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$: CNS 2 Physiology lab - JUdoctors « Medicine lectures ... · CNS 2 Physiology lab 3 | P a g e When we talked about light refraction, accommodation must occur. In order to look at$
CNS 2 Physiology lab

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Presbiopia: is decrease in accommodation of the lens, and it happens

in old ladies

Astigmatism: a problem in one of the focal planes; due to irregularity

(in cornea or lens), one of the focal planes is out of focus compared

to the others. It is tested by the chart shown below:

You ask the patient, which line you see shorter, longer, or thicker than

the others; since only one plane is out of focus and all the other planes

are in focus, one line will be different from the others.

It should be noted that the doctor emphasized that this material is also

considered as continuation of the theory material and is INCLUDED IN THE

THEORY EXAM.


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Confrontational visual field exam is used to test the visual field of a

patient and know if he has quadrant-anopia or bilateral hemi-anopia

Now if you stand in front of the patient and point your finger and

ask him/her if he can see your finger, the patient might move his

head.

To make sure that the test is done properly, you should stand

straight opposite to the patient, leaving some distance, and ask

the patient to cover one eye and you (the examiner) cover the

eye opposite to the patient's covered eye (i.e. if the patient

covers his right eye, you cover your left eye) and make the

patient focus on you, by this you make sure that the patient does

not move his eyes. Now, you start moving your finger around

midway between you and the patient, and ask the patient if he

can see your finger without him moving his eyes away from you.

In this situation, you are the control because when you place your

finger outside the patient's visual field, you know it's outside the

visual field and not a defect because you can't see your finger

either.

After you are done, you examine the other eye by the same way.

The pictures below show you how the test is done:


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When we talked about light refraction, accommodation must occur.

In order to look at a near object, the following occur:

1. Pupil constriction

2. Accommodation of the lens, in which it becomes more dense and

rounded

Even in dim light, if you want to look at a near object, the pupil will

constrict, even though the pupil should be dilated. This is called pupil

near reflex.

Now light coming from the outside can fall anywhere on the retina

depending on its source.

If the light is refracted at the periphery of the lens, it will not give a

good well-formed refraction; it will make a point concentration of

light, but it will be really small; thus, the region that you see is a

really small point.

When light passes through the center of the lens, there will be

more uniform refraction; the light will be focused on a larger point.

In lenses, not only eyes, the distance in which the light is

concentrated on one point is called focal depth.

If a lens's focal depth is small, like 1mm, any point that is 1mm

will be in focus, but anything more than 1mm will be out of focus

If the focal depth is large, like 5mm, any point less than 5mm will

still be in focus.

Q) Why is it important for the pupil to be constricted when looking at

a near object even in dim light?

Ans.) When the pupil is constricted, the light will fall on the center of

the lens and not the retina. Whether the pupil is constricted or dilated

the light will fall on the same point on the retina however, the focal

depth differs when the pupil is constricted or dilated.


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This is important in near vision because 1mm doesn't make a difference

in far vision. When you focus on your finger, you want to see your whole

finger not just the surface or one part of it; in this case, 1mm makes a

difference, in order to do that you must have a bigger focal depth. When

you are reading from a book and not a board in which it has a flat

surface, if you want to focus on a word; if the focal depth is small, the

upper part will be out of focus and all other words will be out of focus

too, so you need to focus on each word by itself.

For this reason, in near vision, even in the dark, there will be pupil

constriction and accommodation of the lens; this results in the passage

of light through the center of the lens and the focal depth is large, and

this allows you to see more than one level in focus together (more than

one dimension).

An example is when you enter a hall; you stand at the front row and

start looking at chairs in the 7th, 8th or 9th row, you can see the numbers

of 3 chairs at the same time; then you look at the 2nd or 3rd row (without

accommodation), you can see the number of one chair only.

This example might be useful:

Figure A shows the situation of an eye looking at a man through a narrow pupil. Everything from close to far away is in focus, because the dispersion of light from each point of the image in front or behind the focal point on the retina is minimal.

Figure B shows the same situation, but with a wide pupil. The image at the focal point on the retina is sharp, but because the dispersion of light from each point of the image is quite large, the sharpness of the image rapidly diminishes at points behind, or in front of the focal point.

Dispersion of light


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Its pathway:

Optic nerve to geniculate nucleus, visual cortex (area 17), secondary

visual cortex (areas 18 and 19) and then signals telling that the image is

out of focus travel down parasympathetic oculomotor to cause

constriction. It differs from the light reflex in its sensory component

only.

This lesion results in what is known as light-near dissociation, where

there is no light reflex but near reflex is present.

A lesion in the posterior part of the midbrain affecting the olivary

pretectal area is called dorsal midbrain syndrome; an example is

Parinaud's syndrome.

Visual acuity is the accuracy of vision, seeing things clearly; it is

determined mainly by refraction errors.

In normal people two points 1.75 mm apart are recognized as 2

separate points from a distance of 6m.

Any person who needs the distance to be less than 6m, like 4 or

5m, in order to recognize them as 2 separate points, then he has

decreased visual acuity

A person who can still see them as two separate points from a

larger distance, like 7m, then he has increased acuity or super

acuity.

If there was a lesion in the posterior part of the midbrain (lesion in

the olivary pretectal nucleus; EW nucleus is intact), will the

pupillary light reflex be lost? The light reflex will be lost since it

interferes with its pathway (olivary pretectal nucleus is damaged);

but the pupil near reflex will not be lost because it will not interfere

with its pathway (EW nucleus is intact), the impulses will travel

directly from higher centers to parasympathetic oculomotor.


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To test the visual acuity, we use Snellen eye chart which is shown below:

You place the chart 6m away from the patient and ask the patient to

cover one eye and ask what letters he sees.

Normal people should be able to see the letter on the line

numbered 6, when the chart is 6m; the vision is 6/6.

If the person is able to see the letters on line numbered 5 then he

has an increased visual acuity because normal people are able to

see them when the chart is 5m away or less.

The + or – depends on the lens, and it indicates the strength of the lens

needed for your vision to become 6/6.

The visual acuity is 2 parts: nearsighted (myopic) and farsighted

(hyperopic).

The Snellen eye chart tests the myopic eye; the Jaeger eye chart tests

the hyperopic eye.

You ask the patient to hold the Jaeger eye chart at a certain level and

ask him to tell you which part of the chart he sees.


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Pupillary light reflex:

As we all know, shining light on one eye will cause light reflex to occur in

both eyes.

Actually 53-67% of fibers cross in the optic chiasma, but for simplicity

we're going to take the average in which 60% of the fibers cross. Due to

this, relative afferent pupillary reflex will occur.

To test the light reflex: make the patient look at you and shine light on

one eye, both eyes will constrict. Do the same for the other eye.

There is swinging light test: you shine the light on one eye, wait for 3

seconds then move the light to the other eye, then move the light back

to the other eye; in normal people, both eyes will be constricted in the

same manner no matter how many times you swing the light.

Sometimes there is a difference in the constriction of each eye that can't

be detected except with swinging light test. When swinging the light the

affected eye will dilate a little, this means that constriction actually

occurred in one eye only; if the affected eye is the right eye, then we say

the patient has positive right relative afferent pupillary defect or positive

true swinging test.

Relative afferent pupillary defect is usually due to before the chiasm

problem:

Retinal detachment: for example, the right eye has 10% retinal

detachment which means that only 90% of the eye is active for

light reflex; this will result in a smaller constriction than the left eye

Ischemic retina

Problem in optic nerve: not a complete cut like ischemia,

compression, neuritis… mainly recovered neuritis because in all the

previously mentioned causes, the vision will be affected, however,

in recovered neuritis, the vision is normal but the nerve is not

completely recovered so there will be a difference in the pupillary

reflex between the two eyes.

Diabetic retinopathy


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Optic track lesion with no visual defects

Cut at B (consider it to be on the right side), left homonymous

hemianopia occurs; light reflex is positive in both eyes, but the

constriction in the right will be more than in the left. In swinging

light test, it will be positive in the left eye because 60% of the

fibers cross to the right side; the positive RAPD test is due to

unequal decussation in the chiasma.

This test was used in the past to differentiate between optic tract

lesion and optic radiation lesion; but now is of less significance

because of MRI.

Unilateral midbrain lesion: no visual defects occur; like we said in

dorsal midbrain syndrome, but here the defect is only on one side

so RAPD will occur.


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Anisocoria is when the pupil of one eye is larger than the other.

In Horner's syndrome, the pupil of one eye is smaller than the

other on the ipsilateral side.

In 20% of people, it is normal physiological thing, which means

that there is no deficit or anything. Anisocoria is abnormal, but

some people have it physiologically with no underlying cause.

Anisocoria is tested in light and dark; and the pupil diameter is

observed in both situations.

Anisocoria is due to a defect in the efferent pathway, either

sympathetic or parasympathetic. If the defect is in sympathetic,

anisocoria is observed in dark; if in parasympathetic in light.

In Horner's syndrome, there is a defect in the sympathetic

neurons; anisocoria is apparent in dark.

Adie's tonic pupil, defect in the parasympathetic postganglionic

neuron degenerates; there is a defect in pupillary reflex, and

light-near dissociation is present because the neurons for

accommodation are more than those for pupillary reflex

Dorsal midbrain syndrome results in light-near dissociation; if it

was on one side, anisocoria results.

Photoreceptors are of 2 kinds:

Rods

Cones: for color vision and has three types of photosensitive

proteins: blue, green and red (in the past, they used to call them

low, medium and high frequency; but it is less accurate). They

allow me to see all colors by combination of their activities.

In Horner's syndrome, the affected eye's pupil will be already constricted, in order to

detect is anisocoria, it must be dark. In Adie's tonic pupil, the pupil will be dilated; in

order to detect anisocoria, there must be light.


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Color blindness is mainly in red and green; there is blue color

blindness but is rare.

Color blindness is X-linked, that's why it is more common in males

than in females.

There may be loss of the green or red color. If the green is lost,

then it called Deuteranope; if red is Protanope.

Blue color: wave

at 450 nm.

There is activation

of 97% of low

frequency (blue

wave) while the

others are zero.

Green color: activation

of 31% red, 36% blue

and 67% green.


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Deuternope

Now if you show a patient with Deuternope a green card, what color will

he see?

He will see the color green, why? The green wavelength is 500nm.

In normal people, the

brain will recognize it by

the activities of 36% for

low frequency (blue),

67% for medium

frequency (green), and

31% for high frequency

(red); so it understands

the color as green

In a person with

deuteranope, the brain

receives the signals as 36

for low frequency and 31

for high frequency but

perceives the color as

green because colors are

already coded by 2 codes only.


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Any region that has no overlap will be a problem; for example,

light of wave 550

Some of the shades of yellow and orange will have the same color

in people with deuteranope.

A normal person with 3

codings will activate high

frequency 85% and

medium frequency 83%

and low frequency 0%.

It is different from 610

wave which has 85% high

frequency and the others

are 0%.

A person with

deuteranope will see

both the same color

because both will

activate high

frequency 85% and

the others 0%.


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Protanope

People with red blindness will not be able to see any wavelength larger

than 610, while normal people are able to see wavelengths reaching up

to 700.

Many people don't know they have color blindness until they test

themselves by chance.

To test color blindness you place colors that may be perceived as the

same color near each other in a chart. Some examples of color test

charts are shown below:

Any wavelength larger than

610 nm will be seen as black

since there is no absorption

of light by the any of the

photoreceptors.

Normal people will see the

number 74.

People with color

blindness will see the

number 21.


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To distinguish between red color blindness and green color blindness,

you use the chart below:

Test for hearing

Sound from outside, reaches the brain and is heard. If the sound

doesn't reach the brain and is not heard then there may be deafness

which is of 2 types:

Conductive deafness: improper conduction of sound from outside

to sensory organ

Sensorineural deafness: Can't convert the sound from vibration

to a neuronal signal or to transmit the neuronal signal to the

brain.

To test for hearing, you use Rinne and Weber test, which depend on the

fact that air conduction is better than bone conduction because there is

amplification of the sound in the middle ear; in bone conduction, there

is no amplification of sound in the middle ear. In normal people, the air

conduction is better than the bone conduction.

To perform the test, you use the tuning fork

Rinne test, you place the base of the tuning fork on the mastoid

process until the patient no longer hears the sound and then place

the tuning fork beside the ear. Normal subjects will hear the vibration

after bone conduction is over. In conduction deafness, air conduction

is not heard after bone conduction is over.

Normal people will see 42

People with green will see 4

People with red will see 2.


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You can use yourself as a control when it comes to air

conduction; if the patient stops hearing the vibration before

you do, you suspect that he has some kind of conductive

deafness and you do further tests.

Now if the patient hears the air conduction better than the

bone conduction, it is not necessarily normal and the patient

may have sensorineural deafness. To make sure that the

patient is normal, we perform Weber's test.

In Weber's test, you place the base of the tuning fork on the vertex of

the skull (you may place it on the forehead, but the vertex of the skull

is better) and the test is performed in a noisy room. Normal subjects

will hear the vibration in both ears equally. Patients with conductive

loss will hear the vibration better in the affected ear because there is

no masking noise. Patients with sensorineural deafness will hear the

vibration better in the normal ear.

I'm sorry for any mistakes

Wish you the best of luck in the exam☺

Your colleague Jumanah Nayef

"Turn your face

towards the sun, and

the shadows fall

behind you"

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