VS 117
Dr. Schor
2/9/99, Tuesday
Notetaker: Kien Phung
Erich Graf's Office Hour: Thurs., 1-2pm in 514 Minor
OUTLINE: (I) Mechanism of Accommodation
(II) Presbyopia
(III) Maddox Components of Accommodation
Comparative Mechanisms of Accommodation
1) Some animals such as birds and reptiles pinch the front surface
of the lens in order to accommodate. This process allows up to
50D of accommodation.
2) In humans, ciliary bodies constrict such that the lens relaxes
to allow accommodation. Young was the first to show that the lens,
and not the increase in axial length or corneal shape changes,
is responsible for accommodation. Helmholtz proposed a passive
mechanical mechanism (the relaxation theory of accommodation).
Lens Anatomy
The lens is supported by suspensory ligaments called Zonule of
Zinn, which attach to the anterior, posterior, and equatorial
regions of the lens. The zonules attach the lens to the ciliary
process anteriorly and extends at a right angle to attach to Bruch's
membrane posteriorly. During accommodation, the ciliary body bulges
up so that the zonules move forward and inward to allow the lens
to round up.
There are elastic forces within the lens, ciliary process, and
Bruch's membrane. When we accommodate, longitudinal fibers of
the ciliary body bunch up anteriorly, allowing the anterior zonules
to relax. Posteriorly, the ciliary body pulls against the elastic
component of Bruch's membrane and the posterior zonules. These
posterior structures, therefore, stretch and they serve as the
antagonist to the longitudinal fibers of the ciliary body. Under
a relaxed state, the elastic restoring force of Bruch's membrane
and the posterior zonules pulls on the ciliary body, which causes
the anterior zonules to be stretched in order to flatten the lens.
Why does the lens round up? Fincham observed the shape of the
lens and noticed that the lens round up when we look far away.
It was noted that the internal components of the lens can be molded
(it is plastic) and that the lens capsule is elastic (it returns
to its original shape after stretching). The shape of the lens
determins which areas become rounded during accommodation. The
anterior pole of the lens is flattest and it becomes more curved
during accommodation. Consider a droplet of water. Due to surface
tension, the droplet of water tends to be round when no external
forces are applied. In the same manner, under relaxed state, the
lens assumes a spherical such that the least curved surface, i.e.
the anterior surface of the lens, changes the most.
When we look at optical infinity, the back surface of the lens
has a radius of curvature of 5mm, while the front surface, 12mm.
In addition, the peripheral lens is more curved than the center
of the lens. Thus, incoming light is bent more at the peripheral
lens so that the retinal image is formed in front of the retina,
resulting in positive spherical aberration. When we accommodate,
the center of the lens becomes more powerful and the pole of the
lens has more power than the periphery. Thus when we view objects
at near distances, we get negative spherical aberration. At 33cm
(3 diopters), no spherical aberration occurs as all rays are focused
on the retina.
Another theory, Tschermack theory, attempts to explain accommodation by proposing that when the lens pulls back, the anterior surface bulges forward. However, no evidence has supported this theory.
Purkinje image three (the image that's reflected off the anterior
surface of the lens) shows that under accommodation, the image
becomes smaller, which proves that the front surface of the lens
does in fact round up during accommodation.
How's accommodation controlled neurologically? Both the parasympathetic
and sympathetic systems of the autonomic nervous system affect
accommodation, although the parasympathetic system has a greater
effect. The parasympathetic system causes constriction, while
the sympathetic system inhibits constriction. Thus, when the eyes
are relaxed, stimulation of the sympathetic system would have
no effect on the eye. However, under accommodation, stimulation
of the sympathetic system would reduce accommodation.
At resting position, the parasympathetic and sympathetic systems
are spontaneously firing, and the balance of their activity determins
the resting focus of accommodation. When we are placed in a dark
room we assume the resting focus and we accommodate at an intermediate
distance (1-1.5 diopters), which represents the balance between
the two nervous systems. Thus, our eyes are most relaxed when
we accommodate by 1-1.5D than at optical infinity.
The motor nucleus begins at the Edinger-Westphal (EW) nucleus,
which is located above the 3rd nerve. When the left half of the
nucleus is stimulated, accommodation on the left eye is affected,
and vice versa. However, EW receives information simultaneously
so that there is a consensual response from both eyes.
Parasympathetic pathway:
EW --> ciliary ganglion --> short posterior ciliary nerve
--> ciliary muscle
( Note that some axons project directly from EW to ciliary muscle.)
Sympathetic (inhibitory) pathway: 8th cervical and 1st, 2nd thoracic regions of spinal cord --> super cervical ganglion --> long posterior ciliary nerves --> ciliary muscles
Function and Aging
At birth, we have 18.5D of accommodation. We lose approximately
1D of accommodation every three years. Hofstetter gathered data
relating aging to accommodation and found a linear relationship
between the two variables. The equation to describe this relationship
is:
Amplitude=18.5-age/3
Q: What's one's amplitude of accommodation at age 33? A: 18.5-33/3= 7.5D
The equation describes the average prediction. Amplitude of accommodation varies from individual to individual.
Functional presbyopia occurs when the patient's near point of accommodation is less than his near working distance. The patient can't accommodate to his near working distance. Since functional presbyopia is a function of one's near working distance, it varies according to the individual's need and occupation.
At a typical reading distance of 40cm, we require 2.5D of accommodation.
At what age is the amplitude of accommodation 2.5D?
2.5D=18.5-age/3 ==> age=48
Thus, at 48 years old, we must accommodate fully to view objects
at 40cm. However, it takes a lot of effort to accommodate completely
and we normally only use half of our accommodative power.
Homework problem:
At what age is our amplitude of accommodation 5D?
This is the age when most people begin to realize that they have problems with accommodation. Problems occur when reading at night or looking at maps, or any conditions in which there is low contrast.
In the future, bifocals will become obsolete because lenses can be replaced by surgery. An artificial, flexible silicon lens can replace the aging lens. Currently, there's been a patient who had her lens successfully replaced so that she is able to accommodate.
What causes loss of accommodation?
1) lens substance becomes more brittle
2) lens constantly growing
Consider the extreme case where the lens diameter thickens to
the point that it touches the ciliary process. In this case, the
ciliary body will no longer have an effect on the lens since the
zonules are basically sitting against the ciliary process.
3) weakening of the ciliary muscle due to aging
4) index of refraction becomes uniform
Absolute presbyopia, when one can no longer accommodate, occurs
at around age 52 (obtained by empirical measurements). Predicted
age (using Hoffstetter's equation) at which absolute presbyopia
occurs is slightly different.
Q: What happens to the delicate balance between sympathetic
and parasympathetic systems when accommodation is gone, as is
the case with absolute presbyopes?
A: Since pupillary constriction still occurs, we know that there
is an effort to accommodate. However, with time, one adjusts such
that one learns not to accommodate, but one is still able to converge
one's eyes. For instance, tropicamide inhibits accommodation.
Initially, attempts to accommodate results in excessive convergence
and a large esophoria. But if tropicamide is applied for two weeks
non-stop, then the eyes would learn to converge by a different
mechanism not related to accommodation to avoid the excessive
esophoria. Similarly, presbyopes wearing bifocals should be very
exophoric due to the loss of accommodation, however they become
orthophoric at near because of adaptation.
Maddox Component of Accommodation
There are different neural mechanisms to stimulate accommodation
and convergence, which can be summarized into four areas.
Unique Stimuli
1) intrinsic innervation: even at resting state, there is constant
spontaneous firing of neurons in the autonomic nervous system.
2) spatio-topic: distance and direction of target are perceived
such that even in complete darkness without any visual stimulus,
accommodation and convergence are stimulated by perceived distance.
3) retino-topic: physical blur of retinal images can stimulate
accommodation
4) cross-coupling: convergence can activate accommodation since
the two motor systems are coupled
Response Mechanism (to each of the four stimuli)
1) tonic convergence and tonic accommodation: responds to intrinsic
innervation
2) proximal accommodation and convergence: visual system responds
to spatio-topic stimuli
3)optical reflex: unconscious response to retino-topic stimulus
4) convergence accommodation to be discussed in chapter 20
Examples of Tonic Accommodation
One becomes myopic when there is too much tonic accommodation.
There are four types of tonic induced myopia (night, space, instrument,
pseudo)
Night myopia is especially common due to two factors:
1) Optical: spherical aberration is manifest by dilated pupils
at night. Dilated pupils allow light to pass through the peripheral
lens which causes positive spherical aberration. (Recall that
the peripheral lens is slightly more powerful than the center
of the lens when we look at optical infinity.)
2) Sensory: Once we dark adapt, rods gain sensitivity relative
to cones. Thus the eyes become more sensitive to short wavelengths
(505nm) than medium wavelengths (555nm) of light. This phenomenon
is known as the Purkinje Shift. Along with the optical component,
Purkinje Shift accounts for 3/4D myopia at night.
Taken together, the real tonic accommodation plus the optical
and sensory components can add up to 1.5D and often patients need
a special pair of night driving glasses to correct the problem.