Refraction and Refractive Errors

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 onto 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.

The process of bending the light rays to focus on a point at the back of the eye is carried out at the air/tear interface (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 shape of the cornea, the depth of the anterior chamber of the eye and the length of the eye from front to back (the axial length). These four elements change over time as the eye grows. Refraction (whether of 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.

The emmetropic eye is able to achieve a perfect focus. Ametropia is the global term referring to any refractive error and clinically, the term refraction refers to the process by which the best possible visual acuity can be obtained for a patient.

The process of refraction has three main aims:

• 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 (e.g. 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 (e.g. preverbal children).
• Binocular balancing: this final step ensures that accommodation and distant viewing is balanced in both eyes.

A note on lenses¹

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. Contact lenses work on the same principle but the space between the lens and the anterior surface of the cornea is reduced to the tear film alone. 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.

Myopia

• Description - myopic eyes have too much optical power given their length 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 UK³ and is said to be mild (up to 3.0D), moderate (3.0-6.0D) or severe / high degree (>6.0D). The latter affects about 200,000 British people and can be associated with degenerative fundal changes. 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:³
  • Prematurity
  • Marfan's Syndrome
  • Stickler's Syndrome
  • Ehlers-Danlos Syndrome
  • Homocystinuria
• Lens correction - a concave (minus) lens is used to correct the problem.

Hypermetropia (hyperopia)

• 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 most young children and this usually resolves by adulthood.³ It 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 (e.g. blackboard) but there is difficulty in viewing objects close-up (e.g. book) which may give rise to eye strain and headache. Although there may be a family history, most cases are sporadic. Other eye conditions associated with hypermetropia include:³
  • Corneal dystrophies
  • Congenital cataracts
  • Retinitis pigmentosa
  • Microphthalmia
• Lens correction - a convex (plus) lens is used to correct the problem.
  
  
  

Astigmatism

• 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:
  • Previous eye surgery
  • Previous corneal injury
  • Corneal dystrophies
  • Congenital cataract
  • Optic nerve hypoplasia
  • Retinitis pigmentosa
  • Albinism
  • Nystagmus
• 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.

Anisometropia

An overview is provided below: please see our dedicated article on surgical correction of refractive errors for further details.

Excimer laser surgery

• Background - the basis of this type of surgery lies in the notion that modifying the shape of the cornea can reduce a refractive error. The three principle techniques used are:
  • Photorefractive keratectomy (PRK) - this is used to treat myopia, astigmatism and low degrees of hypermetropia. A laser is used directly on the surface of the cornea to sculpt it into a predetermined shape.
  • Laser epithelial keratomileusis (LASEK) - this is a modified form of PRK whereby there is a chemical loosening of the corneal epithelium which is then replaced on the corneal surface according to the refractive requirement.
  • Laser in-situ keratomileusis (LASIK) - this involves cutting a flap of cornea and on peeling this back, applying the laser to the corneal stroma (middle layer of the cornea). The epithelial flap is variously vapourised depending on the refractive requirement. This technique has been widely adopted since the mid-1990s.
  This type of surgery is not usually available on the NHS but it may be carried out in some NHS hospitals. It costs ~ £1000 - £1500 per eye.
• Qualifying for surgery - the patient must be over 21 with a stable prescription over 2-3 years and have otherwise healthy eyes. Extremes of refractive errors are usually not considered as the procedure may yield unpredictable results with a higher complication rate. Patients are counselled prior to surgery with regards to the results that can realistically be hoped to achieve in their case. Pregnant or breast-feeding women as well as patients with certain medical conditions (e.g. diabetes, rheumatoid arthritis, systemic lupus erythematosus) will not be able to go ahead with surgery as well as those on certain medications such as steroids.
• Operative considerations - refractive surgery can be done for errors ranging from -10.0D of myopia to +6.0D of hypermetropia and up to 4 cyl of astigmatism. Other factors that will be taken into consideration by the surgeon when considering a patient for refractive surgery is the angle kappa (which measures how close the centre of the pupil is to the central visual axis - the two are not always perfectly aligned) and the pupillary size.
• The procedure - these are day-case procedures done under local anaesthetic (a mild sedative may be used), carried out in specialist centres. As PRK is often painful, usually one eye only is done at a time but LASEK and LASIK can be unilateral or bilateral procedures.
• Post-operatively - recovery can take between days and a few weeks, depending on which procedure is done and whether both eyes were done or not.
• Complications⁶
  • General - under or over correction, astigmatism, anisometropia, presbyopia, glare or halo effects, dry eye symptoms and reduced contrast sensitivity.
  • PRK - more common problems include slow-healing epithelial defects, corneal haze, poor night vision, regression of refractive correction.
  • LASIK - unusually, there can be intraoperative complications with the corneal flap. Post-operatively, complications include residual epithelial defects, corneal distortion, keratitis and more rarely, other problems such as anterior segment ischaemia and optic neuropathy.
• Outcome⁷ - PRK and LASIK have comparable refractive outcomes which tend to be good with LASIK possibly having the upper edge in that visual recovery is faster and less likely to result in loss of best spectacle-corrected visual acuity.

Other types of surgery

• Thermokeratoplasty: altering the corneal surface using heat emanating from an electric wire or a laser.
• Keratotomies: radial or arcuate corneal incisions are made to alter corneal shape.
• Insertion of corrective intraocular lens implants.

Presbyopia

• 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.
• Patient perspective - the patient finds it difficult to carry out near-tasks and describes needing reading glasses (often referred to as 'readers').
• 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 bi- or trifocals. Contact lenses may also be used. Surgical correction with PRK or LASIK is possible although this option needs to be considered in the light of the low but present risk of complications (see above).

Accommodative insufficiency⁹

• 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 (e.g. parasympatholytics and tranquilising drugs). It is characterised by an inability to maintain binocular alignment as an object comes nearer to a patient.
• 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.
• Correction - treatment is with corrective lenses. Exercises for the eyes involving the near point of convergence may also be suggested and very occasionally, there is a role for pharmacological agents. Any underlying condition needs to be addressed.

Accommodative excess

• Description - this arises as a result of ciliary muscle spasm which may arise from ocular disease (e.g. iridocyclitis), drugs used to treat ocular disease (e.g. 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 (e.g. cycloplegics).