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°.
Define presbyopia
Age-related loss of accommodative ability of the eye.
What causes presbyopia?
Either due to an increase in lens size and hardness, or due to ciliary muscle dysfunction.
Causes the lens to not thicken or flatten properly, and thus accommodative power is lost.
Describe the progression of presbyopia in terms of age and accommodative power
At the age of 8 years, accommodative power is 14D.
Presbyopia starts at about the age of 40, and by age 60 the accommodative power is almost completely lost being < 1D.
What is meant by the amplitude of accommodation?
Maximum increase in diopter power the eye can achieve through accommodation.
At least 1/3 of the amplitude of accommodation should be kept in reserve to achieve comfortable vision.
What is the near point of the eye?
Closest point where the image remains clear.
A patient sees clearly at 33 cm but with any less the image becomes blurry. He wishes to see clearly at 25 cm. What power correction does he need?
Answer: +2D correction.
Explanation:
He sees clearly at 33 cm (near point). This means his amplitude of accommodation is +3D (Power = 1/f = 1/0.33).
To see comfortably he needs at least 1/3 of his accommodative amplitude in reserve. This means he can only use +2D (of his +3D power).
To be able to see clearly at 25 cm. He needs +4D power (Power = 1/f = 1/0.25). This means he needs an extra +2D power correction.
Define esotropia
Eye is deviated nasally and moves temporally on cover testing to fixate.
Define exotropia
Eye is deviated temporally and moves nasally on cover testing to fixate.
How are esotropia and exotropia managed?
Measure the angle of deviation objectively via prism testing.
When prescribing prisms, they should be placed for both eyes with the power of prisms split evenly between the two eyes. The apex of the prism should point towards the deviation. For example, in an esotropic eye the apex points nasally.
Define hypertropia
Eye is deviated superiorly and moves inferiority on cover testing to fixate
Define hypotropia
Eye is deviated inferiorly and moves superiorly on cover testing to fixate
How should hypertropia and hypotropia be managed?
Prisms should be prescribed and split evenly between the two eyes. The apex of the prism should point towards the deviation and the prisms should be pointing in opposite directions for both eyes.
E.g. for right hypertropia: right eye has prism base down and pointing upwards, and left eye has the prism base up and pointing downwards.
Which direction does a prism refract light towards?
Prisms refract light towards the base of the prism.
To the observer, the virtual image formed is erect and displaced towards the apex (opposite direction to the direction of light refraction)
What is Snell’s law?
Law of refraction:
When light moves from one transparent medium of higher density to another of lower density, the light refracts.
Define the angle of incidence
Angle which the light travels at as it hits the boundary of another medium
Define the critical angle
The angle of incidence at which the angle of refraction is equal to 90 degrees.
Define the angle of refraction
The angle which light travels at as it crosses the boundary of another medium
What is total internal reflection?
angle of incidence > critical angle:
Light will not pass through the medium, and is completely reflected.
What is the definition of a prism diopeter (PD) and how is it calculated?
One prism diopeter produces a deviation of a light ray of 1cm measured at 1m from the prism.
(a) P = Fd
- P = Prism power in diopeters (PD)
- F = lens power (D)
- d = distance of pupil from optical centre (cm)
(b) P = 2 x angle of deviation
What does 6/6 vision mean in terms of Snellen chart use?
A patient with 6/6 vision can read the letter on the row of the Snellen chart at a distance of 6m, and an average person can read it at 6m.
Describe the duochrome test for visual acuity
Uses chromatic aberrations of the eye to refine the best vision sphere following optical correction.
Uses black letters on a red background and on a green background. Red will focus behind the retina (longer wavelength) and green will focus in front of the retina (shorter wavelength).
One eye is occluded at a time and patient is asked to determine which colour background the black letters look clearer on.
If red, focus is behind retina so there is a degree of hypermetropia —> sphere is either under plus or over minus.
If green, focus is in front of retina so there is a degree of hypometropia—> sphere is either under minus or over plus.
If equal, they have been given the perfect sphere correction.
When is an Isihara chart used?
Screening for red-green colour vision detect.
Isihara plates comprise of plates containing dots of various colours and sizes and other dots which form certain numbers that should be visible to a patient with normal colour vision.
How do Snellen and LogMAR scores compare?
6/6 Snellen acuity = 0 LogMAR
6/60 Snellen acuity = 1 LogMAR
The scoring of LogMAR is from 1 to 0, so each letter read correctly will result in subtraction of 0.02 starting from 1.
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