Refractive errors and their correction

Question 1

What is emmetropia?

Answer 1

Emmetropia refers to an eye that has no visual defects. Images formed on an emmetropic eye are perfectly focused, clear and precise. Eyes that have emmetropia do not require vision correction. When a person has emmetropia in both eyes, the person is described as having ideal vision

Question 2

1. What is myopia?
2. What is the most likely cause?
3. What is the management?

Answer 2

1. Patient can see clearly up close but their vision is blurred at distance. the focal point is in front of the retina. Therefore divergent light rays falling on the retina produce a blurred image.
2. Most commonly excessive axial length and rarely due to too great refractive power (index myopia in cataracts).
3. Concave lenses

Question 3

1. What is hypermetropia?
2. What is the most likely cause?
3. What is the management?

Answer 3

1. Patient can see clearly in the distance but not up close. The focal point is behind the retina. The converging rays that should fall on the retina produce a blurred image.
2. Axial length too short
3. Convex lenses

Question 4

1. What is astigmatism?
2. What is the most likely cause?
3. What is the management?

Answer 4

1. Part of the image in one plane is out of focus due to unequal refraction. The parallel incoming rays deform and do not focus at a single point, causing a blurred retinal image.
2. Corneal curvature
3. Cylinders (toric lenses), corneal surgery or laser

Question 5

1. What is presbyopia?
2. What is the most likely cause?
3. What is the management?

Answer 5

1. Gradual loss of focussing power. the subject is usually 45+ and cannot see clearly to read near type. they progressively hold the type further and further away.
2. There is a normal loss of accomodative range with increasing age, due to decreasing lens elasticity.
3. The reading correction is added to the distance correction

Question 6

1. What is 1 dioptre?
2. How many dioptres does an emmetropic eye have?
3. How many dioptres are provided by the lens and how many by the cornea?

Answer 6

1. 1 Dioptre is the power of a convergent lens to focus parallel light at its focal point 1m behind the lens.
2. 58
3. 43 from the cornea and 15 by the lens

Question 7

1. What is retinoscopy?
2. What is the difference between subjective and objective refraction?
3. What is cycloplegia?

Answer 7

1. Retinoscopy is a technique to obtain an objective measurement of the refractive error of a patient’s eyes. The examiner uses a retinoscope to shine light into the patient’s eye and observes the reflection (reflex) off the patient’s retina. While moving the streak or spot of light across the pupil the examiner observes the relative movement of the reflex or manually places lenses over the eye (using a trial frame and trial lenses) to “neutralize” the reflex.
2. Subjective refraction relies on the patient self-reporting their vision. Objective refraction allows the doctor to check physically.
3. Temporary paralysis of the ciliary body and inhibition of accomodation.