Angle lambda (λ) is defined as the angle between the line of sight and the pupillary axis at the entrance pupil. We previously developed a child-friendly and portable method to measure this angle in daily practice. In a given population, angle λ fluctuates according to age or refractive error. As changes in the pupil diameter induce changes in the location of the pupil centre, it was hypothesised that a given subject will exhibit several angles λ, varying with the luminance level. The study aimed to investigate correlations between angle λ and biometric values and to analyse the effects of pupil dilation and entrance pupil location on angle λ. The study was performed on 70 right eyes from 70 participants (58 women, 12 men; mean age 22.91 ± 2.57 years). Angle λ was assessed under photopic and scotopic luminance conditions. Angle λ was also measured under standard luminance conditions in a subgroup of 15 eyes. Axial length, anterior chamber depth, chord μ (coaxial corneal light reflex position) and the optic disc to fovea distance (OFD) were also measured. The pupil centre offset was quantified by digital analysis. Mean photopic and scotopic angle λ values were +2.81 ± 2.34° and +3.30 ± 2.59°, respectively. A negative correlation was found between axial length, anterior chamber depth or OFD and both photopic and scotopic angle λ. A positive correlation was found between spherical equivalent or chord μ and angle λ. The mean pupil offset was significantly higher under photopic than scotopic conditions and was negatively correlated with angle λ for both luminance levels. This study confirms that angle λ is correlated with biometric values. Furthermore, fluctuation of pupil diameter induces variations in angle λ. Thus, a given subject exhibits several angles λ according to the luminance level.
Rateaux et al. (Thu,) studied this question.
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