Refractive errors and cataracts Flashcards
What is refraction?
In optical physics, the term ‘refraction’ describes the bending of light rays at the interface between two different transparent media.
Refraction is measured in dioptres (D) which describes the power that a structure has to focus parallel rays of light (i.e. bring them to a point) The higher this value, the stronger the focusing ability.
In the eye, refraction happens mainly at the surface of the cornea and at the surface of the lens.
Refraction at the front surface of the cornea accounts for about 80%, with the lens being responsible for most of the rest. The air-tear interface, aqueous and vitreous humours also make a small contribution. The lens is, however, the total source of accommodation (focus on near objects) and can change the focal length of the eye by 7-8%.
What is hand neutralisation?
Hand neutralisation can determine the power of the lens:
If a cross moves against then it is convex
If a cross moves with then it is concave
What determines the accuracy of refraction?
For good vision the point of focus must be on the retina. This accuracy of refraction depends on:
- The curvature of the cornea and lens.
- The axial length of the eye (from front to back).
These change as the eye grows and ages. Refractive development is influenced both by environmental factors and by genetic factors
Why is it important to treat refractive errors?
Uncorrected refractive error accounts for half of avoidable vision impairment globally and nearly a third of avoidable total loss of vision.
Globally, 153 million people have visual impairment or total loss of vision due to uncorrected refractive error, most in low-income countries.
Minor reduction in vision (<6/12) has been associated with an increased risk of death and physical, social and psychological problems in people older than 50 years.
Undetected refractive errors in childhood may lead to behavioural problems and adversely affect social interaction and performance at school.
Under-corrected refractive error may account for up to 75% of all vision impairment in high-income countries.
What is ametropia?
This is a global term for any refractive error.
What is myopia?
Myopic eyes have excessive optical power for the axial length of the eyeball and so focus the image in front of the retina.
What is the cause of myopia?
This arises as a result of physiological variation in the length of the eye or of 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.
What are the complications of myopia?
hHgh myopia can be associated with degenerative fundal changes (Förster-Fuchs spots).
High myopia is associated with an increased risk of retinal detachment, cataract formation and glaucoma.
What is the presentation of myopia?
Patients are said to be near-sighted - distant objects appear to be blurred but, unless severe, close-up objects are in focus.
There may be 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 to worsen pre-existing myopia).
What are the associated conditions with myopia?
Prematurity Marfan's syndrome Stickler's syndrome Ehlers-Danlos syndrome Homocystinuria
What is the lens correction for myopia?
A concave (minus) lens is used to correct the problem
What is hypermetropia?
The eye has insufficient optical power for its refractive length and therefore light from 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.
The focusing power of the eye is too weak or the axial length of the eye is too short.
What are the complications of hypermetropia?
Persistent hypermetropia is associated with an increased risk of glaucoma, squint and amblyopia.
What is the presentation of hypermetropia?
Patients are said to be long-sighted - distant objects are sharply focused but there is difficulty in viewing near objects, which may give rise to eye strain (due to the extra accommodative effort) and headache. There may be a family history but most cases are sporadic.
What are the associated conditions with hypermetropia?
Corneal dystrophies
Congenital cataracts
Retinitis pigmentosa
Microphthalmia
What is the lens correction for hypermetropia?
A convex (plus) lens is used to correct the problem
What is astigmatism?
Light from a point in the visual field has to focus at a single point on the retina. 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) result in rays failing to focus on a single point.
How is the degree of astigmatism measured?
The degree of astigmatism is measured in cylinders (cyl).
What is the presentation of astigmatism?
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.
The brain will try to compensate for distortion but optical symptoms may include:
- Blurry, distorted, or fuzzy vision.
- Difficulty seeing at night.
- Eyestrain.
- Squinting.
- Eye irritation.
- Headaches.
What is the lens correction for astigmatism?
A cylindrical lens is used to ‘neutralise’ astigmatism. The axis of the cylinder depends on the meridian of asymmetry in the patient’s cornea
What is anisometropia?
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.
What is presbyopia?
Accommodation is the process by which the eye adjusts its optical power to maintain a clear image of an object as its distance varies. There are three elements to it: the eyes converge, pupil size reduces and the crystalline lens changes shape and position.
Presbyopia is the gradual loss of the accommodative response due to a decline in the elasticity of the lens.
It is a lifelong process which only becomes clinically significant when the residual accommodative amplitude is insufficient for the patient to carry out near-vision tasks such as reading.
How do you assess for refractive errors? (hx and exam)
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.
Symptoms may be so gradual that altered vision may not be noticed and patients may instead complain of headaches or red, sore, watery eyes. Young children may rub their eyes a lot or turn their heads when looking at things. School-aged children may present with behavioural problems.
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]
Do an examination of the eye. Patients should use their usual distance glasses or be tested with their contact lenses in, as you are looking for deterioration beyond that already diagnosed/treated. Test again using a pinhole: if they do better, an uncorrected refractive error may be present.
Referral to local eye unit based on symptoms.
What is the 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 spherical, cylindrical or a mixture of both.
Contact lenses
Laser surgery
