More on Development

The visual system develops at all levels of the visual pathway after birth. From cortex to retina.

Retina

• At birth perip retina well developed (temporal bit more than nasal)

• Postnatally most important change is @ macula. Recent studies show development of fovea lasts beyond 3rd postnatal yr, when foveal width + cone diameter have reached adult levels.

• Much of the postnatal development of VA results from foveal cone maturation.

Myelination in visual pathways

• Incomplete at birth. Midbrain fully myelinated @ 3 months, Optic nerve/tract @ 2 yrs, extrastriate areas/intracortical neurones @ mid childhood

Cells

• Number of cells is complete @ birth but they grow in size/synapse numbers/interconnectivity especially during first 6 mths. Max density occurs @ 8mths-2yrs then declines to adult levels (60% of max) by age 11

How we objectively assess visual function

• Preferential looking - can do VA, stereo acuity, vernier acuity, colour vision, dark adaptation
• VEPs - w/flash & patterned stimuli, applicable to any age, VEP disappears when pattern can no longer be resolved
• OKN - repetitive eye movements induced by moving visual field VA = finest pattern that induces the movement

VEP data shows better visual performance in general as you aren't relying on the child to do something, merely recording the amount of activity in their cortex. VEP acuity levels are adult like at 6-8mths (PL @ 3.5yrs - correlates well w/data on cone density).

Contrast Sensitivity

• Newborn - 1 month don't show low freq attenuation. Sensitivity greatly reduced
• At 2-3 mths shape of function similar to adult one but is shifted to lower spat.freq. and lower sensitivities
• CSF nearly adult-like at seven months (VEP) or 3-5 yrs (behavioural data)

Refractive error

• Newborn around +2.00 with SD of +2.75
• 6-8 yr old +0.25 SD +1.00
• Passive emmetropisation occurs w/normal eye growth. Optical bits decrease in power to comp for eye growth, reducing Rx
• Active emmetropisation is less understood but it's the role of visual feedback in controlling eye growth. Visual system seems to recognise the value/direction of refractive error and guides the growth accordingly. Can be disrupted by congen.cat, ptosis etc
• 80% of full term kids hyperopic. Range of refractive errors tends to decrease during first year of life. Hyperopia declines in the first year onset 3-8 months. In 82% of children emmetrop. complete within 12 months
• Astig common during first 18 months. Most of it is corneal
• If emmet. fails and rx is +3.50 or more/+0.75 astig into the second year then incidence of amblyopia/strab is greatly increased.
• Anisometropia not the norm in kids older than 2-3months of age.
• Significant Rx after one year of age = +3.50 hyperopia, +1.00D astig, +1.00D anisometropia.
• Best to correct any myopia if significant in the second year. Premature babies tend to be more myopic