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
What are cataracts?
Cataracts are lens opacities. Some are small and do not need treatment but they often become large enough to block light and obstruct vision.
What is the main cause of cataracts in the developed world?
Ageing is the main cause of cataract development in the developed world, although they can also form congenitally and after various forms of injury to the lens.
What are the main causes of cataracts in the developing world?
In the developing world, other factors contribute, including malnutrition, acute dehydrating illnesses and excess ultraviolet (UV) exposure.
Lack of access to treatment makes them a leading cause of blindness worldwide.
What are the risk factors for cataracts?
Age, smoking, DM and systemic steroids are the major risk factors in the developed world.
Other risk factors include eye trauma, female gender, uveitis, UV exposure, poor nutrition, lower socio-economic status, toxins such as drugs of abuse, dehydrating illness crises and alcohol.
In the developing world, the RF includes diet (malnutrition), acute dehydrating diseases and cumulative exposure to sunlight.
What is the protective factor against cataracts?
A protective factor is diet. Vegetarianism is significantly protective against cataract. The lens is particularly sensitive to nutritional deficiencies such as protein, vitamin A, B, E and niacin.
What is the pathophysiology of cataracts?
The lens continues to grow after birth, with the new secondary fibres being added as outer layers. New lens fibres are generated from the lens epithelium. Old fibres are not removed.
Transparency is maintained by the structure of the lens proteins and by the way they are stacked, linked and aligned.
Disruption of the crystallin fibres will affect the integrity of the carefully composed structure, leading to protein aggregation.
Cataracts result from the deposition of aggregated proteins in the lens, causing clouding, light scattering and obstruction of vision.
A second contributing factor that occurs with ageing is an accumulation of yellow-brown pigment in the lens. This does not affect image sharpness, but it affects colour vision and contrast, so may eventually make reading difficult.
What is the presentation of cataracts?
Symptoms include gradual painless loss of vision, difficulties with reading, failure to recognise faces, problems watching TV, diplopia in one eye and haloes.
Many cataracts present before they are symptomatic because they are noticed by an optician at a routine eye check.
Opacities can be seen as defects in the red reflex obtained when the ophthalmoscope is held 60 cm from the eye. This is best seen with a dilated pupil.
The lens may appear brown or white when a bright light is shone on the eye.
Check that:
- Visual acuity is not improved by viewing test through a pinhole.
- The patient can indicate where a light is placed.
- Pupillary reactions are normal.
What is the classification of cataracts?
Nuclear sclerosis
Coritical
Posterior subcapsular
What is nuclear sclerosis cataract?
Nuclear sclerosis is when the cataract is formed by new layers of fibres compressing the nucleus of the lens.
How does nuclear sclerosis cataract present?
Typical symptoms include gradually reduced contrast, reduced colour intensity, reading good for acuity level and difficulty in recognising faces.
What are cortical cataracts?
Cortical is when new fibres added to the outside of the lens age and produce cortical spokes.
How do cortical cataracts present?
These may not produce symptoms unless on the visual axis or the entire cortex is affected when it is ‘mature’:
- Light scatter from opacities.
- Problems with glare when driving, particularly at night.
- Difficulty reading.
- Daytime activity relatively unaffected as the iris is constricted
What is posterior subcapsular cataract?
Opacities in the central posterior cortex.
How does posterior subcapsular cataract present?
This may occur in younger patients and may cause glare ± deterioration in near vision:
- Visually disabling in good lighting - less trouble at low light levels when the pupil is dilated.
- Difficulty in daytime driving.
- Difficulty in reading.
What are paediatric cataracts?
When a baby is born with a cataract it is called a “congenital cataract”. If a cataract develops in the first six months of life it is known as an “infantile cataract”.
What are the causes of paediatrics cataracts?
Congenital: hereditary/genetic, metabolic (e.g.T, galactosaemia), in-utero infection (toxoplasmosis, other, rubella, cytomegalovirus, herpes simplex (TORCH)).- infection by rubella being the most common cause.
Developmental: genetic, metabolic (e.g., galactokinase deficiency).
Acquired: metabolic (e.g., diabetes mellitus), traumatic, post-radiotherapy.
What are the signs of paediatric cataracts?
Opacities can be seen as defects in the red reflex obtained when the ophthalmoscope is held 60 cm from the eye. This is best seen with a dilated pupil.
The lens may appear brown or white when a bright light is shone on the eye.
Check that:
-Visual acuity is not improved by viewing test through a pinhole.
-The patient can indicate where a light is placed.
-Pupillary reactions are normal.
What is the main complication of paediatric cataracts?
If one of your child’s eyes is sending poorly focused, unclear images to their brain because they have a cataract in this eye, their brain will learn to ignore these images in favour of those provided by the other better seeing, or “stronger” eye.
This prevents the visual system from developing properly in the eye which has the cataract. This is known as amblyopia or “lazy eye”.
Amblyopia may result in permanently reduced vision as the visual system has not developed, particularly when the brain doesn’t get a chance to see clear images in the first few months of life.
What are the differentials for cataracts?
Macular degeneration
Presbyopia
Retinal disease
What is the management of cataracts?
There is no proven prevention or medical treatment for a cataract. Modern cataract surgery involves lens extraction and replacement.
The technique can be intracapsular or extracapsular, although intracapsular extraction is now rarely performed. Surgical removal of the cataract is the only effective treatment to restore or maintain vision.
Extracapsular lens extraction involves removal of the anterior capsule and extraction of the lens nucleus and cortex, either manually via a large incision at the limbus, or after phacoemulsification of the lens via ultrasound via a smaller incision. The posterior capsule is left to support the implanted artificial lens.
What are the complications of cataracts?
Trauma to the iris Anterior chamber haemorrhage Vitreous haemorrhage Endophthalmitis Refractory uveitis Protruding or broken sutures. Glaucoma Retinal detachment Age-related macular degeneration
What are the restrictions on driving in patients with cataracts?
Advise the patient not to drive and to contact the DVLA if either of the following apply. It is likely to apply where there are severe bilateral cataracts, or after failed bilateral cataract extractions
What is the eyesight requirement for driving?
To read in good daylight (with the aid of glasses or contact lenses if worn) a registration mark fixed to a motor vehicle and containing letters and figures 79 millimetres high and 50 millimetres wide at a distance of 20 metres, or at a distance of 20.5 metres where the characters are 79 millimetres high and 57 millimetres wide.
Visual acuity (with the aid of glasses or contact lenses if needed) must be at least 6/12 with both eyes open (or in the only eye if you have vision in one eye only).
Note that in the presence of a cataract, glare may affect your ability to meet the number plate requirements, even if your acuity is good enough.
