Refractive Errors

By refraction, the light that enters the eye is focused onto a particular point of the retina. The main refractive components of the eye are the cornea and the lens. When the corneal curvature, the refractive power of the lens, does not match the size of the eye, ametropia or a refractive error occurs. The types of refractive errors include myopia (nearsightedness), hyperopia (farsightedness), and astigmatism (which can occur in both myopia and hyperopia). The use of a proper refractive device helps correct the visual impairment. Laser in situ keratomileusis (LASIK) is the most common corrective surgical procedure.

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Definitions and Epidemiology

Refraction

  • Light entering the eye is bent and focused onto the retina to create an image. 
  • Affects visual acuity
  • Light is refracted as it moves through structures of the eye (listed in order of encounter by light rays):
    1. Cornea 
      • Avascular, transparent structure in the front of the eye
      • Primary site of refraction (⅔ of the eye’s refractive power)
      • Curvature of the cornea must match eye size; if it does not → refractive error
    2. Aqueous humor
    3. Through the pupil (dilation adjusted by iris)
    4. Lens
      • Second greatest site of refraction
      • Able to adjust refraction
      • Accommodation: adjustment in lens shape to change focus from distant to near images (lens shape changed by action of ciliary muscle)
    5. Vitreous body
    6. Retina (inner surface of the eye containing photoreceptors)

Ametropia or refractive error

  • When light entering the eye does not focus on the retina
  • The focusing power of the cornea and lens (corneal curvature, refractive power of lens) does not match eye size (axial length of globe).
  • The condition requires a refractive device.
  • Unit of measurement of refractive error: diopter
  • Types:
    • Myopia: nearsightedness
    • Hyperopia: farsightedness
    • Astigmatism: uneven focus

Epidemiology

  • Refractive error affects approximately ⅓ of the population > 40 years of age in the United States and Western Europe.
  • Myopia, the most common refractive error, decreases with increasing age; high prevalence among Asians
  • Hyperopia and astigmatism increase with age.

Further definitions

  • Emmetropia
    • Normal vision
    • Light is focused on the retina
  • Presbyopia
    • Not a refractive error but affects visual acuity
    • Affected by physiologic loss of accommodation
    • In aging, the lens loses elasticity, thus has less ability to become more rounded → inability to focus at reading distance
  • Anisometropia: each eye with different refractive power

Related videos

Clinical Presentation

MyopiaHyperopiaAstigmatism
Focal pointIn front of retinaBehind retinaMultiple sites
CausesEye too long, cornea with too much curvatureEye too short, cornea with too little curvatureUneven curvature of the cornea
ResultCannot focus distant objectsCannot focus near objectsBlurry vision

Myopia

  • Axial length of the eye is too long or the cornea is too curved.
  • Light is focused in front of the retina. 
  • Ability to focus on near objects only; faraway objects will be out of focus
  • Risk factors: 
    • Genetics
    • Reading (↑ axial length during development)
    • Medications (sulfa, diuretics, cholinergics): transient effect
    • Diabetes mellitus: transient; change in serum osmolarity increases osmotic fluid into the lens → lens becomes swollen → increased refractive power
    • Trauma
    • Excessive accommodation
    • Elevated intraocular pressure
    • ↑ maternal age and maternal smoking during pregnancy
Myopia

This figure illustrates myopia (nearsightedness). The top image shows light as it is refracted too sharply by the cornea, resulting in a focal point in front of the retina.
With corrective negative lenses (bottom image), the light can be dispersed through the concave lens and focus appropriately onto the retina for clear vision.

Image: “myopia” by OpenStax-CNX. License: CC BY 4.0

Hyperopia

  • Axial length of the eye is too short or the cornea is flat.
  • Light is focused behind the retina.
  • Ability to focus on faraway objects only; near objects will be out of focus.
  • Risk factors:
    • Trauma
    • A mass or lesion behind the eye
    • Surgery (lens removal)
    • Medications: anticholinergics
    • Rapid decline in glucose (transient)
Hyperopia

This figure illustrates hyperopia (farsightedness). The top image illustrates uncorrected refraction, where the light is not refracted sharply enough, resulting in a focal point behind the retina.
With corrective positive lenses (bottom image), the light can be dispersed through the convex lens and focus appropriately onto the retina for clear vision.

Image by Lecturio.

Astigmatism

  • Uneven curvature of the cornea (cornea shape is “more like a football than a basketball”)
  • Multiple focal points = different refractive powers along different meridians
  • Results in blurry vision at all viewing distances
Astigmatism

This figure contrasts the normal corneal curvature with that of an astigmatic cornea. The cornea with astigmatism distorts the focus point of light in front of and/or behind the retina.

Image by Lecturio.

Diagnosis

Adults

  • No consensus on routine vision screening for adults > 65 years of age without vision problems
  • Vision is integral to maintaining independence in activities of daily living (ADL), so the need for optical correction has to be determined.
  • An eye specialist referral is recommended for unexplained vision change/loss. 
  • Snellen chart
    • High-contrast symbols (black letters/numbers on white background) with varying sizes read at a standard distance (20 feet (6 meters))
    • Often used to evaluate visual acuity

Children

  • Vision is critical for social relationships and mental development (learning and reading) → screening is recommended to detect visual problems
  • Preferred optotypes for pediatric vision screening (standardized and validated):
    • LEA chart (symbols for children or those with mental disability)
    • Sloan letters
    • HOTV (letter matching using those 4 letters)
  • Screening for children < 5 years old: 
    • Vision assessment in newborns (inspection, ocular motility, pupillary exam, red reflex)
    • Vision risk assessment at all health maintenance visits and vision screening if with risk factors (prematurity, family history of congenital cataracts/retinoblastoma, developmental delay, eye symptoms of systemic disease)
    • Start screening at 12 months of age (photoscreener or autorefractor); refer if child fails photoscreening
    • Vision screening at 3–4 years of age (monocular distance acuity); refer if: 
      • Worse than 20/50 in a 3-year-old
      • Worse than 20/40 in a 4-year-old
  • Screening for children ≥ 5 years old:
    • Check every 12 years
    • Refer if worse than 20/30

Management

Non-surgical treatment

Myopia

  1. Corrected with concave (diverging) lens, which spreads out the light
    • Functions to decrease the focusing power (“minus” power lenses)
    • Eyeglass prescriptions for myopia are in negative diopters (less focusing power), i.e., –3.00. 
  2. In general, school-aged children are treated for myopia of > 1.52 diopters (interferes with education and/or social function).

Hyperopia

  • Corrected with convex (converging) lens, which brings the focus forward onto the retina
    • Functions to increase the focusing power (“plus” lenses)
    • Eyeglass prescriptions for hyperopia are in positive diopters (more focusing power), i.e., +2.00.
  • In children, mild hyperopia is normal. Correction recommended in high hyperopia, i.e. > 4 diopters (interferes with education and at risk for refractive amblyopia).

Astigmatism

  • Corrected with cylindrical lenses (to even out focusing power)
  • Eyeglass prescription for astigmatism indicates the cylinder and axis (orientation of astigmatism)

Presbyopia

  • Use of reading glasses (convex lens)

Eyeglass prescription

  • OS (oculus sinister): left eye
  • OD (oculus dexter): right eye
  • OU (oculus uterque): both eyes
  • SPH: sphere/degree of near- or farsightedness)
    • Numbers represent diopters: the further away from zero, the stronger the prescription is
    • “Minus” sign: myopia, i.e. -3.00 = 3 diopters of nearsightedness
    • “Plus” sign: hyperopia, i.e. +2.25 = 2 and ¼ diopters of farsightedness
  • CYL/cylinder: indicating astigmatic component x AXIS (radial degrees of the cylinder)
    • CYL can be “plus” (farsighted astigmatism) or “minus” (nearsighted astigmatism)
    • AXIS is 0180 degrees; indicates where the difference in curvature is
  • Add: additional lens power, i.e., bifocals (instead of having separate reading glasses)

Example:

A patient with hyperopia, more on the left, also with astigmatism in both eyes, would be given the following prescription:

  • OS +3.25 + 2.50 x 090
  • OD +3.00 + 2.50 x 090

Surgical treatment

  • Typically performed in adults
  • LASIK: laser-assisted in situ keratomileusis 
    • Most widely used procedure
    • Uses excimer laser to reshape the surface of the cornea (epithelium and stroma)
    • Minimal pain
    • For myopia: flattens the cornea
    • For hyperopia: Laser is applied at the periphery to steepen the central cornea.
    • For astigmatism: combines peripheral and central laser treatments
  • LASEK: laser epithelial keratomileusis
    • Corneal epithelium flap is formed with use of alcohol, then laser is applied
    • Mild-to-moderate postoperative pain
    • Slower optical recovery than LASIK 
    • May need postoperative medications for up to 3 months
  • PRK: photorefractive keratectomy
    • The corneal epithelium is removed and then laser treatment is applied.
    • Mild-to-moderate postoperative pain, may require systemic analgesic
    • May take up to 3 months for visual acuity to stabilize

Differential Diagnosis

The differential diagnosis of refractive errors includes the following conditions:

  • Cataracts: A leading cause of blindness worldwide. It is characterized by a decrease in vision due to cloudiness or loss of transparency of the lens, which develops areas of opaqueness that require surgical extraction.
  • Glaucoma: An optic neuropathy with distinctive changes in the optic cup and visual field defect. It is often associated with increased pressure within the eyeball (commonly within the anterior and posterior chambers), which results in gradual vision loss. This can occur acutely due to a blockage of the aqueous humor drainage (acute angle-closure glaucoma) which is an emergency. The majority of cases are due to open-angle glaucoma.
  • Uveitis: acute inflammation of the uvea (vascular layer of the eyeball) and of the ciliary body. Affected patients often suffer from blurred vision, painful (anterior uveitis) or painless (posterior uveitis) redness of the eye.

References

  1. Shahzad, M., Gardiner, M. & Givens, J. (2019). Visual impairment in adults: Refractive disorders and presbyopia. UpToDate. Retrieved September 1, 2020, from https://www.uptodate.com/contents/visual-impairment-in-adults-refractive-disorders-and-presbyopia
  2. Bower, K.; Jacobs, D. & Givens, J. (2020). Laser refractive surgery. UpToDate. Retrieved September 1, 2020, from https://www.uptodate.com/contents/laser-refractive-surgery
  3. Bope, Edward T.; Kellerman, Rick D. (2015). Conn’s Current Therapy 2016. Elsevier Health Sciences. p. 354. ISBN 9780323355353

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