Orthoptics I - Accommodation

What is accommodation?

process whereby the dioptric power of the eye increases to bring near image into focus on the retina

What happens to a near image if accommodation is insufficient?

near image remains blurred, even if distance vision is clear

What is the near triad?

link between convergence and accommodation
- to accommodate, the eyes converge and pupils miose

What is the relationship between accommodation and convergence quantified as?

AC:A ratio and CA:C ratio

How does the nervous system control accommodation?

an involuntary reflex controlled by the parasympathetic nervous system
- oculomotor parasympathetic fibres from the Edinger-Westphal nucleus innervates the ciliary muscle

How does stimulation of the ciliary muscle impact accommodation?

contraction releases zonules of Zinn tension allowing the lens to take a convex form, shorting the focal length, allowing increased refraction

What drives accommodation?

disparity (now believed to be the main cue)
proximity (looming)
blur

What is accommodative demand and how is it calculated?

amount of accommodation required to see clearly for any given task

accommodative demand = 1/object distance(m)

What influences accommodative demand?

refractive error
e.g. uncorrected hypermetropia increases demand

What is the far point of accommodation?

point at which no accommodation is necessary (optical infinity)

What is the near point of accommodation?

closest point at which object can be seen clearly
- changes throughout life and dependent on accommodative ability

What impact do concave lenses have on accommodation?

induces accommodation

What impact do convex lenses have on accommodation?

relaxes accommodation

What is presbyopia?

a natural ageing process where progressive build up of protein in the lens makes it inflexible, resulting in an inability to accommodate

What is an accommodative insuffiency?

occurs when amplitude of accommodation is lower than would be expected for individuals refractive state and age

What are the clinical symptoms of an accommodative insuffiency?

blurred near vision
headaches
eye strain
diplopia

What is ill-sustained accommodation and what does it indicate?

occurs where the accommodative amplitude begins normally but decreases with fatigue, indicating an AI

How do we calculate expected accommodative amplitude?

Hofsetter's formula
- average accommodation(D) = 15 - (0.25 x age)

When is accommodation fully developed by?

3 months

Why do children have an increased capacity for accommodation?

they have an increased accommodative demand due to the emmetropisation process

Why can it be beneficial to undercorrect hypermetropia in a child?

drive emmetropisation without impacting accommodative amplitude

What is accommodative inertia?

difficulty in changing accommodative response from near to distance fixation

What is an accommodative spasm?

spasm of the ciliary muscle causing blurred near vision

What can an accommodative spasm be associated with?

convergence spasm, resulting in pseudo-myopia and diplopia

What is accommodative paralysis?

no accommodative response to any stimulus, resulting from disease or trauma

What is an objective measurement of accommodation?

dynamic retinoscopy

How do you measure amplitude of accommodation with an RAF rule?

small target slowly brought to the subject, who verbally reports blurring, which is considered the point where accommodation has failed
- repeated 3 times BEO, 3 times with RE and 3 times with LE

How can concave lenses be used to test accommodation?

minus lenses introduced and gradually increased to determine the strongest lens at which an image can be clearly perceived

How is accommodative facility/inertia tested?

using flipper lenes (in either 1, 2 or 3 DS+/-)
- how many flips between + and - lenses can the subject continue to accommodate

What is dynamic retinoscopy?

a procedure that uses the retinoscope to determine the dynamics of the accommodative system
- observing ret reflex
- "with": indicates lag
- "against" indicates lead

What is eccentric photorefraction?

use of mirrors to establish optical pathways allos photorefraction to an accommodative target
- refraction estimate can then be translated to units of accommodation

What is the aim of orthoptic exercises to treat accommodative dysfunction?

to normalise accommodation/convergence relationship

What is the aim of the accommodative rock?

to improve accommodative facility by using flippers
- near and target chart
- swapping fixation by using the flippers

What lenses may be prescribed to treat accommodative dysfunction if orthoptic exercises do not work?

convex

What is the relationship between accommodative demand and response?

typical response does not always equal demand
- some lag of accommodation is expected (0.25D -> 0.75D)

What are the implications of managing accommodative dysfunction?

it is difficult to determine the level at which objective accommodative dysfunction requires treatment

How is accommodation implicated in patients with Downs Syndrome?

defective accommodation found irrespective of refractive error
- bifocals advocated for in this population