Accommodative Function Can Decrease When Viewing Smaller Screens

Smart devices encourage users to engage in sustained periods of near focus and less blinking, and the resultant eye strain is prompting more visits to ophthalmologists, according to researchers of a case series published in BMC Ophthalmology. This study is one of the first to compare accommodative differences by screen size using both objective and subjective assessments.

Investigators recruited 46 participants 40 years of age and younger, with best corrected visual acuity of 20/20 and healthy eyes for the prospective study. Each patient watched a 1-hour documentary on the display of a 6.1-inch phone and 9.7-inch tablet. They underwent baseline and follow-up testing with an autorefractor/keratometer device that measured near point accommodation (NPA) in static and dynamic modes. Near point convergence (NPC) and pupil diameter were also examined. 

Participants viewed the smartphone and tablet screens on 2 separate visits 1 week or more apart, wearing any prescribed contact lenses or glasses, with a different video each time. Additional objective tests included measurements of stereopsis, heterophoria, tear film break-up time (TBUT), as well as a subjective survey; the ocular discomfort analog scale (ODAS).

“After using the smartphone and tablet, the post-NPA (increase in distance in cm) decreased to 5.43 cm ± 1.19 cm and 5.35 cm ± 1.01 cm, respectively,” the study says. NPA decreased significantly more after smartphone watching than with tablet use (P =.044). NPC was also reduced notably more after focusing on the smaller display (P =.033). ODAS surveys revealed that fatigue after using a smartphone came on more rapidly than with a tablet (P =.012).

Further, TBUT was significantly shortened after smartphone use (P =.010), but did not considerably decrease with tablet viewing (P =.198). Of note, intraocular pressure actually decreased after the test with a tablet (P =.032), but significantly increased after using a phone (P =.040). Investigators theorize sustained near focus on a smartphone’s small screen stimulated lens thickening, and raised IOP.

Researchers note that screen sizes only differed by 3.6 inches diagonally, but even then, subjective accommodative capacity showed a 2-fold reduction with smaller displays. NPC also decreased substantially; by 2.3% after using the iPhone and 0.9% after watching an iPad, “indicating that the change in accommodation is about 2.5 times more severe when viewing a small display,” the report says.

Limitations of the study included its small sample size, viewing time of 1 hour only, video content not tailored to individual interests, and participant fatigue potentially different between the 2 test days. Overall, participants reported a greater degree of discomfort after sustained focus on the smaller device. “These results may be because excessive accommodative convergence is necessary to form a clear image and the small size of the screen and displayed font induce visual fatigue,” the investigators speculate. 

Reference

Kang JW, Chun YS, Moon NJ. A comparison of accommodation and ocular discomfort change according to display size of smart devices. BMC Ophthalmol. Published online January 18, 2021. doi:10.1186/s12886-020-01789-z