BIOSYSTEMS: SEMINARS (2) - tear quality, bionic eye, colour therapy Flashcards
What are the roles of the tear film? [tear quality #1]
- Protects and lubricates the eyes
- Prevents and reduces the risk of eye infection
- Washes away foreign particles
- Smooth refractive surface for clear vision
- Provides oxygen and nutrients to the surface of the cornea
Describe the chemical composition (in terms of polarity) of the lipid layer of the tear film [tear quality #1]
Hydrophobic outer layer: non-polar lipids (60-70%)
Hydrophilic inner layer: polar lipids (30-40%)
Which layer of the tear film is the main bulk of the tear film? [tear quality #1]
Aqueous layer
What are meibomian glands? [tear quality #1]
Oil glands located in the upper and lower eyelids
Roles of meibomian glands? [tear quality #1]
- prevents tears from evaporating too quickly
- role in increasing surface tension
- protecting against bacterial infection
- help maintain quality of tear film
What is Meibomian Gland Dysfunction (MGD)? [tear quality #2]
Either too much or too little secretion of meibum
How does inadequate meibum affect the eye? [tear quality #2]
Tears evaporate too quickly: get dry eyes
Name the pathophysiological mechanisms of MGD. [tear quality #2]
- eyelid inflammation
- cytokine release onto the cornea
- microbial factors
- lipid deficiencies
List the symptoms of MGD. [tear quality #2]
- dryness
- burning
- itching
- stickiness/crustiness
- watering
- light sensitivity
- red eyes
- foreign body sensation
- chalazion/styes
- intermittent blurry vision
What can cause a corneal infection? [tear quality #3]
- bacteria
- virus
- injury
- allergy
- chemicals
- dry eye
How can MGD lead to a corneal infection? [tear quality #3]
- accumulation of inflammatory molecules at ocular surface
- quantitative reduction or qualitative aberration of tear film
- destruction of epithelial tight junctions resulting in sloughing of ocular surface epithelia
- disrupting corneal epithelium allows opening for microbial invasion
Therefore, if you have MGD induced dry eye and bacterial colonisation: then you have increased risk of corneal infection
List the management strategies for MGD. [tear quality #4]
- warm compress
- gland probing
- emulsion eye drops
- lipiflow
- N-acetylcysteine
- topical azithromycin
- oral supplements (omega 3)
Describe the steps of Gland probing. How effective is it? [tear quality #4]
- numb eye with aesthetic drop
- probe with 2mm probe at slit lamp
- (switch to 4mm probe if persistant tendernes)
24/25 patients had immediate relief (25/25 had long term relief)
When might warm compress be a problem for a px? [tear quality #4]
Might be a problem if px experiences burning eye sensation
Which MGD management techniques are most useful to target burning? [tear quality #4]
Emulsion eye drops and topical azithromycin
List the 10 layers of the retina in order [bionic eye #1]
- RPE
- Photoreceptor layer
- OLM
- ONL
- OPL
- INL
- IPL
- Ganglion cell layer
- Nerve fibre layer
- ILM
Describe the RPE: What constitutes it? Where is it? What is it’s role? [bionic eye #1]
- single layer of melanin containing cells
- between neural retina and choroid
- part of blood-retinal barrier: transports nutrients to outer retina
Describe the barrier function of OLM: barrier between what? [bionic eye #1]
Barrier b/w subretinal space and neural retina proper
How does Retinitis Pigmentosa change the retina? [bionic eye #1]
- impaired function and death of photoreceptors
- bone-spicule pigment: RPE cells around inner retinal blood vessels
- outer nuclear layer thinning
- inner nuclear layer thickness remains the same in some cases (sometimes thickens, possibly due to aqueous flare)
What is the goal of retinal prosthesis? [bionic eye #2]
To restore vision by providing a sensory input to replace impaired/lost photoreceptors in Retinitis Pigmentosa
For retinal prosthesis to work, what must remain intact in RP patients? [bionic eye #2]
There needs to be an intact ganglion cell layer to transmit the signal to the brain
What happened in 1929 that helped lead to the development of retinal prosthesis? [bionic eye #2]
Discovery that electrical stimulation of visual pathways could result in phosphates being seen
What was the first retinal prosthesis? and when was it made? [bionic eye #2]
Argus I, in 2002.
What was the first retinal prosthesis to be approved for retinitis pigmentosa? and when? [bionic eye #2]
Argus II, 2013
Describe the features of Argus I and Argus II prostheses [bionic eye #2]
Argus I: a cochlear implant which ran a cable into the eye to stimulate 16 electrodes
Argus II: uses an external pair of glasses combined with an internal electrode array with a total of 60 electrodes
How does the number of electrodes in a retinal prosthetic implant affect resolution? [bionic eye #2]
More electrodes = greater resolution
Name 4 types of epiretinal implants/prosthetics [bionic eye #2]
- Argus I
- Argus II
- Epi-Ret
- IRIS V2
How does the Epi-Ret differ from Argus II? [bionic eye #2]
Epi-Ret only has 25 electrodes (lower resolution). Although the main difference is that:
- to be able to implant Epi-Ret, you must remove the intraocular lens and vitreous
How many electrodes does IRIS V2 have? [bionic eye #2]
Has 2 versions: 49 electrode and 150 electrode version
Briefly describe the relative success of Argus II vs Epi-Ret vs IRIS V2 [bionic eye #2]
Argus II: moderate success in various aspects of vision
Epi-Ret: no published test results (however have been successfully implanted in humans)
IRIS V2: Subjects were able to perceive basic patterns like horizontal bars
Where are subretinal prostheses implanted, and how can they be divided? [bionic eye #2]
Are implanted in that common location where photoreceptors are typically found. Can be divided into active and passive systems
List one advantage and one disadvantage for suprachoroid retinal prosthesis? [bionic eye #2]
Advantage: minimal surgery needed for electrode array implementation
Disadvantage: Electrodes are further away from target rGCs compared to epiretinal (signal has to travel further)
Where are S, M, and L cones located? [Colour Therapy #1]
S cones: Perifoveal
M and L cones: dense in fovea
Describe the differences between:
Protan, Duetan, Tritan [Colour Therapy #1]
Protan: red-green deficiency (insensitive to red, overcompensate with green)
Duetan: red-green deficiency (insensitive to green)
Tritan: blue-yellow deficiency
What 3 theories work together to explain how colour vision works? [Colour Therapy #1]
- trichromatic theory
- opponent processes theory
- retinex theory
On what chromosomes are the s-cone, m-cone and l-cones located? [Colour Therapy #1]
S-cones: Chromosome 7
M and L cones: Chromosome X
Describe the visual pathway to the Anterior Infero-Temporal Cortex [Colour Therapy #1]
Cones – bipolar cells – ganglion cells – LGN – V1 – V2 – PIT (posterior IT) complex – anterior IT complex
How does the anterior infero-temporal cortex contribute to colour vision? [Colour Therapy #1]
Contains colour selective neurons, presumably connecting from glob cells
Why are M and L cone pigments prone to homologous recombination? [Colour Therapy #1]
Because they are 98% identical
True or False: Tritan deficiencies affect males and females equally [Colour Therapy #1]
True
Where are CVD frequencies highest? [Colour Therapy #1]
In Europe
Describe the ratio of L:M cones in normal trichromats. Do they vary? [Colour Therapy #2]
Generally 2:1, and they are highly variable
Why is it difficult to determine L:M cone ratio? [Colour Therapy #2]
Due to:
- amino acid homology
- large individual differences in the maximum wavelength of the L-cone photopigment
Do L and M cones share a locus control region? [Colour Therapy #2]
Yes they do
What factors influence the ratio of L:M cones? [Colour Therapy #2]
LCR (locus control region), mRNA and epigenetic changes influence this ratio
What might be the reason why there are more L cones than M cones? [Colour Therapy #2]
Closer proximity of L genes to LCR (LCR has a slight bias for L-cones due to this proximity)
Provide evidence for Neural Plasticity mechanisms in colour vision? [Colour Therapy #3]
1) Long term chromatic filter exposure in one eye in adults induced long term changes in colour perception for both eyes
2) and the fact that normal trichromats have variations in L:M cone ratio but still have similar colour perception to each other
- so the visual system uses experience information to compensate for genetic variation
How do you maximise colour vision perception through neural plasticity? [Colour Therapy #3]
By adjusting post-receptoral gains at the nervous system level (R/G channels), based on experiences at the photoreceptor level
How can neural plasticity help anomalous trichromats? [Colour Therapy #4]
can help them maximise their colour perception
What type of anomalous trichromats have an abnormal cone, and what does this result in? [Colour Therapy #4]
X-linked anomalous trichromats have an abnormal cone which has a spectral sensitivity distribution shifted towards that of the other x-linked cone, resulting in decreased sensitivity of stimuli of different wavelengths
How does the ‘gain’ of L cones in anomalous trichromats compare to normal trichromats? [Colour Therapy #4]
L cone gain is greater in anomalous trichromats
Anomalous: L cone gain = 0.92
Normal: L cone gain = 0.64
The result is that the higher gain allows unique yellow to still be perceptible, thus allowing some discrimination of red-green
How can anomalous trichromats distinguish colours that normal trichromats cannot? [Colour Therapy #4]
Neural adaptation allows the perception and differentiation of colours that are otherwise expected to be metamers (based on cone spectral sensitivities)
What is Gene Therapy? [Colour Therapy #5]
Treatment of a disease through the introduction of a functional gene copy into cells
How does gene therapy for colour vision work? [Colour Therapy #5]
A recombinant adeno-associated virus (rAAV) delivers functional L-opsin to cones via subretinal injection
How did the injection of functional opsin gene affect colour vision in monkeys? [Colour Therapy #5]
Injection with enough functional opsin gene was able to achieve a change in colour vision for red-green CVD monkeys
- however changes in colour perception take a bit of time to occur
