Rupali Singh, Bachelor of Optometry

Fellow  Paediatric Optometrist, Dr. Shroff’s Charity Eye Hospital, Delhi.

Myopia is a kind of refractive error in which parallel rays of light are focused in front of the retina of a resting eye .⁽¹⁾ This is not a new term to us as we all know how myopia can affect our vision as well as our routine work. It can cause permanent blindness if left untreated. This blog is about a particular type of myopia which not only affects our vision but also our routine lifestyle known as night or nocturnal myopia.

The human eye can easily cover more than 10 log units in luminance. ⁽²⁾ Due to different eye mechanism under high illumination, quality of vision improves. However, there are also significant changes occurring in the eye in dim light. One such phenomenon that has attracted interest over several centuries is night or nocturnal myopia. ⁽³⁾ It is an increase in power of the eye under low illumination, thus leading to myopic shift of the eye.  ⁽²⁾ Nevil Maskelyne in early 1789 first reported this phenomenon during his attempts to correct it on his own eyes to improve astronomical observations. ⁽²⁾ The magnitude of night myopia appears to be different among individuals and different studies. Values ranging from negligible to as much as −4.00 D of myopia have been reported while the average values in most studies are around −1.5 D. ⁽³⁾ It was concluded that luminance conditions not less than 1 cd/m² were not dim enough to produce significant myopic shifts .⁽³⁾

There have been several theories regarding the causes of night myopia. Spherical aberration was mainly suggested to be responsible because under low luminance pupil dilates which causes spherical aberration in the eyes thus inducing a myopic shift. ⁽⁴⁾ Longitudinal chromatic aberration in combination with the Purkinje effect is another theory leading to this phenomenon. ⁽⁵⁾ Chromatic aberration occurs when the shorter wavelength of light is focused in front of the retina rather than on the retina. Normal eye is emmetropic for light wavelengths of about 550 µm but shifts towards myopic for shorter wavelength of approximately 510 µm (Purkinje shift).⁽⁶⁾ In dim illumination, the shift from cone to rod causes the eye to become more sensitive towards blue light which is predominant towards darkness and thus becomes myopic. ⁽⁶⁾ Another hypothesis included anomaly in accommodation during low illumination. During low illumination, eye would not focus on distant object because of which accommodation is not relaxed and thus leading to myopic shift. ⁽⁷⁾ Campbell et al (1953) suggested that as luminance decreases, the functional amplitude of accommodation decreases. So, under dim illumination, accommodation assumes a fixed focus averaging 1.0 D thus leading to myopic shift. ⁽⁸⁾ The combination of all these factors would lead to night myopia.

People in profession such as driver, astronomer, and pilot are severely affected due to this phenomenon.  Because of night myopia, accidents in the road become more frequent especially if the driver is already a myopic patient. ⁽⁹⁾ Till now no treatment has been seen to manage night myopia but keeping adequate luminance around your surroundings during night-time can help individuals to overcome it.

Reference: 

1. Curtin, B. J. (1985). Basic science and clinical management. The myopias, 177.
2. Levene, J. R. (1965). Nevil Maskelyne, FRS, and the discovery of night myopia. Notes and Records of the Royal Society of London, 20(1), 100-108.
3. Artal, P., Schwarz, C., Cánovas, C., & Mira-Agudelo, A. (2012). Night myopia studied with an adaptive optics visual analyzer. PLoS One, 7(7), e40239.
4. Artal, P., Guirao, A., Berrio, E., & Williams, D. R. (2001). Compensation of corneal aberrations by the internal optics in the human eye. Journal of vision, 1(1), 1-1.
5. Bedford, R. E., & Wyszecki, G. (1957). Axial chromatic aberration of the human eye. JOSA, 47(6), 564_1-565.
6. Otero, J. M., Plaza, L., & Salaverri, F. (1949). Absolute thresholds and night myopia. JOSA, 39(2), 167-172.
7. Leibowitz, H. W., & Owens, D. A. (1975). Night myopia and the intermediate dark focus of accommodation. JOSA, 65(10), 1121-1128.
8. Campbell, F. W., & Primrose, J. A. E. (1953). The state of accommodation of the human eye in darkness.  Ophthalmol. Soc. UK, 73, 353-361.
9. Cohen, Y., Zadok, D., Barkana, Y., Shochat, Z., Ashkenazi, I., Avni, I., & Morad, Y. (2007). Relationship between night myopia and night‐time motor vehicle accidents. Acta Ophthalmologica Scandinavica, 85(4), 367-370.