Exam I - Fixational Eye Movements

Fixation

eye movement whose function is to stabilize a target on the fovea, maintaining visual gaze on a single target. This eye movement is not steady. When the eyes are focused on a single image, small eye movements occur within 0.5 degrees of the fovea.

head movements, 0.75

The biggest challenge to fixation is ______. With an unrestrained head, head movements of _____ degrees may occur during attempted fixation. This head movement decreases gaze stability.

decreases

Head rotation (increase or decreases) gaze stability, but typically in amounts where retinal image motion is below threshold and does not adversely affect VA.

duration

Fixational area increases with fixation _____. Proposes a clinical implication for tests requiring prolonged fixation. Ie) retinoscopy on a child

directional

While fixation is typically central most of the time, the other portion of the time it shows ______ preference

Bivariate normal probability distribution

a three dimensional mathematical plot of fixational eye movements.

negatively

Fixation stability is (positively or negatively) affected by changes in gaze direction away from primary position

dark

Fixation stability is decreased in the ____ due to a lack of a fixation target

upward

gaze most restricted gaze in relation to age related changes in fixation

Midbrain oculomotor neurons

firing rate of these neurons is linearly related and proportional to gaze angle.

Pulse

an increase of innervation used to move the eye rapidly by some specific magnitude during a fixational eye movement

Step

constant innervation signal used to hold the eye at a particular position during a fixational eye movemen

NPH

MVN

nucleus of cajal

three Neurological Substrates of Gaze Holding Functions

Medial vestibular nucleus (MVN)

portion of the brainstem responsible for horizonal conjugate eye movements

Interstitial nucleus of Cajal

portion of the brainstem responsible for vertical conjugate eye movements.

Tremor

a high frequency, low amplitude movement occurring during fixation which is thought to represent the neural noise occurring in the oculomotor system. It occurs due to irregular firing of the brainstem motor neurons, resulting in random fluctuations in extraocular muscle fiber discharges. Has no adverse impact on vision.

brainstem motor neurons

tremor occurs due to irregular firing of the _____, resulting in random fluctuations in extraocular muscle fiber discharges. Has no adverse impact on vision.

inverse

There is an _____ relationship between tremor frequency and amplitude.

drift and microsaccades

Tremor is superimposed on....

Drift

a low frequency, slow velocity movement occurring during 95% of fixation time. Amplitude slightly increases when retinal errors are generated only from the near and far retinal periphery. Believed to be a result of neural noise. Largely error producing, but also error correcting at times.

95%

what percentage of fixation time does drift occur?

error producing

is drift error producing or correcting?

gaze, linear

Drift velocity in the dark is ____ dependent and increases as a ____ pattern

Time constant

the time required to reach 63% of the final intended amplitude. Is about 20 to 100 seconds for drift eye movements with a mean of 70 seconds during darkness.

Microsaccade

a high velocity eye movement occurring during fixation which has a large dynamic overshoot component opposite to the direction of eye displacement. It is always binocular with a high amplitude of correlation suggesting central neurological control. Is largely error correcting in response to drift.

error correcting

is a microsaccade error producing or correcting?

displacement from mean eye position, displacement

The probability of a microsaccade and its amplitude increases with _____ and occurs opposite to the direction of ____

Aberrant tremor

an abnormal fixational eye movement that is not detectable in clinical examination. Reflects the functional state of the brainstem and the degree of disturbance of consciousness.

Slow drift

an abnormal fixational eye movement commonly seen in patients with amblyopia or macula scotomas (patients using eccentric fixation). Not rapid enough to degrade VA

Saccadic intrusion

an abnormal fixational eye movement which is a large fixational saccade that intrudes one's attempt at accurate foveal fixation. Indicates a cerebellar abnormality or some other neural structure.

Square wave jerks

saccadic intrusion where the eye jerks away from the object of interest via a saccade. At about 200 msec later the eye returns back to its original position. Can be observe in patients having normal vision, but also in functional strabismus.

congenital nystagmus

square wave jerk can precede the development of...

square wave jerk

Can be transiently suppressed voluntarily in patients having functional strabismus Ie) during ophthalmic examination such as tonometry

Macro square wave jerks

variation of the square wave jerk but with movement that is slightly larger and with higher frequency, but eye is removed from the target for a shorter period of time.

100 msec

how long is the eye removed from the target during a macro square wave jerk?

Macrosaccadic oscillations

a sequence of saccades that is increasing then decreasing amplitude on either side of the fixation. Have 200 msec intersaccadic pauses. Commonly found in cerebellar disease.

Nystagmus

involuntary rhythmic oscillation of the eye which is usually associated with an ocular anomaly (ie strabismus). Can be congenital or acquired.

50%

what percent of patients with strabismus have nystagmus

15%

what percent of patients with congenital nystagmus have strabismus

oscillopsia

Acquired nystagmus will result in more complaints by the patient due to the presence of _____. Acquired nystagmus is commonly associated with myelin diseases, brainstem strokes, and monocular vision loss.

Null position

the direction of gaze for which the nystagmus intensity is the least. Corresponds to the period of foveal fixation where visual acuity is optimal. Head position of the patient will often be opposite to this position in order to position the eyes for a longer period of time this position.

Pendular nystagmus

nystagmus where the eyes move equally in both directions. Foveal fixation occurs at the peak of the waveform where eye velocity is slowest. Typically seen for congenital cases of nystagmus.

horizontal, albinism

Congenital pendular nystagmus only has ____ motion defects and is often associated with _____

horizontal and torsional

Acquired pendular nystagmus can have both ______ motion defects and is often associated with myelin disease, brainstem strokes, and monocular vision loss

pendular nystagmus and jerk nystagmus

Congenital and acquired ______ respond to eye movement auditory biofeedback therapy.

Jerk nystagmus

nystagmus where there is a slow phase in one direction and a rapid phase in the opposite direction. Foveal fixation occurs immediately after the rapid saccade.

Slow phase

phase of jerk nystagmus where the eye moves away from the object of regard

Congenital jerk nystagmus

jerk nystagmus present at birth. Visual acuity may be affected due to abnormal visual experience throughout development. Treatments include yoked prism, eye muscle surgery, base out prisms to increase convergence, orthoptic contact lenses, and auditory biofeedback.

Convergence

decreases congenital nystagmus

Gaze evoked jerk nystagmus

jerk nystagmus where the velocity of the slow phase wave form is decreasing instead of increasing. Found in patients using drugs, having cerebellar or vestibular problems, or having MS.

1-2 degrees

what is the amplitude of gaze evoked jerk nystagmus?

1-3 Hz

what is the frequency of gaze evoked jerk nystagmus?

Latent nystagmus

a benign form of congenital jerk nystagmus where the patient has normal vision and a nystagmus that appears only when one eye is covered. Slow phase will occur towards the occluded eye, and the fast phase will occur in the opposite direction.

strabismus

Latent nystagmus always occurs with....

Vestibular Jerk nystagmus

jerk nystagmus with central or peripheral vestibular origin characterized by a linear or constant velocity slow phase that moves the eye away from the object of regard. Aspects of this nystagmus type vary widely.

Peripheral

_____ vestibular jerk nystagmus usually affects horizontal eye movements

Central

____ vestibular jerk nystagmus usually affects vertical eye movements

peripheral

Fixation suppresses ______ vestibular jerk nystagmus

head position

Both central and peripheral vestibular jerk nystagmus are exacerbated by changes in...

Alexander's law

law stating that vestibular jerk nystagmus will increase when the patient gazes in the direction of the nystagmus saccades.

microsaccade

which normal fixational eye movement correlates between two eyes?

tremor

which normal fixational eye movement is high frequency but low amplitude?

drift

which normal fixational eye movement is low frequency and slow velocity?

microsaccade

which normal fixational eye movement is high velocity?

30-100 Hz

the frequency value for a tremor eye movement

<0.5 Hz

the frequency value for a drift eye movement

1-2 Hz

the frequency value for a microsaccade eye movement

20 sec (avg, 5-30 sec range)

the amplitude value for a tremor eye movement

1-5 min arc

the amplitude value for a drift eye movement

5 min arc (1-25 min arc range)

the amplitude value for a microsaccade eye movement

30 min arc/sec

the velocity value for a tremor eye movement

5 min arc/sec (1-8 min arc/sec range)

the velocity value for a drift eye movement

1-20 degrees/sec

the velocity value for a microsaccade eye movement

microsaccade

what is the only fixational eye movement that is correlated between the two eyes?