V.S. 117
By Jorge Solorzano (524-1534)

Lecture #5

Accommodation (Chapter 17)
Madox Components: Madox classified stimuli to accommodation into four categories:
1. Tonic Accommodation
2. Proximal Accommodation
3. Optical Reflex Accommodation
4. Cross-coupling Accommodation
Out of these, Proximal Accommodation is not measured in clinic

1. Tonic Accommodation
It is referred to as the resting focus of accommodation
It includes:
A. Night Myopia - occurs in darkness. There can be a difference of 1.5D difference in refractive error between day and night. Sometimes the patients will need an extra night prescription. The factors that contribute to night myopia are:
a. Larger pupil that results in positive spherical aberration.
b. Purkinje shift, which is a shift of visible spectrum at night due to increased rod sensitivity. The dark adapted eye is more sensitive to short wave lengths of light.
B. Space Myopia (also called empty space) - Our ability to detect distant targets decreased after viewing an empty space. For pilots this may be a problem.
C. Instrument Myopia - When looking through a microscope, we tend to over accommodate due to the nearness that is perceived. You form a pinhole pupil.
D. Pseudo Myopia (or accommodation spasm) - occurs when an individual has read for a long period of time. It is called the "law-student syndrome". Patient should be recommended to take breaks when reading or to exercise with flippers for about two weeks.
2. Proximal Accommodation
This stimulus is used 99% of the time to alter accommodation. People use proximal cues at night. (i.e. entering into a dark room and finding the light switch). Blur is judged within 1-2D, but proximal cues have large ranges.

3. Optical Reflex Accommodation
This is the accommodation that occurs in direct response to reduction of retinal image contrast. It is a fine adjustment of accommodation used for small distance errors. As long as the error is less than 2D, we always accommodate in the correction direction.
Odd error signals - helps you to know the direction of accommodation. Three types of odd error signals include:
a. Chromatic Aberration,-Example of an application- we use the Duo Chrom Test in clinic. We ask the patient to focus on the 20/400 "E" letter that is half green and half red. Then you ask patients which side is clearer. Myopes say the red side is clearer because they over accommodate. Hyperopes say the green side is clearer because they under accommodate. Emmetropes find equal clarity. A disadvantage of this test is that it is done under dark conditions, which creates spherical aberration due to large pupils. This chromatic aberration can cue us to the correct direction to accommodate.
b. Nystagmus of the lens - the lens is continually changing power. Power changes over a range of 0.25D. The accommodative fluctuations are called a Hunting Cycle. This can also be explained as a systemic factor (pulse causes nystagmus)
c. Astigmatism - most people have With The Rule. The distortion of the circle of least confusion tells you which way to accommodate 1-2D range
only. Accommodation tries to focus somewhere within the interval of sturm (which is the interval of displacement perceived by an astigmatic person, as he looks at a cross positioned in space… review 100A notes). If a person is over accommodating, he/she will see vertical contours more focused. If a person is under accommodating, he/she will see horizontal contours more focused. This cue can help you know in which direction to accommodate.

4. Cross-Link (convergence) accommodation - when you converge, your eyes accommodate and vice versa. A disadvantage of this response is that it can over-react and produce a mismatch of responses. This can be compensated for with lenses and prisms.

Graphical analysis: assumes accommodation completely responds and there is little or no lag of accommodation. It represents the sum of three of the four Madox components (not proximal0. It assumes the components add linearly to make up the total response.

Accuracy of Accommodation
It is task dependant (looking at car vs. license plate). It is limited by the depth of focus of the eye. We usually have a depth of focus of about 0.5-0.75D.
2mm pupil is the ideal pupil, smaller pupils decrease blur. Larger pupils increase blur, due to the peripheral rays entering the eye. Clinically, doctors tend to over refract the eyes with plus lens. They tend to make all eyes a little bit myopic by prescribing the maximum plus correction that does not produce blur.
The accommodation accuracy depends on pupil size, letter size (task involved), visual acuity (if you are 20/20 you do not need to accommodate as much because you still see well, even with a small amount of blur), and tonic accommodation. Adaptable tonic Accommodation is intrinsically innervated, not visually stimulated. It is a durable response that provides comfort and accuracy, but it is slow. On the other hand, optical reflex lacks comfort and accuracy, but it is fast.

Accommodation Dynamics
Latency - how long it takes to begin accommodation
Velocity - how quickly

Latency: accommodation is very, very slow. It takes 300-400 ms. This slowness can be due to the sluggishness of the ciliary body.

Velocity: our accommodative velocity equals 5D/sec for every diopter of accommodation. We can speed up out accommodation by blinking or by making saccadic eye movements, while trying to focus. This doubles the velocity of accommodation and decreases the latency period. The saccade triggers other motor responses, latency decreases to 150 ms. As future doctors, we need to remember that in the clinic this influence is minimized since all the letters are lined up along the visual axis so that we converge and accommodate without making lateral or vertical gaze shifts..

Accommodation anomalies
Consensual accommodation - Efferent signals from one eye goes to both Edinger Westphal Nuclei, but other things get in the way, such as:
a. in asymmetric vergence, one eye that is closer to the target accommodates more than the other eye
b. anisometropia - myopic eye accommodates less than hyperopic eye
c. unequal amplitude of accommodation in each eye due to age
Monovision - presbyopes who wear contact lenses. Near contact lens for one eye, far contact lens for the other eye. They get good stereopsis because it is not sensitive to blur. Remember to give distance vision prescription to the dominant eye.

Disorders of accommodation:
1. Accommodation Spasm
2. Accommodation Insufficiency (trouble shifting, fatigue). This can be strengthened with flippers. Start with +/- 0.5D all the way to +/- 2D and exercise with flippers (20 cycles per minute). This gives them 1.5 seconds to clear each image. When they succeed, you give them higher power flippers and repeat the exercise. Increase the power until they are using the 2.5 flippers.

Vergence (Chapter 19)

Disjunctive or vergence responses are movements of the eyes in opposite directions. They serve the same function as conjugate movements of the eye, which is to preserve the alignment of the two retinal images on corresponding retinal points.

3 axis of disjunctive movements - horizontal, vertical (skew), torsional
When you look at something in tertiary gaze at a near distance, you must make vertical vergence movements to fuse targets because they are at an unequal distance from the eyes. Cyclo movements can occur when you look up and down. Horizontal movements can occur when you change viewing distance.

Units of measurement
3 types:
a. O.D.
Prism diopters
100 x tan o
b. M.D.
Degree
c. Ph.D.
Meter angle = 1/(viewing distance in meters)

(Meter angle) x (P.D. in cm) = Prism Diopter