mallet unit

24
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

Page 3: Mallet Unit

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

Page 4: Mallet Unit

Distance Unit

Page 5: Mallet Unit

Distance Unit

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

Page 6: Mallet Unit

Near Unit

Page 7: Mallet Unit

Near Unit

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

Page 8: Mallet Unit

Mallett Unit

Page 9: Mallet Unit

Correcting Disparity

Use ∆ before eye with slipAdd enough ∆ to align markers

with XDisparity can be altered using

lenses

Page 10: Mallet Unit

Treating the Disparity

Only correct if patient has symptoms

Add ∆ before eye with slip

If ∆ too high split between eyes

Page 11: Mallet Unit

Exo Disparity

Weakest BI ∆ to eliminated slip

Modify Rx in -ve direction

Orthoptic exercises

Page 12: Mallet Unit

Eso Disparity

Weakest BO ∆ to eliminate slip

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

Orthoptic exercises

Page 13: Mallet Unit

Vertical Disparity

Eliminate any horizontal slip

Check optical centres on specs

Check head position

Any residual vertical slip use ∆

Page 14: Mallet Unit

Objective Measure of FD

Pickwell and Stockley 1960

Page 15: Mallet Unit

Objective Measure of FD

Page 16: Mallet Unit

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

BV

Page 17: Mallet Unit

Management of Heterophoria

Oculomotor MeasuresSize of phoria

Fusional reserves (blur, break, recovery)

AC/A

Fixation Disparity

Page 18: Mallet Unit

Normative Data

Page 19: Mallet Unit

Fixation Disparity Data

Curve typeY interceptX interceptSlope

Page 20: Mallet Unit

Phoria and FD

Page 21: Mallet Unit

Frequency of Curve Type

Type Distance Near

I 60 58

II 30 25

III 7 10

IV 3 7

Page 22: Mallet Unit

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

Page 23: Mallet Unit

Treatment

Vision TrainingLensesPrismsCombination of above

Page 24: Mallet Unit

Summary II

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