Binocular Vision Lecture 2 - Extraocular Movements

Binocular Vision Lecture 2 - Normal Extraocular Movement Part 2

• The lecture is being recorded and will be accessible via Panopto.
• Recorded comments and questions may be included in the recording.
• Students can request pauses if they don't want their questions or comments recorded.
• Learning objectives for the session:
  • Identify positions of rest and fixation.
  • Explain the difference between saccadic and pursuit movements and how each is tested.
  • Understand how to perform extraocular movement examinations.
  • Recognize the muscles and types of movements assessed during ocular motility testing.

Positions of Rest and Fixation

• Anatomical rest:
  • Eye position without muscle tone or postural reflexes.
  • Typically occurs in death.
  • Represents roughly 45° angle between orbital axes.
  • Eyes are generally divergent due to orbital anatomy.
• Physiological rest:
  • Eye position in the absence of a fixational reflex.
  • Determined by postural reflexes and muscle tone.
  • Occurs in total darkness.

Effect of Accommodation

• Accommodation and convergence are related.
• As accommodation increases, convergence also increases.

Fusional Convergence

• Fine-tuning eye movement to fuse a target.
• Retinal images are focused onto corresponding points.

Dissociated (Passive) Position

• Position when fusional convergence is absent, but other factors are present.
• Occurs when one eye is covered (cover test).
• Present in other tests preventing eye fusion (Maddox rod and Maddox wing).

Associated (Active) Position

• Eyes positioned with fusional convergence along with other physiological factors.
• Retinal disparity triggers fusional vergence, positioning the eyes.

What This Means

• Deviation of eye position (heterophoria) exists if associated and dissociated positions are different.
• If no difference, then it is orthophoria.

Categories of Version Movements

• Saccades
• Smooth Pursuit Movements
• Fixational Eye Movements
• Vestibulo-Ocular Reflex
• Optokinetic Nystagmus

Saccades

• Short, rapid, and abrupt movements.
• Occur during reading and fixating between points.
• Rods and cones respond to luminance changes.
• Peak velocity of a 10° saccade is often over 300°/second.
• Vision is suppressed during a saccade.

Smooth Pursuit Movements

• Eye movement while fixating a moving target.
• Smooth eye movement as long as the object's speed is below approximately 40 degrees per second.

Fixation Eye Movements

• Maintaining an image's center within the fovea.
• Quick eye movements (physiological nystagmus).
• Varying amplitude and frequency.
• Prevents retinal image stabilization fatigue, ensuring image clarity.

Vestibulo-Ocular Reflex

• Eyes move opposite to the head's movement.
• Triggered by semicircular canal stimulation.
• Maintains image clarity on the retina during head movement.
• Best responsive with high visual stimulus velocities and frequencies (e.g., spinning).
• Also involves at lower stimulus rates/frequencies (optokinetic reflex).

Optokinetic Nystagmus (OKN)

• Physiological eye movement.
• Called "train nystagmus."
• Occurs when watching objects moving across the visual field.

Ocular Motility

• Assesses a patient's ability to look in all directions, including version movements.
• Evaluates extraocular muscles.
• Determines if eye deviations (comitance/incomitance).
• Comitance - direction unchanging
• Incomitance - angle of deviation changes in different gazes.

Motility Test - Setup

• Routine test.
• Adjust room light to see corneal reflections.
• No glasses.
• Use a pen torch as a target.
• Direct positioning ensuring both eyes are visible to the examiner.
• Explain the test to the patient with instruction to follow the light and keep the head still. Indicate to tell the examiner if any double vision is present.

Motility Test - Procedure

• Hold the pen light 40 cm from the patient.
• Move the light in radial directions across all six cardinal positions of gaze.
• Keep the light within the binocular field of vision.
• Aim for 20-25 degrees from the primary position.
• Observe for corneal reflex misalignments.
• Check for points where the patient experiences double vision.
• Check for 'A' and 'V' patterns (to be further explained later).

The Six Cardinal Positions

• Assess integrity of synergist (yoke) muscles.
• Includes specific eye movements (e.g., RSR, RIO, LIO, LSR, RLR, LMR, RMR, LLR, RIR, LSO, RSO, LIR).
• Testing in the vertical midline is only required to assess 'A' and 'V' patterns.

Muscles Involved with Ductions

• Table detailing agonist and antagonist muscles for different eye movements.

Specific Eye Movements (E.g., Abduction, Adduction, Abduction and Elevation, Abduction and Depression, Adduction and Elevation)

• Specific muscles involved in these movements are examined based on the greatest exertion of action for each movement along with corresponding relaxed muscles for each movement.

Recording Findings

• Record findings if no abnormalities noted - full smooth eye movements.
• In cases of abnormalities, record if any discomfort, jerky, or inaccurate pursuit eye movements occur.
• Note underaction and overaction detection, and if necessary, conduct further duction testing.

Monocular Motility Test (Testing for Ductions)

• Repeat motility test using one eye at a time.
• Determines if restriction is paretic or mechanical.
• If underaction occurs less during version than duction, paresis is often the likely cause.
• If similar underaction occurs for both version and duction movements, mechanical restriction is more likely.