1. Lecture NO ( # )

2. In normal binocular vision the fovea
of one eye corresponds with a small
area centered on the fovea of the
other eye ( Panum`s area)
This point to area correspondence
means that if deviation of one eye
starts to occur , no diplopia will be
seen until the eye has deviated
enough to move the image out of
panum`s area

3.  Panum`s area is small and horizontally oval
 It allow the eye to deviate by a very small
amount from fixation without diplopia
 FD is a slight vergence error when bifixating
and sensorially fusing a binocular contour
 FD is a tendency of the eyes to drift in the
direction of the hetrophoria, while
hetrophoria refers to a fusion free vergence
and FD refers to a small misalignment of the
visual axis under binocular condition , much
smaller than squint

4. While strabismus prevents BSV , FD
keeps it , however it may reduce a
patient`s level of stereopsis
The heterophoric or the intermittent
strabismic with normal retinal
correspondence benefitted from
fixation disparity prisms
The aligning prism is the prism power
that eliminates and neutralizes fixation
disparity

5.  The standard test is Mallett unit , done at
near , but if subjective symptoms are
present , the neutralizing prism power
usually is prescribed for all distances
 The apparatus is designed for use at
distance and near
 There is a central fixation target
 The word OXO seen with both eyes , and
two monocular strips in line with X , one
seen with each eye

6. Dissociation of the monocular marks
is done by cross – polarized filters
In FD the images will be displaced
slightly on the retina , the marks will
therefore misaligned with X
The target with vertical strips detect
horizontal FD and apparatus rotated
90 degree for vertical FD

7. The Mallett unit measures the degree
of prism relief required to restore the
marks aligned with The X
Associated phoria
Dissociated phoria
FD increases under stress working
The presence of FD indicate to
decompensated phoria which should
be confirmed by other methods