Refractive errors are common, and when identified can be easily managed with spectacles and/or contact lenses. Problems may arise when individuals have limited access to services, resulting in delayed diagnosis and management. Such events are associated with morbidity and a reduced quality of life, and although more frequently encountered in the developing world, are not to be overlooked in the UK.
This article aims to provide medical students with an understanding of the commonest refractive errors likely to be experienced in clinical practice.
Review of Anatomy
For a more detailed overview of intraocular anatomy, see the article on Anatomy and Physiology of the Eyeball.
Emmetropia = an eye with no refractive error.
- The human eye is responsible for transmitting visual information to the brain.
- The first step of this process involves refraction of light rays from the outside world on to the retina.
- In a normal eye, light is refracted mainly by the cornea (2/3 of total refraction) and subsequently by the lens (1/3 of total refraction).
- The measurement of the degree of refraction is in Dioptres, written commonly as the abbreviation 'D'.
- The usual refraction of the cornea is 40D and the lens 20D.
- In an emmetropic eye, refraction of light accurately focuses beams on the retina, before the second stage of processing can occur. Here light is converted to chemical signals and sent to the brain along the optic nerve.
- The refraction process is dependent on: (1) the status of the tear film across the surface of the cornea; (2) the shape of the cornea and lens; and (3) the length of the eye, as this determines the distance the refractive beam has to travel.
- If any of these elements are altered or defective, a refractive error will be evident.
- The broad term for any form of refractive error is ametropia.
Myopia = light is refracted in front of the retina, causing short-sightedness.
- This is the commonest refractive error.
- Myopia may result from an eye that is increased in length, a cornea with increased curvature e.g. in the condition keratoconus, or a cornea or lens that are too strong in refractive power.
- As can be seen in the diagram opposite, light beams fall short of the retina in myopic individuals, causing objects in the distant vision to appear out of focus.
- Myopia usually becomes apparent in teenagers or young adults and about 1 in 4 adults are affected in the UK.
- This condition may be benign. It may also be due to underlying pathology; keratoconus, Marfan's syndrome, cataracts or diabetes.
- Myopic individuals are at greater risk of glaucoma, cataracts and detachment of the retina.
- Correction of myopia requires a concave lens. This is indicated in medical notes or on prescriptions with a '-' (MINUS sign) prior to the numbers.
- A concave lens spreads the light beams wider, so they can travel further through the eye before focusing on the retina.
Although considered almost a separate entity from simple myopia discussed above, pathological myopia is a common cause of blindness and around 2% of individuals living in the USA are affected.
90% of cases are diagnosed before the age of 13. Patients with this condition are so profoundly short-sighted that their eye becomes altered in shape. If children with pathological myopia are not identified, they are at risk of amblyopia and even loss of vision.
The pathology behind this condition lies in the alteration in the shape of the eyeball. The eye increases in length, and thus causes tension in the underlying structures. As a result, the back of the eye may swell, leading to the condition staphyloma and eventually a high risk of blindness. Additionally, torsion of the retina can lead to areas of weakness that blood vessels can penetrate through. Fragile vessels may then be prone to haemorrhage, which can cause visual loss if near to the retina. Another possible development at the sites of retinal weakness is retinal detachment. Finally, the altered shape of the eye can narrow the areas where fluid is drained from the eye, leading to the development of glaucoma.
Correction of this condition is most effective with contact lenses, as glasses may require extremely thick lenses in very myopic patients and also do not provide great use for peripheral vision.
Patients with pathological myopia must be closely monitored for the complications mentioned above.
Hypermetropia = light is refracted by the eye to a point behind the retina, causing long-sightedness.
- Hypermetropia is caused by an eye which is reduced in length or has a reduced power of refraction e.g. a lens cataract.
- The beam of light refracted by the front of the eye focuses behind the retina, causing near vision to be blurred.
- Initially, patients may be able to compensate for the hypermetropia by using accommodation to focus on near objects.
- Over-use of accommodation, such as long periods of reading, may cause symptoms such as headaches, eye strain and blurred vision.
- Eventually accommodation is not sufficient to overcome hypermetropia, and the patient experiences blurred vision of near and distant objects.
- Hypermetropia may be benign and sporadic, or it may be benign and hereditary. It may also be due to an underlying condition, such as; lens dislocation, retinal fluid, retinal detachment or choroid tumour.
- Patients with hypermetropia are more at risk of glaucoma, squints and amblyopia ('lazy eye').
- Correction involves spectacles with a convex lens, identified by the prefix '+' (PLUS sign) on prescriptions.
- A convex lens narrows light beams, causing them to focus at a shorter distance within the eye and thus land on the retina.
Astigmatism = Light is refracted to multiple points of focus, causing a generally blurred image.
- Astigmatism is caused by an asymmetrical eye, due to irregular curvature of the cornea or lens.
- When these surfaces are not symmetrical, they refract light to different points of focus, which leads to blurred vision.
- Astigmatism is usually present with some amount of myopia or hypermetropia, due to refraction points falling in front of or behind the retina, respectively.
- The units in which we measure astigmatism are cylinders ('cyl').
- It is usually a benign sporadic occurrence.
- Some patients will develop astigmatism secondary to problems involving the cornea; surgery, trauma, infection, external masses, or problems with the lens such as tumours or cataracts.
- Correction of astigmatism requires glasses with cylindrical lenses. The defective area of the eye causing asymmetry is identified, and the lens is chosen accordingly to balance out this area.
- Alternative options are contact lenses or corneal surgery (see below).
Presbyopia = The gradual loss of ability to accommodate with age, leading to blurred reading vision.
- Presbyopia affects patients over 45 years of age, and is a benign part of the ageing process.
- When reading, patients with normal vision use accommodation to focus. This requires the lens to alter its shape to focus near images on the retina.
- With increasing age, the lens hardens making it less flexible.
- This makes accommodation less effective.
- Patients notice difficulty reading up close due to blurred vision.
- Because the problem with refraction is essentially the same as in hypermetropes (images are focused behind the retina), presbyopia is also corrected using glasses with a convex lens for reading.
- Alternatives to reading glasses include contact lenses, laser corneal surgery and intraocular lens implants.
Alternatives to Spectacles
Although probably the simplest and cheapest method of correcting refractive errors, wearing glasses may not be practical for some individuals.
- Do not affect appearance
- More suitable for those with active jobs or sportspeople
- Co-operation with cleaning and maintenance procedures
- More expensive
- Risk of eye infection
There are a number of surgical procedures available which alter the refractive ability of the eye to correct errors. The 3 main types of surgical approach are:
- Corneal surgery: To alter the shape of the cornea and thus alter its refractive power. This is the commonest type of refractive surgery at the current time. It is colloquially known as "laser eye surgery". There are a number of options available, but the commonest procedure is LASIK (Laser-Assisted In-Situ Keratiomileusis) where a flap of cornea is lifted back allowing a laser to be applied underneath the surface of the cornea. This procedure is popular as it causes minimal disturbance to the corneal surface and reduced discomfort for the patient.
- Lens removal: The patient's own lens is removed and replaced with an artificial one of the correct refractive power. This is infrequently done in current practice, as the invasive nature of the procedure poses the risk of infection and retinal detachment.
- Additional lens: A lens of the correct refractive power is placed in the eye alongside the patient's own existing lens. This is also infrequently done as a procedure, due to invasiveness and also the increased risk of cataract formation.
As LASIK is the commonest procedure in current practice we will consider it in more detail here.
Correcting different refractive errors with LASIK:
- Myopia: the cornea is flattened with the laser to reduce its refractive power.
- Hypermetropia: the edge of the cornea is flattened by the laser to make the gradient of the cornea steeper and increase its refractive power.
- Astigmatism: The laser is specifically applied to the area of asymmetrical cornea responsible for the refractive error.
Possible complications are infrequent (1-2%) but include:
- Greater or lesser correction than required leading to a refractive error
- Glare especially when driving at night
- Flap complications e.g. perforation
- Corneal scarring
Anisometropia = When a person has a different refractive error in each eye.
- In patients with anisometropia each of their eyes have a different refractive error.
- There are different degrees of severity.
- When there is only a subtle difference in the refractive errors, the patient may be unaware. For example if both eyes are myopic but one requires a stronger lens than the other for correction.
- These patients may be asymptomatic, and corrective glasses are usually sufficient.
- When the difference between two eyes is larger than 2D, or the eyes have different types of error such as if one is myopic and the other hypermetropic, this can prove to be more difficult.
- These patients may present with diplopia, problems reading, headaches, photophobia and nausea.
- Management of these patients is not easy and requires specialist input.
- Each patient will require different combinations of lenses depending on their individual defects, and contact lenses tend to be tolerated better than spectacles.
- Anisometropia may become evident in patients who have had cataract surgery in a single eye. In such cases, this can be corrected by completing cataract surgery on the other eye as well.
- In children who have anisometropia, their brains can 'block out' information from one of the eyes. This is when amblyopia ('lazy eye') can become apparent, as visual development is still in progress up to the ages of around 7 or 8. If amblyopia is not detected before this age, visual loss can occur in the eye which is suppressed by the brain. For this reason, pre-school screening for anisometropia in children is common practice in the UK, and this is considered the ideal age for optimum correction of refractive defects.
Aniseikonia = When each of a patient's eyes perceives one image as two different sizes.
- Patients with aniseikonia view one image as a different size in each of their eyes.
- The difference in size of the images between the two eyes is measured as a percentage e.g. an image may be seen 10% larger in the left eye than the right eye.
- Aniseikonia is thought to be noticeable to the patient when the difference between eyes is 3-5%.
- Aniseikonia may be caused by a difference in the two eyes such as their length or power of refraction.
- Patients may present with: head/eye pain, double vision, nausea and dizziness, photophobia and reading problems.
- Causes are many:
- Anisometropia (see above): Commonest cause.
- Aphakia: This is the absence of a lens, commonly due to congenital cataract extraction or trauma.
- Astigmatism (see above)
- Stereopsis: This is a difficulty with being able to perceive depth of objects, and patients with this problem are slightly more likely to develop aniseikonia.
- Strabismus: This is malalignment of the eyes.
- Amblyopia ('lazy eye')
- Aniseikonia can be managed in a variety of ways with contact lenses, glasses and intraocular lenses. The best choice will vary for each individual.
Accommodation and Accommodative Abnormalities
Accommodation is a process that allows us to focus on objects at different distances from the eye.
Accommodation is under paraysmpathetic control.
When an image is perceived as blurry the parasympathetic nervous system intiates accommodation to bring the image into focus.
If the object to be focused on is near, the suspensory ligaments which hold the lens in place relax, allowing the lens to bulge into a rounder shape. This increases the refractive power of the lens and allows the near image to focus on the retina and cease to be blurred.
If the object to be focused on is further away, the suspensory ligaments tighten, and the lens flattens. This reduces the refractive power of the lens and brings the distant object into focus on the retina.
Defects of accommodation exist and an overview of them is given below:
a) Accommodative Insufficiency:
- Commonest defect of accommodation.
- Patients cannot accommodate to the degree expected for their age, due to a defect in initiating accommodation.
- Its cause is largely unknown, but it has been associated with diabetes mellitus and multiple sclerosis.
- Common symptoms: problems with near vision, head/eye pain, double vision and tiredness.
b) Accommodative Excess:
- An episodically continuous state of accommodation which results in over-accommodation for near objects.
- Caused by a higher tone in the ciliary muscles which hold the suspensory ligaments.
- Unknown aetiology, but it has been linked to un-managed hypermetropia.
- Episodes may be instigated by lengthy use of near vision e.g. prolonged reading.
- Patients have blurred vision for near and far objects, due to the inability to stop accommodating.
- Symptoms: difficulty with far and near vision, head/eye pain, double vision and photophobia.
Basic eye anatomy image:
Cochrane, G.M., Toit, R.d., Mesurier, R.T.L., Management of refractive errors, BMJ 2010; 340: 855-860.