05 Flashcards
- Premie babies tend to be (hyperopic/myopic), with (round/flat) lenses positioned very far (forward/backward)
- How about full-term babies?
- At 5 yrs (child), what’s the mean Rx?
- when (at what age) does myopia often begin?
Premies: myopic, ROUNDED lenses (zonules not strong enough to pull), FORWARD.
Term: slightly hyperopic, FLATTER lens
5Y/O: ~+0.50D
-14Y/O: onset of myopia
Which type of accommodation is primarily used (esp in infants), explaining how infants are highly hyperopic that eventually move toward emmetropia?
TONIC accommodation - allows massive amts (~3D) of accommodation while infant is highly hyperopic, until it levels out
What mechanism allows almost all children ~3Y/O to have approximately the same refractive error?
VISUAL FEEDBACK mechanism - majority (80%) of 3Y/Os have +0.50 to emmetropia
If visual feedback is disrupted (but LIGHT is still allowed in to the covered eye), myopia develops solely due to WHAT?
AXIAL ELONGATION (anterior seg still nml)
If the optic nerve is cut and the NASAL RETINA ONLY is deprived of visual feedback, what’s the result?
elongation of the TEMPORAL retina only….STILL HAPPENS w/ cut optic nerve
–suggests the process happens LOCALLY w/i the eye, has nothing to do w/ the brain
Changing the thickness of the ____ can allow for a transient Rx change when looking @ a near target
-what three things make this possible?
choroid
1) quick accommodation
2) thinning of the choroid (hrs-days)
3) elongation of the post. pole of the sclera
T/F: the eye will recover after removal of form deprivation, but the process is slow and takes several months
FALSE - it’s fast! Within a week!
So, the DEFAULT condition for a form-deprived eye is what? What controls it, effectively slowing it?
AXIAL ELONGATION
-visual feedback slows it
If an eye is biased and exposed to a minus lens half the time, then a plus lens half the time, it will end up slightly (myopic/hyperopic)
hyperopic
- if lenses removed altogether, it will return to emmetropia.
- w/ minus lenses - axial elongation
- w/ plus lenses - axial growth stops
- dopamine, acetylcholine, atropine, and pirenzepine act at a ____ level
- retinoic acid acts at a ____ level
- ECM acts at the ____ level, along with its growth factors (VEGF, Zenk)
- retinal
- choroidal
- scleral
T/F: A child’s Rx PRIOR to emmetropization (~3-4Y/O) can predict the amount of future myopia as the kid ages
-tends to follow what type of fxn?
True
-Gompertz function: suggests that mechanism causing myopia is similar regardless of age or amt of myopia**
Does the Gompertz fxn state that the onset of myopia is gradual or rapid? When does leveling off occur?
RAPID; levels off around 12-13Y/O (relatively quick finish also)
- Is there a simple genetic pattern explaining myopia, genetically?
- How do the myopia rates b/w children w/ NO or ONE myopic parent compare? How about two myopic parents?
- what about asian population?
NO
0-1: similar rate of myopia
2 parents: higher correlation (60%)
-with TWO myopic parents (regardless of amt of myopia), whildren are 2.5-3X MORE LIKELY to be myopic
-asians: equal rate of myopia w/ either one OR two myopic parents
Which situation is the only case where genetics alone (no environmental factors) can explain myopia?
HIGH myopia (6D or more)
Does environment play a role in myopia progression?
-Study in the amazon found that 16Y/O amazon teens have an identical refraction to __-__Y/Os in america (as if they never went to school)
absolutely
5-6Y/Os - because they never did go to school; they’re emmetropes
only 1.6% HAVE BILATERAL MYOPIA!! ALMOST NONE! 2 teachers, one seamstress, one extensive reader
T/F: more than 1% of illiterate people living in the amazon become myopic in both eyes
FALSE - LESS THAN 1% of the ILLITERATE people (don’t include the literate ones)
How do aberrations induce myopia?
degrade retinal image timilar to partial pattern deprivation –>breaks fdbk loop, changes the point spread fxn–> increases the eye’s depth of focus–>makes it less sensitive to blur—>reduces fdbk signal even more–>allows MORE axial elongation
T/F: astigmatism rarely causes myopia.
FALSE - HIGHLY correlated to development of myopia!
->1D ATR astig kids 9-12mos develop myopia sooner and in larger amounts, AND their astigmatism increases
During myopia progression, does changing the object distance change the accommodative demand? If not, WHAT DOES?
NO!!
- it’s the STIMULUS itself (lenses, etc.) that cause the eye to accommodate –> NOT the distance of the stimulus
- result: myopes have trouble INTERPRETING blur/responding appropriately
- response to defocus RETURNS TO NORMAL when myopia progression stops
T/F: adult myopes are insensitive to defocus, but have normal accommodation.
If true, HOW is this possible?
TRUE
-neural up-regulation: blur signals are used more effectively than perception
Is accommodative variability greater in adult myopes? Do they have the same accommodative lag as emmetropes?
YES, YES
-myopic accomm fdbk system has REDUCED sensitivity and ELEVATED gain - leads to increased variability
T/F: when given an increasing near demand, myopes have a higher percent of look-aways compared to emmetropes
FALSE - they look away LESS frequently
**T/F: if myopes remain steadily fixated when viewing near text, their feedback control is MORE susceptible to hyperopic defocus
TRUE –important concept
-excessive hyperopic defocus, intense fixation, nearer reading distances, and insensitivity to blur are all explanations as to why some children become myopic (and others don’t)
T/F: the near peripheral retina can guide refractive development
true - can have fovea missing entirely and still get emmetropic eye (monkey)
Emmetropic eye: more ____ (shape); fovea slightly (myopic/hyperopic) and periphery slightly (myopic/hyperopic)
-Myopic eye: more ____ (shape) - what’s the relationship here? Implications of a FULLY corrected myope?
emmetropic: OBLATE (flat in back) - fovea slightly hyperopic, periphery slightly myopic (protective against myopia)
myopic: PROLATE (longer @ PP)- fovea more myopic, periphery more hyperopic - induces further myopia
- fully corrected myopia: peripheral defocus even MORE hyperopic; myopia even more likely to be induced
Ortho-K lenses aim to provide ____ correction, shifting peripheral ____ defocus to ____ defocus, theoretically stopping myopia progression
-what type of aberration do these lenses cause? (INTENTIONALLY?)
optimal
hyperopic peripheral defocus shift to MYOPIC defocus (like in emmetrope) –> halts myopia
-cause POSITIVE SPHERICAL aberration
T/F: daytime illumination has no effect on myopia progression
FALSE: the brighter the light, the SLOWER the progression of myopia - unknown mechanism
Is myopia caused by genetics, or the environment?
trick question –> BOTH
-INTERACTION of genes/env causes myopia
name three drugs used/being tested in treating myopia
atropine, pirenzepine, dopamine agonists
-Note: pirenzepine: DOESN’T affect accomm or pupil size –> mech of action must be related to the CIRCUITRY of the retina, not changes in accomm or pupil size
Which form of atropine therapy is MOST successful? 1%, 0.5%, 0.05%, or 0.01%?
- 01% - ULTRA-WEAK - MOST SUCCESSFUL
- virtually NO effect @ 8 months, but then myopia progression suddenly STOPS, even after DC’ing. And there’s NO REBOUND (works for at least one year)
- generally, strongest percentages have the strongest effect/biggest reversal of myopia, but they all cause some type of rebound, EXCEPT 0.01%
