P-optics Flashcards
Causes of astigmatism
Mechanical: ptosis, lid / orbital mass, ptg, dermoid
K: keratoconus or other ectasia, salzmann’s nodules, pellucid, Terrien, PUK, sutures
lens: cataracts, dislocation, lenticonus
Must not miss: ciliary body melanoma
Does NOT cause: retinal diseases (these cause vision distortions, not astigmatism)
Problems w high powered lenses
- blurry vision
- distortion
- aesthetics
-Blurry vision: Double check refraction, power of the actual lenses, base curve (which can affect distortion and magnification, thus often matched to old prescription) and PD
-High minus power: barrel distortion / minification
-High plus power: pincushion / magnification
-Bifocals: prismatic issues -> lower the optical center, slab off (the least plus lens) technique, separate reading and distance, add fresnel lenses
-Aesthetics: appearance, weight of lens
-Solution: high index materials to decrease weight, flatter base curve for the front of the glasses, bevel edges, thicker frames –> contact lenses or refractive surgery
strategies to keep glasses on child
Double check refraction, prescription, PD
Optimize fit: nose bridge, ears, frame size
Encourage parent and educate about amblyopia and strabismus
In hyperopes, can atropine to increase dependence on glasses.
arm splints
How to measure accomodative amplitude
How to prescribe near add based on accomodative amplitude
- Prince rule. Start at distance when 20/25 clear (a). Slide towards until 20/25 blurry (b). a-b = AA (D)
- Use distance only correction without add. Locate near point of accomodation. 1 / that distance = AA (D).
- Place near card at 40cm and have patient focus on 20/25 line. add plus until blurred, add minus until blurred, the diff is AA (D).
Near add = 1/reading distance - AA/2
e.g. reads at 40cm: 1/0.4-AA/2 = 2.5 - AA/2
How to transpose
e.g. +2.00 + 2.00 x 78
e.g. +2.00 – 2.50 x 105
- add sphere and cyl for new sphere
- flip cyl
- add/subtract 90 for axis (keep within 180)
- sphere = 4.00
- -2.00
- 78+90 = 168
+4.00 - 2.00 x 168 - sphere = -0.50
- cyl - +2.50
- 105-90 = 15
-0.50 + 2.50 x 15
reasons for trouble w new cyl correction and how to improve acceptance of astigmatism correction
Always: double check refraction, prescription, base curve and PD
- meridional magnification (magnification of only certain meridians)
–rotate cyl towards 90/180 as much as possible without compromising vision
–decrease cyl as much as possible while preserving spherical equivalent
–minus cyl form ground to the back of the lens (check using geneva lens clock)
–minimize vertex distance - anisometropia –> Aniseikonia
–minimize difference in prescription between the 2 eyes without compromising vision
If all else is optimized
-reassurance they will get used to it
-contact lens
-refractive surgery
how to adjust IOL power from bag -> sulcus -> ACIOL
bag -> sulcus:
<9D: no change
9.5-17D: reduce by 0.5D
17.5-28D: reduce by 1.0D
>28D: reduce by 1.5D
If optic capture: no need to adjust
bag -> ACIOL:
needs ACIOL specific calculations. Measure WTW and add 1.0mm for sizing.
Duochrome test:
- which wavelengths are bent more?
- how to perform?
- works on color blind individuals?
- shorter wavelengths (blue gets bent)
- monocular, dial in plus until red is sharp, then add in minus until both sides are equally clear (plus first to maximize accommodative relaxation)
- yes
How to reduce unwanted accomodation during MRX?
fogging
plus power build-up until the patients’ visual acuity is reduced by one to two lines. This is followed by defogging, where the plus lenses are reduced until there is no further improvement in vision.
what is it?
What is it used for?
Ok to use on plastic lenses?
Geneva Lens clock: has center pin that moves while 2 side pins are stationary
measures base curve, calibrated for crown glass (n=1.52) so need to convert for other materials with differing index of refraction.
Pins can scratch plastic so beware.
Lens meter
- how does it work
- how to use it for prescription?
- how to use for prism?
-illuminated movable target to determine focal length. Has built in fixed lens of known power to decrease the distance needed to move the target (optometer principle).
- rotate axis until narrow lines are parallel
- focus narrow lines (sphere)
- focus thick lines; difference from 2 is cyl
in keeping w ophtho convention of positive cyl, the drum should be rotated in positive direction from step 2 to 3. If negative, then switch axis by 90 and repeat 2 and 3 - use nonpermanent marker to mark optical center
- center lens
- center of illuminated lines from cross hair is PD, each concentric circle is 1 PD. If lines is nasal to cross hair, then BI, up is BU etc. (e.g. 3PD BI)
patient with glasses unhappy only at night
Likely due to night myopia: mostly due to accommodation (because unable to see distance targets) with some contribution from chromatic aberration (purkinje effect = cone -> rod becomes more sensitive to blue light) and spherical aberration (peripheral lens has greater focusing power)
Add 0.25 minus (can also test under dark settings to customize minus correction, to be worn at night)
How to assess visual potential in a patient with dense white cataract.
What level of visual potential can these techniques determine?
- history: prior diseases / trauma, surgery, amblyopia
- RAPD
- Maddox Rod orientation - potential acuity meter (lighted eye chart projects via pinhole onto the retina through “windows” in the cataract)
- ERG
- Haidiger’s brush: rotate polarized light in front of a blue background to illicit entoptic phenomenon
- Projecting light through colored gel films
- light perception with orientation of projection
20/200
acquired hyperopia: causes?
Acquired myopia: causes?
Acquired hyperopia
- surface: dry eye
- K flattening: scar, RK progression, LASIK overcorrection, CL
- lens: dislocation, silicone oil in phakic eye (PH)
- CB: loss of accomodation (Adie’s tonic pupil) -> uncovering of latent hyperopia
- AL: mass, CME, ON swelling -> flatens globe
Acquired myopia:
- surface: dry eye
- K steepening: ectasias (KCN, pellucid, post-LASIK)
- lens: cataract, DM (wait 3mo, do not update MRX), silicone oil in aphakic eye (AM)
- CB: anterior rotation (antihistamines, topamax)
- AL: retinal staphyloma
ddx and eval for young patient w nyctalopia
- vit A deficiency: bitot spots
- rod dystrophy
–RP: FHx, waxy disc pallor, attenuated retinal vessels, bony spicules -> autofluorescence (picture) and ERG (essential)
–fundus albipunctatus: part of CSNB - night myopia
What is it? What does it do and how?
Distometer
Measures vertex distance (back of lens to K) by measuring to closed eyelid and assuming 2mm eyelid thickness
Prentice rule
e.g. OD: -8, OS: +4
looks down 1cm
Deviation in PD = cm decentration * spherical equivalent D
e.g. OD 8PD BD, OS 4PD BU = 12PD deviation
How to refract w retinoscopy
fixate on distance target -> SPAM: same plus, against minus - with motion requires plus power, again motion requires minus power until neutral. For cyl, rotate beam to be parallel to reflex.
What glasses prescription for this retinoscopy streak?
Assume working distance of -1.5D
OD: -3 +2 x90 -> -4.5 +2.0 x90
OS: -5+2 x90 -> -6.5 +2.0 x90
The cross drawn = direction of movement, not orientation of light beam
orientation of the light beam = axis
OD: -3 lens w horizontal light beam moving up and down; -1 lens w vertical light beam moving left and right
OS: -5 lens moving up an down, -3 lens moving left and right
How to fit contact lenses (ignore power calcs)
Diameter: increase = more puchase = tighter
BC: decrease = steeper = tighter *smaller clothes are tighter and show more bulges
Soft CL
-diameter: K + 2mm
-BC: 0.5D steeper than flattest K (47 / 45 -> 45.5)
steeper curve = bigger D = smaller radius
RGP
- diameter: K - 1.5mm
- optic zone radius: 0.5D steeper than flattest K (if astigmatism >2.5D, then toric lens)
- optic zone diameter: pupil + 1mm
Scleral lens
- read brand fitting guide
- AS-OCT to measure sagital depth; lens should be vaulted off the cornea
- fill lens with saline, place patient face parallel to floor, elevated lens onto eye without spilling or introducing bubbles
- change peripheral curves to adjust fit
Let lens settle for 10min for soft, 20min for hard, 4hrs for scleral -> fluorescein
