This document discusses strabismus and sensory physiology related to binocular single vision. It covers topics such as retinal stimulation, the horopter, Panum's area, fusion, stereopsis, and sensory adaptations that can occur in strabismus including suppression, anomalous retinal correspondence, monofixation, and eccentric fixation. The document is intended to provide an overview of these topics for an ophthalmology resident.

1. STRABISMUS: SENSORY
PHYSIOLOGY AND
PATHOLOGY
Dr Haitham Al-Mahrouqi
OMSB Resident
Oct 2018

2. RESOURCES
AAO, pediatric manual

3. RETINAL STIMULATION
 The retina when stimulated externally by light or otherwise internally
mechanically (e.g. RD), the retinal sensation is that of LIGHT.
 Each photoreceptor have a subjective visual direction in space (i.e.
when stimulated as if light was coming from a specific direction).
 A fixation target stimulating a particular retinal location in each eye,
are said to be corresponding.

4. SENSORY PHYSIOLOGY
 With normal retinal correspondence,
the foveae of the 2 eyes are
corresponding points. Retinal areas in
each eye that are essentially
equidistant to the right or left and
above or below the fovea are also
corresponding points.
 The locus of points in space that
stimulate corresponding points in each
retina is known as the horopter
 Panum’s area stimulate non-
corresponding points but the slight
disparity aid STEREOPSIS.
 Anything outside of Panum’s area is
seen as double.
 Receptive fields explain the wider area
of panum’s area in the periphery. The
ganglion cells have larger receptive
fields in the periphery (larger no. of
cons). Larger receptive fields also

5. RETINA-GENICULATE-CORTICAL
VISUAL SYSTEMS
 Magnocelluar (M)
system: Detects
movement (spatial
perception).
 Parvocelluar (P) system:
Detects resolution and
contrast
 Koniocelluar (K)
system: Detects colour
(especially blue).

6. EFFECTS OF ABNORMAL VISUAL EXPERIENCE
ON THE RETINOGENICULOCORTICAL PATHWAY

7. BINOCULAR SINGLE VISION
Umbrella term for 3 levels:
Simultaneous vision
Fusion
Stereopsis

8. FUSION
 Cortical phenomenon
 The images from both eyes must be similar in shape and size,
otherwise diplopia/suppression.
 Fusion in areas near the fovea (central fusion) tolerates very little
dissimilarity between the images in each eye before diplopia is
elicited, because of the small receptive fields in this region. More
dissimilarity is tolerated in the periphery (peripheral fusion), where
the receptive fields are larger.
 Fusion has been artificially subdivided into sensory fusion, motor
fusion, and stereopsis.

9. FUSION
 Fusion has been artificially subdivided into sensory fusion and motor
fusion
1. Sensory fusion: brain perceives 2 stimuli coming from
corresponding retinal points (or near corresponding as in panum’s
area) singly.
2. Motor fusion: Motor fusion is a vergence movement that causes
similar retinal images to be maintained on corresponding retinal
areas (e.g. when an object is moved from one place to another OR
when using prisms).

10. STEREOPSIS
 Binocular phenomenon.
 Horizontal disparity gives clues for 3D image as well as depth
perception.
 Depth perception can be a monocular phenomenon given clues.

11. ABNORMALITIES IN BSV
 In strabismus, objects do not fall on corresponding retinal points.
This results in either of 2 phenomenon:
1) Confusion: Brain tries to fuse 2 corresponding retinal images.
Phenomenon of extra-foveal areas (fusing 2 different objects).
2) Diplopia: Brain tries to fuse foveal image of one eye and extra-
foveal image from the other eye (fusing 2 images of the same
object).
How do we adapt to the above??

12. SENSORY ADAPTATIONS IN
STRABISMUS
Only develops in the immature visual system (i.e. children).
1. Suppression (> 30PD) or eccentric fixation (> 30PD)
2. Monofixation (<10PD) or Anomalous retinal correspondence (10-
30PD)

13. SUPPRESSION
 Image from one eye is inhibited from reaching the consciousness.
Types
1) Central (suppressing image from fovea of non-fixating eye;
physiological): eliminates confusion VS Peripheral (suppressing the
extra-foveal area in the non-fixating eyes; pathological):
eliminates diplopia.
2) Alternating vs non-alternating
3) Facultative vs constant.

14. ANOMALOUS RETINAL
CORRESPONDENCE (ARC)
 Seen with esotropia of 10-30PD or intermittent exotropia.
 Superior state than suppression.
 Binocular phenomenon
 Basically, the extrafoveal point becomes the ”fovea” and there might
be some degree of gross stereopsis.
 Such patients are at increased risk of paradoxical diplopia (usually
crossed) which normally disappears after few weeks.

15. MONOFIXATION
 Binocular phenomenon
 Seen with microtropias <10PD
 There is central suppression as the image falls also into the fovea
(small tropia only)

16. ECCENTRIC FIXATION
 Monocular phenomenon
 Strabismic eye assumes a permanent new fovea
 Seen with long standing strabismus of > 30PD