Abnormal visual development - genetics Flashcards
(50 cards)
what are the two mechanisms of visual development and when in life do these occur
- nature: innate, mainly pre natal life
- nurture: visual environment, post natal life
what are the properties in the nature aspect of visual development
- gene & molecular expressions
- establish the basic ‘hardwire’ - primitive function connectivity between different pars of the visual system
- rudimentary vision at birth
what are the properties in the nurture aspect of visual development
- experience-dependent visual acuity
- stabilises the exiting ‘hardwire’ and refines the ‘software’ (i.e. need to be exposed to to visual activity), optimises and adapts functional connectivity of the system
- mature perception and fine discriminations
we operate the software, so its user dependent
name a disorder that can be caused by pre natal/genetic causes
albinism
what implications does a disorder that can be caused by pre natal/genetics have
- disrupts the basic ‘hardwire’
- permanent and severe visual defects
- treatment: palliative only (can’t cure them, only help them)
name a disorder than can be caused by post-natal/experimental causes
amblyopia
what implications does a disorder that can be caused by post-natal/experimental causes have
- disrupts the promotional (maintaining) and refinement processes
- arrested or altered/sub-optimal development of visual stimulus
- potential for (partial) recovery
- treatment: visual stimulation-based ‘cures’ e.g. patching in amblyopia
how prevalent is human albinism
~1 : 16,000
what are the genetic and phenotypic variations in, with albinism
variations in melanin synthesis and expression
what does oculo-cutanous albinism OCA affect on a human
affects hair, skin and eyes
what is the cause of oculo-cutanous albinism
the absence of tyrosinase due to a gene mutation, which is the enzyme that produces melanin,
resulting in no melanocytes in the skin
no melanin in the RPE, choroid and iris so can see the blood flow and tissue through the iris
what does ocular albinism OA affect on a human
the pigment inside the eyes only (so not the iris) and is x-linked so affects males only
where is x-linked male only ocular albinism inherited from
from the maternal line
how is melanin defective in ocular albinism
has a problem with packaging melanin into melanosomes, but only with the melanin producing cells derived from the neural tube e.g. the RPE and ciliary retina
so melanocytes of the skin and iris don’t have this problem as they’re not tube derived
how does tyrosine produce melanin (which is defective in oculo-cutanous albinism)
it catalyses the conversion of tyrosine to DOPA and DOPA to dopaquinone
what is a direct consequence of albinism in the eye
absent/reduced ocular pigment
what implications does absent/reduced ocular pigment in the eye have, name two problems
- iris (trans-iridial illumination): causes marked photophobia, so even in low illumination the px has to wear sunglasses
- fundus (no black box effect): light bounced around in the eye, causes visual acuity loss, as there is no pigment to absorb the the light so when light goes into the eye, it is not focussed on the correct optical apparatus e.g. the retina. also there is no fovea or macula
how does embryonic RPE melanin expression begin, and what does it do
normally begins at the eye-cup stages of early embryonic development (even before the RGC’s develop in the neural retina)
it regulates the development of the adjoining neural retinal cells and hence development on the RPE
embryonic RPE melanin expression is absent or reduced in albinos
what are two consequences of an absent or under developed embryonic RPE melanin expression
neural retinal abnormalities:
- fovea is underdeveloped
- retinal pathways are mainly crossed
what is the outcomes of the fovea underdeveloped as a consequences of an absent or under developed embryonic RPE melanin expression (in both OC and OCA), name two
- absent/poorly defined pit + hypoplasia (absence of cone cells in fovea, and reduced amount next to fovea)
- further effects on visual acuity ~ 6/40 both eyes, so cannot drive as they can’t see the number plate at 25 metres
as the pit in our fovea is meant to allow light to go straight onto our foveal outer segments of cones and helps to reduce light scatter and improve our acuity
what is the outcomes of the retinal pathways mainly crossed as a consequences of an absent or under developed embryonic RPE melanin expression (in both OC and OCA)
- temporal axons are misrouted across the midline at the optic chiasm (they cross over with the nasal axons)
- visual pathways are then essentially monocular and dominated by the opposite eye so…
- binocular depth perception is absent or reduced and px will have a strabismus or nystagmus i.e. fixation instabilities as va is so poor and no fovea
what is severely disrupted as a consequence of misrouted temporal axons to the opposite side of the brain and what is the consequence of this
LGN layers and visual maps are disrupted
as a result, the stimulation of nasal or temporal hemi-fields of the left eye causes activity mainly in the opposite right cortex of albinos, and only a bit of activity in the left cortex due to a small amount of axon connection to temporal fibres of the left eye (like its suppose to)
what are the VEP results of an albino due to hemispheric asymmetry
due to chiasmatic misrouting, there will be pronounced contralateral dominance of VEP activity, as temporal as well as nasal axons cross over to the other side of the cortex, e.g. in left cortex, after R eye stimulation and right cortex after L eye stimulation
pattern reversal is also reduced in amplitude and has a longer latency responses via the ipsilateral eye (as the temporal axons do not stay on that side like they should have done)
how do you treat albinism
with low vision aids such as dark glasses, hand held & illuminated magnifiers to combat reduced visual acuity (as albinos can’t read without low vision aids)
this will only be able to improve the quality of life
