EM 3 Fixational EM

Fixational EM

What are fixational eye movements and their main characteristic?

• Micro/miniature eye movements during fixation

• Eyes are never perfectly still even when fixating

• Maintain visual perception during steady gaze

Why are fixational eye movements clinically and experimentally important?

• Help distinguish normal vs abnormal eye movement patterns

• Critical for tasks requiring stable fixation

• Affect accuracy of ocular measurements/instrumentation

In what clinical/imaging contexts are fixational eye movements important?

• Visual field testing

• Corneal topography

• LASIK procedures

• OCT (optical coherence tomography)
  → Precise fixation needed for accurate results

What are the three types of fixational eye movements and their general significance?

• Tremor

• Drift

• Microsaccades

These occur during attempted fixation because the eyes are never perfectly still.

What are the characteristics of tremor in fixational eye movements?

• High frequency: 30–100 Hz

• Very small amplitude: 5–30 arcseconds (30 arcseconds = 1/120 of a degree)

• Generally considered noise

What are the characteristics of drift in fixational eye movements?

• Slow movement during fixation

• Velocity: 1–8 arcmin/sec (~5 arcmin/sec mean)

• Amplitude: ~2–5 arcmin

• Probably noise, but may help with error correction

What are the characteristics of microsaccades in fixational eye movements?

• Occur 1–2 times per second

• Mean amplitude: ~5 arcmin

• Range: 1–25 arcmin

• Rarely >10 arcmin

• Usually error-correcting

Which fixational eye movements are mainly considered noise vs error-correcting?

• Tremor: mostly noise

• Drift: probably noise, but may be error-correcting

• Microsaccades: usually error-correcting

How does the area of fixation variability change with fixation time?

• The area over which fixation varies increases with fixation time

• Longer fixation → greater spread around the point of regard

What is meant by a directional bias in fixation?

Fixation does not vary equally in all directions

What are the key characteristics of variation during fixation?

• Fixation is not perfectly stable

• Variability increases over time

• Variability often has a preferred direction (directional bias)

What happens to a stabilized retinal image over time?

• Initially seen clearly

• After a few seconds, it fades away

• Leaves a virtually blank field

What does the fading of a stabilized retinal image suggest about fixational eye movements?

• Small eye movements are needed to keep the image changing on the retina

• Without retinal image motion, perception fades

• Therefore, fixational eye movements help maintain visual awareness

What is the Troxler effect?

• Described by Troxler (1804)

• Stationary retinal images fade from perception during steady fixation

• Demonstrates perceptual fading when visual input is not refreshed by eye movements

What do stabilized retinal image experiments show about tremor and drift?

• Simulated tremor and drift contribute little to visibility

• They do not significantly improve perception of a stabilized image

Which fixational eye movement contributes most to improving visibility in stabilized retinal image testing?

• Microsaccade-like movements

• They produce much greater improvement in visibility than tremor or drift

What do stabilized retinal image studies suggest about the function of microsaccades?

• Microsaccades help refresh the retinal image

• They are likely the most important fixational movement for maintaining visibility

• Supports their role in preventing perceptual fading

What are the 3 major categories of fixation abnormalities?

• Slow drift

• Saccadic intrusions

• Nystagmus

What should be done if it is unclear whether a fixation abnormality is acquired or congenital?

• Refer for additional testing

• Distinguishing acquired vs congenital is clinically important

What are common causes of acquired fixational abnormalities?

• Stroke

• Tumor

• Aneurysm

• Infection

• Multiple sclerosis (MS)

Which lesion locations are especially associated with acquired fixation abnormalities in MS?

• Brainstem

• Cerebellum

• Vestibular system

What is slow drift as an abnormal fixational eye movement, and what condition is it commonly associated with?

• A type of abnormal fixation instability

• Common in amblyopia

• Typically a monocular phenomenon

What are the typical amplitude and velocity of abnormal slow drift?

• Amplitude: ≤ 1°

• Velocity: < 3°/sec

How is abnormal slow drift usually corrected?

• Probably corrected by saccades

• Meaning: the eye slowly drifts off target, then a corrective saccade brings fixation back

How does abnormal slow drift differ from normal fixational drift?

• Normal drift: tiny, slow fixational movement during normal fixation

• Abnormal slow drift: larger, pathologic fixation instability

• In abnormal slow drift: amplitude can be up to 1°, often seen in amblyopia, and is corrected by saccades

What are saccadic intrusions, and when should they be referred?

• A category of abnormal fixational eye movements

• Refer if not long-standing

• Important because they may indicate neurologic disease

What is a square-wave jerk, and what is its clinical significance?

• A type of saccadic intrusion

• Seen in 25–60% of normal individuals

• Amplitude: 0.5–5°

• May indicate cerebellar disease or multiple sclerosis (MS)

What is a macro square-wave jerk, and what are the common causes?

• Larger form of a square-wave jerk

• Amplitude: 5 to 15°

• Most commonly associated with cerebellar disease and MS

What is macrosaccadic oscillation?

• A sequence of saccades to either side of fixation

• Amplitude first increases, then decreases

• Represents an abnormal oscillation around the fixation point

How do square-wave jerk, macro square-wave jerk, and macrosaccadic oscillation differ?

• Square-wave jerk: 0.5–5°, can occur in normals

• Macro square-wave jerk: 5–15°, more pathologic

• Macrosaccadic oscillation: back-and-forth saccades with increasing then decreasing amplitude around fixation

What is nystagmus as an abnormal fixational eye movement?

• Rhythmic oscillation of the eyes

• Can be congenital or acquired

• Congenital nystagmus is also called infantile nystagmus syndrome (INS)

What are the 2 main types of nystagmus?

• Pendular nystagmus

• Jerk nystagmus

Can pendular and jerk nystagmus be congenital or acquired?

• Pendular nystagmus: can be congenital or acquired

• Jerk nystagmus: can also be congenital or acquired

How is congenital nystagmus commonly labeled?

• CN = congenital nystagmus

• Also referred to as infantile nystagmus syndrome (INS)

What is the characteristic waveform of jerk nystagmus?

• Constant-velocity slow phase with a corrective fast phase

• Produces a saw-tooth waveform on position vs time tracing

What are the major types/causes of jerk nystagmus shown on this slide?

• Vestibular nystagmus

• Optokinetic nystagmus (OKN)

What are the physiologic forms of vestibular jerk nystagmus?

• Post-rotary nystagmus → occurs after spinning/rotation

• Caloric nystagmus → induced by warm or cold stimulation in the ear

Can vestibular jerk nystagmus be physiologic or pathologic?

• Yes

• Can be physiologic (post-rotary, caloric)

• Can also be acquired/pathologic

How do you recognize jerk nystagmus on a tracing?

• Slow drift in one direction

• Followed by a rapid corrective jump

• Repeats rhythmically → classic saw-tooth pattern

What nystagmus waveform is characterized by an accelerating slow phase?

• Accelerating slow-phase nystagmus

• Usually congenital

• Commonly associated with congenital nystagmus / infantile nystagmus syndrome (INS)

How does an accelerating slow phase appear on a position-vs-time tracing?

• The eye drifts away from fixation with increasing velocity

• Followed by a quick corrective fast phase

• Produces a repetitive jerk waveform

How does accelerating slow-phase nystagmus differ from constant-velocity slow-phase jerk nystagmus?

• Accelerating slow phase: slow phase speeds up over time; usually congenital

• Constant-velocity slow phase: slow phase stays uniform; often vestibular/optokinetic

• Both are jerk nystagmus waveforms

What jerk nystagmus waveform is characterized by a decelerating slow phase?

• Decelerating slow-phase jerk nystagmus

• Includes gaze-evoked nystagmus

• Also seen in latent nystagmus / manifest latent nystagmus

What is gaze-evoked nystagmus, and what is its movement pattern?

• A form of decelerating slow-phase jerk nystagmus

• Eye drifts toward primary gaze

• Then a corrective saccade brings the eye back to fixation

What are important causes/associations of gaze-evoked nystagmus?

• Drug induced (especially alcohol)

• May be physiologic as endpoint nystagmus

When should gaze-evoked nystagmus be considered suspicious for an acquired abnormality?

• If it occurs at less extreme gaze angles

• Endpoint nystagmus at extreme gaze can be physiologic

• Earlier onset in gaze = more concerning for acquired pathology

What is another nystagmus associated with a decelerating slow phase?

• Latent nystagmus

• Also called manifest latent nystagmus

What proportion of patients with nystagmus also have strabismus?

About 50% of patients with nystagmus have strabismus

Is congenital nystagmus usually idiopathic or associated with other disorders?

• Can be idiopathic

• Can also be associated with multiple disorders

What disorders are commonly associated with congenital nystagmus?

• Albinism

• Congenital cataract

• Retinal disease

• ONH (optic nerve hypoplasia)

When does congenital nystagmus typically present?

Usually appears within the first few months of life

What is the null position in congenital nystagmus?

• The gaze direction where nystagmus intensity is least

• Intensity = amplitude × frequency

• At the null position, visual acuity is optimal

What is periodic alternating nystagmus (PAN) in congenital nystagmus?

• A form where the null position shifts over time

• Shift occurs over several minutes or longer

How does convergence affect congenital nystagmus?

There is often a reduction in nystagmus with convergence

What are the key clinical characteristics of congenital nystagmus?

• Appears in the first few months of life

• Usually has a null position where intensity is lowest and vision is best

• PAN may cause the null to shift over time

• Often improves with convergence

What are foveation periods in congenital nystagmus?

• Brief periods when the eyes are relatively stable

• Allow the image to fall closer to the fovea

• Important because they are the moments of best visual function

Do patients with congenital nystagmus usually experience oscillopsia (a visual symptom where stationary objects or your surroundings appear to continuously shake, bounce, jiggle, or vibrate)?

• No

• Patients with congenital nystagmus typically do not perceive the world as moving

What is the typical abnormal head posture seen in congenital nystagmus, and why does it occur?

• Patient turns the head opposite the null position

• This keeps the eyes positioned in the null direction when looking straight ahead relative to the head

• Purpose: reduce nystagmus and optimize vision

What is the usual direction of movement in congenital nystagmus?

Typically horizontal

What is the key rule regarding acquired nystagmus?

• Acquired nystagmus is not normal

• If you cannot prove it is congenital, then refer

What is the clinical recommendation if nystagmus cannot be shown to be congenital?

• Refer the patient

• The slide emphasizes: “If cannot prove it’s congenital, refer!!”

What findings favor congenital nystagmus over acquired nystagmus?

Congenital nystagmus:

• Null point present

• Decreases with convergence

• Usually horizontal

• No oscillopsia

• Foveation periods present

• Abnormal head posture common

Acquired nystagmus:

• No null point

• No improvement with convergence

• Direction can be any

• Oscillopsia present

• No foveation periods

• No abnormal head posture

What feature of nystagmus direction may suggest congenital nystagmus?

• Congenital nystagmus is usually not two-dimensional

• So nystagmus that is not two-dimensional is more consistent with congenital forms

What additional consideration is mentioned for a child with nystagmus?

Even if the nystagmus seems congenital, in a child you may still consider a brain scan

How can prism be used to manage congenital nystagmus with a null point?

• Use prism to shift the image toward the null point

• Goal: reduce abnormal head turn and improve comfort/vision

Example:

• If null point is to the right → prescribe base-left prism

Why is prism prescribed toward the null point in congenital nystagmus?

• Moves the visual target into the gaze position where nystagmus is least

• Helps the patient use the null position without turning the head

• Therefore decreases abnormal head posture

How can base-out (BO) prism help in congenital nystagmus?

• BO prism increases convergence demand

• Since congenital nystagmus often decreases with convergence, BO prism may help reduce nystagmus

What is the goal of surgery in the management of congenital nystagmus?

• To move the eyes toward the null point

• More accurately: shift the null point toward primary gaze

• Main benefit: decreases abnormal head turn

What surgical approach is described for congenital nystagmus with an abnormal head posture?

• A 2-muscle procedure

• Designed to make the patient use the null position in primary gaze

• Goal: reduce head posture by relocating the functional null point

If the null point is to the right, what muscle surgery can be done?

• Weaken right LR + left MR

• This makes the patient need to pull more to the right to keep the eyes straight

• Effectively helps place the null point in primary position

What is an alternative surgical strategy if the null point is to the right?

• Could also strengthen right MR + left LR

• Same principle: shift the eye position demand so primary gaze aligns more with the null point

What is tenotomy surgery in the management of congenital nystagmus?

• A 4-muscle procedure

• Involves detaching all 4 horizontal recti and reattaching them

• May be done in the same position or combined with repositioning

Which extraocular muscles are involved in the 4-muscle tenotomy procedure for congenital nystagmus?

All horizontal recti:

• Right medial rectus

• Right lateral rectus

• Left medial rectus

• Left lateral rectus

How can a combined tenotomy procedure differ from a standard 4-muscle tenotomy?

• Standard tenotomy: muscles are cut and reattached in the same place

• Combined procedure: tenotomy is done with repositioning of muscles, similar to a 2-muscle null-point procedure

What is latent nystagmus, and how is it usually classified?

• Almost always congenital

• Often associated with strabismus

• It is a jerk nystagmus with a decelerating slow phase

When is latent nystagmus typically elicited, and what is the direction of the fast phase?

• Occurs when one eye is covered

• The fast phase is toward the viewing eye

What is manifest latent nystagmus (MLN)?

• Called manifest latent nystagmus because there is often a small nystagmus even under binocular viewing

• This binocular nystagmus is often subclinical

Does latent nystagmus usually require treatment, and why is it still clinically important?

• Usually no treatment required

• Important because it can affect the eye exam, especially when covering one eye

What is the clinical significance of acquired nystagmus?

• Almost always requires referral

• Often indicates an underlying neurologic, vestibular, toxic, or drug-related cause

What are the major neurologic causes of acquired nystagmus?

acq:

• Stroke

• Tumor

• Aneurysm

• Infection

• Multiple sclerosis (MS)

Key MS locations:

• Brainstem

• Cerebellum

• Vestibular system

What toxic or medication-related causes can produce acquired nystagmus?

• Alcohol intoxication

• Phenytoin (Dilantin)

• Other anti-seizure medications

• Sedatives

What type of nystagmus is gaze-evoked nystagmus?

• A jerk nystagmus

• Has a decelerating slow phase

What is the normal/physiologic form of gaze-evoked nystagmus?

• Endpoint nystagmus

• Also called physiological nystagmus

• It is still a jerk nystagmus

What does pathologic gaze-evoked nystagmus suggest, and what are common causes?

• Suggests a neurologic disorder

• Common associations:

  • Multiple sclerosis (MS)

  • Cerebellar disease

  • Vestibular system disorders

How do physiologic endpoint nystagmus and pathologic gaze-evoked nystagmus compare?

• Both: are jerk nystagmus with a decelerating slow phase

• Physiologic form: endpoint nystagmus at extreme gaze

• Pathologic form: due to neurologic disease (e.g., MS, cerebellar, vestibular lesions)

How is horizontal gaze nystagmus (HGN) used in alcohol testing?

• Used as a roadside sobriety test

• Helpful when direct blood alcohol concentration (BAC) testing is not practical

• Alcohol can produce horizontal gaze-evoked jerk nystagmus

What is the relationship between angle of onset of nystagmus and BAC in alcohol intoxication?

• Earlier onset of gaze-evoked nystagmus is associated with higher BAC

• Tharp finding:
  Angle of onset ≈ 51° – 105(BAC)

• Reported that HGN testing could identify BAC ≥ 0.10% about 77% of the time

How was gaze nystagmus scoring used in the Dr. Good study?

• Highway patrol examiners scored gaze nystagmus on a 0–6 scale

• 4 or greater = failure

• Used to help predict whether BAC was ≥ 0.10%

What is visuoscopy and what instrument is used?

• A method to assess fixation location on the retina

• Performed with a direct ophthalmoscope using the fixation target setting

What does the patient see and what does the examiner see during visuoscopy?

• Patient: sees the fixation target

• Examiner: sees the target imaged on the retina

How does visuoscopy determine whether fixation is central?

• View the macula with a direct ophthalmoscope fixation target

• Have the patient look at the center of the target

• If fixation is central, the foveal reflex aligns with the center of the target

What are the main purposes of visuoscopy when evaluating fixation?

• Determine whether fixation is steady or unsteady

• Determine whether fixation is central or eccentric

• If eccentric, determine the direction and magnitude of eccentric fixation

How do you interpret the rings in a visuoscopy target when measuring eccentric fixation?

• The center ring has a radius of 1 prism diopter (pd)

• Each successive ring increases by 1 pd

• This lets you estimate the magnitude of eccentric fixation in pd

What is an important distinction regarding eccentric fixation?

• Do not confuse eccentric fixation with eccentric viewing

• Eccentric fixation = abnormal retinal locus used for fixation

• Eccentric viewing is a different concept

What is eccentric fixation?

• Patient believes they are looking straight at the target

• There is a change in monocular visual direction

• Uses a nonfoveal retinal point for fixation

• Clinically, we try to eliminate eccentric fixation

What is eccentric viewing?

• Used by patients with a central scotoma

• Patient intentionally looks away from the target

• Purpose: place the image on a healthier nonfoveal retinal area

• Clinically, patients may be taught eccentric viewing

How do eccentric fixation and eccentric viewing differ in patient awareness and mechanism?

• Eccentric fixation: patient thinks they are looking directly at the target; abnormal monocular visual direction

• Eccentric viewing: patient knowingly looks off to the side because straight-ahead viewing does not allow the target to be seen

How can best visual acuity be approximated in a patient with eccentric fixation?

• Use the formula: MAR = EF + 1

• MAR = minimum angle of resolution (in arcminutes)

• EF = magnitude of eccentric fixation (in prism diopters, pd)

What is MAR, and how does it relate to Snellen acuity?

• MAR = minimum angle of resolution in minutes of arc

• It is the reciprocal of the Snellen fraction

Examples:

• 20/20 → MAR = 1

• 20/40 → MAR = 2

How is fixation assessed with a penlight?

• Patient monocularly fixates a penlight held by the examiner

• Examiner sights over the penlight

• Note the position of the first Purkinje image (corneal reflex) relative to the center of the entrance pupil