Binocular Vision Lecture 7

Binocular Vision - Lecture 7

• Lecture focuses on convergence and accommodation
• Students will learn to measure near point of convergence and jump convergence in patients.
• Students will also learn to measure accommodation using various methods.

The Near Vision Triad

• The near vision triad encompasses the synkinetic actions of accommodation, convergence, and pupil response.
• These actions occur together during near vision.
• A later section of the lecture will delve deeper into the pupils.

Convergence

• Problems with convergence can lead to near-vision issues.
• Two key types of convergence movements are:
  • Pursuit convergence: Moving an object slowly towards the eyes.
  • Jump convergence: Changing fixation to an object at a different distance.

The Near Point of Convergence (NPC)

• NPC is a measure of pursuit convergence.
• It's defined as the closest point where binocular single vision is still maintained.
• Measurement is taken from the nose in centimeters.

How to Measure NPC

• Routine testing is crucial.
• Room lights should be on.
• Use near correction if necessary (for presbyopia).
• Common targets include the RAF line or a pen tip.
• Hold the target slightly below primary position for natural near-vision simulation.
• Instruct the patient to tell you when the target becomes double.
• Explain that blur is not a factor for recording.
• Move the target towards the patient's nose.
• When diplopia or divergence is reported, note the distance in centimeters.
• If the patient can converge to their nose, record as "NPC: to nose."
• Otherwise, record in centimeters (using the RAF rule's centimeter scale).

Jump Convergence Test

• This measures the ability to quickly switch focus between targets with differing convergence demands.
• The test is subjective, evaluating movement speed and quality.
• To perform:
  • Use distance target (letter chart) one line above worst eye acuity
  • Use pen tip 15cm away from patient's primary position as near target
  • Have patient repeatedly switch fixation between the targets
• If both eyes move smoothly and quickly, record as "Jump good to 15 cm."
• If movement is jerky or slow, record as "Jump poor to 15 cm."
• If one eye shows no movement, record as "No jump to 15 cm."
• Note that over-convergence followed by corrective movement is a normal response.

Accommodation

• Accommodation is central to near-vision issues.
• Several assessment methods are utilized.

Methods For Assessing Accommodation

• Clinically, common methods include:

  • Push-up test for amplitude of accommodation
  • Accommodative facility tests
  • Dynamic retinoscopy

• In research settings, methods such as:

  • Defocus curves (covered in OPT107).
  • Open field auto-refractometry
  • Open field aberrometry are used

Push-up test for amplitude of accommodation

• A subjective measurement of accommodation impacted by depth of focus.
• Defined as the closest point where focus remains maintained.
• Measured in diopters from the nose.
• Procedure:
  • Measure the right eye first (occluding the left).
  • Select the smallest readable word.
  • Slowly move it towards the patient.
  • Ask them to report when it blurs; that's the blur point.
  • Note the value in diopters.
  • Move the target back until it clears again (recovery point); note this value in diopters.
  • The average of the two values represents the accommodation amplitude.
  • Repeat for the other eye and binocularly.

Push-up test for amplitude of accommodation Results

• Compares results to expected ranges based on patient age.
• Patients with greater depth of focus show better accommodation amplitudes.
• If AOA is significantly lower than expected (~1.50 Diopters or greater) it suggests accommodative insufficiency.
• Expected values vary with age and are based on Hofstetter's formula (a relevant formula referenced by the notes).

Accommodative Facility

• Measures the rate of change of accommodation.
• It doesn't measure the total amount of accommodation; instead, it assesses how quickly an accommodative response changes.
• It is a valuable tool for identifying accommodative issues.
• Requires using ±2.00DS flippers.
• To perform:
  • Place the distance target at 40cm from the patient and their distance correction as appropriate.
  • Direct patients to stare at the smallest text they can comfortably read.
  • Start with +2.00DS lenses infront of the eye and swap to -2.00Ds. Have patient tell you when words are clear again.
  • Cycle between the two powers and record the number of complete cycles per minute (cpm).

Dynamic Retinoscopy

• Used to assess both distance and near vision.
• Two techniques exist:
  • Estimate technique (MET)
  • Nott method

Dynamic retinoscopy - MET

• The target should be an accommodative stimulus (letters) at 40cm.
• The lights should be dim enough to see the reflex but bright enough for the patient to see the stimulus.
• The patient's distance correction should be in place.
• Do not fog the other eye.

Dynamic Retinoscopy - NOTT Method

• Target stays at 40cm
• Move the retinoscope instead of the lenses to adjust for any accommodative lag or lead.
• Record movements in diopters.

Homework

• Students are assigned reading material in specific chapters from textbooks on Binocular Vision Assessment and Ophthalmic Clinical Procedures.