropia or smartphone presbyopia in young people. However, paper media and electronic media may have different effects on near response. In subsequent studies, we will conduct a similar study using tablet devices.