VS 117
Tuesday, January 20, 1998
Dr. Schor
COOPS: Heidi Fahringer
Syllabus review
Course Overview

The course is split into two portions:

I. Accommodation and Convergence
Accommodation , Focusing, Convergence and their interactions
The simultaneous focusing (accommodation) and the binocular eye alignment process (convergence) is called the near response. To see things at a near viewing distance one focuses and converges his eyes to see singly and clearly. Of all the health professions, ours is the one that knows most about this subject. A synergistic interaction (performing one motor response causes another motor response to occur) occurs between accommodation and convergence. When the eyes accommodate they automatically converge. When the eyes converge, they automatically accommodate. This interaction helps us to see singly and clearly but also can cause problems. If farsighted, focus excessively to see near, can cause the eyes to turn in too much = cross-eyed = esotropia = strabismus, due to an abnormally high relationship between these two mechanisms. When a child has an eye turn, one eye tends to be favored. The eye not favored is suppressed, turned off, so the child doesn’t see double. Vision is not fully developed at birth. With one eye is turned off, development is not completed - has immature sight. This reduction in acuity is called amblyopia. Amblyopia can be prevented by spotting the risk factors (strabismus or the potential for strabismus such as farsightedness is the optometrist’s job). Optometrists also evaluate reading disorders; headaches (may be corrected by using lenses and prisms) and ergonomics. Computer displays and computer usage cause visual stress, workers take few breaks. Virtual reality devices have head mounted displays which cause convergence and divergence but not a corresponding change in focus. A mismatch between convergence at near and focus at near occurs causing headaches and discomfort and complaints.
The aging process of accommodation (= presbyopia caused by lens becoming brittle with age) leads to bifocal usage in the early to mid forties depending upon the amount of near work we do. Accommodation is diminishing every day. It drops by a third of a diopter every year and eventually we are unable to focus at a working distance.
Clinical methods for analyzing the interactions
Gain conceptual knowledge in this class

II. Neurology
A new subject in optometry has come about due to our new partner in the health profession, the insurance company. To minimize costs, all patients must go through a general practitioner, also called the gatekeeper, to screen people to determine if a specialist such as a neurologist, radiologist, surgeon or optometrist is necessary. Optometry decided to become a gatekeeper to insure referrals. Consequently, we need to know more about general health disorders: systemic, neurologic, and metabolic illness. Ocular motility is a way of conducting a neurological examination. Is the cerebellum, motor cortex, and brainstem intact? An oculomotor examination may pinpoint where a problem is in the brain based upon knowledge of ocular motor pathways and eye movement disturbances. It is analogous to an ophthalmoscope examination which may reveal blood in the eye indicating a systemic problem such as diabetes or an aneurysm, or other systemic disorders.

In this course we will learn about the pathways for eye movements and their characteristics and how to distinguish normal and abnormal eye movements.
Four areas of abnormalities disorder
1. stability, steadiness nystagmus
2. restriction of eye movement palsy or paresis
3. eye turns strabismus
4. rapid eye movements inaccurate saccades
Is the problem life threatening or benign? referral or counseling?

Categories of Neurological Disorders
1. acquired: recently acquired, dangerous
2. congenital: albino nystagmus
3. developmental: amblyopia develops due to strabismus or anisometropia. easy to predict by risk factors. Screen for risk factors to easily prevent. Its easy to prevent and difficult to treat.
The same disorders with a different history determines action to be taken.
Neurologic disorders are rare and subtle, and were unlikely to see them in clinic. With a gross neurologcial disorder a patient is more likely to self refer and see a physician. An optometrist will mainly see very early stages of very subtle signs and symptoms such as ocular flutter which could indicate a serious problem such as a cerebellar tumor. Legally we are responsible to recognize these problems.

Taxonomy Categorization or description of eye position and movement (Chapters 1 and 2)
FUNCTIONAL CATEGORIES
1. Stabilization reflexes: These are postural reflexes that are automatic. They operate at a subconscious level similar to knee jerk response. Their purpose is to allow us to see stationary objects better while we walk or move around. As we move the gaze direction of the eyes tends to remain stationary in space.
This happens due to the vestibular ocular reflex (VOR): tip head to the side, activates three sets of fluid filled canals, the inertia of the fluid in the canal lags behind the movement of the head, tends to press on hair cell sensors at the end of the canal, The motion of the hair cell causes a bipolar cell to depolarize, and it sends an action potential to the brain (vestibular nuclei) causing the eyes to move opposite the head rotation direction. The VOR responds to all directions of head rotation, even torsional (roll about the visual axis) yaw (rotation about a vertical axis) and pitch (rotation about the interaural axis). Pitching forward causes the eye to elevate and depress called the Doll’s head reflex. The eyes stay steady in space while the head moves up & down.
Optokinetic nystagmus (OKN): visual reflex following response includes a slow following and a quick snap back (saccadic reset) example: following BART train travel out of the station. Sets up a jerk nystagmus including a ramp portion called the slow phase and a saccadic movement called the fast phase. It is easier to see the eyes reset than to see the eyes follow. Clinically we describe this movement by the direction of the fast phase. A quick jerk to the right is called a rightward jerk nystagmus. OKN allows for stabilization of slow changes to the retinal image that occur with body sway. It keeps eyes locked in a constant direction of gaze. If moving at a constant velocity, the OKN is working. VOR only works during acceleration. Graph of left-right eye movements vs time illustrating slow phase and fast phase:

Demonstration of VOR and OKN:
1) shake head left to right, eyes jerk
2) follow rotation of striped drum, eyes jerk at a rate of about 3 times/sec
rotation to the left, to the right, up and down
3) rotation of a pattern causes eyes to spin, about its visual axis (torsion movement).
See blood vessels moving
Easy to observe quick phase, difficult to observe slow phase.
The optokinetic system attempts to stabilize the eye for all degrees of freedom: laterally, vertically, and torsionally. The optokinetic reflex works for any motion of the body. In torsion, the eye is twisting around the visual axis. It works for spinning and rolling. Works when objects pass by the front of your eye, laterally and vertically. Large field optokinetic stimuli give you a sense of body movement when there is no movement. Sitting in traffic, adjacent car moves forward, you feel like you are rolling back. The visual system is tricked = vection = self-motion.
Stabilization reflex is one class of eye movement, very reflexive like a knee jerk reflex, found in every vertebrate, the most fundamental oculomotor response, allows locomotion through space with ability to see. A lot of animals don’t have voluntary eye movements like rabbits. Have only stabilization reflexes because that’s all they need to survive. The oculomotor reflex initially developed to stabilize the eye. Then the fovea developed. Fovea requires not only stabilization of the retinal image of a stationary world, but it also requires that we be able to track or follow small moving targets with respect to the background. Stabilization reflexes will not work for this task because the small object doesn’t occupy much of the retina. Most of the retinal image is the background and the background doesn't move. The small object will not trigger OKN. Once the fovea developed, another eye movement system had to be developed involving tracking of small targets independent of the background.

2. Tracking 2 developments to track
1) pursuit: allows the fovea to follow small slow continuous movements
2) saccade: the term is derived from the French word meaning rapid twist of the horse’s head. Bridling effect of pulling the eye to one side by tugging on one muscle and relaxing the other. Allows for a rapid shift of gaze from one location to another, to place the fovea on one target and then another. Unnecessary for a rabbit due to the lack of a fovea, all parts of the retina see equally well.

Demonstrations:
1) pursuit: follow pencil with eyes - up, down, left, right - results in a smooth movement. Cannot pursue torsionally. Rabbits cannot pursue. Attempt to slowly move eyes from left to right: voluntary without a moving visual stimulus and pursuit is not possible. Instead you observe small jerky movements (cog-wheel pursuits). A visual stimulus is necessary. A feedback system uses the motion of the retinal image to guide the slow movement.
2) saccade: two pencils as targets - move eyes left and right- eyes rotate at about 1000° per second. Both horizontal and vertical. Upper eyelid exaggerates visibility of vertical movements, moves more than the eye. Eyelid movement is used clinically during cover paddle test to diagnose strabismus, vertical nystagmus.
3. Fixational Eye Movements
Demonstration: attempted fixation: tiny quick, saccades, involuntary eye movements occur a few times per second. Eye is always drifting around and making Fixational eye movements even if attempting to hold eye still.

Summary of three classes of eye movements:
1) Stabilization
2) Tracking
3) Fixation

ANATOMICAL CATEGORIES
1. Fovea present or not. If present, have all three categories of eye movements. If not, only stabilization reflex is present.
2. Placement of the eye in the head:
lateral forward
no fovea fovea
grazing animals predators
stabilization have all three functions: stabilization, tracking, fixation
panoramic vision lacks panoramic vision - eye and head movements extend visual field
detection only stereopsis
mostly eat allows for tool usage, prey capture

QUANTITATIVE ASPECTS (handout #2)
The eye doesn’t move (translation) but it does rotate (=turn), is a ball & socket, rotates around a single point called the center of rotation. No physical socket supports the eye, supportive tissue acts like a socket.
Centrode refers to the range of movement (translation) of the center of rotation. It may shift by as much as 1 mm.
BALL & SOCKET ROTATIONAL REFERENCES)
Primary direction of gaze - describes the initial position of the eye from which all other positions are referenced. It is an arbitrary reference point.
Visual axis (=line of sight) is directed straight ahead in primary gaze, and the two eyes are parallel in terms of their visual axes, head is erect, and visual axes are parallel to the ground. Zero rotation.
Secondary direction: straight up, pure vertical movement or pure lateral movement
Tertiary direction: combination of vertical and horizontal movements
Strabismus = misalignment of the two eyes. One eye is turned away from the primary position while the other is not.

CARTESIAN DESCRIPTION OF GAZE DIRECTION FROM PRIMARY POSITION
I. Horizontal and vertical components
A. Monocular eye position - duction
adduction - turned toward the nose
abduction - towards ear
supraduction - upward eye movements
infraduction - downward eye movements

B. Binocular eye position

1. Conjunctive or conjugate or yoked
Hering’s Law: Hering observed synchronized movement of the eyes, as if they were yoked symmetrical, conjugate.
Yoke = version
dextroversion - right
levoversion - left
supraversion - up
infraversion - down

2. Disjunctive movement of eyes in opposite directions = vergence
Convergence and divergence also obey Hering's law because the eyes rotate symmetrically.
Demonstrations:
1) near and far jerk
2) move pencil from far to near - smooth movement, pursuit
3) monocular - only the left eye can see the pencil but the right eye converges also, illustrates the accommodative interaction between focusing and convergence
horizontal: convergence & divergence
vertical: the neurological term is skew movement, hyperskew (= elevation of one eye relative to the other) or hyposkew . Optometrists refer to hyperdeviation or hypodeviation. A strabismus, one of the oblique muscles, causes the eye to go up or down. Damage to the superior oblique causes the eye to be up, a hyperdeviation.

II. Torsional or screw movements- spins around the visual axis, screw movement. Anomalies, for example, superior oblique palsy is associated with an outward torsional rotation of one eye (extorsion). Described by the position of the top of the vertical meridian

A. Monocular
extorsion - rotated out toward the ear, outward tip
intorsion - tipped inward, toward the nose

B. Binocular
1. Conjugate
levotorsion - both eyes to the left
dextrotorsion - both eyes to the right, happens with head roll to the left shoulder to keep the vertical meridian of the retina vertical in space. Eyes twist as much as 10°, roll head 60 °, eyes counter-roll about 10 °.
2. Disconjugate
binocular vergence component
incyclovergence upper meridian forms an "A" frame, intrusion, both in, upward gaze
excyclovergence move outward like a "V", extorsion, both out, downward gaze
difficult to measure clinically, so concentrate on vertical and horizontal components

C. Velocity Categories
1902 Raymond Dodge first to quantify eye velocity of eye movements, used a photographic technique
Saccades: fast velocity: rotational velocities of >50°/sec, small fixation flicks/smallest saccades possible
searching velocity are about 15 ° in amplitude, 100°/sec to 1000°/sec
Pursuits: slow velocity saccades, slow phase of nystagmus
The neural circuitry to control saccades is different from the circuitry controlling the slow following eye movements. There are two brain strategies to rotate the eye, one for quick, one for slow smooth.

D. Magnitude Categories not strict quantified categories
small 2.5° fixational , barely detectable, 1-2 ° is threshold for detection
medium 2.5° to 15° most common
large >15°
Eye movements rarely exceed 15°
Greater eye movements require head movement. As people age they tend to move their heads more than their eyes. High index glass spectacles have tremendous aberrations in the periphery, and people wearing them tend to move their head more than their eyes. Head movements can be disorienting. Important for driving and sports.