Describe prenatal development with timeframes
Optic Groove (Day 22)
Retinal Receptors: Precursors (3mths), Inner Segs (5mths), Outer Segs + Macular Differentiation/Myelinatin of ON (7mths)
Eyes open at 26wks
What ocular structures form from the neuroectoderm, surface ectoderm and mesoderm?
Neuro: retina, glia, iris dilator, sphincter pupillae
Surface: lens, corneal epithelium, conjuctival
Mesoderm: lens capsule, vascular/support tissue
How much mature is the newborn visual system’s retina, myelination, LGN and striate cortex?
Retina: periphery relatively adult (esp. nasal)
Myelination: incomplete all levels, ON starts at 7mths
LGN: 50% adult, immature neurons, M/P cells at 60%
Cortex: 25% low synaptic density
What are the possible sequellae of prematurity?
ROP
IVH (Intraventricular Brain Haemorrhage)
Papilloedema -> Optic Atrophy
Refractive Error (high myopia)
Ocular movement disorders (squint, nystagmus, palsies)
Reduced Vision
Name 6 risk factors of IVH/ROP?
Extreme prematurity (<28wks or 3lbs)
Duration of O2 administration/assisted ventilation
Apneic spells
Blood transfusions
Seizures
Give 4 births defects of ophthalmological interest
abnormal face appearance/skin tags
extra fingers/toes
absence of red pupil reflex (opacities)
How does binocular spatial resolution and contrast sensitivity between newborn and adult vision? What age are adult levels reached?
Newborn:
Spatial Res. 1 cycle/deg
CS 10% or 10 at best (logCS=1)
Adult: Spatial Res. 30-50 cycles/deg
CS 0.3% or 333 (logCS=2.5)
Reached at 5-6yo
Describe newborn visual field and compare to childhood VF?
Bino VF < 30* each side
Temp. develops more than nasal (monocular)
VF shape similar to adults
1-2yrs: horizontal meridian adult like, inferior field worse, slight expansion into adulthood
Describe 5 anatomical changes in the eye/visual pathway and it’s physiological consequence for vision
Macular differentiation (foveal cone diameter decreases as packing density increases) - increased resolution/Nyquist limit
Outer-seg foveal cones lengthen - increased retinal + contrast sensitivity
Retinal (eye) expansion - increased VF
Increased neuron myelination - increased nerve impulse conduction
Synaptogenesis in LGN/cortex improves binocularity owing to cortical changes
At what points in the pathway does myelination complete?
Subcortical (3mths)
Optic Nerve + Tract (2yrs)
Extrastriate + Intracortical neurons (Mid-childhood)
Describe the early prenatal development of ocular motor control components
EOMs (4wks)
Orbit angle 180 to 50 in neonate (45 in adult)
Movements (slow from 16wks, fast 18-20wks, decline 32-40wks, behavioural state 36wks)
IOMs (6-8mths)
Describe the perinatal state of the visual system
EOMs functions to produce eye movements (slow better)
IOMs can adjust pupil size/accommodation (iris/ciliary)
Orbit angles slightly > than adult (harder to converge)
Describe 4 perinatal risk factors
Prematurity: squint, gaze, saccadic palsies, nystagmus
Birth Asphyxia + Neurological impairment: EOM disorder
Stimulus deprivation: nystagmus/fixation instability (infantile cataract)
Describe post natal states of convergence, saccades & smooth pursuits
Convergence: appropriate vergence movements < 2mths, well developed by 6mths
Saccades: initially multiple hypometric with head movements
Smooth pursuit: poor, tendency towards saccades, best for slow targets (<10deg/s)
Describe the state of post natal optokinetic responses
Binocular OKN present from birth
Monocular OKN initially asymmetric (T-N > N-T)
Symmetric monoc. OKN emerges with binocularity depending on binocular subcortical mechanisms
How do children and adults prevent retinal image blur?
Adults use VOR, optokinetic response and smooth pursuit to account for <2.5deg/sec retinal slip for clear vision
Children depend mostly on VOR
How do newborns and early infants accommodate? Why is this not as problematic for focussing?
Newborns can adjust accom. most accurate if target is within 75cm, by 3-4mths it’s reasonably accurate over a wider range
precise focusing less important due to greater depth of focus
Name 7 pre-requisites for good stereoacuity
Bi-foveal fixation
Cortical binocular cells
Good/Equal mono VAs
Focussed retinal images of similar sizes
Fusion capacity/reserves
Convergence
Explain the significance of binocular findings up to 9mths
<3mths transient ocular misalignment normal
6mths 95% show constant binocular alignment
9mths misalignment indicates probable abnormality
Describe 8 stages of binocular development
Rudimentary bin. fixation at birth
Responses to kinetic flow fields at 4wks
Appropriate vergence + no cortical binocularity <2mths so initial inability to suppress?
Stereopsis by 6mths
Crossed disparity 4wks before uncrossed
SA < 1 min arc within 5wks of onset
Describe the sensitive period for binocular development
begins several months post birth
peak sensitivity in first 2yrs remaining high in preschool years
realistic upper limit ≈ 7 years
