The purpose of the globe is to receive light from the outside world and transmit it to the brain for processing. There are two aspects to this function. In the first instance, the light rays have to be correctly focused on to the back of the eye. Then, this information has to be converted to electrochemical signals by the cells within the retina and transmitted to the brain.
In optical physics, the term 'refraction' is used to describe the bending of light rays as they pass across a particular medium. Refraction - whether by natural eye tissue or of an artificial lens - is measured in dioptres (D) which describes the power that a structure has to focus parallel rays of light. The higher this value, the stronger its focusing ability. In the eye, refraction happens at the air/tear interface on the surface of the cornea (the most important site of ray refraction), by the cornea and by the lens. The accuracy of this process depends on:
- The integrity of these structures.
- The shapes of the cornea and lens.
- The depth of the anterior chamber of the eye.
- The length of the eye from front to back (the axial length).
These four elements change over time as the eye grows (eg axial length) and matures in later years (eg quality of tears which affects the air-tear interface). The emmetropic eye is able to achieve a perfect focus. Ametropia is the global term referring to any refractive error.
Importance of refractive errors
- Refractive errors are important because they account for half the cases of avoidable vision impairment globally (153 million people).
- Undetected refractive errors in childhood may lead to behavioural problems and adversely affect social interaction and performance (academic or sporting) at school.
- It has been found that a minor reduction in vision has been associated with an increased risk of death and physical, social and emotional problems in people over 50 years.
- Under-corrected refractive error may account for up to 75% of all vision impairment in high income countries.
Interventions to treat refractive errors (eg spectacles) are simple and cost-effective.
- Description - myopic eyes have too much optical power and so focus the image in front of the retina. This arises as a result of the physiological variation in the length of the eye or an excessively curved cornea. This common condition affects about 1 in 4 adults in the UK and tends to manifest itself in adolescence or early adulthood. It is said to be mild (up to 3.0 D), moderate (3.0-6.0 D) or severe/high degree (>6.0 D). The latter affects about 200,000 British people and can be associated with degenerative fundal changes (Forster-Fuchs' spots). It is also associated with an increased risk of retinal detachment, cataract formation and glaucoma.
- Patient perspective - patients are said to be near-sighted: distant objects appear to be blurred and, unless severe, close-up objects remain in focus. They may have a family history of myopia and there is some evidence to suggest that children who do a lot of close-up work are more likely to become myopic (or worsen pre-existing myopia). Other associations include:
- Lens correction - a concave (minus) lens is used to correct the problem (see diagram and explanation below).
- Description - this is the opposite problem to myopia. In this case, the eye does not possess enough optical power for its refractive length and therefore an object is focused behind the retina, so giving rise to a blurred image. Mild hypermetropia is a common finding in babies and very young children and this usually resolves by about 3 years of age. Persistent hypermetropia is associated with an increased risk of glaucoma, squint and amblyopia.
- Patient's perspective - patients are said to be long-sighted: distant objects are sharply focused but there is difficulty in viewing objects close-up, which may give rise to eye strain (due to the extra accommodative effort) and headache. Although there may be a family history, most cases are sporadic. Other eye conditions associated with hypermetropia include:
- Corneal dystrophies
- Congenital cataracts
- Retinitis pigmentosa
- Lens correction - a convex (plus) lens is used to correct the problem (see diagram and explanation below).
- Description - not only do light rays have to focus at the level of the retina (as opposed to in front or behind it) but also on a single point. This is achieved through the symmetry of the corneal and lens curvatures around their circumference. In astigmatism, variations in the symmetry of these curvatures (usually corneal) results in rays failing to focus on a single point. The degree of astigmatism is measured in cylinders (cyl). Astigmatism is often present in association with some degree of myopia or hypermetropia. A mild degree of astigmatism is relatively common in childhood and resolves in a number of cases. More severe astigmatism may lead to amblyopia, especially if there is an associated squint.
- Patient's perspective - there is blurring of vision that is not necessarily associated with obvious far-/short-sightedness, although distant viewing is usually the more problematic of the two. Most cases are sporadic but there may be a family history or a background of:
- Lens correction - a cylindrical lens is used to 'neutralise' astigmatism. The axis of the cylinder depends on the meridian of asymmetry in the patient's cornea. Where there is associated myopia or hypermetropia, a spherocylindrical lens is used.
- Description - this refers to the situation where there are unequal refractive errors between both eyes. This may be mild with limited consequences, eg different degrees of myopia in each eye - a relatively common situation. However, a problem arises where there are large differences (some clinicians reserve this term for a ≥2.0 D difference), eg one eye is myopic and the other hypermetropic. In childhood, this can usually be fully corrected with spectacles and without side-effects but severe cases may be associated with amblyopia. It is more unusual and problematic in adulthood but can occur following trauma, refractive or cataract surgery. The most extreme form occurs where there is unilateral aphakia (one lens is missing or has been removed).
- Patient's perspective - differing refractive states result in very slight differences in image sizes (aniseikonia refers to a difference in size or shape of images formed in the two eyes). Where the refractive difference is very small, this is imperceptible by the patient and not a problem. Where the effect is magnified due to a large refractive difference, the patient may experience diplopia, headaches, photophobia, reading difficulties, nausea, dizziness and general fatigue. It is for this reason that in childhood, the brain suppresses one of the images and hence amblyopia develops in the eye where the image has been suppressed.
- Lens correction - this is tricky and usually involves various types of spherical and cylindrical lenses depending on the type of anisometropia. However, the prismatic effects of the lenses often vary in different positions of gaze, giving rise to further symptoms (collectively known as anisophoria: a lens-induced aniseikonia) and many patients tolerate the lenses even less well than the original problem. Contact lenses often offer a better solution. The management of these patients remains within the remit of specialist optometrists.
Assessment of refractive errors
In primary care
Although the diagnosis of a refractive error can only accurately be made when the patient is refracted (see next subheading below), you can get a sense of whether this might actually be a problem of refractive error:
- The symptoms - commonly mentioned symptoms tend to relate to activities of daily living and may include difficulty with driving, reading (particularly small print such as bills or medication instructions) and preparing meals. The symptoms may be so gradual as not to be perceived by the patient as relating to their vision and they may actually come to you complaining of headaches or red, sore, watery eyes. Young children may rub their eyes a lot or turn their heads when looking at things (eg television) and school-aged children may present with behavioural problems.
- Associated conditions - enquire about other ocular or systemic conditions (see list of associations above).
- The prescription - if a patient brings along their optometrist's prescription for you to consider, it may be helpful to have a sense of the annotation used. Essentially, the prescription tells you how strong a lens is needed to bring the eye back to emmetropia. The larger the numbers, the stronger the lens required.The format of the prescription is:
[figure indicating degree of myopia/hypermetropia]/[figure telling you how astigmatic they are] x [meridian in which astigmatism lies]So, for example, a patient with a prescription of -1.25/-1.00 x 180 has a refractive error requiring a 1.25 D lens to correct the myopic element and a 1.00 D lens to correct the astigmatic element (at 180° which tells the optometrist in which plane the astigmatism lies). If the prescription was 0.00/-2.00 x 180, the patient has some astigmatism but no myopia or hypermetropia. Understanding refraction and prescriptions can be tricky but having a some sense of what is being indicated can be helpful when considering the patient's problem.
- Assessing for refractive errors - test each eye separately for near and distance vision (see separate article Examination of the Eye). They must use their usual distance glasses or be tested with their contact lenses in. Try using a pinhole: if they do better, a refractive error may be at least part of the problem.
- Referring - depending on the nature of the presentation, you may wish to refer to your local eye unit (acute presentation or presence of other symptoms, eg changes in colour perception, visual field or physical appearance of the eye) or, if this seems to be a long-standing problem, for an optometrist assessment (they can refer on directly if need be). Consider:
- A semi-urgent referral in anyone unexpectedly scoring 6/60 or worse.
- A non-urgent referral where:
- Children managing 6/9 or worse or adults managing 6/12 or worse.
- There is a two line difference in visual acuity between the eyes.
- You see or suspect strabismus (see separate article Eye Problems in Babies for more detail relating to strabismus in very young babies).
- There is difficulty in achieving a reliable visual acuity measurement
- A non-urgent referral where the near acuity is less than N5
Assessment of a refractive error can only really be done via the process of refraction (generally carried out by an optometrist). In clinical ophthalmology, 'refraction' is actually an abbreviation for 'refractive correction' and it refers to the process by which the best possible visual acuity can be obtained for a patient. Refraction has three objectives:
- It measures the patient's refractive error.
- It determines the optical correction required for focusing on distant and close objects.
- It provides the patient with appropriate corrective spectacles/lenses.
Refraction has several components to it:
- Objective refraction: this uses a special instrument (retinoscope) devised to determine the approximate nature of the patient's refractive error (eg myopia, astigmatism) and does not require any input from the patient.
- Subjective refraction: this uses a series of lenses to refine the measurement of the refractive error. It requires active patient participation and therefore is difficult to perform in patients with limited communication (eg preverbal children).
- Binocular balancing: this final step ensures that accommodation and distant viewing are balanced in both eyes.
Management of refractive errors
These may be managed by optometrists (specialists in the diagnosis and management of refractive errors), orthoptists (specialists in ocular motility problems and assessment of refractive errors in very young children) or ophthalmologists (medically qualified physicians or surgeons).
Spectacles are the simplest, safest and most cost-effective way or managing refractive errors. Lenses may be spheres, cylinders or a mixture of both. Spherical lenses are characterised by a constant curvature over the entire surface and may be convex (converge light rays, known as plus lenses) or concave (diverge light rays, known as minus lenses). Cylindrical lenses have focusing powers in one meridian only, the orientation of which depends on the patient's problem. The power of a spectacle lens can be measured using an instrument known as a lensmeter. Lenses may have one or more refractive components to them, the latter being known as multifocal lenses. The power needed for each component can be assessed and prescribed separately.
Contact lenses work on the same principle as spectacle lenses but the space between the lens and the anterior surface of the cornea is reduced to the tear film alone. Orthokeratology is an emerging technique used, whereby a rigid contact lens is fitted to distort the corneal shape in a controlled fashion overnight, so reducing symptoms of myopia during the day. See separate articles Contact Lenses (Types and Care) and Contact Lens Problems for more information about these.
This may entail microsurgery (such as lens implants) or laser treatment to the cornea. Go to our separate article Surgical Correction of Refractive Errors for details.
There has been evidence to suggest that outdoor activity may reduce the risk of children developing myopia later on. Myopia may also be slowed with orthokeratology (see separate Contact Lenses (Types and Care) article). Other than that, screening is the only method to pick up and correct refractive errors.
Asymptomatic, low-risk patients (no ocular comorbidity or family history) should have eye examinations at least:
- Every 10 years (19-40 years old).
- Every 5 years (41-55 years old).
- Every 3 years (56-65 years old).
- Every 2 years (>65 years old).
Patients at risk of visual impairment (eg diabetics, those with cataracts, macular degeneration, glaucoma or a significant family history of these) should have eye examinations at least:
- Every 3 years (>40 years old).
- Every 2 years (>50 years old).
- Every year (>60 years old).
Accommodation is the process whereby changes come about in order to increase the refractive power of the eye and enable near vision. There are three aspects to it: the eyes converge, pupil size reduces and the crystalline lens changes shape and position.
- Description - this is the gradual loss of the accommodative response due to a decline in the elasticity of the lens. It is effectively a life-long process but only becomes clinically significant when the residual accommodative amplitude is insufficient for the patient to carry out near-vision tasks such as reading. These symptoms most commonly occur after the age of 40 although this varies depending on pre-existing refractive error, pupil size and the patient's usual visual tasks. There are an estimated 517 million cases of uncorrected presbyopia world-wide, 410 million of whom would be restricted in doing basic tasks.
- Patient perspective - the patient finds it difficult to carry out near-tasks and describes needing brighter lighting conditions for these or needing reading glasses (often referred to as 'readers'). They may also complain of an accommodative lag: there is a slowed recovery time changing from distance to near tasks (and vice-versa). Individuals may also report tiring with continuous close work.
- Correction - if there is no pre-existing eye problem or refractive error, many manage perfectly well with over-the-counter glasses. In this case, the patient should be advised to take the glasses off when not carrying out near-tasks. However, if there was a pre-existing refractive error, prescription glasses are required; these may be bifocals or trifocals. Contact lenses may also be used. Surgical correction is possible although this option needs to be considered in the light of the low but present risk of complications (see separate Surgical Correction of Refractive Errors article).
- Description - this effectively describes a premature form of presbyopia. The exact problem is not clearly understood but is probably neuronal in origin. It is found in a number of neurological conditions, such as encephalitis and closed head trauma. It is also seen in patients with current or past debilitating illness and it may be induced by certain medications (eg parasympatholytics and tranquilising drugs). It is characterised by an inability to maintain binocular alignment as an object comes nearer to a patient. This condition is relatively common (3% to 5% of the population) and particularly affects individuals who have high visual demands (a lot of close work such as teenagers, college and others studying), in periods of illness, anxiety or fatigue.
- Patient perspective - initially, patients may complain of asthenopia - the vague discomfort associated with using the eyes (headache, 'eye strain', brow ache), particularly when trying to accommodate. Ultimately, near vision becomes blurred or there is diplopia associated with bringing objects closer. Symptoms vary from mild to severe but are few or absent in distant vision.
- Correction - treatment is very successful with corrective lenses or exercises for the eyes involving the near point of convergence. Very occasionally, there is a role for pharmacological agents. Any underlying condition needs to be addressed.
- Description - this arises as a result of ciliary muscle spasm which may arise from ocular disease (eg iridocyclitis), drugs used to treat ocular disease (eg anticholinesterases used in the treatment of glaucoma) or it may arise due to uncorrected refractive errors (usually hypermetropia). It can also occur after prolonged periods of close work.
- Patient perspective - the symptoms characteristically include headache, brow ache, variable blurring of distance vision and an unusually close near point.
- Correction - lenses and eye exercises are prescribed but, occasionally, the patient may need pharmaceutical help too (eg cycloplegics).
Further reading & references
- Cochrane GM, du Toit R, Le Mesurier RT; Management of refractive errors. BMJ. 2010 Apr 12;340:c1711. doi: 10.1136/bmj.c1711.
- American Academy of Ophthalmology - Basic and Clinical Science Course Section 3: Clinical Optics (2005-2006)
- About Aniseikonia, opticaldiagnostics.com
- Wilson FM: Practical Ophthalmology (5th ed.), 2005. American Academy of Ophthalmology
- Care of the Patient with Accommodative and Vergence Dysfunction, American Optometric Association (Last updated 2006)
- Bartiss MJ; Convergence Insufficiency, Medscape, Feb 2011
|Original Author: Dr Olivia Scott||Current Version: Dr Olivia Scott|
|Last Checked: 22/06/2011||Document ID: 3896 Version: 24||© EMIS|
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