WEEK 3: BVS and Fan and Block Flashcards
What is the aim of BVS?
Achieve maximum VA by placing the smallest blur circle on to the retina, whilst keeping
accommodation relaxed.. Trying to find the BVS of the COLC
List the 3 methods for BVS
a. Maximum Plus for Maximum VA
b. Plus/Minus sphere
c. Red/green side of the duochrome chart
What is the purpose of MPMVA?
- Maximise the plus power given, without causing a reduction in VA
- This ensures accommodation is relaxed (ie focal point is in front of the retina)
- Why is this important?
During refraction, relaxed accommodation is required for an accurate result
What are the cons of Plus/Minus?
Plus/minus technique is more efficient in a trial frame, but has the potential to over minus
What is the con of Red/Green duochrome technique?
It can be unreliable for BVS in some patients
Read over the BVS techniques
Read over the BVS techniques
What are the principles of the Duochrome (Bichromatic) Chart?
The duochrome chart is based on the principle of axial chromatic aberration – shorter wavelengths are refracted more by the eye than longer wavelengths. The duochrome chart features black figures/letters, with half the chart background using a red (~620nm) and half using a green (~535 nm) background. The dioptric distance
between these wavelengths is approximately 0.44DS.
What does it mean when the patient can see the RED side of the duochrome chart more clearly?
What about the green?
When the RED side is clearest, the patient needs more MINUS. (Relative myopia)Adding more minus (divergence) shifts the system back (Posteriorly). When the two sides are equally clear, you have an
approximation of the BVS*. When the GREEN side is clearest, the patient needs more
PLUS POWER. (Relative hyperopia). Adding more plus (convergence) shifts the system
forward. (Anteriorly)
What if ‘equal’ never arrives? (when doing BVS with DUOchrome)
Leave slightly hyperopic, ie ‘green is clearer’ – this means slightly posterior to retina, which can be remedied by accommodation, OR you can change technique to MPMVA.
What if ‘equal’ has more than one option? (when doing BVS with DUOchrome)
- A larger depth of focus (smaller pupil) will lead to several lenses looking ‘equal’!
Choose the ‘equal’ closest to ‘green is clearer’ (slight bias to relative hyperopia – Max plus for best VA!)
What are the Optical principles of the Fan & Block technique?
- Start with the Mean Spherical Equivalent to the px’s Best Vision Sphere (BVS)
- Fog the patient - with both focal lines anterior to the retina, the orientation of the axis of the astigmatism may be determined. The focal line closest to the retina (the most posterior focal line) indicates the AXIS of the clearest lines on the fan chart
- Reduce the Interval of Sturm – move the anterior focal line posteriorly, so the two focal lines exist at the same location.
a.Cylindrical/Toric lenses allow us to target a single plane with power, whilst having no effect on the perpendicular plane
b. The anterior focal line is targeted by applying negative-cyl in the appropriate orientation (Fan and arrowhead used to identify orientation of neg-cyl axis)
c. The anterior focal line is shifted towards the posterior focal line location by applying the correct magnitude of negative cyl (blocks used to determine neg cyl power)
d. When focal lines are at the same location, you now have just a blur circle- Defog – using spherical lenses, move the blur circle posteriorly on to the retina
Px should still be fogged, hence reduce plus till MPMVA
- Defog – using spherical lenses, move the blur circle posteriorly on to the retina
What Situations that may benefit from use of the stenopaiec slit?
- High astigmatism – poor entering VA
- Opacities – block off scatter rays from optical opacities to focus on plans that may provide better vision
- Keratoconus – frequently possess irregular astigmatism. Upon identification of one of the principle meridians, rotate slit again to find the other principle meridian (often not perpendicular)
Difficulty trying to locate a principle axis with other methods
READ FAN and BLOCK Method
READ FAN and BLOCK Method
What is the purpose of the stenopaiec slit?
The stenopaiec slit isolates planes of vision, so that refraction is broken down into planes of spherical correction.
- When the slit is aligned with either of the principle meridians, it blocks-out rays from perpendicular meridian. - The slit reduces the effective pupil size in the meridian perpendicular to the slit. The focal line in the plane perpendicular to the slit is effectively perceived as a small blur circle
