OPT503 Lecture 11 Handout - Binocular Vision - University of Plymouth

Binocular Vision Lecture 11 – Adaption to strabismus Professor Phillip Buckhurst  This lecture is being recorded as part of Plymouth University's Content Capture project and will be available via the Panopto block located on your module DLE pages shortly. Please note - If you ask a que...

Binocular Vision Lecture 11 – Adaption to strabismus Professor Phillip Buckhurst  This lecture is being recorded as part of Plymouth University's Content Capture project and will be available via the Panopto block located on your module DLE pages shortly. Please note - If you ask a question or make a comment it may appear on the recording, you can ask me to pause the recording if you do not wish you question to be recorded. The PAHC building is a NUT FREE zone Some of our students and staff have allergies so please think before you bring food and drinks into the building Development of normal binocular vision Visual direction monocularly Oculocentric – Monocular vision Principal – Each point on the retina has its Visual own visual direction which direction Secondary passes through the nodal point f Visual direction of the eye o – The principal visual direction refers to the visual axis where the light passes through the nodal point to reach the fovea – The secondary visual direction Nodal refers to where light passes point through the nodal point to reach any other point on the o’ retina f’ (fovea)  Visual direction monocularly EGOCENTRIC - Binocular vision We see through two eyes each with its own oculocentric frame of reference. So how do we see? We see as if from an imaginary cyclopean eye located between our two real eyes The two foveae (fL and fR) have identical visual directions (both focusing on F) we can represent these as a single cyclopean fovea (fC) fL fR fC  To find your Cyclopean eye With both hands and both eyes open point to the spot – Point fingers at distant object hands together and arms at full stretch Without moving your hands close your right eye Without moving your hands close your left eye Observe the alignment of your eyes to the target You may find that with one eye the fingers will align with the target and with the other it won’t – this means you have a dominant eye If equal dominance then you have a true cyclopean eye and through the right eye and left eye the target will be misplaced by the same distance  The Horopter F For a given focal point, the O location of all the corresponding points is known as the horopter In this case O is on the horopter of point F oL fL oR fR oC f C  Vieth-Muller circle F In 1818 Veith and Muller O gave a definition of the shape of the horopter and proposed that it passed though the nodal points of each eye and the fixation point This circular shape of the Horopter is now known as the Veith-Muller circle In reality the horopter is likely to be eliptical oL fL oR fR oC f C  Corresponding points F An object falling on the O horopter has the same visual direction in each eye and is therefore at a corresponding point It is seen as a single object oL fL oR fR oC f C  Disparate points F Any object not falling on the horopter has a different visual direction in each eye D and is therefore called a disparate point It is then seen as double This double vision is called physiological diplopia dL fL dR fR dL dR fC Panums fusional area F Panums fusional Space Each point on one retina actually corresponds with a larger area in the other eye This means there is some tolerance and single vision still occurs provided an object falls within this area called the Panums fusional area Physiological diplopia only occurs outside of Panums fusional Space fL fR fC  Crossed Diplopia F The object lies in front of the horopter Object seen in crossed diplopia The image from the O right eye is seen on the left side of fixation The image from the left eye is seen on the right side of fixation OL OR fL fR OL f OR C  O Uncrossed Diplopia F The object lies behind the horopter Object seen in uncrossed diplopia The image from the right eye is seen on the right side of fixation The image from the left eye is seen on the left side of fixation OL OR fL fR OR fC OL Binocular single vision and strabismus  O Right exotropia The left eye is fixated on the target The right eye is exotropic The image falls on the fovea of the left eye The image falls on the temporal retina of the right eye So, what does the Px see? fL fR This depends on the level of sensory adaption One of three things can happen if the patient has a strabismus 1. Diplopia and confusion 2. Suppression of the binocular visual field of one eye 3. Abnormal retinal correspondence 1. Diplopia and confusion Diplopia – The retinal image of an object falls onto the fovea of one eye but onto the peripheral retina of the other eye – The retinal images fall onto non-corresponding points – Patient sees two images (diplopia) Can be very distressing for patients 1. Diplopia and confusion Confusion – Different retinal images will fall onto the foveas of both eyes – The fovea's are corresponding points and so two different images appear to share the same visual space – The brain cannot fuse these different images Also very distressing for patients Note: Confusion and diplopia occur simultaneously  O Exotropia The image falls on the fovea of the left eye The image falls on the temporal retina of the right eye Therefore the right eye sees the image on the left side of the right eye image Crossed diplopia fL fR Retinal image from left Retinal image from right eye eye falling on the fovea falling on temporal retina  O Esotropia The image falls on the fovea of the left eye The image falls on the nasal retina of the right eye Therefore the right eye sees the image on the right Uncrossed diplopia fR fL Retinal image from left Retinal image from right eye eye falling on the fovea falling on nasal retina Hypertropia The image falls on the superior retina of the hypertropic eye Therefore the hypertropic eye sees the image below that O of the fixating eye fR Hypotropia The image falls on the inferior retina of the hypotropic eye fR Therefore the hypotropic eye sees the image above that O of the fixating eye What can be done about diplopia Px may adapt to the diplopia by adopting a compensatory head posture Prisms – This is the main treatment for diplopia – Prisms may not always work Intractable diplopia Surgery – Not great for restoring BSV – Generally used for cosmetic benefit Note: The amount of prism required to achieve BSV can be achieved in a number of ways e.g. prism bar cover test (more on this later) What happens if intractable diplopia Occlusion of the eye – Occlusive contact lens – Occlusive spectacle lens – Eyelid occlusion Botulinum toxin – Eye patch Suppression A condition that occurs during binocular vision The image from one eye is not perceived under binocular viewing conditions 2. Suppression of the binocular visual field of one eye To stop from having diplopia the brain “ignores” (suppresses) the image from the strabismic eye This sensory adaption occurs if the patient is within the critical period – Occurs if the strabismus is present in childhood The critical period From birth until around 8 to 10 years of age (still lots of debate over the exact critical period) Neural connections are still able to be modified If a strabismus occurs within this period then – Sensory adaption is more likely to happen – Treatments are more likely to be successful Full suppression Under binocular conditions the brain ignores the image of the non-fixating eye This situation is not ideal as depth perception is reduced Suppression and amblyopia During the critical period vision develops For vision to develop normally there needs to be sufficient sensory stimulation If the visual field of one eye is ignored (suppressed) then that eye is deprived of a image – Therefore the vision will not develop properly Amblyopia – reduced vision – A common occurrence with strabismus if developed within the critical period What do we need to know need to know: – which eye? – area of suppression? – is it constant or intermittent? suppression tests must present monocular and binocular information to both eyes in the presence of binocular viewing What would we notice on our routine assessment of vision Binocular balancing would not work – Patient is not binocular Px would have no stereopsis The subjective methods of assessing phorias would also not work – Maddox rod – Mallet unit – Maddox wing  Maddox rod If a patient has diplopia caused by a right exotropia what will they see on the Maddox rod Right eye sees Left eye sees red line spot of light Maddox rod If a patient has a right exotropia but suppresses the visual field of the right eye what will they see on the Maddox rod Right eye is suppressing and so does not see a red line Left eye sees spot of light  Mallet unit If a patient has diplopia caused by a right exotropia what will they see on the mallet unit Left eye Right eye sees sees a another mallet unit mallet unit  Mallet unit If a patient has a right exotropia but suppresses the visual field of the right eye what will they see on the Mallet unit upper strip (nonius target) seen by one eye (e.g. LE) central targets and peripheral surround (including rest of chart and testing OXO room) serve as fusion locks lower nonius is suppressed  Specific tests to investigate suppression Worth 4 dot test Stereopsis tests Mallet unit Bagolini lenses Neutral density filters Worth 4-Dot Test Set up Red/green goggles worn over Rx – Red in front of RE – Green in front of LE – Do NOT show Px the test lights before goggles worn Test at 6m and 40 cm (separate units available for distance and near) with appropriate Rx worn – Px should adopt slight downward gaze at near Room lights off, test held so that red light on top Ask “How many dots do you see? What colour are they?” Worth 4-Dot Test (how it works) In an eye with normal binocular vision: – The right eye sees the top and bottom circular targets – The right eye sees the left, right and bottom target The number of dots the patient sees indicates if they have binocular single vision, diplopia or suppression Right eye sees Left eye sees  If the patient sees four dots 4 dots: – normal second degree fusion – HARC if tropia present (next lecture) If the patient sees two dots 2 dots: – suppression LE If the patient sees three dots 3 dots (C): – suppression RE  IF the patient sees five dots 5 dots: a – Diplopia – a uncrossed diplopia – b crossed diplopia b Worth 4-Dot Test Area of suppression If 2 red or 3 green lights seen, suppression scotoma big enough to cover all of the lights seen by suppressing eye Indicate size of scotoma (area of suppression) – Angle subtended by dots becomes smaller as test moved further away – Suppression may thus be recorded at 6m but not at 40 cm – Could start test at 40 cm and gradually move it away until suppression first reported (report this distance) Not generally done Mallet suppression test Near Mallett unit (33cm) Polaroid visor worn – RE sees letters to left – LE sees letters to right – both eyes see: central ‘O’s (foveal lock) vertical lines (paramacular lock) Letter size varies to test area of suppression from 5 to 20 mins arc LOT 20 O 15 O 10 O 7 CLOVEN 5  Suppression Recording findings Record test: W 4-dot or Mallett Record suppression: – Eye: RE or LE – Area: testing distance (W 4-dot) or mins (Mallett) – Constant or intermittent W 4-dot: fusion @ 6m & 40cm  Depth of Supression To assess depth of suppression you can use a Neutral density filter First place a red filter over the patients good eye – The patient will see one red spotlight Introduce the Neutral density filter and increase the depth of density until the patient either sees – A pink spot – A white spot – Or two spots (one red and one white) Record the density of neutral density filter required to remove suppression  Stereopsis testing If total suppression is present then the patient will have no stereopsis If stereopsis is present in a patient with a strabismus then it would imply that there is some binocularity (HARC more on this next week)  Lang stereotest Can be used to assess stereo of small children with preferential looking Star is always visible – Elephant 600'' Truck 400'' Moon 200''  Frisby Stereotest Three plates – 6mm, 3mm, 1.5mm Stereo is worked out by the distance the test is conducted and the thickness of the plates Usually quoted in octave steps (doublings of threshold; for example, 200 to 400 arcsec) Disparity 6mm 3mm 1.5mm (Sec arc) 600 30cm 300 42cm 150 58cm 42cm 80 82cm 58cm 42cm 40 82cm 58cm 20 82cm 10 115cm 5 165cm  Titmus stereotest Uses polarised glasses to assess stereopsis Ranges from 400-40 seconds arc using the circles 400-100 seconds arc on the animals Uses contour targets (has localised cues)  TNO stereotest The TNO stereo test uses a random dot pattern. Red and green glasses are required Measures disparity values down to 15 seconds of arc Binocular Vision Lecture 10 – Adaption to strabismus Professor P Buckhurst Session: OPT503 Adaption Lecturer: Phillip Buckhurst