Refractive errors+ visual acuity

Question 1

What does the term visual acuity mean?

Answer 1

Visual acuity refers to an angular measurement relating testing distance to the minimal object size resolvable at that distance

Question 2

How does a Snellen chart works?

Answer 2

The traditional measurement of distance acuity refers to a visual test in which a target subtends a visual angle on the retina of 5 minutes of arc when a subject is 20 feet (6 m) away from that target

Question 3

What is important to know about measurement notation?

Answer 3

@ The top number refers to the testing distance from the eye to the chart

@ The bottom number refers to the distance at which a subject with unimpaired vision can read the same figure

Question 4

How can a Snellen fraction be converted to a decimal notation?

Answer 4

@ Snellen 20/20 = decimal 1.0

    @  Snellen 20/30  = 0.7

    @. Snellen 20/40 = 0.5

Question 5

What does the logMAR notations mean?

Answer 5

@. It expresses the visual acuity as the logarithm of the minimum angle of resolution

    @  The minimal angle of resolution is the  inverse of the Snellen fraction

    @. Snellen 20/20  = logMAR 0

    @  Snellen 20/50 = logMAR 0.4

    @. Snellenn 20/200 = 1.0

Question 6

How can the near visual acuity be measured?

Answer 6

The Jaeger (J) notation

Question 7

What is the difference between the following:

Answer 7

• . genotype
  - phenotype
  - optotype

    @  genotype =
  
    @. phenotype =
  
    @ optotype = each individual letter, number or picture on a testing chart

Question 8

Which letter is the hardest letter for patients to recognize?

Answer 8

The letter B

Question 9

Which letter is the easiest letter for patients to recognize?

Answer 9

The letter L

Question 10

What does a visual acuity of 15/40 -2 mean?

Answer 10

The patient misses 2 letters on the same line but was only 15 feet away from the chart

Question 11

Tips for testing the Snellen Distance Visual Acuity (AAS)

Answer 11

@ Determine if the patient is familiar with the optotypes being used

    @  Make sure that the patient occludes one eye completely with no peeking and not pressing on the eye

    @  Traditionally, the left eye is occluded first while the VA of the right eye is tested first

    @  Test the VA:

            - in both eyes
             - without correction (sc)
             - with correction (cc)
              - with the pinhole
               - ask the patient to read all the letters

Question 12

What is important to know about the Pinhole Acuity Test?

Answer 12

@ The pinhole admits only central rays of light, which do not require refraction by the cornea or the lens

    @  If the pinhole improves the patient's VA by 2 lines or more, it is likely that the patient has a refractive error

    @  If poor uncorrected VA is not improved with the pinhole, reduced VA is likely due either to an extreme refractive error or to non-refractive causes (eg cataract)

    @. The pinhole of no more than 2.4 mm in diameter should be used

    @. The patient should wear their habitual optical correction

Question 13

What is important to know about the Near Acuity Test?

Answer 13

@ if the patient uses near spectacles, the patient should wear them during near visual testing

    @  The test is usually performed at 40 centimeters

Question 14

What is important to know about testing for Near Point of Accommodation (NPA)?

Answer 14

@ The NPA is the nearest point at which the eye can focus so that a clear image is formed on the retina

    @  With increasing age, the NPA recedes due to presbyopia

    @  With the patient wearing full distance correction and the one  eye occluded, place the near testing card at a distance of 40 cm from the patient

    @  Ask the patient to read the 20/40 line with the unoccluded eye

    @  Move the test card slowly toward the patient as you ask the patient to state when the letters have become blurred

    @. Record this distance in centimeters

    @  Repeat for the  other eye

    @ The RAF rule can be used for doing this test

Question 15

What is important to know about the Near Point of Convergence (NPC) ?

Answer 15

@ With the patient wearing appropriate correction and with neither eye occluded, hold a pencil at a distance of 40 cm from the patient and ask the patient to fixate on it

    @  Move the object slowly forward and ask the patient to tell you when the object doubles

   @   Observe whether both eyes are converging

    @  Note the position at which the image doubles or one eye deviates away from the fixation target.

    @  The NPC is that point where a single image can no longer be maintained

    @  Record the distance in centimeters between that point and the upper bridge of the nose at the midpoint between the eyes

    @  The normal NPC is between 6 cm and 10 cm. irrespective of age

Question 16

In infants and preverbal children, an estimation of visual function can be made based on:

Answer 16

• the ability to look directly at (fixate) a visual object
  - follow the object
  - maintain steady fixation

Question 17

When should horizontal Optokinetic nystagmus be present and when vertical Optokinetic nystagmus?

Answer 17

• Horizontal OKN = should be present before 3 months of age

- Vertical OKN = might not be elicited until a child is approximately 6 months of age

Question 18

Name the possible causes of near acuity being poorer than distance acuity

Answer 18

@ Presbyopia/premature presbyopia

    @ ubdercorrected or high hyperopia

    @  Overcorrected myopia

    @  small, centrally located cataracts

    @  accommodative insufficiency

    @. Systemic or topical drugs with anticholinergic effect

    @. Convergence insufficiency (applies to binocular visual acuity)

    @  Adie's pupil

    @  Functional visual loss

Question 19

Define amblyopia

Answer 19

@ Amblyopia, when unilateral, is a visual disorder defined as a difference in optically correctable acuity of more than 2 lines between the 2 eyes

    @  that results from abnormal visual input in early childhood.

    @. Normal development of vision occurs early in life through ongoing stimulation of vision-receptive cells in the brain

Question 20

Name the causes of unilateral amblyopia

Answer 20

@ Anisometropia

    @. Strabismus

    @. Unilateral media opacities, such as monocular congenital cataracts

Question 21

Name the causes of bilateral amblyopia

Answer 21

@. Longstanding uncorrected refractive errors

    @  Nystagmus

Question 22

Describe testing for stereoacuity with the Titmus stereopsis test

Answer 22

@ Place the polarizing glasses on the patient.

    @. If glasses are usually worn, place the polarizing glasses over them

    @  Hold the fly image facing the patient approximatelyn 40 cm away, with the surface of the page parallel to the surface of the glasses

    @. Ask the patient to touch or pinch the wings of the fly

    @. If the fly test is positive, show the patient 3 rows of animal figures.  Ask which figure in each row is coming forward or is above the page

    @  After noting the responses to the animals, direct the patient''s attention to the squares with the 4 circles in each.  Ask the patient to tell you which circle is coming forward in each square or push the button that is popping up

    @  Score the response as the last correctly identified before 2 consecutive circles are missed

    @  Record the stereopsis as seconds of arc as designated in the instruction booklet

    @  Be sure that the patient has both eyes open while doing the test

    @. Some patients may choose monocular clues  instead of stereopsis: turn the book upside down and ask them to describe the images.

    @. If they do not describe the images as going behind or sinking into the page, then you have confirmed your suspicion that they were relying on monocular clues instead of stereopsis

Question 23

When asked to do the VISION of a patient:(AAS)

Answer 23

@ Do the VA for distance

    @  Do the VA for near

    @  Do the central  vision (Amsler chart)

    @  Do the colour vision

    @  Exclude double vison

    @  Do the NPA

    @  Do the NPC

    @. Do the Titmus test for streopsi