Introduction to Visual Acuity and Neuro Ophthalmlogy(2).doc

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Introduction to Visual Acuity and Neuro-Ophthalmology

Introduction to Visual Acuity:

Visual acuity is a measurement of a patient’s ability to see letters, objects, etc.

It is tested one eye at a time with several different methods depending on the
ability of the patient to communicate with the tester.
Usually visual acuity is denoted in the form of 20/(another number), which may
be interpreted as follows:

20/20 is considered “perfect” vision.
The bottom number is a comparison between the patient you are testing and a
person with perfect vision. For example:

20/40 vision means that the person with 20/20 (perfect) vision can see an
object from 40 feet away that a person with 20/40 vision can only see from 20
feet. The larger the bottom number the larger the letters must be for the
patient to see them.

The following is a list of different methods used to test visual acuity:

1. Snellen letter chart
2. Tumbling E’s
3. Pictures
4. Central, Steady, Maintained
5. Fix and follow
6. Reacts to light or threat

Let’s discuss each method including when and how they are to be
used:

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Methods:

⮚ Snellen letter chart:

The Snellen letter chart consists of letters of increasingly smaller size.
The size of the letters is important and the chart must be calibrated
appropriately to accurately measure the vision. The 20/400 E
subtends an angle of 5 minutes at any distance and it is the largest
letter on most Snellen charts. If the chart is not properly calibrated
for the distance the patient will be sitting from the chart then the
measurement will not be accurate as the letters on the chart must be
the same size at a distance of 20 feet for all patients. Patients who
will be sitting closer than 20 feet to the chart will need to see smaller
letters than those who sit at 20 feet. In general the exam room will be
set up with mirrors to create the effect that the patient is sitting 20
feet from the chart and the size of the letters will be calibrated based
on this effect.

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Each block on the chart above represents 1 minute, the whole E is comprised of 5 blocks X5
blocks with each part of it subtending 1 minute and each leg of the E is separated by 1 minute.

⮚ The chart then has 20/200, 20/100, 20/80, 20/70, 20/60, 20/50, 20/40,
20/30, 20/25, 20/20, and 20/15 lines which are increasingly smaller in
size. Each line usually has the corresponding bottom number next to the
letters. You usually start measuring visual acuity in the right eye (with
the left eye covered with an occluder.) Make sure that the left eye is in
fact completely covered so that the patient is not peeking around the
occluder. Show the patient the lines until they can no longer read the
letters on a particular line. You would record the vision as the last line
they read. For example: if the patient reads the 20/25 line you would
record the vision as 20/25. If the patient was able to read only 2 of the 4
letters on the 20/40 line you may record the vision as 20/40-2. If the
patient could read the 20/30 line and one letter on the 20/25 line you
could record the vision as 20/30+1. If the patient is unable to read
better than 20/40 you should pinhole them and see if they can see any
better. To pinhole slide the pinhole apparatus on the occluder over the
opening and instruct the patient to move the occluder around until they
can see through one of the holes. Have them read you the letters on the
chart and record this vision as PH 20/xx. PH stands for pinhole and xx for
the number corresponding to the smallest line on the visual acuity chart
that they can read, if they are not able to see any better than without the
pinhole record: PH NI (for no improvement).
When measuring visual acuity with this chart and you find that the
patient is unable to tell you what the 20/400 letter is it is important to
find out if the patient can actually read. You may go about this by
asking the patient if he or she reads often. If you find that the patient
cannot read or you are unsure, you should use the tumbling E’s chart.
If the patient can in fact read but cannot see the 20/400 E hold up
several fingers in front of the patient at a distance and keep changing
the number of fingers while moving them farther away. Once the
patient can no longer see your fingers you record the visual acuity as
CF @ _ft. (for count fingers; fill in the blank with the farthest number
of feet that you could hold your fingers up with the patient still being
able to see them). If the patient is unable to count your fingers right
in front of him/her, move your hand around and see if they can see the
movement. If they can see your hand moving, record the visual acuity
as HM (for hand motion). If they are unable to see the movement of
your hand, shine a light in the eye and see if they can detect the light
(turn the light on and off and ask the patient to tell you what is
happening or to tell you when they see the light go on or off. If the
patient can see the light record the visual acuity as LP (light

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perception). If the patient is totally unable to see any light
whatsoever record the visual acuity as NLP (no light perception).

⮚ Tumbling E’s chart: This chart is used on patients who cannot read or

who do not speak a language that consists of the Alphabet (for example
Japanese or Chinese). This chart also has letters (all E’s in capital form),
which subtend an angle of 5 minutes, and the size of the letters is
important for accurate measurements. The method of testing is the same
as with the Snellen chart: you start with the right eye (left eye covered)
and you ask the patient to tell you which way the legs on the E point,
they may show you this with their hands. You will record the last line
that they correctly identified as their visual acuity. As with the Snellen
chart you may use + or – if they are able to identify a few on one line but
not the whole line.

⮚ Pictures chart: This chart is primarily used for children who can talk and

will tell you what the pictures are. Vision is tested the same way as with
the Snellen letter chart and the tumbling E’s chart. The bottom number
of the visual acuity is printed on each line as with the other charts. There
are cards with the pictures on them that you can use to go over the
pictures with the patient prior to checking vision. This way you will know
what words the child uses to identify each picture. For example- it is not
necessarily important that the patient tells you the picture is a horse-
they may identify it as a cow or dog. As long as you know what that
patient is going to call that picture then you are able to check her vision.
Then each time the horse picture is on the screen and your patient says
“dog” you will know they are seeing the picture that they identified to you
as a dog.

⮚ Central, Steady, Maintained (CSM): This method is used for nonverbal

patients (Children and adults who cannot or will not communicate with
you). The procedure is as follows: Cover the left eye and shine a light
at the patient while watching the right eye to see if the light reflex is in
the center of the pupil (if it is it is considered to be central). Watch the
right eye now to see if it stays still or if it shakes or oscillates (if it stays
still it is considered to be steady; if there is an oscillation or shaking the
patient probably has nystagmus). Now take the cover off the left eye
while still shining the light at the patient and see if the light reflex stays
in the center of the right eye (if it does it is considered to be maintained;
if the patient’s eye shifts and the reflex is no longer in the center it is
unmaintained). Now repeat this process on the left eye. If the visual
acuity is central, steady, and maintained in each eye then you record it as
CSM for each eye and you can assume the vision is roughly equal in both
eyes. If one of the three parts is not normal (not central, not steady, or
not maintained) then you would record that part as follows: UC

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(uncentral), US (unsteady), or UM (unmaintained). This method of
documenting vision can look like any of these: CSM, CUSM (central,
unsteady, maintained), CSUM (central, steady, unmaintained), or UCSM
(uncentral, steady, maintained) or it may have more than one element
that has the “U” in front of it.

⮚ Fix and Follow (F&F): This method tests both eyes at the same time.

This method is used on patients who are not able to sit still enough to
look at your light for CSM. This will primarily be used on young babies.
Hold an object that will attract the patient’s attention, (possibly use one
of the baby’s toys), and slowly move it watching to see if the patient’s
eyes follow it. You can also check versions on the patient while doing this
test to save time. If the patient is able to fixate on an object and follow it
with his/her eyes then you record the visual acuity as fixes and follows
(or f & f) ou. If the patient will not fixate on an object it is important to
make sure that it is due to lack of vision and not lack of interest. Babies
may not be interested in the toy you are using so you may have to try
several different objects. Often a bright light such as the muscle light or
direct ophthalmoscope can be used to get the child’s attention. Putting
your finger over the light is fascinating to many babies as they like to
watch your finger glow. You can even ask the baby to do it too and that
will keep his attention while you evaluate his eyes.

⮚ Reacts to light or threat: This method is used on very young babies who

cannot yet fixate and follow an object. Shine a light in the patient’s eyes
and see if they turn away or shield their eyes or close their eyes. If you
notice a reaction that is directly related to the light then record the visual
acuity as reacts to light (RTL). For patients who you believe are
malingering (faking not being able to see) you may flick your fingers in
front of the patient’s eyes and see if they flinch. If the patient responds
record the visual acuity as reacts to threat (RTThreat). Another version
of this is the patient objects to cover of one eye more so than the other.
You may be trying to do CSM (central, steady, maintained) visual acuity
checking and notice that when you cover the right eye the patient looks
at your target light without problems. However, when you cover the left
eye the patient moves his head to get away from your cover. This could
be denoted as “objects to cover OS.” This would indicate that there is
poorer vision in the right eye (as that is the eye being used when OS is
covered) than the left eye.

Examples:

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Now that you have read about these methods lets go over some
examples.

1. You call back a 35year old female to work her up for the doctor:
You will start with the Snellen letter chart. She tells you that she
is not able to see the big E. What should you do? Question her
to find out if she can in fact read. For example ask her, “Do you
see ok with your glasses to read?” She responds that she
actually has had trouble lately reading books and that she loves
to read. Assume then that she can read and hold up your
fingers in front of her face. She correctly identifies that you
have 2 fingers up so you move your hand back about 1 foot and
she tells you correctly that you have 1 finger up. Again move
your hand back 1 foot and change the number of fingers you are
holding up. She tells you that she thinks that you have 4 fingers
up (you have 2). Record her vision as CF @ 2 ft.

2. Your next patient is a 1month old baby whom the parents are
saying can not see well. She doesn’t seem to recognize their
faces. You attempt to have her fixate on a small toy but she
won’t. What should you do? Ask her parents if they have a toy
of hers that she likes to play with. If they do, use that, if not
try another toy of yours. You find that you are unable to get her
to fixate on any toy, what do you do next? Try shining a light in
her eyes- when you do this she cries and closes her eyes. You
can record the vision as RTL and it is also a good idea to notate
in the chart that you tried but were unable to do F&F.

3. The next patient you take back is a mentally challenged 7 year
old. The first thing you should do is ask his parents if he knows
his letters. They tell you no, so you should try the tumbling E’s
chart. He does not understand what you want him to do when
you ask him to show you which way the E’s are pointing. You
should then use the pictures chart.

4. Your next patient is a 15-year-old female who correctly identifies
3 of the 5 letters on the 20/30 line using the Snellen chart. You
would record her vision as 20/30-2.

5. The next patient is a 40-year-old male who tells you he has
been in a car accident and he cannot see out of his right eye at
all. You try the Snellen chart first, and he says he cannot see
the big E. You try count fingers and he says he can not see your
hand when it is right in front of his face so you wave your hand

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back and forth in front of him and he says he still can not see
anything. You shine a light in his eye but he tells you that he
cannot see anything, however you notice a slight closing of his
eyelid when the light is shone directly in his eye. Try flicking
your fingers right in front of his right eye to see if he flinches, he
blinks and moves slightly back. You know that he can see
something or he would not have reacted to the light or your
fingers. Note in the chart that the patient responded to light and
to threat.

Introduction to Neuro-Ophthalmology:

Introduction:

Neuro-ophthalmology deals with the brain and the nerve
pathways from the brain to the eye.

The system works in basically this way: The eye detects
light and this stimulus is carried away from the eye to the
brain by way of an afferent nerve pathway. In the brain
the stimulus is analyzed and then a message is sent to
certain muscles (depending on what action the brain wants
the eye to do) by way of an efferent nerve pathway.

The input of the stimulus to the brain is carried by the
afferent pathway. The output from the brain is carried by
the efferent pathway.

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The neuron is the basic unit of the nervous system and it
carries messages to and from the brain. There are afferent
nerves which carry messages to the brain or central
nervous system from an organ. There are efferent nerves
which carry messages from the brain or central nervous
system to an organ.

The neuron is composed of the following parts:

Dendrite- the first part of the neuron to receive the
impulse, this receives the impulses from the neighboring
neuron
Cell body- the impulse travels through the cell body after it
has been received by the dendrite
Axon- this carries the impulse away from the cell body
toward the synapse.
Synapse- At the end of the axon there is a space between
the axon and the neighboring nerve’s dendrites. This space
is the synapse. The nerve impulse must “jump” across the
synapse.

See the diagram below for more information on the parts of
the nerve we just discussed:

Nerve Impulse Conduction:

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Nerve impulses must travel from the dendrites to the cell
body then down the axon and across the synapse to the
next nerve cell. When a stimulus excites an axon, it
causes a chemical reaction which then causes the signal to
continue down the axon toward the synapse. At the
synapse a neurotransmitter is produced and it travels
across the synapse to the next neuron where it causes the
signal to continue along that neuron.

See this diagram which shows how an impulse travels:

The activity of one neuron affects the next neuron. Actions can
either cause the impulse to travel on or to stop. These actions are
described here:

Excitatory actions- this type of action by one neuron causes the
impulse to continue to travel on to the next neuron.

Inhibitory actions- this type of action by one neuron causes the
impulse to stop- it will not be conducted on to the next neuron.

For an impulse to travel from the presynaptic neuron across the
synaptic cleft and on to the postsynaptic neuron a certain chemical
(which is called a neurotransmitter) must be released and travel
across the synapse. Neurotransmitters are discussed below.

Neurotransmitters

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The autonomic nervous system contains both the sympathetic
and parasympathetic systems. Each of these systems has
their own neurotransmitters.
Sympathetic system- This system is activated in fright or
flight situations. This system controls dilation of the pupil.

Parasympathetic system- This system is activated in more
relaxed situations. This system controls constriction of the
pupil.

Both the sympathetic and parasympathetic systems send
messages along their neurons by way of neurotransmitters-
chemicals which take a nerve impulse from one neuron to the
next. These are represented by the yellow circles with black
dots in the diagram of the nerve impulse conduction above.

2 neurotransmitters in the autonomic nervous
system are:

Acetylcholine- A short acting transmitter that works mainly
on parasympathetic nerve pathways. This is abbreviated as
ACH

Norepinephrine- A longer acting transmitter that works
mainly on the sympathetic nerve pathways.

Enzymes are also present to break up the neurotransmitters.

The enzymes that break down the neurotransmitters above are listed
here:

Cholinesterase- breaks down acetylcholine.

Catechol-o-methyl transferase (COMT)- breaks down
norepinephrine.

Monamine oxidase (MAO)- breaks down norepinephrine.

When a signal is carried down the axon of a neuron and a
neurotransmitter is released into the synapse, the signal can
either be stopped by one of the enzymes listed above or sent on.

The Central Nervous System

The central nervous system contains the brain (the cerebrum
and brainstem) and the spinal cord.

All of these parts are protected and supported by:

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1. Meninges- Layers of tissue which cover the central
nervous system parts. There are 3 layers that make up
the meninges:
a. Dura mater
b. Arachnoid
c. Pia mater

2. Cerebral spinal fluid- a fluid that acts as a cushioning
for the brain and protects it from quick back and forth
motions.

3. Bony skull

The Cerebrum:

The function of the cerebrum is to gather afferent nerve impulses and
to send efferent messages to body systems in order to control motor
function and to store memories.

It is also the origin for the supranuclear pathways- which are pathways
that connect the 2 eyes together. These are discussed in detail in
Advanced Neuro-Ophthalmology II.

The Brainstem:

The brainstem connects the cerebrum to the spinal cord. It is
composed of 3 parts:

1. Midbrain
2. Pons
3. Medulla Oblongata

Cranial Nerves

There are 12 cranial nerves which start in the central nervous system
(they have their nuclei in the central nervous system) and travel
throughout the body to their destinations. They may be either sensory
nerves (control one of the 5 senses) or motor nerves (control muscle
movement). They are listed in order here with their cranial nerve
number and name:

1. Olfactory
2. Optic
3. Oculomotor
4. Trochlear
5. Trigeminal
6. Abducens
7. Facial

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8. Acoustic
9. Glossopharyngeal
10. Vagus
11. Spinal Accessory
12. Hypoglossal

Ophthalmology concerns itself mainly with the following
cranial nerves: 2nd, 3rd, 4th, 5th, 6th, and 7th. These are
discussed below. A helpful mnemonic to use to remember the
names of these nerves is: oh oh to touch a feather. The first
letter of each word in that sentence corresponds to the first
letter for the nerve.

Optic Nerve (2nd cranial nerve):

The optic nerve is not a true cranial nerve in that it is actually
composed of axons only in the central nervous system. Its
cell bodies are the ganglion cells in the retina so they are
found in the eye.

Oculomotor Nerve (3rd cranial nerve):

The Oculomotor nerve originates in the midbrain. It is responsible for
innervating the Superior Rectus, the Levator, the Medial Rectus,
Inferior Rectus, and the Inferior Oblique muscles as well as the
pupillary sphincter. The 3rd nerve splits into an inferior and superior
division as it enters the orbit through the superior orbital fissure.

Trochlear Nerve (4th cranial nerve):

The Trochlear nerve which is the 4th cranial nerve originates near the
bottom of the midbrain. It innervates the Superior Oblique muscle.

Trigeminal Nerve (5th cranial nerve):

The Trigeminal nerve which is the 5th cranial nerve originates in the
Pons. It splits into 3 divisions:
● V1 (this is also known as the ophthalmic division).
● V2 (this is also known as the maxillary division).
● V3 (this is also known as the mandibular division).

Abducens Nerve (6th cranial nerve):

The Abducens nerve which is the 6th cranial nerver originates in the
Pons. This nerve innervates the Lateral Rectus muscle.

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Facial Nerve (7th cranial nerve):

The Facial nerve which is the 7th cranial nerve originates in the Pons.
It innervates muscles which control facial expression (which involves
lid closure), salivary glands, taste, and the anterior 2/3 of the tongue.

Eye movement Systems:

There are 5 different eye movement systems whose goal together is to
position an object on both foveas and to keep it there. We will discuss
the 2 major ones called saccades and pursuits.

Saccades:

Saccades are defined as fast, voluntary, accurate movement of both
eyes together to focus on an object. This involves innervating different
muscles to move the eyes to the same gaze position.

Pursuits:

Pursuits are defined as slow smooth involuntary following of an object.
The eyes will automatically move to follow an object of regard to keep
it on the fovea.

NOTE: Saccades and Pursuits are independent of each other.

Nerve Nuclei Pathways:

There are 2 pathways in the brainstem that connect the 3rd, 4th, and 6th
nerve nuclei. One is the Medial Longitudinal Fasciculus (or MLF)
and the other is the Paramedian Pontine Reticular Formation (or
PPRF). These pathways are used to help the nerves “talk” to each
other when saccades and pursuits are needed.

Palsies:

A palsy is basically the inability of a muscle to function. A palsy may
be Neurogenic (the nerve is at fault) or Myogenic (the muscle is at
fault).
If the palsy is Neurogenic, it may be from a lesion at the nucleus or a
lesion located somewhere along the pathway of the nerve from the
nucleus to the muscle. Here we will discuss the 3rd, 4th, and 6th nerves
in regards to palsies.

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3rd nerve: Since the 3rd nerve innervates several different muscles

and has several different “divisions” it is easier to localize where a
lesion might be along its pathway. A 3rd nerve palsy may cause some
or all of the following symptoms:

● Eye on the affected side would have exo deviation (be
turned out)

● Eye on the affected side would have decreased ability to
look down (inferior rectus affected)

● Bilateral Ptosis (Levator affected)
● Eye on the affected side would have dilated pupil

(pupillary sphincter affected)
● A 3rd nerve palsy is most commonly caused by Diabetes

or aneurysm

4th nerve: The 4th nerve innervates only the Superior

Oblique muscle. It is not possible to localize a lesion along the
pathway of the 4th nerve since it has no divisions and
innervates only one muscle. A 4th nerve palsy would cause
the following:

● Diplopia

● Often a 4th nerve palsy is due to trauma.

6th nerve: The 6th nerve innervates the Lateral rectus. It is

not possible to localize a lesion along the pathway of the 6th
nerve since it has no divisions and innervates only one
muscle. A 6th nerve palsy would cause the following:

● Diplopia (especially in the gaze toward the side of the
affected eye, possibly not noticed in primary gaze.)

● No abduction of the affected eye
● Often a 6th nerve palsy is due to a tumor in the Pons.

Lesions:

Lesions in different areas will cause different types of
problems. Listed here are some lesions and what you might
see with each.

Supranuclear Lesion: (Lesions that involve more than one
nerve nucleus). These cause:

● Gaze palsies- inability to look to left gaze or right gaze
or up gaze or down gaze

● May affect saccades and pursuits

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Infranuclear Lesion: (Lesions that involve the nerve
pathway or the muscle). These types of lesions cause:
● Diplopia in different gaze positions
● May be due to a lesion on the nerve or in the muscle

Frontal Lobe Lesions: (Lesions found in the frontal lobe of
the brain). These types of lesions will cause:
● Defect in saccades to the side opposite the lesion because

the saccades pathway starts in the frontal lobe

Occipital Lobe Lesion: (Lesions found in the occipital lobe
<back> of the brain). These types of lesions will cause:
● Defect in pursuits to the same side as the lesion
● Vertical visual field defect

This is the end of Introduction to Visual Acuity and
Neuro-Ophthalmology.
You are now ready to take the post test Successful completion of
the post test is required to earn JCAHPO credits.

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