the astigmatic reflex

Question 1

what happens to reflex in astigmatism with spot RET ?

Answer 1

. reflex becomes elliptical - “ band” reflex in spot RET

Question 2

what does a eccentric movement or twist of the reflex tell us ?

Answer 2

. eccentric movement or twist of the reflex tells us that there is a difference in movements between the two principal meridians and this causes the eccentric movement or twist we see - clue that we are dealing with an astigmatic eye

Question 3

what happens to reflex speed in astigmatism ?

Answer 3

• reflex speed will be different along the two principal meridians

Question 4

what is astigmatism ?

Answer 4

two different degrees of spherical ametropia along the two principal meridians

Question 5

what is mixed astigmatism ?

Answer 5

• in mixed astigmatism - one eye is myopic and other is hypermetropic

Question 6

what happens if we got myopic meridian along 90 deg in mixed astigmatism ?

Answer 6

• this means that as we sweep along the 90 deg meridian we would get an against movement
• far point with working distance in place is between the observer and eye
• the ray reflected from retina is filling pupil of eye but by the time it comes back to the observer , some rays fall outside the bottom of sight hole - this happens as we move RET up along 90 deg meridian
• point on retina moves up and what happens with rays falling outside sight hole - rays originating from top part of pupil vignetted , the shadow moves down across the top of pupil and we get against movement

Question 7

what happens if we got a hypermetropic meridian along 180 deg in mixed astigmatism ?

Answer 7

• if we sweep right along 180 deg we get with movement
• location of far point with working distance is located behind RET - all light originating from fundus will focus on a point at far point plane
• some rays fall outside the sight hole
• as we sweep right shadow moves in same direction with RET

Question 8

what do we correct first in astigmatism 180 hypermetropic meridian or 90 myopic meridian ?

Answer 8

• we correct astigmatic eye we correct most positive / least negative meridian
• we will correct 180 hypermetropic meridian first
• add plus to get 180 meridian closer to reversal - this causes 90 meridian to be more myopic as we added positive spheres to correct hypermetropia in 180
• far point of 90 deg meridian moved closer to the eye
• rays from 180 meridian are focussing on sight hole this is when we get fast , flashy reflex
• 90 deg meridian rays are focussing closer to the eye and some of those rays are falling outside sight hole - we end up with elliptical shape reflex ( band reflex )
• therefore when one meridian is closer to reversal , the other meridian is far from reversal and that causes vignetting creating band reflex

Question 9

what happens in band reversal in spot ret ?

Answer 9

• when one meridian is closer to reversal , the other meridian is far from reversal and that causes vignetting creating band reflex

Question 10

why is the band reflex not always visible when preforming RET on astigmatic eye ?

Answer 10

. the band reflex isn’t always visible when preforming ret on astigmatic eyes
. the band reflex can be masked by strong spherical ametropia - as SPH component is very dominating
- it may be necessary to reduce the level of SPH component before the effects of astigmatism are seen

Question 11

what does band reflex have an effect on ?

Answer 11

• the band reflex also has an effect on how accurately we can determine the principal meridian

Question 12

why is it difficult to know if there is a strong amount of astigmatism to be precise about the meridian ?

Answer 12

• it becomes difficult if there is a strong amount of astigmatism to be very precise about the meridian because the reflex appearance can be same when sweeping down meridians that are close to each other

Question 13

what kind of prescription that might mask the principal meridians by having a strong band reflex ?

Answer 13

-0.50/-3.50 x 20
. we got a weak SPH and a moderate level of astigmatism at -3.50
- this level of astigmatism is sufficient to give a reasonable amount of banding of reflex
- appearance of the reflex appears the same along both meridians ( 20 and 110 )
- a strong band reflex can mask the axis of the principle meridians

Question 14

explain the eccentric reflex movement that we observe in astigmatic eyes ?

Answer 14

• this can be explained using vector diagrams
• we will start with eye which has got compound myopic astigmatism - this means that both meridians are myopic but one is closer to reversal than another
• both meridian will give against
• horizontal = myopic closer to reversal = faster against movement
• vertical = myopic = slower dull movement
• if we sweep ret along 45 meridian - we can split ret movement into two movements along the two principal meridians - one along horizontal meridian and other along vertical meridian
• as we sweep along vertical , slow against movement
• as we sweep along horizontal we will get a fast against movement
• we can combine both responses ( fast against and slow against ) into an overall movement for the reflex
• add two vectors together and that produces resultant movement of reflex which is a fast against movement

Question 15

what does the direction in which the reflex is pulled indicate ?

Answer 15

.the movement of reflex tends to be pulled closer to the meridian that is closer to reversal because there is a much faster movement and that has a greater pull on direction of reflex pulling it in that direction
. the direction in which reflex is pulled can indicate the meridian is closest to reversal in astigmatism