Refractive Errors

Overview

Eye Cells

Video 6: Normal accommodation​

Normal accommodation

  • Light rays from objects successively cross four transparent zones of the eye:
    • The cornea,
    • The aqueous humour,
    • The lens,
    • The vitreous body

before arriving at the retina, the sense organ of vision.

  • Whenever a light ray crosses one of these zones, it undergoes a deviation: refraction.
  • The totality of such refractions result in convergence of light rays on the retina (1).
  • Deformity of the lens makes it possible to “focus” and to ensure clear vision of the object, an object that is near, as well as one that is far away:
    • Distant vision: the lens is very elongated and little convergent. (The eye is at rest.)
    • Near vision: the lens is protruding and convergent at a maximum.
    • This change in shape is called accommodation.

Ametropia

  • Whenever light rays converge in front of or behind the retina and no longer on the retina, perception of the image is blurred.

We then refer to errors of refraction or ametropia.

  • Among these errors of refraction, include the following:
    • myopia,
    • hypermetropia,
    • astigmatism,
    • presbyopia.
  • Primary symptoms:
    • Distant vision is blurred.
  • Symptoms that can be concomitant:
    • headache,
    • eye dryness, caused by excessive fixation of objects, leading to eye irritation, itching, visual fatigue, sensation of a foreign or eye redness
    • in children, excessive blinking or rubbing of the eyes (2).
  • The world-wide incidence is difficult to estimate (ranges between 800 million and 2.3 billion (3)). According to WHO, 153 million persons live with a visual disorder related to an uncorrected error of refraction and 35 million persons suffer from decreased vision which would require management (4).

Clinical Presentation

Definition & symptoms 

  • Not an eye disease strictly speaking: a visual defect.
  • Characterised by:
    • Blurred vision of distant objects
    • Near vision is not affected.
  • Can result from:
    • A too high axial length of the eye,
    • A cornea which is too convex.
  • These defects lead to convergence of light rays in front of the retina (2).

Video 7.1: Myopia: illustration of symptoms

Video 7.2: The myopic eye with a too long axial length

Video 7.3: The myopic eye with a too convex cornea​

Epidemiology & Risk factors

  • Prevalence:
    • About 25% of the population over 40 years of age is affected in countries of Western Europe and in the USA. (5)
    • In France, 39% of the population may have myopia.
    • Is variable depending on:
      • Ethnic population
      • Geographic location
        • Is wide-spread in the industrialised areas of eastern Asia.
    • Has been on the increase for the last half century
  • Results from complex interaction between:
    • A genetic predisposition 
    • Environmental exposure.
  • Environmental risk factors:
    • Socio-economic status
    • Lifestyle (Role of visual work is suggested) (6).

Outcome & Complications

  • It can progress according to different modes:
    • Occurrence at school age and stabilisation at around 15 -17 years.
    • Occurrence in young adults (7).
  • In case of severe myopia, increased risk of complication such as:
    • Retinal detachment,
    • Sub-retinal neo-vascularisation,
    • Cataract,
    • Glaucoma (6).

Definition & Symptoms

  • Characterised by:
    • poor vision of objects that are near
    • in adults, vision of distant objects, as well as that of near objects is blurred.
    • Children and young adults with slight hypermetropia can see clearly as result of their capacity for accommodation.
  • Can result from:
    • A too short axial length of the eye,
    • A too flat cornea.
  • These defects are at the origin of convergence of light rays behind the retina (2).

Video 8.1: Hypermetropia illustration of symptoms

Video 8.2: The hypermetropic eye with a too short axial length 

Video 8.3: The hypermetropic eye with a too flat cornea 

 

Epidemiology & Risk factors

  • Prevalence:
    • About 10% of the population over 40 years of age in countries of Western Europe and in the USA may have hypermetropia. (8)
    • In France, 9% of the population may have hypermetropia.
  • A complex interaction of biological, environmental and behavioural factors.
    • Role of heredity (5).

Outcome & Complications

  • Risk, in particular, for children presenting with severe hypermetropia of developing:
    • strabismus
    • amblyopia or "lazy eye” (9).
 

Definition

  • Characterised by:  
    • Blurred and imprecise vision of near, as well as of distant objects.
    • Abnormal curvature of the cornea: an oval instead of a round shape.
    • The light rays focus on different points, both behind and in front of the retina:  
      • Deformity of the image (2).

Vidéo 9.1 : Astigmatism: illustration of symptoms

Video 9.2 : The eye with astigmatism​

 

Epidemiology & Risk factors

  • In France, 15% of the population may have astigmatism.

Outcome & Complications

  • Can exist from birth or correspond to tissue deformity acquired following:
    • An injury,
    • Inflammatory episode,
    • Degenerative dystrophy (keratoconus). (10)
  • Generally is weakly progressive
  • In case of rapid succession, keratoconus should be suspected.

Video 10.1: Presbyopia: illustration of symptoms

 

Video 10.2: the eye with presbyopia​

Definition & Symptoms

  • Characterised by:
    • increasing difficulty in near  vision.
  • With age, progressive loss of elasticity of the lens (becomes increasingly rigid).

-> Loss of power of accommodation (mechanism that ensures clarity of the image according to distance by a change in curvature of the two sides of the lens)

Epidemiology & Risk factors

  • In Europe and in the USA, about 90 million subjects suffer from presbyopia;
  • In France, about 27 million subjects, about 19 million of whom are between 45 and 70 years of age (11).
  • Decisive role of age
  • Other factors can be involved:
    • Uncorrected hypermetropia,
    • Close visual work,
    • Eye disease and injuries affecting the lens,
    • Existence of diseases such as diabetes or multiple sclerosis, etc. (12) 

Outcome & Complications

  • Ability for accommodation decreases with age:
    • At 6 months, it is 18.5 dioptres, 
    • At 30 years: 7 dioptres,
    • At 40 years: 4 dioptres,
    • At 45 years: 3,
    • At 60 years: 1 dioptre
    • Above 65 years of age: < 1 dioptre (13).
  • Presbyopia tends to stabilise after age 65 years. 

Diagnostic

World Cells

Diagnostic approach

  • Possible orientation by description of the visual impairment experienced in adults.
  • Measurement of visual acuity:
    • Reading, with one eye or both, of letters of decreasing size appearing on an illuminated panel.
  • Determining precise type and degree of ametropia:
    • Refractometry
    • Skiascopy
  • (Measurement of length of the eye by biometry.)

(Pre-operatory examination to choose the most appropriate implant)

Refractometry

  • Measuring the spherical refraction of the eye, which is altered in persons who have:
    • myopia,
    • hypermetropia.
  • Measuring cylindrical refraction of the eye, altered in case of:
    • Astigmatism.
  • Use of a computerised instrument called an automatic refractometre.
  • It is possible to instil a cycloplegic eye drop to suspend the process of accommodation and obtain greater precision in measurement.
  • Recording of 3 consecutive measurements in each eye.
  • A painless, rapid and precise technique. 

Skiascopy

  • Generally use of a self-illuminated electrical skiascope.
  • Principle:
    • Observing movement of reflection of an area in the fundoscopy which illuminates by means of light of the skiascope.
      • If there is no movement, there is no error of refraction;
      • A movement going in the direction of the light bundle indicates hypermetropia;
      • Movement going in the reverse direction suggests myopia, etc.
    • Studying the speed of movement of reflection:
      • The slower the speed, the more the error of refraction is high.
      • Inter-placing lens of variable power up to neutralising the reflection
      • Enables to determine the power of correction necessary. (14)

Treatment

Eye Cells

Eyeglasses

  • Myopia: corrected by concave lenses enabling to push back the image on to the retina in order to restore good distant vision.
  • Hypermetropia: corrected by convex lenses, which advance the image onto the retina to remedy poor near vision.
  • Astigmatism: corrected by tonic lenses whose curvatures compensate for those of the cornea.
  • Presbyopia: different types of  lenses are available:
    • Bifocal lenses: the lower convex part to correct defective near vision and upper part to correct distant vision.
    • Progressive lenses: make it possible to see at all distances without separation in the middle of the lens (2).

Contact lenses

  • Better correction of visual acuity and of peripheral vision than corrected lenses.
  • Myopia and hypermetropia: soft or hard lenses.
  • Astigmatism: hard lenses or tonic soft lenses
  • Importance of instructions concerning hygiene and handling provided by your ophthalmologist (2).
  • Replacement systems exist for a day, a week, a month or for 6 months. 

General considerations

  • Is designed to reduce dependence on eyeglasses or contact lenses.
  • Different techniques:
    • Laser technique consisting of modifying curvature of the cornea.
    • Placement of an intraocular implant.
  • Choice of method is up to the surgeon and depends, in particular, on:
    • on type of ametropia to treat,
    • its intensity,
    • morphology of the cornea.

Laser techniques

Photorefractive keratectomy (PKR)

  • Principle:
    • Sculpting the anterior layers of the cornea after removing it from its epithelium (superficial corneal remodelling).
  • Objective:
    • A flatter cornea in patients with myopia
    • A more protruding cornea in patients with hypermetropia.
  • Results:
    • After surgery, almost 95% of patients see with 5/10 or more and no longer wear eyeglasses (2).

LASIK or laser keratomileusis in situ

  • Principle:
    • Decreasing the thickness of the cornea in order to modify its capacity of refraction.
    • Creating a corneal tissue flap in order to enable in-depth remodelling of the corneal curvature with a laser.
  • Indications:
    • myopia, astigmatism and hypermetropia.
  • Results:
    • Rapid recovery of vision and majority of persons report a major improvement in their vision starting the next day (2).

LASEK or laser epithelial keratomieusis

  • Technique intermediate between PKR and LASIK.
  • Recovery generally similar to that obtained in PKR (2).

Intraocular implants

  • Optical contact lenses made of synthetic material.
  • Implanted as a replacement for the lens or in front of the lens.
  • Enables to restore near or distant vision, depending on the defect to be corrected.
  • Generally reserved for patients with severe myopia and with hypermetropia or with the cornea which is too thin to be treated with laser therapy.
  • Ultrasound control measurement:
    • Before surgery, to verify that the capsule of the lens is intact.
    • After surgery, to verify the proper position of the implant. 

References

Cells
  1. Albou-Ganem C, Saragoussi JJ. Qu’est-ce que la réfraction. Chirurgie réfractive. Bash 2008, p. 32-3.
  2. Porter RS, Kaplan JL. Chapter 61. Refractive Error. The Merck Manual of Diagnosis & Therapy, 19th Edition,  Merck Research Laboratories 2011, p. 669-671.
  3. Dunaway D, Berger I. Worldwide Distribution of Visual Refractive Errors and What to Expect at a Particular Location. InFOCUS Center for Primary Eye Care Development 2005.
  4. Organisation mondiale de la santé. Qu’est-ce qu’un défaut de réfraction ? Questions-Reponses. http://www.who.int/features/qa/45/fr/#. 10 Octobre 2013.
  5. Wojciechowski R. Nature and Nurture: the complex genetics of myopia and refractive error. Clin Genet. 2011 April ; 79(4): 301–320.
  6. Foster PJ, Jiang Y. Epidemiology of myopia. Eye (2014) 28, 202–208.
  7. Saw SM et al. Myopia: attempts to arrest progression. Br J Ophthalmol. Nov 2002; 86(11): 1306–1311.
  8. Kempen JH, Mitchell P, Lee KE, et al. The prevalence of refractive errors among adults in the United States, Western Europe, and Australia. Arch Ophthalmol. 2004;122:495–505.
  9. Jones-Jordan L et al. Spectacle correction versus no spectacles for prevention of strabismus in hyperopic children. Cochrane Database Syst Rev. ; 2014 : 8.
  10. Touboul D. Correction des astigmatismes irréguliers par anneaux intra cornéens. 2012 : 35 (3) 212-219.
  11. Gilg AN. Traiter la presbytie. Collection : Optique et Vision. Editions Lavoisier 2009
  12. American Optometric Association. Optometric clinical practice guideline - Care of the patient with presbyopia. 2011. p. 6.
  13. Duncombe-Poulet C. La presbytie : bases cliniques et physiopathologies. Tropique 2005. p. 4-10.
  14. Damanakis A. La  Skiascopie. Éditeurs: A & J Pechereau (2007).