mallet unit

Measurement of Fixation Disparity

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Page 1: Mallet Unit

Measurement of Fixation Disparity

Page 2: Mallet Unit

Mallett Units

Most popular method in UKDoes not measure physical

disparityMeasures amount of ∆ to reduce

disparity to zeroUsed at distance and near

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Ideal Parameters

Normal visual environmentCentral and peripheral fusion locksDetect small disparitiesMonocular markers not easily suppressedMonocular markers close to central

fixation pointDetect horizontal and vertical disparitiesEasily understood by patients

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Distance Unit

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Distance Unit

Red markersFoveal fixation 10 min arcCentral lock OXOPeripheral lock room featuresUse with distance RX

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Near Unit

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Near Unit

Green markersFoveal fixation 18 min arcCentral lock OXOPeripheral lock text on page Use with near Rx

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Mallett Unit

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Correcting Disparity

Use ∆ before eye with slipAdd enough ∆ to align markers

with XDisparity can be altered using


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Treating the Disparity

Only correct if patient has symptoms

Add ∆ before eye with slip

If ∆ too high split between eyes

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Exo Disparity

Weakest BI ∆ to eliminated slip

Modify Rx in -ve direction

Orthoptic exercises

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Eso Disparity

Weakest BO ∆ to eliminate slip

Not possible to alter Rx at distance but may be possible at near

Orthoptic exercises

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Vertical Disparity

Eliminate any horizontal slip

Check optical centres on specs

Check head position

Any residual vertical slip use ∆

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Objective Measure of FD

Pickwell and Stockley 1960

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Objective Measure of FD

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Summary I

Size of phoria not necessarily related to presence of FD

Size of phoria not always related to symptoms

FD not always related to symptomsAmount of ∆ not related to size of FDUse in conjunction with other tests of


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Management of Heterophoria

Oculomotor MeasuresSize of phoria

Fusional reserves (blur, break, recovery)


Fixation Disparity

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Normative Data

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Fixation Disparity Data

Curve typeY interceptX interceptSlope

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Phoria and FD

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Frequency of Curve Type

Type Distance Near

I 60 58

II 30 25

III 7 10

IV 3 7

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Discriminative Value

All Subjects XOP SOPSheards Sheards Phoria

FDC Slope Y Intercept FDC Slope

Opposing Vergence X Intercept Recovery Range

Recovery Range Opposing Vergence Break Range

Break Range Vergence Recovery Opposing Vergence

90% correct 89% correct 89% correct

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Vision TrainingLensesPrismsCombination of above

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Summary II

FD Useful additional testDon’t FD use in isolationToo much emphasis on FDClinical judgement required