Orthoptics I - AC:A

What are the 4 types of convergence?

proximal, tonic, fusional and accommodative

What is accommodation measured in?

dioptres

What is convergence measured in?

metre angles

How to calculate accommodative demand and convergence?

1/object distance (1m)

Is accommodation:convergence a 1D:1MA relationship?

no - each individual responds to a unique stimulus of accommodation (D) with an individual amount of accommodative convergence
- either less or more than the demand

Define AC:A ratio

refers to the amount of accommodative convergence per dioptre of accommodation

True or false: 1PD causes 1cm of light displacement at 1m

true

What base direction is the measurement for convergent fusional range?

base out

How do you calculate the convergence requirement (in PD)?

MA x IPD(cm)

Calculate the convergence requirement for an object at 0.5m for a patient with an IPD of 65mm

1/0.5 = 2MA
2MA x 6.5cm = 13PD

Does AC:A ratio change?

no

What are the two types of AC:A ratio?

stimulus and response

What does stimulus AC:A ratio assume?

observed changes in deviation are due to changes in demand

What does the response AC:A ratio refer to?

change of deviation as a function of actual accommodative response exerted to ophthalmic lenses

What are the methods of assessing stimulus AC:A ratio?

near gradient
distance gradient
heterophoria
synoptophore

How are exo and eso deviations denoted in an AC:A ratio?

eso = +
exo = -

What is a gradient AC:A ratio?

a method of assessing stimulus AC:A
- uses concave lenses at 6m and convex at 0.3m
- changes the amount of accommodation to be exerted

What types of convergence do gradient AC:A ratios minimise?

proximal and tonic

What is the gradient AC:A formula?

AC:A = (+L-(-L))/D

+L = PCT on accom
-L = PCT s accom
D = spherical lens used

Describe the method of gradient AC:A assessment

1. px wears rx
2. deviation measured
3. +/-3.00DS placed on top rx
4. allow px to clear vision
5. remeasure deviation

What is the accepted normal gradient AC:A ratio?

4:1 or less

What is the heterophoria AC:A?

assessing stimulus AC:A
- uses IPD, near/dist devuations and accommodative stimuli
- assumes no accommodation is exerted at distance
- assumes all near convergence requirement is fulfilled by accommodative convergence

What is the heterophoria AC:A ratio formula?

AC:A = IPD x (n-d)/D

What is the advantage of the heterophoria AC:A ratio method? What is the main downside?

rapid as no additional lenses are used
but, proximal convergence is uncontrolled

Between the gradient and heterophoria AC:A ratio method, which one produces higher results?

heterophoria

Describe the synoptophore AC:A ratio method

1. px c rx
2. use simultaneous perception slides
3. measure IPD
4. measure deviation
5. insert -3.00DS
6. allow px to clear
7. repeat deviation measurement

What is the synoptphore AC:A ratio formula?

AC:A = deviation on accommodation - deviation without accommodation/lens power

How is response AC:A ratio calculated?

using photorefraction - not used in a clinical environment

In what conditions is it essential to measure AC:A ratio?

accommodative convergence excess esotropia

IDEX (intermittent distance exotropia)

Does AC:A account for all convergence?

no - normal ratio is too low to account for the vergence response needed for accurate binocular fixation
- fusional convergence, driven by retinal disparity, makes up the excess

What is the CA:C ratio?

the ratio of convergence initiated accommodation (D) per prism dioptre of convergence

What relationship does the CA:C ratio have with the AC:A ratio?

reciprocal - one is high when the other is low

When does convergence induced accommodation occur?

in the absence of blur cues

What is the accepted CA:C ratio?

0.04D/PD - 0.20D/PD

What is an important requirement of measuring CA:C?

accommodation must be open loop - allow for measurements of accommodation without influencing it
- difficult to achieve in a clinical setting as blur cues have to be removed

How can you open an accommodative loop?

no clinically routine way - has to be done in a lab
- achieved using binocular pinhole of Difference of Gaussian (DoG) targets

What are the methods of objectively measuring accommodation?

eccentric photorefraction, autorefraction and dynamic retinoscopy

What is dynamic retinoscopy?

a procedure that uses the retinoscope to determine the dynamics of the accommodative system - lag or lead (with and against)

What movement denotes accommodative lag?

with movements

What movement denotes accommodative lead?

against movements

What does a neutral reflex when performing dynamic retinoscopy indicate?

the patient is appropriately accommodating for the accommodative demand

What are two ways to quantify the lag of accommodation?

Nott and MEM

What is Nott retinoscopy?

objective measurement of accommodative lag/lead

What are the required conditions for Nott retinoscopy?

use a reduced block of 6/6 optotypes, at 40cm, in a well-lit room with refractive correction on

What is indicated by a neutral Nott retinoscopy reflex at 50cm (2.00DS) with a 40cm (2.5DS) target?

0.5DS of accommodative lag

What is MEM retinoscopy?

a technique used for observing a patients spontaneous accommodative response to a detailed target at a normal working distance

What is added onto the retinoscope for the MEM technqiue?

a 12x12cm white card with letters/pictures

What lens is briefly added over the eye is accommodative lag is detected in MEMR? Why briefly?

+ lens (convex)
- to avoid changing accommodation

What is eccentric photorefraction?

uses mirrors to separate optical pathways to allow photorefraction to an accommodative target

What is cycloplegic retinoscopy?

retinoscopy done with accommodation paralysed

How do cycloplegic agents inhibit/paralyse accommodation?

anticholingeric drug that inhibits the action of the ciliary muscle

What is mohindra retinoscopy?

alternative to cycloplegic retinoscopy
- near retinoscopy performed in the dark with one eye occluded