In Optician's April issue, Wendy Sethi offered an overview of the key points from the BCLA CLEAR publication, which focused on the interactions between lens materials, care products and the tear film, looking at the biochemistry and compatibility of the various elements involved in successful contact lens wear.
Two important areas discussed, which are both of clinical relevance when undertaking an aftercare assessment, were the provision of oxygen to the cornea via a contact lens and the wettability of the contact lens material during wear.
This exercise was designed to explore how changing a contact lens material for one reason, such as improving oxygen transmissibility, may have an impact upon wettability.
Case scenario
A patient attends your practice for the first time. They are new to the area and know that, as a full-time contact lens wearer, they are due an aftercare. Their last aftercare was one year ago and they report that, at that time, no problems or concerns had been identified by their last eye care practitioner.
They are currently wearing monthly replacement lenses and use a multipurpose solution (MPS). The exact nature of their lenses is unclear, though the patient admitted that they had previously been advised to keep daily wear time to eight hours maximum and to have ‘one day of rest a week’. The MPS used varied according to whichever supermarket brand they happened to see.
The patient professed to keeping to the time limits of wear previously recommended.
Consider the following questions for your discussions:
- What questions would you ask to check on compliance with lens care and wearing habits?
- What signs might you see to suggest that the patient’s current lenses were not allowing adequate oxygen to the cornea?
- You decide to change the lenses to a monthly replacement silicone hydrogel material. Might this affect the wettability of the lenses and, if so, how might you confirm this in future?
Your Responses
Compliance
With regard to checking compliance, most respondents rightly pointed out the need to ask ‘real-life’ questions relating to everyday activity, as opposed to blandly reciting textbook style questions, typically beginning with ‘how many hours…?’
Here are some examples:
- ‘Questions one would ask about compliance and care habits are: do you wash your hands before removing and inserting contact lenses?
- Do you leave your contact lens case to air dry in the day time? Do you rub your lenses when cleaning them? How long do you wear your lenses for?
- Do you sleep or swim in your contact lenses?
- How often do you change your contact lenses for a new pair? How long do you use your contact lens solutions once they have been opened?
- Do you go to your contact lens checks regularly?’
Another systematic approach included the following questions:
‘When do you wear your contact lens? Do you wear them for work? Do you wear them for social occasions? Which day do you not wear them, if any? These questions will help check the amount of days the patient wears the lens and if they really have a day off.
‘When do you typically insert your lens? Before or after breakfast? What time? When do you take them out? Before or after dinner? What time? This will help check how many hours they wear them for.
‘How do you use your solution? Do you rub the lens? How long for? How long is the lens left in the solution? Overnight? Do you use any eye drops? This will check the cleaning schedule to make sure any deposits left on the lens are cleaned off properly.’
And another: ‘We would ask the patient whether they could comfortably wear the lenses until they went to bed as eight hours is not really practical for the average human being. Also, if they ever slept or napped in them. It would be useful to know if they used a flat case or a barrel case to identify what cleaning regime they were using and if they rub-rinsed the lenses prior to soaking overnight.’
Early hypoxic signs
Abnormal new vessels can invade the corneal stroma from pre-existing peri-corneal structures as a result of a disruption in the balance of angiogenic and antiangiogenic factors that normally preserve corneal transparency and subsequently lead to corneal neovascularisation (figure 1).
Figure 1: Early corneal vascular encroachment
However, since the advent of silicone hydrogel, the days of significant corneal vascular invasion are long gone in everyday contact lens practice. For this reason, it is important for practitioners to be able to identify earlier, perhaps less obvious, signs of a cornea struggling for oxygen.
These might range from the easy-to-see limbal hyperaemia to the hard-to-see epithelial microcysts.
Here is one of the better responses: ‘The signs most commonly seen when a contact lens is not allowing adequate oxygen to the cornea are blood vessels growing in the cornea. With a slit lamp examination one can see: microcysts and vacuoles in the corneal epithelial layer and epithelial oedema; neovascularisation and stromal oedema; endothelial blebs, which may be cause by hypoxic stress; polymegathism/pleomorphism.
‘If the contact lens is not allowing enough oxygen to the cornea, one can change the lens to one of a material that allows more oxygen to penetrate, and also to change the wear time schedule.’
Wettability considerations
The introduction of silicone hydrogel lenses brought greater oxygen permeability to contact lenses, but also more deposition of tear film components (figure 2). Manufacturers made modifications to lens surfaces to reduce this, and while the lenses appear to have better wettability and compatibility as a result, a link to improved comfort remains unclear.
Figure 2: Deposits and poor wettability on a silicone hydrogel reusable lens. Image from Dr Keyur Patel
One of the more comprehensive responses: ‘A monthly replacement silicon hydrogel contact lens will affect the wettability of the contact lens. Wettability is the ability of a liquid to spread and maintain contact with a surface, therefore this refers to how much of the lens surface is covered by the tear film.
‘This can be altered by addition of other substances, either on the lens surface or in the associated liquid. The surfactants lower the surface tension of a liquid. Therefore, this causes the ability to spread and penetrate.
‘The wettability of a contact lens depends on the chemical property of the contact lens and that of the surface coating and wetting agents that are added to the lens. The initial wettability of a reusable lens may change when exposed to lens care solution and tear film components.
‘There are several factors that can affect the wettability of a contact lens in the eye; tear composition, blinking and tear evaporation, ocular temperature, type of lens material worn, duration of lens wear, lens replacement schedule.
‘Wettability of a contact lens in the eye can be assessed indirectly by assessing the pre-lens tear film with slit lamp microscopy and specular reflection at high magnification, interferometry observation techniques (figure 3) and grading scales. The most common method of measuring contact lens wettability in practice is non-invasive break up time.
‘This covers a large amount of lens surface and is mainly unaffected by eye movement. Wettability can be assessed by gently pressing the contact lens on the eye and seeing how much it moves. Normally about 0.3-0.5mm. Silicone hydrogel lenses will move more than other lens materials due to its wettability properties.’
Figure 3: Interferometry can help assess wettability
