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
