Myopia: Nature vs. Nuture

Question 1

Emmetropia

Answer 1

Refractive state of the eye in which the conjugate focus of the retina is at infinity, with relaxed accommodation.

Generally -0.25 to +0.50/+0.75 (inc.)

Question 2

Myopia

Answer 2

Refractive condition of eye where distant objects are focused in front of the retina.
D vision blurred, N is clearer
In general, eye has grown too long for the remaining refractive components
Concave spectacles used to correct

Generally ≤ -0.50

Question 3

Hypermetropia

Answer 3

RC of eye where distant objects are focused behind the retina when accomm. is relaxed.
Eye too short, however image can be brought to focus on retina with accommodation (prov. AA sufficient)

Greater difficulty at N in moderate and high degrees of hypermetropia, BV disorders and age

Risk: convergent strabismus & amblyopia developing in infancy

Question 4

Astigmatism

Answer 4

RC where image of point object is not single point, but 2 perpendicular lines at different distances from the optical system

Caused by ≥1 toroidal refracting surfaces of the eye

Question 5

What are the refractive components of the eye?

Answer 5

1. Cornea: power +43D (41-45)
2. Lens:
   3: ACD: 3-4mm
3. VCD: 16.5mm
4. AL: 23.5mm (22-25) - 16.5mm neonate - 14mm in utero

Question 6

How do we achieve emmetropia?

Answer 6

85% of ocular enlargement occurs in the 1st 3 years of life, remaining 15% over next 10-15yrs.

At birth, we are hyperopic. Eye is programmed to achieve emmetropia in youth (school aged children) and maintain this during early adulthood. Axial length contributes most to em/RE.

The refractive components change in a coordinated manner to achieve emmetropia and any breakdown of this growth coordination results in ametropia.

Question 7

Population endpoint refractive state distribution

Answer 7

Young adult population:
Emm = 54%
Myopes = 27%
Hyperopes = 19%

Leptokurtotic distribution (large peak)

Question 8

Relative distribution of RE in 40-90yo

Answer 8

Hyperopic shift as we age 40+

Myopic shift @ 80+ as lens becomes less flexible/formation of cataracts

Question 9

Prevalence of myopia

Answer 9

Varies with ethnicity & location

Taiwan: 60% 12yo
AUS: 20% population; 1.5% 6yo, 13% 12yo
Singapore: 29% 6-7yo (cf. Sydney 3%)

Question 10

Classification of Myopia

Answer 10

Degree of myopia: low, medium, high
Descriptive: association with pathology & age of onset
Cause: structural, mechanism

Question 11

Significance of myopia

Answer 11

Very costly to healthcare system
Ocular elongation and stretch is associated with sight threatening sequelae (RD, tears/holes, CRA, glaucoma, cataracts, etc.)

Question 12

Environmental contributors of myopia

Answer 12

Modification of visual environment
Higher education levels
Higher individual income
Professional/office-related professions
Better housing
Urban vs. rural living
Increased near work
Decreased outdoor activities

*low outdoor + high near work = 2-3x myopia risk
larger effect for outdoor than near
≥14hrs/wk of outdoor activity negated impact of having 2 myopic parents

Question 13

Genetic contribution to myopia

Answer 13

High heritability of RE and ocular components:
- high concordance for twins (mono > dizygotic) & siblings
BUT also shared environment

FOH (AD, AR, X-LR)
Increased # of myopic parents increases:
prevalence, degree of myopia, AL, VCD, progression and AL growth

> 18 c’somal regions implicated in high myopia

Question 14

Significance of hyperopia

Answer 14

Vision impairment
Strabismus/amblyopia
Reduced stereoacuity
Astigmatism, anisometropia
Learning difficulties, decreased academic performance
Asthenopia
Genetic and environmental factors
Syndromic conditions - crowding of ocular structures
ACG
AMD
DR?