mallet unit
DESCRIPTION
TRANSCRIPT
Measurement of Fixation Disparity
Mallett Units
Most popular method in UKDoes not measure physical
disparityMeasures amount of ∆ to reduce
disparity to zeroUsed at distance and near
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
Distance Unit
Distance Unit
Red markersFoveal fixation 10 min arcCentral lock OXOPeripheral lock room featuresUse with distance RX
Near Unit
Near Unit
Green markersFoveal fixation 18 min arcCentral lock OXOPeripheral lock text on page Use with near Rx
Mallett Unit
Correcting Disparity
Use ∆ before eye with slipAdd enough ∆ to align markers
with XDisparity can be altered using
lenses
Treating the Disparity
Only correct if patient has symptoms
Add ∆ before eye with slip
If ∆ too high split between eyes
Exo Disparity
Weakest BI ∆ to eliminated slip
Modify Rx in -ve direction
Orthoptic exercises
Eso Disparity
Weakest BO ∆ to eliminate slip
Not possible to alter Rx at distance but may be possible at near
Orthoptic exercises
Vertical Disparity
Eliminate any horizontal slip
Check optical centres on specs
Check head position
Any residual vertical slip use ∆
Objective Measure of FD
Pickwell and Stockley 1960
Objective Measure of FD
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
Management of Heterophoria
Oculomotor MeasuresSize of phoria
Fusional reserves (blur, break, recovery)
AC/A
Fixation Disparity
Normative Data
Fixation Disparity Data
Curve typeY interceptX interceptSlope
Phoria and FD
Frequency of Curve Type
Type Distance Near
I 60 58
II 30 25
III 7 10
IV 3 7
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
Treatment
Vision TrainingLensesPrismsCombination of above
Summary II
FD Useful additional testDon’t FD use in isolationToo much emphasis on FDClinical judgement required