COOPS#5
VS117 2/17/98
Stephanie Clark-Rayess
Announcements: Well have lab 2/27 and 3/13.
Quiz on Thursday 3/10.
Midterm right before Spring Break.
Todays lecture covers Chapter 18
Ernst Maddox wrote a wonderful book entitled The Clinical Use of Prisms that had a profound effect on the field of optometry. He discussed how convergence and accommodation would interact and their various components. That evolved into the basic case analysis of optometry that is performed on all patients in the Clinic. When you do a work-up on a patient, you use the principles developed by Maddox.
4 Maddox Components of Accommodation:
STIMULI RESPONSE MODE
1. Intrinsic Tonic
2. Spatio-Topic Proximal
3. Retino-Topic Optical Reflex
4. Cross-Coupling Convergence Accommodation
Intrinsic Stimuli - internal stimuli generated within us; intrinsic baseline
firing rates of the CNS at resting level
Spatio-Topic Stimuli - perceptual, what we see out in space, that perception
in space (perceived distance) that drives accommodation; 90% of what we
do is responding to perceived distance, very little of it is optically
driven
Retino-Topic Stimuli - low-level information, blur of retinal image that
we respond to; the motor system is more sensitive than the sensory system
so even if we dont see blur and our brains send out commands to
clear it up
Cross-Coupling Stimuli - synergistic response. Activating either accommodation
or convergence causes a consensual response from the other components of
the "Near Triad" (accommodation, convergence and pupillary constriction).
This is because of collaterals sent out by groups of neurons to the other
Near Triad components. The downside is that this synergy can overreact
and produce a mismatch of responses (accommodation is less than convergence).
These overactions can be remedied by bifocals or prisms.
Tonic Response Mode - environmental conditions, called "myopias"
manifest as
3 Myopias:
I. Dark Focus/Night Myopia - due to low light levels at night. There
can be up to a 1.5 D difference in refractive error between night and
day. Sometimes patients need separate night prescription.
3 factors contributing to night myopia:
1. Purkinje Shift - shift of visible spectrum at night due to increased
rod sensitivity. At night, peak spectral sensitivity shifts from 555nm
to 505 nm. The eye is more myopic to blue region of the spectrum
and when our sensitivity changes to this region of the spectrum we have
to accommodate more to see blues clearly. We must over accommodating
approx. 0.5D to see blues at night.
2. Tonic accommodation - caused by mostly empty field due to increase
in parasympathetic innervation. It is approx. 0.5 - 0.75D
3. Positive Spherical Aberration - due to larger pupil; has approx. 0.5D
effect.
Night Myopia can be dealt with by prescribing a separate pair of glasses
for night vision.
II. Space/Empty Field Myopia - decreased ability to detect distant targets
after viewing an empty field. With no accommodative stimulus, youll
automatically focus at around 1 meter (resting focus). This results from
a balance point between sympathetic and parasympathetic innervation.
Accordingly, we must exercise "negative accommodation" to see
distant objects.(Pilots suffer from this.)
III. Instrument Myopia - happens while looking through a microscope due
to accommodating to the perceived nearness of microscope slide; its
a type of proximal accommodation
IV. Pseudo Myopia or Accommodative Spasm - the "law student syndrome"
Reading for long periods of time without breaks causes an increase in
tonic innervation. Phasic (fast) accommodation is quick but easily tired,
whereas tonic accommodation is slower but longer lasting. Tonic compensates
for phasic accommodation, makes reading more comfortable but also hard
to focus at far. Prescribe eye exercises and work breaks.
Proximal Response Mode - This is a coarse adjustment of mechanism for accommodation. Proximity stimulus is used 90% of the time to alter accommodation. Blur isnt used as much, until the focus error is within 1-2D.
Optical Reflex Accommodation - A "fine" adjustment mechanism of accommodation, used for tiny errors; as long as blur is less than 2D you always accommodate in the correction direction, positive or negative. We know which way to accommodate by odd-error signals.
Types of Odd-Error Signals:
I. Chromatic Aberration - the direction of it tells you which way to
accommodate
In clinic, we use the Duo-Chrom Test (see figure 1). You have the patient
focus on the edges of the 20/400 letter E. Ask which side is clear, red
or green. If the two sides arent equally clear, adjust lenses in
front of the eye until theyre equally clear. Its a good way
of refining your distance refraction. Myopes say the red side is clearer
because they overaccommodate. Hyperopes say the green side is clear because
they underaccommodate. Emmetropes find equal clarity. The downside is
that youre testing in a dark room which causes spherical aberration
due to the larger pupil. Therefore, youll tend to overcorrect in
the minus direction.
II. Nystagmus of the lens - the lens is continually changing power twice
each second. This is called the Hunting Cycle. The eye is looking for which
way to accommodate; its a trial and error process. (see figure 2)
Figure 2 compares the temporal frequency of the hunting cycle with a normal
and a pinhole pupil. With the normal pupil, accommodation oscillates quickly,
to hunt for which direction will clear the image. The pinhole pupil has
slower oscillations. (An aside, the nystagmus is somewhat coordinated with
heartbeat so some of it is mechanical instability.)
III. Astigmatism - spatial or meridional aberration of refractive power.
With the rule (WTR) astigmatism has more power along the vertical meridian
and against the rule (ATR) astigmatism has more power along the horizontal
meridian. (see figure 3) Figure 3 shows the perception a person with astigmatism
will have, of a cross positioned in space. The horizontal and vertical
bars of the cross are displaced from one another. They will not focus in
the same image plane. This displacement is called the Interval of Sturm.
Accommodation tries to focus somewhere within this interval (circle of
least confusion). If youre overaccommodating, youll see vertical
contours most focused. Likewise, if youre underaccommodating, youll
see horizontal contours most focused. You can use this cue to know in which
direction to accommodate. Its natural for people to have a small
amount of astigmatism. WTR is thought to be in the lens.
ACCURACY OF ACCOMMODATION
The eye has a depth of focus (DOF). DOF has optical and perceptual components.
We usually have a DOF of about 0.5 - 0.75D. Most of it is due to optics,
some is due to the perceptual task. The optical components that lead to
our insensitivity to detect blue are primarily pupil size. Smaller pupils
decrease blur due to the paraxial ray bundle. Larger pupils increase blur
due to the peripheral rays entering the eye. Clinically, we tend to overrefract
the eyes by half the DOF. We tend to make all eyes a little bit myopic
by prescribing the maximum plus correction that does not produce blur.
Therefore, our far point isnt at infinity. Its about 0.5D in
or 2 meters from infinity.
In clinic we manipulate the location of the patients far point, pulling it in from infinity 0.5 - 0.75D so as to increase the patients range of clear vision. We are able to do this because of our depth of focus. What you do is have the patient read the lowest line on the chart possible. Then add plus lenses until the image gets blurred. What youre doing is bringing the image up into the vitreous until its one depth of focus inside the eye, one depth of focus "overpowered". The depth of focus makes it clear. This extends the range of clear vision in front of, as well as behind the retina.
Accordingly, while viewing a far target, you have a lead of accommodation,because of the refraction plus lens bias (see figure 3). Looking at a near target, you tend to bring your accommodation in just enough so your conjugate point is close enough to resolve the target. Youre not accommodating completely, and theres a lag of accommodation. For example, if you stimulate 4D of accommodation, youll only respond around 3.5D. Far targets (targets at infinity) usually focus in front of the retina, whereas near targets focus behind the retina and we focus. The targets look equally clear because its within your depth of focus. If you increase the accommodative stimulus above your near point amplitude, your accommodative response will actually drop. This happens because there is so much blur, you cant maintain your full amplitude of accommodation. You use a little bit of retinal blur to stimulate more effort to accommodate, to help you hold your accommodation at near. The closer you get, the more blur you need to maintain the response. This contributes to the increased lag of accommodation with nearness of the target. With near targets have a need to exercise a lot of accommodative effort and to do that we use a little bit of blur to stimulate that extra effort.
DYNAMICS OF ACCOMMODATION
There are some clinical anomalies of accommodation called accommodative
insufficiencies. In these cases, accommodation is really sluggish, it takes
them 3-5 seconds to focus something up close rather than the usual 1 second.
It normally takes about 1/3 of a second to initiate accommodation and 1
second to complete the response. The accommodative latency is usually 300
msec. Accommodation has the longest latency of all the visual motor mechanisms.
Compare this to 200 msec latency for a saccade and 150 msec latency for
a vergence eye movement. The reason it takes so long to accommodate is
that it takes a really big force to change the shape of the lens"jelly
bag" . Also, whenever we accommodate we also converge, but we converge
150 msec before we accommodate, even though the response was stimulated
by monocular blur.
Once accommodation begins to respond, it takes nearly one second for accommodation to come to rest at a new target distance, after its started responding (see figure 4). Compare this to a saccade, which comes to its position of rest in approx. 50 msec, thats 20x faster. Accommodation starts off at a high velocity, then slows way down. It does this to avoid overshooting the target. The velocity of accommodation is related to the amplitude of the accommodative response. Our accommodative velocity equals 5 D /sec for every diopter of accommodation. We can speed up our accommodation by blinking or by making saccadic eye movements, while trying to focus. This does two things, it decreases the latency period from 300 msec to 200 msec, and it doubles the velocity of accommodation. This works well for strabismics, especially exotropes.
Saccades trigger the "clutch" of the brain stem. This "clutch" is the omni pause neuron. It gates the activity of multiple simultaneous actions. When Pause cells are active, all the burster cells in the brain are suppressed. So when you turn off the pause neurons, the bursters are then released to have an effect of triggering several motor system. Quick movements like blinks and saccades turn off the "pausers". You are effectively "popping the clutch" of the brain stem.
Lesson: you can shorten the latency of accommodation and double its velocity with blinks and saccades.
DYNAMIC DISORDERS
Treatment of Accommodative Insufficiency
For a patient with slow accommodation you can quickly and easily treat it
with a pair of accommodative flippers (see figure 5). There are plus lenses
on one side of the handle and minus lenses of equal power on the other side.
They vary in power from 0.5 - 2.5D. You start off at the low powered flippers.
Have the patient look through one pair of lenses, as soon as the image
is clear, have them flip them and repeat. Have the patient try to do 20
cycles/min, up and down. This gives them 1.5 sec to clear each image.
Initially, it will take the patient about 5 sec to clear each image. With
daily practice, though, theyll soon be focusing the images in 1.5
sec. When they succeed, you give them higher powered flippers and repeat
the exercises. You increase the power until theyre using the 2.5D
flippers. Accommodative insufficiency is one of the easiest things to treat,
it only takes about one month to cure. For some reason, though, it doesnt
correct itself, even though its so easily correctable.
CONJUGACY OF ACCOMMODATION
1. Consensual accommodation
Does Herings Law apply to accommodation? Yes! The default response
is consentual accommodation, equal responses in the two eyes. Consentual
accommodation is stimulated the eyes looking straight ahead but not in lateral
eccentric viewing. In eccentric viewing, the ipsilateral eye is closer
to the target than the contralateral eye. If the accommodation response
is consensual in eccentric gaze, the eye farther away from the eccentric
target will overaccommodate, so only one eye will see the target clearly
at a time. This is only a problem for viewing distances within 20 cm. E.g.
it can create problems for asymmetric tasks such as encountered by draftspersons,
violinists.
There are two responses to anisometripic blur. The first is to tune out
the blur from the out of focus image. One is called "monovision"
suppression. Its called monovision because of a treatment for presbyopes
who wear CLs. Bifocal CLs are not well developed and an alternative
is to wear one CL for distance over one eye and a different one for near
on the other eye. You effectively suppress the blur in the out of focus
eye but still retain stereopsis. You still fuse well because of the coarse
features present in the target. Patients will either adapt to the monovision
correction within an hour or theyll never adapt to it. Always use
the sighting eye as the far eye, otherwise they could have trouble driving,
etc.
2. Differential accommodation
Some of us do something a little different in response to anisometropic
blur. We focus the two eyes independently. With training, you can exercise
0.5 - 0.75D unequal accommodation. Its not optically driven but rather
a voluntary response. It takes 10 - 15 sec to shift it so it only works
for a long-standing anisometropic refractive errors.
Pretend youre 0.5D anisometropic. One eyes always 0.5D more powerful. This anisometropic accommodation is ideal for it because it allows you to make a small correction and hold it. Its not good for sudden shifts of gaze, however, where each eye needs to be more powerful successively.
It may also guide infants to becoming isometropic. Many infants are born anisometropic, but by 1 year they have equal power in the two eyes. The two eyes could be adjusting independently or they could be using this anisometropic accommodation to straighten things out.