2 Chapter 2: Myopia, Hyperopia, Presbyopia and Astigmatism Definitions, Epidemiology and Ageing

2A. Refractive Errors – The Basics

[1.] Spherical Refractive Error

Sphere power refractive error is dependent upon where the image is focused relative to the retina. This is defined by two main categories in the eye.  First, the anterior segment optics determines the total refracting power of the eye.  This includes the cornea that accounts for approximately 2/3rd the power and the lens that accounts for the remaining 1/3rd.  There are other refractive elements such as the tear film, anterior chamber depth and cornea posterior surface that have effects on the power, but we generally ignore those at a basic level.  The second part is the length of the eye after the optics.  this is called the axial length of the eye, and goes from the posterior surface of the lens to the photoreceptors of the retina.  Any mismatch between these two components results in the focus being either in front or behind the retina.  This mismatch is called REFRACTIVE ERROR.  

When the eye’s optics are perfectly focused on the retina, the eye is considered to be EMMETROPIC.  If the eye has more power than needed or longer than its power, the image is focused IN FRONT of the retina, resulting in MYOPIA (middle image).  If the eye has less power or is too short for its power, the image is focused BEHIND the retina, resulting in HYPEROPIA (bottom image).  

Real World Examples How Refractive Error Can Develop

Diabetes: The lens provides approximately 1/3rd the refracting power of the eye.  When diabetics have poorly controlled blood sugar, excess sugar (through the sorbitol pathway) can enter the lens.  This draws in water into the lens, causing it to swell.  This swelling has two effects. First, it typically causes the lens to become more curved, thereby increasing the power of the eye.  Second, the anterior surface of the lens moves closer to the cornea (it has to, it has gotten bigger), which also causes the total power of the eye to increase.  Because the axial length of the eye has not changed, the increased power moves the focal position in front of the retina and causes a myopic shift.  Diabetes can also cause changes in astigmatism and hyperopia depending upon the swelling, but that is for later in this course.

Keratoconus: Keratoconus is a collagen disease causing the cornea to thin and weaken.  This causes distortion in the cornea, but it also causes cornea to steepen (become more curved).  This again, increases the power of the optics of the eye while having no effect on the axial length.  Increased total power causes the focal position to be in front of the retina, thereby resulting in more myopia.

Central Serous Retinopathy: Central Serous Retinopathy is a swelling behind the retina (more in ocular disease)  that pushes the retina forward.  In this case, the optics have stayed the same total power, but the retina has moved forward and reduced the axial length.  Due to the new focal position being behind the retina, it has induced a hyperopic shift.

What is the take home point?  Correct focus of the eye is dependent upon the refractive power of the eye (cornea and lens) being matched to the exact axial length to focus light on the photoreceptors of the retina.  

[2.] Emmetropia

Emmetropia is when the eye’s refracting power and axial length match perfectly.  An emmetrope with a flatter cornea than normal will have a corresponding longer eye to compensate for the less power and vice versa.  

[3.] Myopia

As stated above, myopia is caused by the eye having MORE power than it needs for its axial length.  To diagnosis someone with myopia, they need to have -0.75 D or less power in their prescription.  Anything between +0.75 and -0.75 D is considered essentially emmetropic.  Now that we know the myopic eye has MORE power than it needs, it is straightforward to see why negative lenses are used to correct it.  They are decreasing the total power of the eye to bring the focus back to the retina.

Most everyday myopia is caused by the eye having a longer axial length than the refractive power of the anterior optics (more on that in later weeks) and is the type that develops between the ages of 8-12.  Myopia later in life is often due to changes in the refracting power of the eye’s optics.

[4.] Hyperopia

As stated above, hyperopia is caused by the eye having LESS power than it needs for its axial length.  To diagnosis someone with hyperopia, they need to have +0.75 D or more power in their prescription.  Anything between +0.75 and -0.75 D is considered essentially emmetropic.  Now that we know the myopic eye has LESS power than it needs, it is straightforward to see why positive lenses are used to correct it.  They are increasing the total power of the eye to bring the focus forward onto the retina.

2B. Refractive Errors – Myopia

[1.] Prevalence and Incidence

Prevalence is the percentage of the population with a disease at any given disease.  This can be done for the population as a whole or by individual groups such as age, gender, race, socioeconomic status, education level, etc.

Incidence is the percentage of the population that will DEVELOP a disease at any given time (typically by year.)

We will largely be talking about prevalence in this page.

[2.] Myopia and the World

The prevalence of myopia at birth is between approximately 5-20% depending upon the study, but quickly drops to about 5%% at the age of 1 years old.  Globally, on average the prevalence is approximately 30% in 2020 (see figure below).  The most important part of the figure below is the rapid increase in myopia over the past 20 years and the projected increase over the next 30 years.  Just 20 years ago, only 22% of the world population was myopic compared to 30% now.  At the current incidence rate, we can project out that the prevalence of myopia will be around 50% worldwide by 2050.

A large driver of this increase is in Asian and industrialized nations like the USA.  In some Asian countries, prevalence of myopia is approaching 90% of the population.

Figure 2. Myopia prevalence has been increasing steadily since the 1950’s, especially in Southeast Asian countries. Reproduced with permission from Dolgin, E. 2015 [2]. Figure source: Ian Morgan, Australian National University.

[3.] Myopia in the USA

Over the past 20 years, myopia has skyrocketed in the general population from around 30% in the 1990’s to just over 50% in 2020.  Due to the rapid change in prevalence, genetics alone can not explain this increase (more on that in other sections.)  If we look at myopia in school age children, the prevalence (and in this case incidence) are relatively low under the age of 5, but rapidly increase from ages 6-13 years of age.  The prevalence continues to increase, but at a slower rate, into the early 20’s.  In the study below, by the end of childhood/teenage years, 59% of the population has myopia.  The grandparents of this generation only had 30% prevalence of myopia (the parents somewhere in between.)

[4.] Gender

Gender also shows some differences in prevalence of myopia.  On average, 10% more female patients will have myopia than males patients.  This is largely between the ages of 12-60.

[5.] Education

 Education and socioeconomic status seem to play a role in myopia development.  The higher levels of education result in increased levels of myopia.  Those with no high school education tend to have very low prevalence of myopia.  As education increases, the prevalence of myopia increase with college graduates and graduate degree recipients having very high prevalence of myopia. Two mechanisms have been proposed for this, one that light levels (time spent indoor/outdoors) are a contributor and the other accommodative demand due to near work.  Theories of myopia development will be discussed later.

2D. Astigmatism

Astigmatism Prevalence and Change with Age