By Jamie Chang, Yong Loo Lin School of Medicine, National University of Singapore
Ms Sri Yudah, optometrist in Optics House, was invited to give an insight into the “Fundamentals of Subjective Refraction” during the Singapore Primary Eye Care Symposium (SPECS) 2019 between 23-24 July 2019 at One Farrer Hotel, Singapore. She reminded eye care practitioners to ensure that when viewing near target through a determined prescription, the accommodative stimulus should be equal in both eyes so that refractive correction can be worn comfortably under binocular viewing conditions.
With the misalignment of visual point in both eyes, there is a fixation disparity causing a reduced or unbalanced stereopsis, an imbalance in the perception of depth produced by the reception in the brain of visual stimuli from binocular vision.
Hence, a large emphasis is placed on performing subjective refraction accurately. Ms Yudah highlighted the two scenarios of over-minusing and under-plusing. Over-minusing will lead to the patient accommodating during refraction. The chronic accommodative tone will lead to eye fatigue and eye strain when reading. On the other hand, under-plusing will lead to the patient being misdiagnosed with early “presbyopia” with insufficient accommodation for reading which causes the blurring of vision and discomfort.
Binocular subjective refraction should be considered as well as it has no disadvantages when compared to monocular refraction although some patients who are strongly dominant in one eye cannot be refracted binocularly. The plethora of advantages binocular subjective refractions entail not needing separate binocular balancing which is time efficient and speeds up the routine. Also, accommodation is suspended with a plus sphere and tied to convergence and latent nystagmus is reduced if present.
Monocular subjective refraction results in a larger pupil and wavefront aberration as compared to binocular subjective refraction. Eyes with larger wavefront aberrations and/or pupil diameters are more significantly affected by this change. Change in pupil diameter would affect measures of subjective visual acuity and contrast sensitivity. Hence, spherical aberrations can be better controlled by carrying out binocular refraction.
Therefore, Ms Yudah espoused that binocular subjective refraction is better and faster than monocular subjective refraction.