02 Optics & Refractive Errors Flashcards
Define axial length of the eyeball
Distance between the corneal surface and the retinal pigment epithelium (RPE)/Bruch’s membrane
When does the majority of axial lengthening of the eye occur?
First 3 - 6 months of life
What are the axial lengths of the eyeballs for newborns, 3 year olds, and adults (13 onwards)?
Newborns: 16mm
3 years: 22.5mm
13+ years: 24mm
What is the length from the lens to the retina in adults?
17mm
Which part of the eye is responsible for the greatest refraction of light?
Cornea (40D)
Lens is 20D
D = power in diopters
Define myopia
Principle focus of light lies before reaching the retina.
Low: -6D
What are the causes of myopia?
Large eyes (axial length > 24mm) —> Axial myopia
High refractive power (e.g. in keratoconus, nuclear sclerotic cataract)
—> Index myopia
List some methods that can be used to slow myopic progression
Atropine and pirenzepine drops
Outdoor activity (less near work)
Bifocals and progressive lenses
Describe the management options for myopia
Spherical concave lenses - glasses or contact lenses
Keratorefractive surgery - laser ablation of the central corneal tissue to make the central cornea flatter
—> PRK, LASIK or LASEK
Define hypermetropia
Principle focus of light lies beyond the retina.
Low: +5D
What are the causes of hypermetropia?
Small eyes (axial length < 24mm)
Low refractive power
—> flat corneas
—> aphakic patients (absence of lens)
Associations: esotropia, angle-closure glaucoma, retinoschisis, uveal effusion syndrome (nanophthalmos), ambylopia
How is hypermetropia managed?
Spherical convex lenses (glasses or contact lenses)
Keratorefractive surgery - laser ablation of peripheral corneal tissue resulting in a steeper central cornea
What are plus lenses?
Convex lenses, increase refractive power (converge light)
Used in hypermetropia
What are minus lenses?
Concave lenses, reduce refractive power (diverge light)
Used in myopia
How do you calculate the power of a lens?
Reciprocal of the focal length (f) in metres.
Power = 1/f
Then determine if + (plus lens) or - (minus lens)
What is an astigmatism?
Occurs when the refractive power of the eye is not the same in all meridians (directions) due to a change in the shape of the lens or in the curvature of the cornea.
Cornea curvature often described as ‘rugby-ball shaped’
Which letters are difficult to distinguish between in patients with an astigmatism?
O and C
How does corneal shape change in normal eyes across the day?
Normal eyes can exhibit diurnal variations in corneal shape:
Flattest in morning as a result of changes in eyelid pressure and muscle tension
Define regular astigmatism
Principal meridians (termed steepest and flattest meridians) are 90° from each other.
A) With-the-rule astigmatism —> occurs when the vertical meridian (90°) is the steepest
B) Against-the-rule astigmatism —> occurs when the horizontal meridian (180°) is the steepest
C) Oblique astigmatism —> occurs when the principle meridians are neither at 90° or 180°
Define irregular astigmatism
Principle meridians are not perpendicular to each other.
Occurs in conditions such as keratoconus or corneal ulcers.
How are astigmatisms managed?
Soft toric lenses - combination of spherical and cylindrical lenses
RGP contact lenses - usually used for irregular astigmatism
How do spherical and cylindrical lenses differ?
Cylindrical lenses focus the light into a line, whereas spherical lenses focus the light into a point.
Spherical lenses have the same power in all meridians but cylindrical lenses have power in one meridian only.
What is transposition of prescription glasses or lenses?
What are the steps of transposing?
Converting a minus cylindrical lens to a plus cylindrical lens, and vice versa. Does not change the optical properties.
Step 1: Add the cylinder and sphere power; this becomes your new sphere power.
Step 2: Change the sign of the cylinder.
Step 3: Change the axis by 90°: If the axis is ≤90° then add 90°, but if it is
>90° then subtract 90°.
A patient has the following lens prescription: +3DS/–1DC at 90°. What is the transposition equivalent?
The +3DS is the power of the spherical component of the toric lens; in this case, we can tell it is a convex lens.
The –1DC is the power of the cylindrical lens.
The axis, 90°, describes the lens meridian that contains no cylinder power;
in other words, it is perpendicular to the meridian that contains cylinder power.
Using the steps of transposing, you will reach +2DS/+1DC at 180°.