Independent Subjective Refraction

How do you refine BVS without a retinoscopy starting point?

> Occlude left eye

> Measure unaided vision

> if possible estimate ametropia

> in myopia position of true far point can be used to estimate refractive error

> e.g person with -8.00D myopia sees clearly if target placed at approx 12.5cm from eye

Relationship between acuity and refractive error

• 6/6 → <0.25 DS

• 6/9 → <0.50DS

• every line on chart equal to 0.25DS or more

for 6/18 or 6/24 use pinhole to see if vision improved

Steps for refining BVS without ret starting point?

• add +1.00D sphere

• is vision worse

• NO → add more plus spherical power until vision blurs , from blur point reduce by +0.25DS

• BVS should be max plus that eye can tolerate without causing blur on letter chart

• if YES (vision is worse) → add MINUS spherical power until best line resolved

• make sure each addition does increase and letters not just smaller

• adjust final sphere on letter chart and/or duo chrome using ±0.25DS

• Record VA

• Occlude right eye & repeat procedure for left eye

How would you refine BVS for patients with poor VA?

• px with poor VA may have difficulty in determining small changes in lens power + clarity

• often necessary to make large power changes → bracketing techniques e.g use a +2.00DS trial lens and compare this with -2.00DS trial lens

• if px able to differentiate between the two, lens giving better vision can be added and technique repeated using a +1.00DS and a -1.00DS lens

• may be necessary to refract patient with chart placed at 3m or less

• if done result is over-plussed as chart acts as near object and adds vergence of about -0.30D + correction should be made

• use of PINHOLE essential when vision poor, in order to attempt to differentiate refractive error from active pathology

Duochrome

• only monocular

• duo chrome is of no real value until ametropia has been corrected to point where acuity is at least 6/12

• +0.25/-0.25 spheres are used, usually mounted on a handle

• Targets equal : add +0.25 DS → targets on red clearer

• Targets equal :add -0.25 DS → targets on green clearer

How to use the Stenopeic Slit to Identify Astigmatic Axis?

• Stenopeic slit: narrow 1-2mm slit in an occluder used in trial frames

• functions like elongated pinhole, allowing only narrow band of light through

• helps identify astigmatic axis by aligning with the meridian of clearest vision

• px views through slit; image sharpest when slit aligns with one of the principle meridians of the Conoid of Sturm

• useful for isolating & refining c

Set up for determining astigmatic axis with stenopeic slit

• px wears trial frames with stenopeic slit over test eye

• occlude other eye

• display 6/18 or 6/24 snellen line

> Patient’s role:

• rotate stenopeic slit using trial frame knob

• stop when image is sharpest

> Outcome:

• slit aligns with one principle meridian of the Conoid of Sturm → astigmatic axis

• other meridian is at 90 deg to first

What is the procedure of the Fan and Block?

• begin with px monocular BVS in place

• Add FOGGING lens to eye being tested (depends on cyl size → if patient estimated 1D of cyl then +0.50DS should suffice) to slightly blur image & prevent over-minus responses

• display fan dial chart at 6 metres

• Ask patient which group of lines apparent clearest or darkest

Astigmatism with Circle of Least Confusion

> fogging lens moves circle of least confusion in front of retina

• prevents over minus

• relaxes accommodation

Understanding Fan Chart Axis Interpretation

• when px views Fan Chart , orientation of lines they see more clearly will appear 180 degrees opposite to what practitioner observes

• e.g if px reports clearest lines at 65 degrees, one meridian is:

• adjustment accounts for reversed perspective between px + practitioner

• cyl axis inserted perpendicular to this, so inserted at 25 deg

• so 180-65=115

• 115-90 =25

What is the Procedure of Fan Chart

• insert a trial cylinder (commonly -0.50DC) at calculated axis

• observe if vision improves → if line clarity becomes more equal , on correct axis

• if distortion worsens, axis may need further adjustment

• rotate axis in 5-10 deg steps & ask px which orientation yields the most uniform clarity

• Refine Cyl power & Axis :

• if blur remains on certain lines , increase cyl power in -0.25 DC steps

• after each change in cyl apply spherical compensation to maintain circle of least confusion → adding +0.25DS for every -0.50DC added

What is the Clock Dial Technique for Detecting and Refining Astigmatism (Elite Chart)?

• used in subjective refraction to determine both axis + presence of astigmatism

• visual analogue of the fan method , but stimulus more closely resembles face of a clock

• allows optoms to localise the clearest + blurriest meridians by using px perception of line clarity , serving as effective tool in refining cyl correction

• if clearer at 1 times by 30

• 7 corresponds to 1 so times by 30 up to 6