How does body height relate to myopia?

Paper Title: Change in body height, axial length and refractive status over a four-year period in Caucasian children and young adults.

Authors: Stephanie Kearney (1), Niall C Strang (1), Bastian Cagnolati (2), Lyle S Gray (1)

1. Department of Vision Sciences, Glasgow Caledonian University, Glasgow, UK
2. Optometrie Cagnolati GmbH, Duisburg, Germany

Date: January 2020

Reference: Journal of Optometry 2020 [Link to open access paper]

The axial length of a child's eye will increase as part of their normal growth and although this is as normal as them growing taller as they get older, excessive axial length of the eye leads to high myopia.  

This longitudinal review sought to discover an association between changes in height, refractive status and axial length over time.  Over the 4yr period of the study 140 participants, aged between 5 and 20yrs old, had their refractive error, height and ocular parameters measured at 3 timepoints.

The results of this study were mixed for the correlation of height and axial length.  The authors found an association between body height growth and axial length increase for emmetropes where a 1cm change in height correlated with a 0.03mm increase in axial length.  However, the axial length increase was out of proportion to the body height growth for the myopic participants where axial length accelerated faster with an increased length of 0.15mm per 1cm of body height for persistent myopes and 0.14mm for progressing myopes.

The authors reached the conclusion that the growth mechanisms for body growth and axial length growth were likely to be different in origin.  For emmetropes an associated correlation can be seen but once myopia is present, the axial length appears to have a separate drive for growth.

• Can changes in body height as a child grows be a reliable predictor of axial length increase and refractive status change?
  • The results of this study have suggested that this might be true for emmetropes but not for myopes
  • Myopes will generally have longer axial lengths than emmetropes and a faster elongation rate during childhood
  • It is unlikely that measurement of body height would assist in myopia management, but knowledge of a recent 'growth spurt' could be relevant in pre-myopes as an indicator of elevated myopia onset risk.

• The number of participants was low
  • A small study sample means that the results can only be extrapolated to larger populations with caution. 
• The cohort was limited to Caucasian participants
  • Repeating the study in non-European countries would give results more applicable to different ethnicities, including any effects of varying myopia prevalence and environment.
• Age range of participants
  • Although the ranges of the participants was large (5-20 years) those children who were 5yrs old at the first timepoint (t1) wouldn't have started puberty by the end of the 4yr study and those who were 20yrs old could have finished puberty before the study began and so the growth rates would have varied.
  • Future research could collect data from smaller, specific age ranges in order to identify the age at which in which axial length growth begins to outstrip the height growth, which will occur at the time of fastest myopia progression.  
• Non-cycloplegic refraction results:  
  • Methodology using fogging instead is understandable as cycloplegia is not permitted in German primary eye care setting, where the study data was collected. Fogging has been found comparable to cycloplegic refraction in adults¹ and non-strabismic children.²
• Axial length may not be the only indicator of future or high myopia
  • In this study, corneal curvature was measured but no significant correlation with change in height was found.
  • Tideman et al discussed using an axial length to corneal radius (AL-CR) ratio as a predictive in their 2018 study and concluded that children with a higher AL-CR ratio were more likely to benefit from myopia management interventions
  • This ratio could be calculated in the present study to explore correlations with height and refractive status.
• Future research could tell us how important changes height will be in identifying those at risk of becoming myopic compared to readily established risk factors such as parental myopia, prolonged near work and less time spent outdoors.

Children and young adults were followed over a period of four years to investigate associations between changes in body height, refractive status and ocular measurements.

This study used a longitudinal design to collect data from 140 participants aged 5 to 20 years of age (mean age 13.0) recruited from the patient database of an optometric practice in Western Germany.  Those who wore rigid gas permeable lenses or had an ocular or systemic disease, anisometropia or astigmatism of 2D or more, visual acuity of less than 6/6 or strabismus were excluded.  Data on the participant's refraction, axial length, height, corneal curvature and anterior chamber depth was recorded at three different time intervals during the 4yr study period.

The refraction and biometry measurements were taken at the initial visit (t1) and at two year intervals (t2 & t3) across four years.

Refraction

Non-cycloplegic refraction was performed with a Shin Nippon NVision-K 5001 infrared binocular open-field auto-refractor.  Accommodation was relaxed by using a +3D Fresnel fogging lens mounted on the auto-refractor (cycloplegia was not permitted for optometric practice) and subjective refraction was carried out.  The right eye's spherical equivalent refraction (SER) was used in analysis.

Ocular biometry measurements

The axial length, anterior chamber depth and corneal curvature were measured using Zeiss IOL Master. An average of 4 measurements was used for axial length and an average of at least 3 for corneal curvature and anterior chamber depth.

Body height

This was measured against a wall-mounted tape measure (without shoes on).

Refractive groups

The SER values were split into refractive groups to reflect changes between the timepoints:

• Persistent emmetropes (PE): SER between -0.50D to +1.00D
• Persistent myopes (PM): SER ≤ -0.50D at each timepoint
• Progressing myopes (PrM): initial SER ≤ 0.50D and an increase of ≤ -0.50D between timepoints
• Incident myopia (IM): SER end value ≤ -0.50D
• Persistent hyperope (PH): SER >+1.00D at each timepoint
• Emmetropising hyperope (EH): SER initial value > +1.00D and subsequent SER between -0.50D and +1.00D.
  
  

Refractive groups

The 140 children and young adults who took part in the study were 99% Caucasian.  The myopic prevalence at baseline (t1) was 35% increasing to 38.1% at the 4 year visit (t3). No differences for gender were found at the t2 or t3 timepoints. The emmetropising hyperopes were omitted from the analyses as there were too few participants in each group (1 in t1 and 4 in t3 respectively).

Change in height relating to axial length

There was a correlation for the persistent emmetropes (PE) and the incident myopes (IM) for between timepoints t1-t2, but not for the other groups.  With every 1cm extra height change between t1 and t2, there was a corresponding change of 0.03mm in axial length for the PE group and 0.11mm for the IM group.  For t3, change in height was only statistically correlated with the PE group.

It was found that the male participants grew taller faster over the period of the study compared to the females but that this did not translate to any significant difference in the axial length when comparing males and females.

Change in height relating to anterior chamber depth (ACD) and corneal curvature

The persistent emmetropic (PE) and progressing myopes groups showed correlation for body growth change and anterior change depth between t1 and t2 timepoints.  However, it was only statistically significant for the PE group for t2-t3 (as with the axial length).

There was no statistically significant change found for the corneal curvature in relating to change in height over the 4yr period in any group.

Statistical analysis

When the authors conducted their cross-sectional analysis, they found the following:

• Those in the progressing myopes group were taller than those in the persistent emmetropes group at t2 and t3, but that there was no significant relationships in other groups
• When the axial lengths were compared across the refractive groups, it was seen that:
  • At t2 and t3, the progressive myope and persistent myope groups had significantly longer axial lengths compared to those who weren't myopic.
  • The axial lengths of the incident myope group were longer than those in the persistent hyperope group in t2 but not significantly different to the axial lengths of the persistent emmetrope group in t2, or to non-myopes in t3
    
    
• The anterior chamber depth was larger in the progressive myope and persistent myope groups compared to non-myopes at t2 and t3 but this wasn't the same for the incident myope group where no significant difference was found.
  
  

The results of this study have shown there is a proportional association between body height increase and axial length increase for emmetropic children and young adults as they grow. 

However, this correlation doesn't hold true for progressing myopes.  They were found to have a faster rate of axial elongation compared to emmetropes that was also out of proportion to their own height increase.

This suggests that there may be separate mechanisms responsible for axial length growth and height growth which have a corresponding relationship for emmetropes, but not for myopes.  

Title: Change in body height, axial length and refractive status over a four-year period in caucasian children and young adults

Authors: Kearney S, Strang NC, Cagnolati B, Gray LS.

Background: Body height and axial length (AL) increase during childhood with excessive axial elongation resulting in myopia. There is no consensus regarding the association between body growth and AL during refractive development. This study explored the association between change in body height, AL and refractive status over 4-years in children and young adults.

Methods: Measures were collected biennially (timepoints: t1, t2, t3) (t1 n = 140, aged 5-20years). Non-cycloplegic autorefraction was obtained using the Shin-Nippon openfield autorefractor. AL, corneal curvature (CC) and anterior chamber depth (ACD) were measured by IOL Master. Body height (cm) was measured using a wall mounted tape measure. Refractive status was classified using spherical equivalent refraction (SER): persistent emmetropes (PE) (-0.50D to +1.00D), persistent myopes (PM) (≤-0.50D), progressing myopes (PrM) (increase of ≤-0.50D between timepoints), incident myopes (IM) (subsequent SER≤-0.50D) and persistent hyperopes (PH) (>+1.00D).

Results: Change in AL and change in height were correlated in the PE (all t:p ≤ 0.003) and the IM (t1-t2 p = 0.04). For every increase in body height of 1 cm: t1-t2: AL increased by 0.03 mm in the PE, 0.15 in the PM, 0.11 mm in the IM, 0.14 mm in the PrM, -0.006 mm in the PH. T2-t3: AL increased by 0.02 mm in the PE, 0.06 in the PM, 0.16 mm in the PrM, 0.12 mm in the IM and -0.03 mm in the PH.

Conclusion: In emmetropia body growth and axial elongation are correlated. In participants with myopia, body growth appears to stabilise whilst axial elongation continues at a much faster rate indicating dysregulation of normal ocular growth.

Link to open access paper is here.