Refraction

Refraction is the measurement of the eye's optical power, and the finding of the lenses that give the clearest vision. It identifies short sightedness (myopia), long sightedness (hyperopia), astigmatism, and presbyopia, and gives the prescription for glasses or contact lenses.

Two methods are used. Objective refraction measures the refractive error without the patient's response: retinoscopy (a streak of light is moved across the pupil and the reflex is neutralised with lenses), and autorefraction, where a machine measures the refraction automatically.

Subjective refraction refines the result with the patient's responses: lenses are placed before the eye in a test frame or phoropter, and the patient chooses the clearest option ("better one or two"), and the cross-cylinder method finds the cylinder axis and power. The result is written as sphere, cylinder, and axis (for example, -2.00 / -0.75 x 180), with an addition for near vision.

In children and in patients with accommodation problems, cycloplegic drops (for example, cyclopentolate) paralyse accommodation and show the full refractive error. The test takes 10 to 30 minutes. Refractive errors are the most common cause of visual impairment: myopia affects about 30% of the world's population, and is rising rapidly, expected to reach about 50% by 2050, and presbyopia affects almost everyone after about 45 years.

Indications.

• Reduced vision that improves with pinhole.
• Headache, eye strain, and difficulty reading or with night driving.
• Screening in children, school age vision checks, and in children with squint, amblyopia, or a family history of high refractive error.
• Before cataract and refractive surgery, and when glasses or contact lenses need updating.
• Rapidly changing refraction, for example in diabetes, keratoconus, and cataract.
• Monitoring of myopia progression in children.
• Work related or driving vision requirements.

Not a substitute for a full eye examination, since normal acuity with glasses does not exclude eye disease.

Contraindications and precautions.

• Cycloplegic drops are avoided or used with care in patients at risk of angle closure (narrow angles, shallow anterior chamber, hyperopes over 50 years), in infants younger than the age allowed for the drug, and in those with known allergy. Depending on the drug, blurred vision and sensitivity to light last for hours to days.
• Pupil dilation delays driving for several hours, and the patient is warned.
• Heart disease, and in infants, drops are used with caution because of systemic absorption, with pressing the tear duct to reduce it.
• Contact lenses are removed for a set period before testing, since they change the corneal shape.
• The test in uncooperative patients, and in those with poor attention, needs adapted methods.

Preparation and technique.

Visual acuity, ocular alignment, pupil responses and an anterior segment check are done first. For adults, an autorefractor reading or retinoscopy gives a starting point, and the patient looks at a distant target with the other eye fogged to relax accommodation.

Subjective refinement follows with sphere first (the most plus lens that gives best acuity), then cylinder axis and power with the cross-cylinder, and then sphere again, and finally the two eyes are balanced. Near addition is tested with a reading card at the patient's working distance. In children, cycloplegic drops are given 30 to 40 minutes before retinoscopy.

Normal appearances.

An emmetropic eye focuses distant light on the retina with no correction (plano), and most children are mildly long sighted (hyperopic, about +1 to +2 dioptres) that falls toward emmetropia during childhood. Adults after 40 to 45 years need a near addition of about +1.00 to +3.00 dioptres, increasing with age as accommodation falls.

Interpretation.

• Myopia: a minus sphere (for example, -2.50 dioptres). Mild is up to about -3.00, moderate -3.00 to -6.00, and high over -6.00 dioptres, which carries a higher risk of retinal detachment, glaucoma and myopic maculopathy.
• Hyperopia: a plus sphere. High hyperopia in a child is linked to squint (accommodative esotropia) and amblyopia.
• Astigmatism: a cylinder, with an axis. Irregular astigmatism points to keratoconus or corneal scarring.
• Anisometropia: a difference between the two eyes of over about 1 to 2 dioptres, which gives amblyopia risk and symptoms when corrected with glasses (aniseikonia).
• Presbyopia: a near addition, and, in a younger person, early presbyopia suggests a cause (drugs, diabetes, illness).
• Changes from the last prescription: a myopic shift in diabetes with high glucose, cataract, and drugs. A hyperopic shift is seen after glucose is lowered.
• Best corrected visual acuity that remains poor shows disease, and further tests are needed.

Critical values and findings needing immediate action.

• Best corrected acuity much poorer than expected, or a sudden change in refraction: examine for eye disease (keratoconus, cataract, macular disease, retinal detachment).
• Acute angle closure after dilatation (pain, red eye, halos, nausea): urgent treatment to lower the pressure.
• High hyperopia or anisometropia in a child at risk of amblyopia: prompt correction and monitoring within weeks, since the window for treatment is limited.
• Rapidly progressive myopia in a child: referral for control of progression.

Limitations and artefacts.

• Accommodation can hide hyperopia and cause overestimation of myopia, particularly in children and young adults, so cycloplegia is used when needed.
• Autorefractors can give false readings in children, with media opacities, small pupils, irregular corneas and unstable fixation. They overestimate myopia (instrument myopia).
• Subjective results depend on the patient's attention, understanding and fatigue, and a very small step in lens power can change the answer.
• Contrast, lighting, and pupil size affect the result. Night myopia is a difficulty in low light.
• Glasses prescribed may not be tolerated in the first days, particularly with large cylinder changes. Test frames and a settling period help.
• A refraction result does not show eye health.

Special groups.

• Children: cycloplegic refraction is the reference, with attention to amblyopia, squint, and myopia control (outdoor time, specialised lenses, low dose atropine drops).
• Presbyopes: near and intermediate addition for working distance, and progressive lenses as an option.
• Patients with keratoconus: contact lenses may be needed, and glasses give a poor correction.
• Cataract and refractive surgery patients: accurate measurements are needed for lens calculation.
• Diabetic patients: refraction varies with the blood glucose, and is repeated when levels are stable.
• Regions: uncorrected refractive error is the leading cause of vision impairment in many regions, and glasses are the most cost effective eye intervention. Trained refractionists, autorefractors, and ready made reading glasses extend access. Self-refraction with adjustable lenses and screening in schools are in use.

Follow up tests.

• Slit lamp examination and fundoscopy, to exclude disease.
• Corneal topography for astigmatism, and keratoconus screening.
• Axial length measurement for myopia.
• Orthoptic assessment for squint and binocular vision.
• Intraocular pressure measurement, and optical coherence tomography.
• Review of the glasses after 4 to 6 weeks if not tolerated, and at regular intervals (every 1 to 2 years, and more often in children).
• Referral for contact lens fitting or refractive surgery assessment.