Refractive Errors Flashcards
What is a refractive error?
A problem with focusing light accurately onto the retina due to the shape of the eye
What is ametropia?
The global term for any refractive error
What are the different types of refractive errors?
- Myopia
- Hypermetropia
- Astigmatism
- Presbyopia
- Ansiometropia
What is myopia also known as?
Short-sightedness
What is the problem in myopia?
Excessive optical power for axial length of eyeball, so focus image in front of retina
What can myopia be classified as?
- Mild
- Moderate
- Severe (high-degree)
What is hypermetropia also known as?
Far-sightedness
What is the problem in hypermetropia?
The eye has insufficient power for it’s refractive length, and therefore light from an object is focused behind the retina, giving rise to a blurred image
What is the problem in astigmatism?
Light does not focus evenly on the retina
What is the degree of astigmatism measured in?
Cylinders (cyl)
What is presbyopia?
Gradual loss of accommodative response
What is the problem in presbyopia?
Decline in elasticity of the lens
What is accommodation?
The process by which the eye adjusts its’ optical power to maintain a clear image of the object as its distance varies
What are the elements to accommodation?
- Eyes converge
- Pupil size reduces
- Lens changes shape and position
What is the timescale for presbyopia?
It is a lifelong process that is only clinically significant when reaches certain point
At what point does presbyopia become clinically significant?
When patient cannot carry out near-vision tasks such as reading
At what age does presbyopia most commonly occur?
After 40 years old
What is anisometropia?
When there is unequal refractive errors between both eyes
What level of difference is required to define as anisometropia?
Generally 2 diopters
Give an example of when anisometropia is mild with limited consequences
Different degrees of myopia in each eye
Give an example of when anisometropia can cause problerms
When there are large differences in childhood, especially if one eye is myopic and the other hypermetropic, may be associated with amblyopia
How important are uncorrected refractive errors as a worldwide issue?
Accounts for half of avoidance vision impairment globally
What is the purpose of the globe?
Receive light from the outside world and transmit it to the brain for processing into a visual image
What are the elements to the function of the globe?
- Image has to be correctly focused onto back of eye
- Information has to be converted to electrochemical signals and transmitted to the brain
What unit is refraction measured in?
Dioptres
What does the unit of dioptres describe?
The power that a structure has to focus parallel rays of light, i.e. bring them to a point
What does a higher value of dioptres mean?
The stronger the focusing ability
Where does focusing happening in the eye?
- Surface of the cornea (80%)
- Surface of lens
Where must the point of focus be in the eye for good vision?
The retina
What does the accuracy of refraction depend on?
- Curvature of cornea and lens
- Axial length of the eye (front to back)
What causes the factors affecting the accuracy of refraction to change?
The eye growing and ageing
What can myopia arise due to?
- Physiological variation in the length of the eye
- Excessively curved cornea
What does hypermetropia arise due to?
Low converging power of the eye lens because of weak action of ciliary muscles, or abnormal shape of the cornea
What does astigmatism arise due to?
Variations in the symmetry of the corneal and lens curvature (usually corneal)
How do variations in the symmetry of the cornea/lens curvature cause problems in astigmatism?
Light from a point in the visual fields has to focus at a single point on the retina. This is usually achieved by symmetry of the coeval and lens curvatures around their circumference. In astigmatism, variations in the symmetry results in rays failing to focus on a single point
When is anisometropia uncommon?
In adulthood
What can cause anisometropia in adulthood?
Can occur following trauma and/or refractive/cataract surgery
What is the most extreme form of anisometropia?
Unilateral aphakia
What is unilateral aphakia?
When one lens is missing or has been removed
What are the risk factors for myopia?
- Family history
- Prematurity
- Marfan’s syndrome
- Stickler’s syndrome
- Erlers-Danlos syndrome
- Homocysteinuria
Is hypermetropia inherited?
Some cases have a family history, but most are sporadic
Does astigmatism occur sporadically?
Most cases are sporadic, but may be family history or background of certain conditions
What conditions might astigmatism occur with?
- Previous eye surgery
- Previous corneal injury
- Corneal dystrophies
- Congenital cataracts
- Optic nerve hypoplasia
- Retinitis pigmentosa
- Albinism
- Nystagmus
How does myopia present?
Said to be ‘near-sighted’, whereby distant objects appear to be blurred by close up objects are in focus (unless severe)
How does hypermetropia present?
Said to be ‘long-sighted’, whereby distant objects are sharply focused but there is difficulty in viewing near objects
What might difficulty in viewing near objects give rise to in hypermetropia?
- Eye strain
- Headache
What other eye conditions are associated with hypermetropia?
- Corneal dystrophies
- Congenital cataracts
- Retinitis pigmentosa
- Microphthalmia
How does astigmatism present?
Blurring of vision that is not necessarily associated with obvious far/short-sightedness, although distant viewing is often the more problematic
What will the brain try and do in astigmatism?
Compensate for the distortion
What are the optical symptoms of astigmatism?
- Blurring, distorted, or fuzzy vision
- Difficulty seeing at night
- Eyestrain
- Squinting
- Eye irritation
- Headaches
What other refraction errors might astigmatism be associated with?
Myopia or hypermetropia, and more severe astigmatism may lead to amblyopia
What causes of the symptoms of anisometropia?
Differing refracting states results in a slight difference in image size. Where the refractive difference is very small, this is not perceived by the patient, but where there is a large difference, the patient experiences symptoms
What is the term for when differing refractive states results in a slight difference in image size?
Aniseikonia
What are the symptoms of anisometropia?
- Diplopia
- Headaches
- Photophobia
- Reading difficulties
- Nausea
- Dizziness
- General fatigue
What happens when there is anisometropia in childhood?
The brain is more likely to suppress one of the images, so ambylopia develops in the eye in which the image has been suppressed
How does presbyopia present?
- Patient finds it difficult to carry out near tasks, and may need brighter lighting conditions for these, or need reading glasses
- Accommodative lag
- Tiring with continuous close work
What is accommodative lag?
Slowed recovery time changing from distance to near tasks, and vice versa
What should you assess each eye separately for?
Near and distance vision
Should patients wear their normal glasses/contact lenses when being assessed for near and distance vision?
Yes
Why should patients wear their normal glasses/contact lenses when being assessed for near and distance vision?
As you are looking for deterioration beyond that already diagnosed/treated
What should be done if a patient shows deterioration when testing near and distance vision?
Test again using pinhole
What does it mean if the patients vision improves when using a pinhole?
Uncorrected refractive error may be present
How can assessment of refractive errors be done?
Via the process of refraction
Who performs testing for refractive errors?
Optometrist
What does the glasses prescription tell you?
How strong a lens needs to be to bring the eye back to emmetropia
What does a larger prescription mean?
A stronger lens is required
What format is a glasses prescription?
[degree of myopia/hypermetropia]/[degree of astigmatism]x[meridan in which astigmatism lies]
I don’t understand this just rote learn it lol
How is myopia managed?
A concave (minus) lens is used to correct the problem
How is hypermetropia managed?
A convex (plus) lens is used to correct the problem
How is astigmatism managed?
A cylindrical lens is used to ‘neutralise’ the astigmatism
What does the axis of the cylinder depend on in astigmatism?
The meridian of the asymmetry in the patient’s cornea
What is used to correct astigmatism when there is associated myopia or hypermetropia ?
A spherocylindrical lens
What is the problem with the management of anisometropia?
Lens correct is difficult
What does lens correction of anisometropia usually involve?
Various subtypes of spherical and cylindrical lenses
What does the lens used in anisometropia depend on?
The type of anisometropia
Why is lens correction difficult in anisometropia?
The prismatic effects of the lenses typically vary in different positions of gaze, giving rise to further symptoms
What are symptoms caused by the lenses in anisometropia collectively known as?
Anisophoria, which is lens induced aniseikonic
What is the result of symptoms caused by lens correction anisometropia?
Many patients tolerate the lenses even less well than the original problem
What is often a better solution for the correction if anisometropia?
Contact lenses
When do patients often managed will with OTC glasses for treatment of presbyopia?
If there is no pre-existing eye problem or refractive error
What should patients be advised to do when using OTC for presbyopia?
Take them off when not carrying out near tasks
When will prescription glasses generally be required in presbyopia?
If there are pre-existing problems
What type of glasses may be used in presbyopia?
Bifocal or trifocal
Can contact lenses be used in presbyopia?
Yes
Is surgical correction possible in presbyopia?
Yes, but small risk of complications
What complications might high-grade myopia be associated with?
- Degenerative fundal changes
- Increased risk of retinal detachment, cataract formation, and glaucoma
What complications is persistent hypermetropia associated with?
Increased risk of;
- Glaucoma
- Squint
- Amblyopia
What are the categories of screening patients for visual impairment?
- Asymptomatic, low-risk patients
- Patients at risk of visual impairment
What is considered to be a low risk patient on eye screening?
No ocular co-morbidity or family history
How often should asymptomatic, low-risk patients have eye screening?
- Every 10 years at 19-40 years old
- Every 5 years at 41-55 years old
- Every 3 years at 56-65 years old
- Every 2 years at >65 years old
What patients are considered to be at risk of visual impairment for screening?
- Diabetes
- Cataracts
- Macular denigration
- Glaucoma
- Significant family history
How often should patients at risk of visual impairment have screening?
- Every 3 years at >40 years old
- Every 2 years >50 years old
- Every year >60 years old
