CLs & Oxygen Flashcards
1
Q
Describe what CLs must provide and what you do not want iwth CLs?
A
- Contact lenses must provide acceptable:
o Vision
o Comfort
o Usability -> handling
o Physiological response - Do not want:
o Mechanical or lens surface effects (such as papillary conjunctivitis, superior epithelial arcuate lesions (SEAL) and conjunctival staining)
o Hypoxia-related adverse responses (such as limbal redness, some types of corneal staining and, most seriously, microbial keratitis.)
2
Q
What are the main sources of oxygen to the cornea?
A
- Cornea requires constant supply of oxygen, but it is avascular
- Cornea receives oxygen from 3 main sources:
1. The atmosphere – O2 disolving into tearfilm & then passing through tearfilm to cornea via tearfilm
2. Aqueous humour
3. Perilimbal vasculature
4. [+ palpebral conjunctiva (BVs from here will supply some oxygen) – during eye closure]
3
Q
Why does cornea need oxygen and why is the oxygen important?
A
- Epithelium and endothelium control amount of water in cornea
o They need oxygen to pump water in and out
o Maintains corneal transparency - Active process -> need nutrients like glucose but also need oxygen to run process
- If they don’t have oxygen, cornea SWELLS. It gets THICKER – called Corneal Oedema
- One of the measures of the physiological performance of a CL, relates to amount of oxygen which reaches ocular surface from atmosphere
- Given this, desirable to have some measure to indicate oxygen performance of a CL
- Best-known measures in this area are the oxygen permeability of materials from which lenses are fabricated (property of lens material), and oxygen transmissibility (the oxygen performance of a finished, manufactured lens – relates to other properties of lens e.g. thickness)
4
Q
Describe corneal oedema?
A
- Epithelial oedema: manifests self as clouding of epithelial tissue
- Usually centrally, can spread out to periphery (see image)
- Commonly associated with punctate epithelial staining
o Sometimes steepening of corneal curvature
o & Possible irregularities in corneal surface
5
Q
What is oxygen permeability (Dk)?
A
- For oxygen to pass through a CL material, molecules must 1st dissolve into material then travel through it
- P = D k
o P is oxygen permeability
o D is diffusion coefficient
o k is solubility coefficient of oxygen in given material (solubility of oxygen in CL material - highly soluble (lots of oxygen can dissolve into polymer) to poorly soluble)
Oxygen Permeability (D):
* Think of no. of cars get through motorway in hour depends on amount of cars & how fast each car travels -> think of straight path rather than meandering path & how fast the O2 pass through
* In high O2 permeability – oxygen doesn’t really interact with material -> gets in polymer and passes through – doesn’t get bound by polymer
6
Q
What would the perfect CL consist of in terms of D & k?
A
Want D & k to be high -> have high permeability (oxygen passes freely through it) -> have high solubility, easy for oxygen to dissolve into polymer -> Dk will be high
For two lenses of same thickness:
High Dk – more oxygen through lens
Low Dk – less oxygen through lens
7
Q
Describe oxygen transmissibility (Dk/t)?
A
- Product of Dk divided by thickness of material
- Degree to which oxygen passes through particular material of a given thickness
- Equal to oxygen permeability divided by the thickness of material under specific conditions
- Oxygen transmissibility (T) = Dk/t
o Dk is oxygen permeability
o t is thickness of a material - 2 lenses made w/ same material, but one is twice thickness of other oxygen transmissibility will be half
o t↑ - T↓ - Dk/t = oxygen transmissibility
o Takes material & design into account
o Better measure of how much oxygen is getting to eye
o Better as know what actual lens performance will be, given thickness required to give function
8
Q
Describe the effects of lens power on transmissibility?
A
- Product guides quote Dk/t for a -3.00DS lens
- Any ↑ in lens thickness reduces Dk/t (oxygen transmission) even if material (Dk) is same
- Because higher power lenses (both plus & minus) are thicker than lower power lenses, they will have lower Dk/t -> less oxygen will get through
- For a given design & material, higher power lenses often have less oxygen transmissibility
9
Q
How much oxygen is enough for a CL?
A
- Oxygen requirements to avoid signs of corneal oxygen deficiency are high
- Daily wear – short days/ part time wear – Dk/t of 24
- Extended or continuous wear – Dk/t of 125
- Not all soft CLs meet criteria
o Some pxs may use CLs longer than should at once or occasionally nap in lenses
o Pxs experience increased oxygen needs resulting from environmental conditions (altitude, dryness etc) - Dk/t for daily wear schedules should be much higher than 24
10
Q
What will happen to the eye is it does not get enough oxygen?
A
- If oxygen is blocked to cornea with a low Dk/t (low oxygen transmission) CL:
o Eye become red & bloodshot
o Endothelial layer of cornea is damaged
If one cells dies, adjacent cells enlarge to fill gap – polymegathism (many shapes)
o BVs grow from limbus into cornea – neovascularisation as result of O2 deprivation
o Eye becomes more vulnerable to infection
11
Q
Describe neovascularisation as a result of CL wear?
A
- Hypoxia produces stromal oedema with/causes release of vaso-stimulatory agents cause BVs to grow from limbus into peripheral cornea
- No significant symptoms
- Signs include new BV from limbal vessels will see on SL BVs going into cornea
- More common in superior cornea due to presence of upper lid
- Manage by improving Dk/t, reducing wearing time (or convince px to stop wearing CLs)
- Prognosis good -> with treatment
12
Q
Describe polymegathism?
A
- Long term hypoxia causes structural damage to endothelial cells endothelial cell can die
o As cell dies, adjacent cells enlarge to fill the space - No real symptoms although may develop sudden CL intolerance with reduced VA and photophobia
- Signs include differing cell sizes in endothelium
- Manage by refitting with a higher O2 lens, change to dailies (rather than EW), reduce wearing time
- Prognosis often not great – endothelium never fully ‘heals’
o Long term it can lead to endothelial decompensation
13
Q
Describe myopic creep in CL wear?
A
- Wear of standard design contact lenses may change aberration profile leading to a greater degree of optically stimulated myopic increase than if spectacles were worn
- This effect may be counterbalanced in case of the wear of higher modulus silicone-hydrogel lenses (quite rigid) -> which will have a tendency to mechanically flatten central cornea region
14
Q
Describe corneal oedema - SEAL?
A
- Superior Epithelial Arcuate Lesions
- Caused by mechanical trauma due to inflexible nature of some CL material (e.g. w/ high modulus)
- Symptoms may include FB sensation
- Signs show arcuate staining 1mm from superior limbus
- Manage by removing lens until cleared lubricants may be required
- If problem reoccurs, change lens design (flatter) and material or try RGP
15
Q
Describe corneal oedema - striae?
A
- Often seen in diabetics, older patients and keratoconics
- Caused by hypoxia
o Lactic acid accumulation in cornea causing an osmotic shift in stroma and corneal oedema - Patient is usually asymptomatic
- Signs include white, vertical lines in posterior stroma
- Manage by switching to dailies, increase Dk/t of material
