4- Ophthalmology (Refractive error) Flashcards
Refractive error
Background
Most common reversible visual impairment
- Glasses
- Contact lenses
- Refractive error
Refractive power
- Is measured in Dioptres (D)
- The typical eye has a total power of 58D
- Cornea accounting for 2/3rds of total refractive power (43D)
- Lens around 1/3rd (15D)
Investigations
- Snellen chart
classification of refractive error
Emmetropia (normal)
Ametropia (refractive error present)
Emmetropia
Ideal refractive
- Parallel light rays from an infinite distance are refracted perfectly by the cornea and the lens to that it converges to a point directly on the retina
- Convergence on the retina is what allows us to see clearly
- Objects look distinct and sharp, instead of blurry and out of focus
Ametropia (refractive error present)
Broad term which just means ‘there is a refractive error present’
- Myopia
- Hypermetropia
- Presbyopia
- Astigmatism
myopia
short sightedness
myopia pathophysiology
In Myopic eyes, the parallel light rays converge too anteriorly, falling in front instead of on the retina, therefore the perecieve image is blurry
- Eye ball too long
2 Causes
- Axial myopia: Myopic eyes are bigger -> less refractic powers of the cornea and lens are too strong, so light converges too soon
- Refractive myopia when refractive power of the eye is stronger than normal, which can happen in conditions that increase the curvature of the cornea (keratoconus), or thickness of the lens (cataract)
Correcting myopia/ short sightedness
- Use of concave lens, which decrease the overall refractive power of the lens (which is too strong in myopia)
- Therefore concave lenses diverge light to decrease the overall refractive power of the eye -> known as a minus lens
Hypermetropia (far-sightedness)
Hyperopia or hypermetropia
Pathophysiology
- Eyes are small and short
- Causes a weak refractive power (opposite of myopia), therefore the light converges behind the retina
Correcting hyperopia
- Aim is to increases the overall refractive power by using a convex lens
- Convex lenses converge light to bring the focus of the light forward onto the retina
- Plus lenses
Astigmatism
Pathophysiology
The surface of the cornea is rarely perfectly spherical, in most people, it is actually shaped like a rugby ball, with one axis more curved than others. This means that the refractive power of the cornea isn’t uniform: the power is greater where the axis is more steeply curved, and weaker where it is shallower.
- Can make wearing glasses tricker- a normal spherical lens only correct for one overall refractive error
- Need to make the lens have varying powers corresponding to the shape of the individual cornea
correcting astigmatism
Correcting astigmatism
- Need to make the lens have varying powers corresponding to the shape of the individual cornea
- This is done by superimposing cylindrical lenses (simply known as cylinders) onto a spherical lens. In practice, there needs to be a careful assessment to work out the exact power and axis the cylinder needs to be placed to provide good vision.
Presbyopia
‘ Presbyopia is a refractive error that makes it hard for middle-aged and older adults to see things up close. It happens because the lens (an inner part of the eye that helps the eye focus) stops focusing light correctly on the retina (a light-sensitive layer of tissue at the back of the eye)’
- Assuming we have an emmetropic eye (an eye without any refractive error) which eye focuses light from the distance perfectly onto the retina
- However this does not mean it is easy to see things close up – presbyopia
- Occurs usually due to old age
- Lens rather than eye shape problem
Pathophysiology
is the irreversible loss of the accommodative ability of the eye that occurs due to aging.
Pathophysiology and management of presbyopia
- As we get older, our lenses lose elasticity and become unable to change shape, hence why older people need reading glasses, which are convex spherical ‘plus’ lenses to add refractive power
