Optics and refractive errors Flashcards
Length of the eye?
- Newborn: 16 mm
- 3 years: 22.5 mm
- 13 to 18 years (adult length): 24 mm
- Length from lens to retina is 17 mm in adults.
Types of Myopia?
Axial Myopia - large eyes (most common cause)
Index Myopia - high refractive power e.g. keratoconus or nuclear sclerotic cataract
Things which slow myopic progression
Use of atropine
Outside work (i.e avoiding work in an office)
Bifocals and progressive lenses
Management of Myopia
Glasses - concave
Surgery - e.g LASIK LASEK
Hypermetropia causes
Small eyes
Low refractive power: absence of lens, low refractive power
Management of Hypermetropia
Spherical convex lens
Surgery - similar to myopia. Peripheral corneal tissue is ablated
How to calculate power of a lens
reciprocal of focal length in metres
1/f
Calculate strength of lens needed in someone with Hypermetropia struggling to read at 33cm
1/0.33
= 3D
Plus lens needed for Hypermetropia
+3D.
What is Astigmatism?
Refractive power of the eye is not the same in all meridians (directions) due to a change in the shape of the lens or cornea’s
curvature, frequently described as ‘rugby ball-shaped’.
Types of Astigmatism
Regular astigmatism: The principle meridians, termed steepest and flattest
meridians, are 90° from each other
Irregular astigmatism: Principle meridians are not perpendicular to each
other. Occurs in conditions such as keratoconus or corneal ulcers
Management of astigmatism
Soft toric lenses: Combination of spherical and cylindrical lenses
RGP contact lenses are usually used for irregular astigmatism.
What is a cylindrical lens?
Lenses that contain a cylinder in a single plane surface, and
unlike spherical lenses, they focus the light into a line rather than a point
What is transposition of a lens?
Transposition of prescription lenses/glasses refers to converting a minus
cylindrical lens to a plus cylindrical lens and vice versa.
Steps of transposing
● 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
Answer: The first thing to understand is what the components mean:
● 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 above, you will reach +2DS/+1DC at 180°. This
is similar to saying +3DS/–1DC at 90°
What is Presbyopia
Age-related loss of accommodative ability of the eye
Starts around 40
Accommodation lost by 60
Calculate amplitude of accommodation
To achieve comfortable vision, at least 1/3 of the amplitude of
accommodation should be kept in reserve.
26 Optics and refractive errors
SCENARIO
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.
What is a prism?
A prism is a transparent medium bound by two planes that are at an angle to each
other. They do not focus light. They bend light (refraction) towards the base of
the prism
Angle of incidence?
The angle the light travels as it hits the boundary of another medium. The angle of refraction is dependent on the angle of incidence and is the angle the light travels as it crosses the boundary.
Critical angle?
When the angle of refraction is equal to 90°, the angle of incidence is named
the critical angle.
Total internal reflection?
Occurs when the angle of incidence is greater than the critical angle; the light will not pass through the medium, that is, it is completely reflected.
Power of Prism worked calculations
SCENARIO 1
A patient has a 15° deviation esotropia of the right eye. What prism power and
direction should be prescribed?
● Answer: 15PD base out (temporally) for both eyes.
● Explanation: P = 2 × angle of deviation = 30PD. Prisms should be split equally
between the two eyes and the base should be away from the direction of squint.
SCENARIO 2
A patient has a right hypertropia and requires 30PD correction. What prism
power and direction should be prescribed?
● Answer: 15PD base down right eye and 15PD base up left eye.
● Explanation: The power should be split evenly across the two eyes and the
bases should be opposite directions.
SCENARIO 3
A patient complains of vertical diplopia. His glasses prescription is +2D (OD)
and +1D (OS). His eyes are 5 mm down from the optical centre. What prism
prescription should he have?
● Answer: 1PD base up right eye and 0.5PD base down left eye.
● Explanation: In hypertropia/hypotropia, the bases of the prisms should be in
opposite directions.
● OD: P = Fd = 2 × 0.5 = 1PD
● OS: P = Fd = 1 × 0.5 = 0.5P
Power of prism
One prism diopter (PD) produces a deviation of a light ray of 1 cm measured at
1 m from the prism. It can be calculated using one of two equations:
P = Fd
Where P is the prism power (PD), F is the lens power (D) and d is the distance (cm)
of the pupil from optical centre.
P = 2 × angle of deviation
Where the angle of deviation is measured in degrees
Ways of measuring visual acuity
Snellen chart - letters down card
Duochrome test - look at letters on red and green cards. Should be same. If see better on red side suggests hypermetropia
Ishihara Chart - This is a test used to screen for red-green colour vision defect. The test comprises 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 vision.
What is retinoscopy?
Light shone onto patient’s retina at a certain distance
Observing the red reflex:
Myopic - direction of reflex against direction of light
Hypermetropic - direction of reflex is with direction of light
Visual acuity development in children
By 3 months infant can fixate and follow objects
By 5 months they develop full colour vision
Optic of slit lamps
This is a test used to screen for red-green colour vision defect. The test comprises
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.
Slit lamp illumination
Direct illumination - beam focuses on part of the eye being examined (most common)
Diffuse illumination - general illumination of eye
Retroillumination - Uses the light reflected from the iris to look for corneal opacities, or from the fundus to examine the red reflex, patency of iridotomies and lens opacities.
Specular reflection - Best way to view corneal endothelium, for example, in
Fuchs’ corneal dystrophy.
Sclerotic scatter - Light is directed at the limbus, which is in turn scattered through the cornea. Used to evaluate general corneal opacities
