HHM And Ms Flashcards
How much magnification would you predict is needed to read 2M print? BCVA 0.4/6M
6/2=3X
An object is 40cm from the eye. If the object is moved to 20cm from the eye and then a +20D lens is inserted one focal length from the object, how much mag is achieved
M=rF
R=reference distance
F=power of the lens
M=rF
M=.04(20)=8X
Important factors for HHM and microscopes
- near acuities must be best corrected including an add for presbyopes
- distance acuities should not be used to predict near mag
- distance and near VA are not always the same
- some people use Kestenbaum’s rule for their actual prediction of the near add, but it is NOT the preferred method
- use Kestanbaums rule as a starting place so that we know patient will be bale to see the MNRead card reasonably well, but not for actual mag prediction
Kestenbaum’s rule
We dont really use this
Near add=reciprocal of distance VA
Near add= snellen denominator/snellen numerator
20/200
200/20=+10.00D
Problems with kestenbaums rule
-it almost always underestimates how much add the pateitns needs-often doubling th result of Kestenbaums rule works better
20/100 VA, 100/20=5 for Kestenbaum, 5x2=10D
Remember that distance VAs are single letter acuities, so it makes snes that the patient would need more hope for fluent reading
Clinical pearl for kestenbaums
-if you do not have a reading acuity chart (only have single letter near charts), make your goal 2x smaller than the patient’s actual goal, i.e. if they want 1M, make them see 0.5M letters and then confirm with a newspaper
Predicting near lens power
M=rF
R=reference distance needed for acuity measurement
F=lens power (D)
What lens is required to achieve 4X magnification for a patient how near VA measured 0.4/2M?
M=rF
4=0.4F
F=+10D
Convex lenses
- objects one focal length from the lens produce a virtual, erect image located at optical infinity (parallel light rays exit lens)
- objects less tha one focal length from the lens also produce a virtual, erect image, but divergent light rays exit lens (this is the situation with s tank magnifier)
Total magnification
M=rF
Microscopes
-high plus lens in the spectacle plane (think high powered reading glasses)
-power is the primer left over after patient’s spectacle correction is considered
-M(total)=(r/d)x(1+hF)
(RDM)x(angular mag)
-provides mainly RMD (and a little angular mag)
Advantages of microscopes
- hands free
- more conventional. Patients tend to understand them well
Disadvantages of microscopes
- close working distances
- may not have sufficient illumination
Formats of microscopes
Monocular
Binocular
Prism compensated half eyes
- available in +4,5,6,8,10,12,14D
- practical limits of binocularity is about +10D
- rule of thumb for prism: power of the lens +2 BI in each eye. +8D half eyes have 10BI OD and 10BI OS
- binocular microscopes
- available in zyl or metal
- high index or CR-39
Clinical use of microscopes
- demonstrate to every patient to discover what equivalent power they need
- based on their response and goals, we will then choose the appropriate magnifier to trial (also, if they like the microscope, they will likely be part of the treatment plan)
When in doubt of binocularity
- COVER TEST
- total convergence demand (c) demands on patients distance pd (DPS), focal length of add (f), and distance from center of rotation of eye to lens (d)
C=DPS x (1/(f+d))
Lens decentration
- BI prism can be obtained by decentering the plus lenses
- use prentice’s rule: prism=d(in cm) X F
- would you decentering in or out to achieve BI prism in a prefabricated (not custom) microscope? In
Clearimage II (designs for vision)
- up to 40mm lens
- distortion free from edge to edge
- can incorporate astigmatic corrections
- +8 to 32D (in 4D steps)
- high power, prismatic, and telephoto versions also available
Univision press on add
- aspheric optics
- 6,8,10,12,16,20,24,28,32,36D
- flexibility positioning
- applications can be readily replaced and repositioned
- excellent cosmetic appearance, light weight
- instock avaialbiltiy
- not dependent on Rx
Pre fab microscopes
- monocular or binocular
- may be full diameter, half eye or lenticular
- wide range of powers and sources available
- affordable
Prescribing for NVO
-custom microscopes will be based off of the distance Rx, exactly as custom NVO glasses are
-a patient that has an Rx of:
OD: +2.00 -5.00 x 123
OS: balance
-that needs +10 microscope over that Rx will need a microscope with:
OD: +12.00 -5.00x123
OS: balance
BVA: NLP OD; (-5.00 DS OS) 20/400 OS; near VA (with +2.50 add): 0.4/8. What add would you predict would enable this patient to read 1M?
What microscope power would you predict to enable this patient to read 1M print?
What you have/what you need 8/1=8X M=rF 8=0.4F F=+20D
20+-5
+15D power needed
Clip on adds
So they can move it out of the way when they need to
