Accommodation

• What is accommodation in the context of vision?

  Accommodation is the process by which our visual system provides the retina with a clear and sharp image when viewing near objects.

  
• At what age is accommodation fully developed?

  Accommodation is fully developed by around 6 months of age.
• What physiological changes bring about accommodation?

  Accommodation is brought about by changes in the ciliary muscle, suspensory ligaments (zonules), and the lens of the eye.

  
• What are the four major factors that stimulate accommodation?

  • Blur
  • Proximal awareness
  • Vergence movements
  • Tonic accommodation

  
• What is the role of the afferent pathway in accommodation triggered by blur?

  The afferent pathway involves blur, the lateral geniculate nucleus (LGN), and the visual cortex.
• What is the motor efferent response in the accommodation process?

  The motor efferent response is mediated by the Edinger Westphal nucleus.
• Which cranial nerve is involved in accommodation?

  The oculomotor nerve (III Nerve) is involved in accommodation.
• How does parasympathetic innervation affect accommodation?

  Parasympathetic innervation (Ach-mediated) facilitates accommodation.
• What is the role of sympathetic innervation in accommodation?

  Sympathetic innervation (noradrenaline-mediated) is antagonistic to the parasympathetic system and assists in re-establishing distance focusing after prolonged near activity.
• What factors influence accommodation?

  • Blur of the object being viewed
  • Target size
  • Depth of focus/pupil size
  • Spatial frequency
  • Flicker frequency
  • Luminance (tonic accommodation)
  • Proximity of the target
  • Chromatic aberration
  • Convergence of the eyes
  • Age

  
• How does age affect the amplitude of accommodation?

  Age affects the amplitude of accommodation, with a decrease in capacity as one gets older.

  
• How does ametropia affect the amplitude of accommodation?

  Ametropia affects the amplitude of accommodation, with myopes having greater amplitude than emmetropes, who have greater amplitude than hyperopes.
• How does spectacle correction impact amplitude of accommodation?

  Spectacle correction affects amplitude of accommodation significantly if the prescription is greater than ±5D by around 0.25D.
• How does luminance affect the accommodative response?

  In dim light, the accommodative response is reduced relative to the accommodative demand.
• What neurological deficits can affect accommodation?

  Third Nerve (CNIII) problems can affect accommodation, leading to other deficits.
• What are the features of accommodative function that can be measured clinically?

  • Posture of accommodation (lag/lead)
  • Amplitude of accommodation (NPA)
  • Accommodative facility
  • Relative accommodation (NRA/PRA)
  • Convergence accommodation (AC/A)

  
• How is the posture of accommodation measured?

  The posture of accommodation is measured using the Monocular Estimate Method (MEM) and the binocular cross cylinder technique.
• What does the amplitude of accommodation measure?

  The amplitude of accommodation measures how much accommodation capacity the system has.
• How is accommodative facility measured?

  Accommodative facility is measured using near accommodative facility/flippers.
• What factors affect clinical tests of accommodation?

  • Age
  • Refractive error
  • Ocular conditions (e.g., amblyopia)
  • Systemic conditions (e.g., head injury)
  • Medications
  • Lighting environment
  • Test type
  • Target size
  • Patient engagement/cognition
• What are the expected values for accommodative testing using the MEM?

  The expected value for MEM is +0.50 +/- 0.25 SD lag.
• What is the expected value for monocular accommodative facility?

  The expected value for monocular accommodative facility is 11 cpm +/- 5 SD.
• What is the expected value for binocular accommodative facility using the cross cylinder test?

  The expected value for binocular accommodative facility using the cross cylinder test is +0.50 +/- 0.50 SD lag.
• What is the expected value for the push-up test for amplitude of accommodation?

  The expected value for the push-up test is 18-1/3 age +/- 2 SD.
• What is the expected value for the minus lens test?

  The expected value for the minus lens test is 2 D less than the push-up test.
• What are common accommodative dysfunctions and their characteristics?

  Common accommodative dysfunctions include insufficiency (high lag, low amplitude), excess (variable VA, no lag), spasm (reduced VA, lead), and ill-sustained (high variable lag).
• What are the key objectives at the end of the lecture on accommodation?

  1. Describe the anatomic and neurologic elements of ocular accommodation.
  2. Classify and describe how accommodation is measured clinically.
  3. Describe factors influencing measurement of accommodation clinically.
  4. Compare normative values of ocular accommodation with actual values.
  5. Classify and describe various presentations of accommodation disorders and their prevalence.
• What is vergence in the context of vision?

  Vergence is the simultaneous movement of both eyes in opposite directions to obtain or maintain single binocular vision.
• How is vergence movement measured?

  Vergence movement is measured in prism dioptres (^) and is the angle formed by a shift of 1 cm at a distance of 1 m in both eyes simultaneously.
• What stimulates vergence eye movements?

  Vergence eye movements are stimulated by displacement or movement of a target toward or away from the subject.
• What factors can affect the vergence system?

  Anatomical factors, neurological lesions, fatigue, alcohol, tobacco, medications, and prisms in glasses can affect the vergence system.
• What are the types of vergence responses?

  • Fusional/Disparity
  • Proximal awareness
  • Tonic
  • Voluntary
  • Vergence adaptation
• What is tonic vergence?

  Tonic vergence is the physiological position of rest, sitting at 17^ beyond parallel when all innervation to the extraocular muscles is cut.

  
• What features of vergence function can be measured clinically?

  • Vergence posture (Heterophoria)
  • Vergence ranges (PRC, NRC)
  • Vergence facility (BI, BO challenge)
  • NPC (Near Point of Convergence)
• How is vergence posture measured?

  Vergence posture is measured using tests such as the Prentice card, Maddox rod, Von Graefe, cover test, and Maddox wing.
• What does the amplitude/range of vergence measure?

  The amplitude/range of vergence measures how much capacity the vergence system has to converge and diverge on demand.
• How is vergence facility measured?

  Vergence facility is measured using prism facility/flippers (12^ Base out & 3^ Base in).
• What are the normative values for vergence function?

  • Near posture/phoria: 3 exo +/- 3^
  • Near PRC minimum expected: 10/16/10
  • Near NRC minimum expected: 10/16/10
  • Near facility: 15 cpm
• What is the expected value for near heterophoria using the von Graefe technique?

  The expected value for near heterophoria using the von Graefe technique is 2 exo +/- 4.
• What is the expected value for near fusional vergences using a horizontal prism bar?

  The expected value for near fusional vergences is BI 17^ (+-4) and BO 25^ (+-9).