CO-OP NOTES – VS 117
2/10/98
Shawna Lovering

Office hours for GSI's: Erich – T 10-11 392 Minor
Shahram – Th 1-2 412 Minor

TOPICS OF DISCUSSION:
1) Neurological control of accommodation
2) Presbyopia
3) Lab preparation – Lab #3: Binocular Accommodation

Neurological Control of Accommodation

The accommodative system is controlled by the autonomic nervous system which has two branches operating under a push-pull mechanism:
1) Parasympathetic branch – causes an increase in accommodation (ciliary constriction)
2) Sympathetic branch – causes a decrease in accommodation (ciliary dilation)

Parasympathetic pathways to accommodation have a final common pathway or motor nucleus (final point in the brainstem that drives this muscular mechanism) located near the third nerve nucleus, referred to as the Edinger-Westphal nucleus. There is an EW nucleus located on each side of the brain and each nucleus controls accommodation in the ipsilateral eye. Accommodation is a consensual response because each EW nucleus receives input from both eyes. Projections from the EW nucleus to the eye first go to the ipsilateral ciliary ganglion, to the short posterior ciliary nerves, to the ciliary body.

Sympathetic pathways are referred to as the inhibitory branch of the autonomic nervous system with respect to accommodation. This pathway originates in the eighth cervical and first two thoracic regions of the spinal cord. These axons begin at the superior cervical ganglion, pass through near the ciliary ganglion without synapsing, enter the eye through the short posterior ciliary nerves which innervate the dilator of the iris and the ciliary body. The main action of the sympathetic system is to counteract or inhibit the action of the parasympathetic system. By itself, the sympathetic system does not do much.

_ Clinical application: As optometrists, we will learn to manipulate these two pathways with certain drugs that can inhibit parasympathetic and stimulate sympathetic activity, thereby reducing accommodation.

Presbyopia –
Functional Presbyopia- a condition where the near point of accommodation is beyond the individual’s working distance
Absolute presbyopia – total absence of the ability to accommodate or change power of the eye.

Hofstetter’s Rule: amplitude of accommodation = 18.5 – age/3
(see graph "Time Course of Presbyopia" – Chap 16, p 11).
Linear regression analysis of amplitude vs age:
y = ax + b y = amplitude of accommodation
a = slope of line
x = age of individual
b = amplitude of accommodation at birth

_ We lose 1/3 Diopter of accommodation for each year of life. An individual’s amplitude of accommodation continues to decrease until age 55.5 years.
_ Assume that the average reading distance is 40cm. Therefore, in order to read, the average individual requires +2.5D accommodation. Be able to answer the following type of question for quizzes and exams:
Q: At what age is our amplitude of accommodation +2.5D?
A: 2.5D = 18.5 – age/3
solve for age: age = 48 yrs
However, it is very uncomfortable to use your full amplitude of accommodation in order to perform a task. Instead, you always want to have an equal amount of accommodation you are using in reserve. In other words, we are able to comfortably use half of our available accommodation.
5 D = 18.5 – age/3
solve for age: age = 40.5 yrs
Interestingly, this is the age that most people begin wearing bifocals for near activities performed at 40 cm.

What causes a decrease in amplitude of accommodation?
The lens becomes stiff and larger in diameter making it more difficult for the lens to change shape (become more rounded anteriorly). Due to an increase in lens diameter, the lens also becomes closer to the ciliary body (the lens zonule or suspensory ligament is slackened). As a result, when the ciliary body constricts and moves toward the lens, there is less change in the tension on the zonules and, therefore, less of a change in the lens capsule. Due to the proximity of the lens and the ciliary body, even following constriction of the ciliary body, the zonules are already fully relaxed. There is no stretching force because the lens has assumed the entire aperture within the ciliary aperture when it is dilated.

Lab Material- Optometers
Devices that measure the optical power of the eye. They consist of a lens before the eye through which virtual images are viewed over a large range of distances.

1) Simple optometer – The optometer lens is in the spectacle plane of the eye (anterior focal point of eye). Compute the distance of the image from the spectacle plane.

Gauss equation: 1/O + 1/I = 1/f
(both object and image distance are measured from the lens)

EX: with a +10 D lens: if O = 5 cm then 1/O = +20 D

1/I = 10D – 20D = -10D
(a 10D myope at the spectacle plane)

EX: for contact lens correction compute the refractive error at the cornea (effective power): Far point from spectacle plane 10cm = - 10D
Far point from the cornea 11.5cm = -8.7D
*we must correct for effective power of the correction at the cornea

The Phoropter is an example of a simple optometer– it places lenses at the anterior focal point of the eye. This also happens to be the spectacle plane of the eye (1.5 cm). Don't confuse this with the anterior focal point of the optometer lens. If you place an object inside the anterior focal point of the optometer lens, it will form a virtual image in front of the eye. Thus myopes will set the object inside the anterior focal point of the optometer lens and hyperopes will set the object beyond the anterior focal point to form an image at their far point which is behind the eye.

The disadvantage of the simple optometer is that image size increases as the object is brought closer to the eye. Both blur and image size stimulate accommodation. If you want to examine the affects of blur on accommodation without changing image size you need a different type of optometer ( a telecentric system) that keeps image size constant while varying dioptric distance of the image. Two such systems are possible. These are the Badal optometer that places the lens so that its posterior focal point coincides with the spectacle plane (anterior focal point of the eye) and the Nagel optometer that places the optometer lens so that its posterior focal point coincides with the nodal point of the eye. Since we usually prescribe for the spectacle plane we will use the Badal system.

How to use a Badal Optometer

Newton’s Thin Lens Formula: X’X" = f’f" X’ = distance from object to
(lens makers equation used for lensometer) anterior focal point of lens
X" = distance from image to
posterior focal point of lens

Conjugate focus (CF) of eye in diopters = 1/X"(meters)
find 1/X": X" = f’f"/X’ 1/X" = P2X’ P = power of lens

In lab, compute X’: X’ = f’ – Ob 1/X" = P2(f’ – Ob) = P(Pf – PO)
where O= object distance to the Badal lens. Since Pf = 1.0, then 1/X" = P – P2O
If the lens = 10D then 1/X" = CF= 10D – (100D)(O)

EX using a 10D Badal Lens
for O = 10cm = .1m 10 – 100(0.1) = 0D(collimated)
for O = 9cm = .09m 10 – 100(.09) = 1D
for O = 8cm = .08m 10 – 100(.08) = 2D

Therefor there is a linear change of diopters and distance. 1D = 1cm

EX using an 8D Badal Lens
for O = 12.5cm = .125m 8 – 64(.125) = 0 D (collimated)
for O = 11.5cm = .115m 8 – 64(.115) = .64D = 1cm
for O = 10.5cm = .115m 8 – 64(.105) = 1.28D = 2 cm

Note: In the lab the object distance of the stigma to the Badal lens is measured off of a scale that does not register zero at the lens. This means we need to subtract a correction factor from the scale measures of the object distance. The scale correction factor will be noted on your Lab optometer. Some of you will use 10D Badal optometers and others will use 8D optometers.

In lab we will use a type of mirror stereoscope (Wheatstone stereoscope) that contains two stigmascopes ( a point source Badal optometer).
Wheatstone Stigmascope – has beam-splitters and Badal prisms that allow us to measure an individual’s accommodation binocularly as they view an acuity chart at various distances

stigma – a point source of light that is moved before to the Badal lens within the instrument until the stigma is in focus. We use this stigma to monitor accommodation while the patient views the acuity chart (target). In effect, we are moving the stigma to the same conjugate focus distance of the retina while the subject accommodates in response to the acuity chart.
The magnitude of the conjugate focus that is measured in the lab depends upon three variables.
Cf = AR + RE + L Cf = conjugate focus
AR = accommodative response
RE = refractive error
L = lens correction
We solve for accommodative response after measuring conjugate focus, refractive error and knowing the lens power of additional lenses placed in the spectacle plane. The lens correction does not refer to the power of the Badal lens.
AR= CF - RE - L