07 1.7 Fan and Block - Lecture notes 7 PDF

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07 Lecture 1.7 | PRECLINICAL OPTOMETRY | Determination of Astigmatic Refractive Error: The Fogging Method (Fan & Block) Lecture Objectives:   

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Revise the location of IS & COLC for various refractive error states Revise how you would determine the BVS of a Px Describe how you would determine your Px’s monocular refraction using the fan & block methods, instructions, set-up, technical aspects & end-point checks o BVS, Fog, identification of posterior focal line, collapse of the Interval of Sturm (IS), removal of fog Describe the change in position of the COLC, position of the focal lines & the size of the IS with each step of the fan & block method

Revision – Spherical Refractive Error  

Image of a point object is at a point but it’s not focused on the retina Spherical lenses are used to focus the image point onto the retina

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Revision – Astigmatism  

A point image does not exist for a point object In astig, two focal lines are formed along the two principle meridians o The two focal lines are separated by the ‘Interval of Sturm’

Astigmatic Imagery  

Light travelling in the vertical meridian forms the horizontal focal line and vice versa “With the rule” astigmatism (CHECK THE RECORDING WHAT IT’S DEFINED AS)

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In Rx, we always refer to the AXIS of the astigmatism, not the power of the atigmatism

Astigmatic Imagery   

In the image below, the vertical focal line is closer to the retina therefore if they view the cross target (a), the image will appear as (b) The crosshair must be aligned with the two principal meridia An astigmatic lens collapses the Interval of Sturm (to form a point image)

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Within the IS, images are in the form of blur ellipses except at the dioptric midpoint between the two focal lines The closest thing to a point image is the COLC, which occurs at that dioptric midpoint

Classification of Ocular Astigmatism

Correction of Astigmatism   

Introduce cylindrical lens to reduce IS to zero such that a focus point can be obtained If point focus isn’t on the retina  use spherical lenses to move it onto the retina Thus, we use sphero-cylindrical lenses to correct astigmatism o i.e. in ret astigmatism, we do a sphere correction of one axis first then use a cyl lens to correct the other meridian

Finding Astigmatic Error with Retinoscopy  

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Move the ret up/ down to see if the axis is at 90 and 180 (which are the most common axis) An astigmatic Px will have differing movements o i.e. Could have both ‘with’ movements but one could be bright and one could be dull, or one might be ‘with’ and one could be ‘against’ Neutralise the most positive meridian first with a positive SPHERE lens o With/With = neutralise the dullest/slowest/thickest movement o With/Against = with/against o Against/against = neutralise the fastest/brightest/thinnest Then neuralise the other meridian with a negative cylinder lens (it’ll always be an against movement if done right)

Beginning Subjective Refraction – Determine BVS Usually After Ret  

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Trial lens set or phoropter can be used Trial lens set – better control over vertex distance o However, it’s slower, and has less wow factor o Good for Px with high Rx, as you can control the vertex distance Phoropter – fast change of lens o However, proximal accommodation, harder for patient to follow what’s going on? Remember Px PD, vertical alignment of phoropter (bubble) or trial frame Do all this in normal room illumination o Px will use glasses in normal light conditions

Best Vision Sphere   

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The spherical lens that gives the best vision Places COLC on the retina, so anterior and posterior focal lines are equidistant from the retina BVS = spherical equivalent (SE) of the sphero-cylindrical lens required to correct that patient’s astigmatic refractive error o Want to give them the best lens to see at distance, but also not too much minus (which would make them have to accommodate) o We want the COLC to straddle the retina i.e. focal points in front and behind the retina +2.00/-5.00 x 90  SE +2.00 – 2.50 = -0.50 D

BVS Example

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The likely amount of astigmatism can be estimated from the VA obtained with the BVS o ROUGH rule of thumb:  Snellen denominator / 9  Or -0.50 ~ 1 line o Example: If you were having a Px with 6/9 with BVS, then we can expect an 0.50 difference

Best Vision Sphere 

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Plus/Minus technique to best VA o Px history (PC & previous Rx help determine) o Level of unaided vision o Refine ret findings Poor vision  large steps, good vision  small steps o Go positive then negative e.g. +0.50 DS then -0.50 DS Instructions critical o Minus: clearer or just blacker and smaller o Plus: Same, or worse Most plus/least minus that gives best VA End-point checks (think about responses – do they make sense?)

Determination of Astigmatism 

Two main clinical techniques used to determine astigmatism in subjective refraction: o The Fogging Method (done today in this lecture) o The Jackson Cross Cylinder Method

Astigmatic Charts 

Variety of methods available for use for the fogging method

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Clinic (Verbaken) charts: fan and o There’s a fan and there are blocks, so it’s called fan and block

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The Fogging Method   

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Begin with BVS so that COLC is placed on the retina When the COLC is on the retina  all lines on fan chart should be equally blurred Fogging method aims to add +ve spherical lenses to move COLC and IS forward relative to the retina o Move it until the posterior focal line is then nearer the retina than the anterior focal line o After fogging, the px will see whichever orientation the posterior focal line is at as the clearest lines on the fan Aim of the fogging method: place both focal lines in front of the retina  proceed to move the anterior focal line back using cylindrical lenses o Essentially collapsing the IS (and the Px will see the vertical lines being clear as well)

How Much Fog?  

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The positive sphere required to place both focal lines in front of the retina is the ‘fogging lens’ Fogging: o If estimated astigmatism (from level of VA after BVS) is -1.00DC or lessfog VA by 2 lines (usually +0.50DS)  Most people are within 1D of astig o If MORE THAN -1.00DC, then need to add +0.50DS plus half the Px’s estimated degree of astigmatism as your fog o Alternatively, determine the BVS after ret so that COLC is on retina, then remove cyl determined from ret (and add +0.50) You want the posterior focal line to be ON the retina or even a little in front of the retina o Better to have a little too much fog than not enough

Fog Examples 

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If VA with BVS is 6/9 for a 20yo, what is the expected amount of astigmatism, and how much fog is required? o A Px with 6/9 might have a bit more astigmatism than 1D o A 6/7.5 Px will have around 1D of astig o 6/6 will have around half a diopter o Use +0.50 fogging If VA with BVS is 6/15 for a 20 yo, what is the expected amount of astigmatism, and how much fog is required? o +0.50 plus a half of the px's estimated astig (which is about 1D) o So, it's +0.50 + 0.50 (so we need 1.00 fog o The worse the vision is, the more the fog o These are more outlier cases, usually we just add +0.50 over our BVS

Axis Determination: The Fan Chart    

With fogging lens added to BVS, ask Px: “Do any lines on the fan appear blacker and clearer?” If all equal, means no cylinder OR over-fogged OR under-fogged (BVS inaccurate?) If single line, indicates the meridian of the posterior focal line If multiple lines, indicates the meridian of the posterior focal lies in the middle (lines usually 10o or 30o apart)

Axis Refinement: The V Chevron 

Locate ‘V’ such that it points to the meridian of the posterior focal line as indicated by the fan chart o Essentially refines the axis of a Px o If you have high astig, then the 10 degree diff on the fan is not accurate enough, will need the V for refinement  Essentially need more accuracy of the axis the higher the astig is  As we rotate the V, the blocks rotate as well, once the blocks are in the same direction as the clearest lines of the fan, then so will the clarity of the lines orthogonal to that

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Ask Px, “Is one of the limbs of the V clearer and darker or do they appear the same?” If on-axis, then limbs will be equally blurred If off-axis, then one limb will be clearer

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Move V in direction of blurred limb until both limbs are of equal clarity Use bracketing technique to accurately locate position of posterior focal line o The anterior focal line will be perpendicular to this meridian Next, get the Px to look at the block chart

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Power Determination: The Block    

The lines in the block aligned with the meridian along which the V is pointing will be clearer and blacker than the other -ve cylinder is added with axis at 90o to clearest meridian Continue adding cylinder until Px reports the lines in the other block are clearest and blackest Note that the lines on the 1st fan chart are labelled with the axis of the required correcting -ve cylinder – mostly not the case

Power Determination – Maintaining Fog   

For every -0.50 cylinder added (DC), add +0.25DS sphere As you add cyl, COLC and IS are slowly pushed behind the retina, so need the sphere to push the points back onto the retina Can ask the Px if the blocks near the circle or near the line is clearer instead of using ‘horizontal’ and ‘vertical’

NOTE: The preclin fans are 15o apart

Examples

Refine Result    

Reduce cylinder by 0.25D (expect reversal) Return to V and confirm/refine axis If change of axis, then return to block and confirm power Once completed: confirm fog has been maintained

Confirm Fog    

Add +0.50 DS to confirm fog has been maintained and IS is minimized: all lines on the fan chart should be equal If lines marked with V become clearer, then increase cylinder to equalize If lines along cylinder axis become clearer then decrease cylinder to equalize Recheck fog again (recheck axis)

Finally  

Reduce fog in 0.25 D steps until best vision is achieved Use same techniques as in determining BVS to prevent over-minusing

Key Points:    

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BVS must be accurate Must have accurate estimate of astig (based on VA or ret) Fog vision so that COLC is on the retina, & the posterior focal line is on the retina How much fog: o If -1DC or less: go +0.50 o If more than -1DC: go +0.50 plus half of the Px’s estimated astig For each -0.50D cylinder added, add +0.25DS to keep COLC in front of retina (maintaining fog) What do you do if you start with ret? (aim to have an understanding of this by the end of semester)...