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23-11-2011 | General practice | Article

NITM additive in nature among myopic individuals


Free abstract

MedWire News: Nearwork-induced transient myopia (NITM) is primarily a lenticular-based phenomenon reflecting transient changes in crystalline lens thickness among individuals with myopia, US suggest study findings.

The results confirm findings from earlier studies suggesting that NITM increases significantly following continuous nearwork, and that this may be an additional environmental factor to consider in myopic refractive error development.

"This is the first study showing directly, via objective means, lenticular-based NITM," say Balamurali Vasudevan (Midwestern University, Glendale, Arizona) and Kenneth Ciuffreda (State College of Optometry, New York).

For the study, the researchers measured axial lenticular thickness in the right eye of 12 individuals with early-onset myopia (EOM), 11 with late-onset myopia (LOM), and 12 with emmetropia (EMM) using A-scan ultrasonography before and immediately after 1 and 2 hours of continuous reading at a distance of 35-40 cm.

All participants underwent an evaluation of baseline vision and oculomotor function to ensure normal vision prior to testing.

The group mean increase in axial lenticular thickness was 0.025 mm and 0.035 mm after 1 and 2 hours of reading, respectively. The mean group increase after the second hour was found to be significantly larger than that seen after the first hour.

When the researchers assessed results according to refractive subtype, they found that only individuals with EOM and LOM showed significant increases in lenticular thickness after 2 hours of continuous reading.

The authors say this finding provides "direct physiological evidence for lenticular-based NITM additivity, with this occurring only in myopes."

No significant correlation between changes in lenticular thickness and refractive error magnitude after the first and second hours of reading was observed among all individuals.

The authors suggest that the mechanism responsible for lenticular-based additivity of NITM may involve the presence of a sympathetically-based inhibitory dysfunction that would produce a relative increase in accommodative response, resulting in an increased myopic shift.

"Closely-spaced, repeated periods of nearwork may produce successive, transient increases in NITM that would be additive in nature, and hence speculated potentially to be myopigenic," write the authors in the journal Optometry.

By Ingrid Grasmo

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