Optics Flashcards
What are causes of astigmatism?
External factors: ptosis, lid masses, pterygia
Corneal issues: Keratoconus Corneal dermoid Salzmann's nodules Peripheral corneal problems (Pellucid, PUK, Terriens) Ectasia (post lasik)
Lens: Lens coloboma Lenticular astigmatism Lens dislocation (Marfan's, trauma) Lenticonus
- *Life threatening cause: ciliary body melanoma.
- Retinal disease does NOT cause astigmatism.
How can you improve upon appearance of high minus lenses?
Ensure refraction is correct.
Patient’s with high minus lenses complain of barrel distortion, minification, appearance.
Help minimize these issues by:
- Smaller frames
- Use thicker frame design
- Flatter base curve for front of the glasses
- Higher index material to decrease weight
- Consider discussing contact lenses or refractive sx
How can you help a child keep glasses on?
Evaluate the fit of glasses
- Focus on fit around ears, inspect skin for pressure points
- Inspect nasal bridge - may not be sitting correctly
- Consider utilizing optician who specializes in child frame fitting
- The frames may be too large
- Check that optical centers are positioned on the visual axis
Check the glasses Rx to ensure it is same as written (make sure they were made correctly)
Check by repeating cycloplegic refraction
Educate patient/parent about importance of wearing the glasses to maximize vision
Consider cylcloplegics for hyperopes to force them to wear the glasses
Worst case scenario: arm splints
What is amplitude of accommodation (AA) and how is it measured?
Amplitude of accommodation (AA) is measured in diopters. Defined as the total number of diopters that an eye can accommodate.
Perform this by putting the patients refraction into the phoropter. Then place the near card at 40cm and have them focus on the 20/25 line. Then add plus sphere until blurred, then add minus sphere until blurred. The difference is accommodative amplitude.
Another way: place the patient in their full distance correction without an add. Use the prince rule to locate their near point of accommodation. The dioptric value of the NPA is then equal to the AA.
How do you convert from plus cyl to minus cyl?
Example: +2.00 +2.00 x 78
- add sphere and cylinder
- convert cylinder sign
- add 90 to the axis
start with +2.00 +2.00 x 78 (plus cylinder)
+2 +2 = 4
+4.00 - 2.00 x (78+90)
answer +4.00 -2.00 x 168 (minus cylinder)
How do you deal with a patient who is not accepting spectacle correction of astigmatism?
Suspect the rx likely has oblique cylinders in both lenses and patient is bothered by meridional magnification. Other possibility is the patient has anisometropia with anisekonia.
First, always check the ordered rx has been correctly made (check wearing with rx)
Check placement of optical centers of the lens
Check that base curve of new glasses matches base curve of old glasses. If these parameters check out then improve symptoms by:
- Reassurance the patient will get used to it (esp younger pts with smaller cyl)
- Minimize vertex distance to decrease magnification
- Be sure the lenses are of MINUS cylinder type (cylinder is ground on to the BACK of lens
- Rotate cylinder toward 90 or 180 meridians as much as possible without sacrificing vision
- Attempt to decrease the cylinder while keeping the spherical equivalent constant.
- Discuss alternatives, such as contact lens/refractive sx when appropriate
How do you calculate spherical equivalent?
Example: -3.00 +1.00 x 180
Sum of sphere power with half the cylinder power:
-3.00 + 1.00 x 180
-3.00 + 1/2 (1.00) = -3.00 + (0.5)
Answer: -2.50 spherical equivalent
How do you adjust IOL power from in the bag to in the sulcus to an ACIOL?
Bag to sulcus Rule of 9s: weaken power (so subtract) 0 -- 9: No change 9.5 -- 18: Subtract 0.5 18.5 -- 27: Subtract 1.0 If > 27.0 : Subtract 1.5
Bag to ACIOL: always refer to IOL calculation sheet.
Use white to white measurement and add 1mm
Rule of 9s: 0 -- 9: Subtract 0.5 9.5 -- 18: Subtract 1.0 18.5 -- 27: Subtract 1.5 If > 27.0 : Subtract 2
What is the duochrome test and how does it work?
Chromatic aberration is related to prismatic effect of lenses - this is basis for duochrome test.
Different wavelengths are bent to a different degree by eye’s optical system. Shorter rays (blue) are bent more and longer rays (red) are bent less. Difference between green and red rays is 1.00 diopters.
Duochrome test is done monocularly. Add plus until the red background/black letters is in focus. Then (RED ADD MINUS - RAM-GAP), add minus until both red/green backgrounds with black letters are clear. This maximuses white light conditions for that eye. Repeat with the other eye.
*Patients can still accommodate during this test. Fogging technique to reduce unwanted accommodation during manifest refraction is still best.
Review Lensmeter and how to use it. (Iowa curriculum Lensmeter)
read up on this.
A patient is unhappy with glasses, what do you do?
First: LISTEN TO THE PROBLEM! Determine if issue is with acuity at near/far or both, problem with glare, night vision, fit, or distortion?
Check current glasses rx and check against written rx to see if it is wrong glasses prescription. If so, then check refraction again and perform trial test. If better, write new Rx.
Check fit:
- Check location of optical centers for induced prism using lensmeter
- Check segment height
- Check frame alignment
- Check alignment of bifocal segments
- Compare base curve of new lenses to that of old lenses
- Check cylinder form (front versus back cylinder lens?)
*Problem at night? See if a night myopia adjustment was made. Typically add 0.25 minus to compensate for Purkinje effect of night myopia (this occurs when patients over accommodate at night due to no target and thus induces myopia).
Distortion? Look at amount and axis of cylinder. Decrease power and rotate as close to 90 or 180 degree meridian as patients can have distortion from oblique astigmatism when cylinder is significant. Patients tolerate meridinal magnification more when axis is 90 or 180.
Induced prism? First make sure optical center of lens corresponds with pupillary diameter (PD). For induced prism in downgaze, consider lowering optical center or grinding lens edge (slab off), separate reading/distance glasses or contact lens wear.
What are causes of acquired hyperopia?
- Changes in refractive index at air/tear film/cornea interface - Dry eye syndrome
- Flattening of cornea- over correction from refractive surgery, progression of effect of RK, loss of corneal tissue from central corneal ulcer
- Loss of lens - aphakia, dislocated lens
- Decreased accommodative tone - latent hyperopia from loss of accommodation (MOST COMMON), Adie’s pupil
- Changes in refractive index of vitreous - silicone oil (in phakic eyes)
- Shortening of axial length - choroidal masses, orbital masses, macular edema, posterior scleritis, serous elevation of the retina
Med student is having difficulty seeing at night, especially while riding her bike home in the evening after long day studying in library. Why is she having trouble seeing at night?
First, want to rule out organic pathology - such as vitamin A deficiency or RP (retinitis pigmentosa)
Take a complete family history. Full eye exam, looking for Bitot spots. DFE looking for waxy pallor of optic disc, bone spicules, attenuated retinal vessels. If suspect RP, get ERG and autofluorescence.
If no retinal pathology consider refractive etiology.
Night myopia shd be considered as possible cause
What is with the rule astigmatism?
WTR astigmatism indicates cornea is steeper vertically than horizontally - thus steepest K reading is within 20 degrees of 90.
WTR astigmatism is corrected by adding plus cyl with the axis at 90.
Alternatively WTR can be corrected with minus cyl with axis at 180.
WTR is assoc with younger individuals due to tightness of the eyelids. Older people tend to have against the rule astigmatism due to decreased eyelid muscle tone.
What is definition of legal blindness in the US?
BCVA of better seeing eye is 20/200 or less
OR
visual field of 20 degrees or less in better seeing eye.
