Refractive Errors
For impaired vision, see Impaired Vision.
Essentials
- A refractive error is caused by an imbalance between the refractive power of the optical system of the eye and its axial length, rarely to altered refractive power of the cornea (keratoconus, cornea plana) or the lens (nuclear sclerosis, spherophakia).
- A refractive error may exist in the form of
- hyperopia (far-sightedness)
- myopia (short-sightedness) or
- astigmatism
- Some eye diseases are more common in
- severe myopia: peripheral retinal tears and retinal detachment
- hyperopia: angle closure glaucoma.
Physiology
- The most important refractive elements of the eye are the cornea (43 diopters, D) and the lens (23 D). They cause parallel rays of light coming from far away objects to converge so that a sharp image falls on the retina, which is located 23-24 mm behind the cornea in a normal (emmetropic) eye.
- The refractive power of the lens can be increased by active work of the ciliary muscle (accommodation).
- When looking into the distance, the ciliary muscle is at rest, the suspensory fibres of the lens, the zonules, are tight and the lens is thin and stretched and its refractive power at its lowest.
- During accommodation the ciliary muscle contracts and the zonules relax, the convexity of the lens increases due to the elasticity of the lens material, and its refractive power increases.
- Accommodation decreases gradually by age, causing symptoms (presbyopia) at the age of 40-45 years.
- The refractive power of the cornea cannot be changed by ocular muscle activity.
- A so-called stenopic hole can be used to distinguish between refractory error and an ocular disease as the cause of visual impairment.
- Stenopic hole often improves vision that is impaired due to reflactory error, but worsens vision that is impaired due to an ocular disease.
- A hole made on a piece of paper with a needle serves as a stenopic hole. The patient looks at an eye chart first without the hole and then brings the hole close to his/her eye and looks through it.
Hyperopia (far-sightedness)
Essentials
- The eye is too short in relation to the lens system.
- The condition may be compensated for by accommodation, i.e. the eye is capable of focusing both far and near with the help of the muscles that adapt the ocular lens.
- Children are hyperopic at birth.
- Children have an accommodative capacity of almost 20 diopters.
- By the age of 7 years, the eyeball usually reaches its full length of 23-24 mm.
- A hyperopic child may develop convergent strabismus (esotropia), particularly if one eye is more hyperopic than the other (anisometropia).
- Convergence associated with accommodation pulls the eye to esotropia.
- Strong hyperopia is rarer than myopia, but more unpleasant, as it prevents from seeing objects clearly at any distance.
- Mild hyperopia is easily compensated with accommodation (latent hyperopia), but if hyperopia is strong or when the accommodation capacity decreases with age, patients will eventually get eyestrain (asthenopia), first in close-up work where the need for accommodation is greatest, but later also when looking far away.
- In children, the symptoms may become aggravated by prolonged looking at close distance (reading, continuous use of a mobile device).
- Conditions that are associated with macular oedema, e.g. diabetic maculopathy, age-related macular degeneration and central serous retinopathy, decrease the effective axial length of the eye, making it hyperopic.
Symptoms
- Symptoms are highly variable:
- frontal headache and/or pain around the eyes
- headache
- burning of the eyes, foreign body sensation, the eyes feel tired (asthenopia)
- possibly also redness of the eyes
- reduced visual acuity when looking close and, when the hyperopic refractive error is large, also when looking far.
Differential diagnosis
- Dry eye syndrome (the Schirmer test and artificial tears) Dry Eye Syndrome
- Latent strabismus and convergence insufficiency
Treatment
- Convex spectacles (plus power) that converge rays of light and compensate for the lacking refractive power of the lens system of the eye, thus forming an image on the retina. This makes it possible for the patient to see far with the eyes at rest.
- Younger persons are first able to do also close-up work with the same glasses without any problems.
- With advancing age, the patient will need bifocal or multifocal (non-progressive or progressive addition) lenses that correct both distance vision and compensate for the weakened accommodation reserve in near vision.
- Bifocal glasses are also used for accommodative convergent strabismus.
Myopia (short-sightedness)
Essentials
- Every third adolescent or young adult is myopic.
- The eye is too long for its refractive power.
- Light rays from infinity are focused in front of the retina.
- One millimetre excess in length corresponds to 2-3 diopters of myopia (axial myopia).
- In emmetropic eyes, the far point plane of the eye is at infinity (6 metres for practical purposes). In -3 diopter myopia the eye sees clearly at a distance of 33 cm (the power of a lens, in diopters, is 1/focal length in metres). In this example the focal length is 0.33 m, and 1/0.33 m = 3 D). The eyes of a person with a -3 diopter myopia are at rest when reading text that is 33 cm away. Any objects beyond this far point plane are not sharp. Screwing up one's eyes makes the object sharper. The narrow palpebral fissure works like the aperture in a camera: the smaller the aperture, the greater the depth of focus. Objects closer than the far point plane are seen sharply with accommodation.
Symptoms
- Distance vision is blurred.
- Children prefer to watch TV closer than usual.
- School children cannot see the blackboard, especially if they are tall and thus sit at the back of the classroom.
- Students have difficulties in distinguishing small text projected in the front of a lecture room.
- Problems recur when glasses become weak.
- The problem is aggravated in dim lighting.
- Drivers notice difficulties seeing in the dark or they do not see traffic signs even in broad daylight.
Differential diagnosis
- Keratoconus Keratoconus (Conical Degeneration of the Cornea)
- Increased refractive power of the lens associated with cataract may cause myopia Cataract.
- A presbyopic person with cataract may be able to read again without spectacles (so-called "second sight").
- Acute swelling of the lens in diabetic hyperglycaemia or epidemic nephropathy may cause transient myopia.
- Spasm of accommodation
- Long working that involves looking at close distance may cause a ciliary muscle spasm, which changes the form of the lens in a way that causes transient myopia.
- Vision is blurry at a distance. The visual acuity may vary. The pupils may be decreased in size.
- Within an examination by an ophthalmologist, a spasm of accommodation can be induced by cyclorefraction, where the ciliary muscle is relaxed using cycloplegic drops.
Treatment
- Concave (minus power) spectacles or contact lenses that diverge the rays of light so that when they enter the eye its own lens system converges them and a sharp picture falls on the retina.
- .
- Currently, refractive surgery is popular. The refractive power of the cornea may be weakened by relaxing incisions (less common) or by flattening its surface with an excimer laser and related techniques.
- Surgery involves the risk of permanent injury to the cornea: scar formation leading to haze that disturbs vision . Therefore it is not the treatment of choice for myopia that can be treated totally risk-free with spectacles.
Astigmatism
- The refractive power of the cornea is not uniform, because it often has an uneven curvature, it is steeper in one direction and flatter in the other (regular astigmatism) or even its surface is uneven (irregular astigmatism). In regular astigmatism there are two main different radii of the curvature of the cornea: smaller and larger.
- The steeper surface refracts the rays of light more than the flatter one, so that the image of an object is never sharp on the retina.
Symptoms
- Patients may experience that the vertical and horizontal elements of the text or pictures are not sharp at the same time, part of it being sharp and the other part not. In addition, monocular double images may also occur especially when reading.
- May cause headache and eye strain.
Treatment
- Cylinder lenses in which the refractive power is present only at an angle of 90 degrees towards its axis (regular astigmatism).
- Irregular astigmatism cannot be corrected with spectacles. It occurs in keratoconus and scarred corneas and can be corrected with rigid contact lenses, as long as the cornea itself is clear. If not effective, invasive treatment of the cornea (penetrating keratoplasty) is indicated.
Presbyopia
- Accommodative amplitude is almost 20 diopters in children but decreases gradually during the subsequent decades.
- By the age of 45 years, the loss of accommodative power begins to cause symptoms, since one third of it should be unused.
- Virtually no accommodation is left by 60 years of age.
Treatment
- Plus correction, which is added to the glasses needed to correct far vision. The amount of this "addition" is about +1.0 D for those aged 45, and a maximum of +2.5 D for those aged 60 years or more.
- Stronger additions act as near vision aids for the visually handicapped. The need for magnification is determined with a simple formula of 1/visual acuity at distance (e.g. for a distance acuity of 0.2, the addition needed is 5 D).