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.