Refractive Error in Children Study Notes

Refractive Error in Children

Overview of Refractive Error

  • Refractive Errors: Irregularities in the focusing of light on the retina caused by the shape of the eye. Includes:

    • Spherical Errors: Myopia, Hyperopia, Presbyopia

    • Cylindrical Error: Astigmatism

  • Focusing Error: A problem with the focusing of light due to the shape of the eye which results in light not focusing correctly on the retina.

    • Common types of refractive errors:

    • Myopia (nearsightedness)

    • Hyperopia (farsightedness)

    • Presbyopia (difficulty focusing on nearby objects, often due to aging)

    • Astigmatism (where light doesn't focus evenly on the retina)

Detailed Overview of Astigmatism

  • Astigmatism: A type of refractive error where the eye does not focus light evenly on the retina, causing distorted vision or blurred vision at all distances.

    • May be asymptomatic in some cases, while higher degrees can lead to symptoms such as:

    • Blurry vision

    • Squinting

    • Eye strain

    • Fatigue

    • Headaches

  • Types of Astigmatism:

    • Regular Astigmatism: Principal meridians are perpendicular.

    • With-the-rule Astigmatism: Vertical corneal meridian is steepest.

    • Against-the-rule Astigmatism: Horizontal meridian is steepest.

    • Oblique Astigmatism: The steepest curve lies between 120° and 150° and between 30° and 60°.

    • Irregular Astigmatism: Principal meridians are not perpendicular.

Specifics of Regular Astigmatism

  • With-the-rule Astigmatism (WTR): Steeper vertical corneal meridian; resembles a rugby ball lying on its side.

    • Retinoscopy shows against movement of the retinoscopy streak in the horizontal (180°) axis.

    • Correction: A minus (negative) cylinder placed on the horizontal (180°) axis.

  • Against-the-rule Astigmatism (ATR): Steeper horizontal corneal meridian; resembles a rugby ball standing on its end.

    • Retinoscopy shows against movement in the vertical (90°) axis.

    • Correction: A minus (negative) cylinder placed on the vertical (90°) axis.

Classification Based on Focus

  • Simple Astigmatism:

    • Simple Hyperopic Astigmatism: One focal line on the retina, the other behind it.

    • Simple Myopic Astigmatism: One focal line in front of the retina, the other on it.

  • Compound Astigmatism:

    • Compound Hyperopic Astigmatism: Both focal lines behind the retina.

    • Compound Myopic Astigmatism: Both focal lines in front of the retina.

  • Mixed Astigmatism: Focal lines on both sides of the retina, i.e., one in front and one behind.

Emmetropization Process

  • Emmetropization: A developmental process whereby the components of the eye coordinate to achieve a near-emmetropic or low hyperopic refractive error.

    • Characterized by:

    • Balancing the optical and axial components of the eye.

    • In newborns, axial length is approximately 16 mm (90 D); in adults, it measures around 24 mm (60 D).

    • Clinical Understanding: Refraction falls within a range of low hyperopia to emmetropia (+0.50 D to +1.00 D), with a standard deviation of ±1.00 D.

Mechanisms of Emmetropization

  • Greatest emmetropization occurs within the first two years of life, influenced by retinal blur and its location (either anterior or posterior to the retina).

    • The visual cortex detects blurriness and prompts anatomical adjustments to reduce it.

  • Emmetropization tends to be hindered by extreme refractive errors at birth (>5.00 D or <-4.50 D).

  • Statistical Data:

    • Myopia prevalence increases during school years: 6% of 6-year-olds and 15% of 15-year-olds are myopic.

    • Approximately 80-85% of children between +0.50 D to +3.00 D remain hyperopic.

Implications of Emmetropization

  • Understanding the trend of refractive error changes in preschool years is critical.

    • Required knowledge includes:

    • Normal refractive error by age.

    • Expected changes with age.

    • Implications for prescribing lenses or refraining from doing so.

Refractive Errors in Premature Infants

  • Definition of Prematurity: Gestation period of less than 37 weeks, affecting refractive development.

  • At Birth: Typically exhibit high myopia, astigmatism, or anisometropia but reach similar levels of hyperopia and astigmatism as full-term infants by 12 months.

    • Birth Weight Classifications:

    • Low Birth Weight (LBW): <2500g

    • Very Low Birth Weight (VLBW): <1500g

    • Extremely Low Birth Weight (ELBW): <1000g

  • Retinopathy of Prematurity (ROP): A severe condition affecting premature infants, primarily those born before 31 weeks, and can lead to blindness due to abnormal retinal blood vessel growth.

    • Strong correlation between the severity of myopia and:

    • Prematurity

    • Low Birth Weight

    • Advanced ROP stages

Average Refractive Error Findings

  • Premature Infants: Average refractive error at birth is approximately -0.50 D with an SD of +2.80 D.

  • Full-Term Infants: Average refractive error at birth is about +2.00 D with an SD of +2.00 D.

  • Refractive Error at Age 1: Average of +1.00 D with an SD of +1.10 D.

  • Refractive Error at Age 3: Average of +0.95 D with an SD of +1.00 D.

Trends in Refractive Errors

  • School Children: Myopia increases with age, while hyperopia tends to decrease. Refractive error distribution becomes steeper and more bell-shaped into adulthood.

    • Emmetropization leads to different distributions among various racial and ethnic groups:

    • Higher prevalence of hyperopia among Native Americans and Blacks.

    • Myopia more prevalent in Jewish, Japanese, and Chinese groups, correlated with increased academic work.

  • Astigmatism: Affects 50% of newborns leading to less prevalence with age, dropping to adult levels by age 5.

Summary of Important Concepts

  • Significant refractive errors are common among full-term children.

  • The process of emmetropization reduces refractive errors by school age.

  • Refractive states of children transition through various shapes of distribution from a bell-shaped curve to heavier leptokurtotic shapes typical of adults.

  • Knowledge of changing refractive error trends is necessary for optometrists to provide appropriate care, particularly during formative years.

Study Questions

  1. Define refractive error and explain the difference between Myopia and Hyperopia.

  2. What is astigmatism? What are its consequences?

  3. Describe the two types of astigmatism and how they differ.

  4. Define and differentiate WTR and ATR astigmatism.

  5. Explain Emmetropization and its clinical implications.

  6. Discuss the expected refractive statuses based on birth weight classifications.

  7. Evaluate how the refractive error distribution changes from infancy into school age and beyond.

  8. Describe the trends of astigmatism prevalence from birth to age 6.