Download Overnight wear with silicone hydrogel contact lenses

Contact lenses
Overnight wear with silicone
hydrogel contact lenses
Jill Woods describes how overnight wear lenses can be an important
part of a contact lens patient’s life
T
he
innovation
of
combining
silicone
compounds with hydrogel
materials has had many
positive impacts on
contact lens practice,
one of which has been the resurgence
of interest in wearing contact lenses
overnight. Practitioners should take
time to learn about this modality, so
they are able to counsel their patients
on all aspects of this mode of wear.
Food and Drug Administration approving the use of cosmetic soft contact lenses
for continuous wear in 1981. In the
following years much was learned about
the physiological requirements of the
cornea, including the hypoxic stress that
the cornea undergoes beneath a contact
lens and the relationship this stress has
with infection risk. Indeed, by 1989 this
association was so closely proven2,3 that
the FDA changed their approval for
cosmetic overnight lens wear from 30
consecutive nights of wear to just six.
In 1987 the Acuvue lens was the first
‘true’ disposable lens to be launched and
it was initially marketed for weekly
(six nights) overnight wear. Due to the
reduced handling requirements and
the fact the lens was initially designed
for single use, it was widely expected
that the introduction of these disposable contact lenses would reduce the
infection risk associated with overnight
wear. However, reports of infection
linked with this modality continued. In
the UK, practitioners remained nervous
about prescribing overnight wear, and
the Acuvue lens was subsequently
Definitions
The definition of extended wear (EW) is
widely accepted to be six nights followed
by a night of no lens wear.
Continuous wear (CW) is widely
accepted to be up to 30 nights followed
by a night of no lens wear.
History of overnight wear
The overnight wear of soft contact
lenses was spearheaded in the UK in
the early 1970s by John de Carle, who
first published his experiences with
Permalens.1 Similar positive experiences
and publications in the US led to the
marketed and used as a daily wear lens
to be replaced every two weeks. About
10 years later, a large Dutch study
confirmed these fears and supported
the hypothesis that a disposable modality had done nothing to reduce the risk
of corneal infection when soft contact
lenses were worn overnight.4
Evolution of silicone hydrogel
materials
By the late 1980s, it was clear that
corneal health was closely linked to
adequate oxygenation of the cornea. Soft
lens materials had relied on their water
content as the transport mechanism to
provide the cornea with oxygen, with
higher water content materials providing the greatest amount of oxygen.5
However, it was evident that in many
cases, hydrogel lenses were not capable
of containing sufficient water to provide
the cornea with the level of oxygen
required to maintain normal metabolic
activity during daily wear, let alone
overnight wear. The relatively high
infection rates measured with extended
wear disposable lenses spurred the indus-
Table 1
Properties of silicone hydrogel lenses
Acuvue Advance
Acuvue Oays
Air Optix Night
& Day
Air Optix
PureVision
Biofinity
Dk
60
103
140
110
91
128
Dk/t (-3.00)
86
147
175
138
110
160
H2O content
47
38
24
33
36
48
Modulus
0.43
0.72
1.52
1.00
1.1
0.75
Surface treatment
PVP in lens matrix
PVP in lens matrix
Plasma coating
Plasma coating
Plasma oxidation None
Diameter
14.0
14.0
13.8
14.2
14.0
14.0
Base curves
8.3, 8.7
8.4
8.4, 8.6
8.6
8.6
8.6
Life cycle
2 weeks
1-2 weeks
4 weeks
4 weeks
4 weeks
4 weeks
Approved for
Daily wear only
up to 6 nights
Up to 30 nights
Up to 6 nights
Up to 30 nights
Up to 30 nights
38 | Optician | 04.05.07
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Contact lenses
try into developing new products. The
incorporation of silicone compounds
into hydrogels seemed a natural step
to increase oxygen transport, as it had
proved successful in increasing permeability for gas permeable lens materials.6 However, adding silicones to act as
‘super-transporters’ of oxygen proved
difficult. Perseverance paid off some
15 years later, when the problem of the
inherent silicone property of hydrophobicity was overcome by paying careful
attention to surface treating lenses to aid
wettability.
The first silicone hydrogel lens to
be released commercially was the
PureVision lens by Bausch & Lomb,
which was closely followed by the
Focus Night&Day from CIBA Vision.
Currently there are six silicone hydrogel lenses on the market: PureVision
(Bausch & Lomb), Air Optix Night&Day
(CIBA Vision), Air Optix (CIBA Vision),
Acuvue Advance (Johnson & Johnson
Vision Care), Acuvue Oasys (Johnson
& Johnson Vision Care) and Biofinity
(Cooper Vision).
Demands on Dk
The ease with which oxygen penetrates
a material is termed its permeability
(Dk). The term Dk/t provides a more
clinically relevant number because it
relates to the amount of oxygen that can
penetrate through a lens of specific thickness (t). It has been reported that, for the
daily wear of a contact lens to have no
impact on corneal metabolism, its Dk/t
must be between 24 and 35 x 10-9, with
overnight wear raising this minimum
Dk/t to between 87 and 125 x10-9.7,8
Less oxygen supply than this minimum
has been shown to cause corneal swelling at greater levels than those found in
non-lens wearing eyes. It is important
to consider that these values are for an
‘average’ cornea, meaning that there
is a significant proportion of the lenswearing population with corneas with
above-average oxygen requirements.
Lens options
Solving the initial manufacturing difficulties of incorporating silicone components was just the beginning, since
these new lenses also had very different mechanical and surface properties from conventional HEMA-based
lenses. They demonstrated a higher
rigidity due to their higher modulus of
elasticity, making the fit less forgiving,
as the draping property of conventional
hydrogel lenses is lost.9 To improve the
wettability of the lens surface Air Optix
Night&Day, Air Optix and PureVision
all undergo surface plasma treatments.
These surface changes are an integral
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Night & Day
175
160
Biofinity
147
Acuvue Oasys
138
Air Optix
PureVision
110
86
Acuvue Advance
19
SofLens 38
Focus Monthly
15
1 Day Acuvue
21
Acuvue 2
21
Proclear Sphere
28
Focus Dailies
26
0
20
40
60
80
100
120
140
160
180
200
Dk/t
Figure 1 Oxygen transmissibility of silicone hydrogel and hydrogel lenses
part of the lens rather than a coating.10,11
Johnson & Johnson Vision Care chose
to incorporate an internal wetting
agent, polyvinylpyrolidone, (PVP),
into the lens matrix which acts as a
shield to cover the hydrophobic silicon
on the lens surface.12 The Biofinity
lens by CooperVision is unique in that
it shows good wettability but has no
surface treatments. Table 1 details the
major properties of the currently available silicone hydrogel lenses.
Oxygen flow
Since there is no longer any doubt that
the cornea relies on an adequate oxygen
supply to maintain a healthy metabolism, then every patient deserves to be
fitted with a lens that allows high levels
of oxygen to reach the cornea. There has
been much debate about ‘how much
oxygen is enough’,13-15 and though this
debate continues, practitioners should
remember there is no evidence that
providing more than enough oxygen
to the cornea is a cause for concern.
There is little reason, therefore, for not
refitting successful hydrogel wearers
into one of the silicone hydrogel lenses,
and conversely, there are many reasons
for refitting the unsuccessful hydrogel
wearer into these new materials. The only
caution is that with some of the current
lenses available, the higher oxygen flow
is accompanied by a higher modulus of
elasticity, which may lead to mechanical
side effects in some patients.16
Infection risks
The driving force for the development
of silicone hydrogel lenses was the desire
to reduce the rate of infection during
overnight wear of contact lenses. So did
this happen?
Previous work17,18 has shown that
Pseudomonas aeruginosa – a microorganism commonly associated with microbial
keratitis – shows a marked reduction in
its ability to bind to the epithelium of
corneas wearing lenses of higher permeability. Recent prospective studies19,20
have concluded that the risk of microbial
keratitis is no lower with silicone hydrogel lenses than it is with hydrogels during
overnight wear. This finding was disappointing to eye care professionals and the
contact lens industry alike. However, the
data did show that, despite wearing the
lenses for longer periods than conventional lenses, the severity of infection
was reduced and the recovery rate from
infection was faster in patients who
wore silicone hydrogel lenses.19,20
Other potential complications
of overnight wear
Hypoxia and corneal acidosis
Hypoxic responses are inevitable with
hydrogel overnight wear and even with
hydrogel daily wear for some patients,
because the Dk/t of these lenses barely
meets the transmissibility required for
daily wear, and falls far short of that
required for overnight wear, as shown
in Figure 1. Conversely, most silicone
hydrogel lenses exceed the required
transmissibility for even overnight wear,
thus providing a much higher level of
safety against hypoxic corneal responses,
whichever mode of wear is practiced.
As a direct result of their higher transmissibility, silicone hydrogel lenses
are rarely associated with the hypoxic
complications that have become familiar with hydrogel use, namely epithelial
and stromal oedema, limbal hyperaemia, corneal vascularisation, endothelial
polymegathism and myopic prescription
shifts.21,22
Inflammatory responses
As with hydrogel lens wear, the incidence
of inflammatory responses increases
04.05.07 | Optician | 39
Contact lenses
in overnight wear compared to daily
wear, regardless of lens type. While the
incidence of inflammatory responses in
overnight wear has changed little with
the introduction of silicone hydrogel
lenses, their severity seems to be much
less and generally a faster recovery rate
is experienced by wearers of silicone
hydrogels. 21-23 Despite this, complacency
should be avoided and occurrences of
complications such as infiltrative keratitis, contact lens acute red eye (CLARE)
and contact lens peripheral ulcer (CLPU)
should always be monitored closely until
resolution.21,24
Mechanical
Common complications attributed to
mechanical effects include contact lens
papillary conjunctivitis (CLPC), superior
epithelial arcuate lesions (SEALs) and
mucin ball formation.16,21 The first two
examples are arguably more prevalent
with silicone hydrogel than hydrogel
lenses and both are associated with overly
mobile lenses. Though hypoxia may
play a role in their development with
hydrogel wear, material rigidity is more
likely the cause with silicone hydrogels.
Refitting with a different lens design
is frequently required to avoid future
repetition. Mucin balls are primarily a
phenomenon associated with silicone
hydrogel lenses, more so with the higher
modulus and exaggerated when the lens
fit is loose.25 It is thought that the lens
movement shears the tear film to create
balls of mucin under the lens which can
create transient depressions in the epithelial surface.25
While unlikely to cause symptoms in
daily wear, these mucin balls can, in some
patients, increase to significant numbers
during extended or continuous wear. In
such cases using lubricating drops prior
to sleep or removing the lens to rub
and rinse is sufficient to manage this
problem.25
Overnight wear or daily wear?
Overnight wear lenses should not be
offered as a ‘safer’ option to a patient
who is non-compliant with their lens
care regime. A patient who ignores the
rules of lens care will probably ignore the
rules of lens replacement and will cease
lens wear when symptoms are present,
both of which may lead to more serious
complications in a continuous wear
modality. There are situations, however,
when an absence of good hygiene
can make extended wear a favourable
option. Consider the patient who has a
pastime of hiking, or who is engaged in
the armed forces. Clean water supplies
for hand washing may be scarce and in
such situations the risks associated with
40 | Optician | 04.05.07
Figure 2 Corneal vascularisation in an eye wearing
a hydrogel lens
Figure 3 Corneal vascularisation in the same eye
one month after refitting with a silicone hydrogel
overnight wear may be less than the
risks associated with lens insertion with
dirty hands. Similar risks may apply to
specific occupations which involve hand
contamination.
Good reasons for considering an
overnight wear modality:
● Poor handling ability: this could be
due to physical or mental disabilities,
high refractive errors or poor visual
acuity
● Lifestyle: outside activities such as
hiking, mountain climbing, camping
etc
● Work habits: shift workers, armed
forces, emergency workers, long and/or
irregular work hours
● New parents: babies and young
children with irregular sleep patterns
demand a functional parent within
seconds of waking
● Therapeutic/bandage
lens
requirements
Good reasons for avoiding an overnight
wear modality
● Non-compliant patients: the consequences of being non compliant in
extended wear mode are potentially
higher than in daily wear mode
● Smoking: ulcers and infiltrates are
more prevalent in the smoking population compared to the non-smoking
population26
● Poor general health: patients with
diabetes (particularly those who are
insulin dependent) or health problems
which lead to an increase in the risk of
inflammation should be advised against
continuous wear due to their higher
risk of developing inflammatory and/or
infective adverse events27
● Patients with chronic blepharitis or
meibomian gland dysfunction: these
patients typically have a higher bacterial
load (especially gram positive organisms) on the ocular surface28,29 which
may increase the risk of developing
corneal infection or inflammation. Extra
care even in daily wear contact lenses is
advised5
● Chronic desiccation staining: this
chronic break in the epithelial barrier can
lead to corneal infection at a higher rate
in overnight wear than in daily wear.
● History of inflammatory response:
Once there has been one corneal inflammatory response event there is a much
higher risk of the patient developing a
repeated inflammatory event24
Patients who fall into the ‘avoid
overnight wear’ groups listed above
should not be left to a fate of problematic conventional hydrogel lens wear.
Silicone hydrogel lenses are not solely
prescribed for overnight wear and just
because a lens is licensed for overnight
wear does not mean that it can’t be worn
as a daily wear lens, indeed Acuvue
Advance does not even have approval
for overnight wear.
Silicone hydrogel lenses can offer
huge benefits when prescribed in daily
wear modality and are fast becoming the
first choice of material for new fits and
refits.30,31
The advantages of switching from
hydrogel to silicone hydrogel regardless of mode of wear have been well
documented and include:
● Increased oxygen supply to the
cornea
● Reduced hypoxic events15,32
● Whiter eyes33,34
● Reduction of neovascularisation35
● Improved comfort36
● Reduced dryness36,37
● Facilitation of a longer wearing
time38
● Potential increase in ‘comfortable’
wear time
● Facilitation of short naps or flexible
wear.
Fitting silicone hydrogel lenses
Due to the higher rigidity of some
silicone hydrogel materials, the fit may be
less forgiving than a hydrogel lens. Lens
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movement is required for flushing toxins
and debris from underneath all lenses but
excessive movement in these materials
may increase lens awareness and lead to
tarsal plate changes.21,39,40 A flat lens is
likely to exhibit lens edge fluting, which
will reduce comfort, as typically the lens
will irritate the lower lid. A lens which
is centrally steep may lead to fluctuating vision which can be solved with a
flatter base curve (Figures 4-6). High
molecular weight sodium fluorescein is
an extremely useful agent for assessing
the fit of these stiffer lenses.
It is common for patients to undergo a
period of adaptation before fully acceptable comfort is achieved.
This adaptation may take as long as
four weeks but in most cases lasts for only
three to seven days. Typical complaints
may include the lens feeling ‘heavy’,
having an awareness of the lens edge,
dryness and sporadic blurring of vision.
These symptoms can be due to the very
different surface properties of the lens,
its rigidity or even the rapid oxygenation of the cornea. Such initial adaptation
problems are less of an issue when fitting
a neophyte with silicone hydrogel lenses
because they have no previous experience
to compare with and will not experience
re-oxygenation symptoms.
To facilitate an adaptation period the
first review should not be scheduled too
soon. The review should be planned to
coincide with two to four hours of lens
wear, so that vision and lens fit are stable
and any potential solution incompatibility staining will be evident.41,42
It is highly recommended to counsel
the patients being refitted on the potential comfort problems they may encounter during the first few weeks. This will
help them to feel more confident during
the adaptation period. When fitting
lenses for overnight wear it is important to discuss the schedule of review
appointments and related costs as well
as providing counsel on how to deal with
adverse events.
Figure 4 A steeply fitting lens, illustrated
by high molecular fluorescein
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Switching patients to
overnight wear
For those patients who are experiencing difficulties with their current
contact lenses, it is not a difficult task
to persuade them to try something new,
particularly as trial lenses are easily
accessible. It is recommended that the
symptomatic daily hydrogel lens wearer
should be switched to a silicone hydrogel
yet remain in daily wear mode to begin
with, so the only change is the lens. This
way any new benefits or symptoms can
be attributed specifically to the change
in lens rather than creating uncertainty
between the lens and the modality. It
is when the lens parameters have been
finaliSed that the options of flexible,
extended or continuous wear can be
broached. On the other hand, persuading the happy, symptom-free hydrogel
wearer to switch lens type can be quite
challenging. In these situations the key
is to educate the patient on the benefits
of a more highly permeable lens.
When discussing the possibility of
overnight wear to patients it is important to provide a carefully balanced
argument. Patients will appreciate the
honesty of being told about the potential downsides of switching to extended
or continuous wear, as well as being
informed of the positive aspects. In an
age of ever-increasing awareness of
the legalities of ‘informed consent’, a
document that provides information on
all aspects of contact lens overnight wear
is a useful tool.
Patients commonly believe that if a
lens type or wear modality has not been
suggested or offered by their eye care
practitioner, they must not be suitable.
It is helpful here to use the marketing power that the manufacturers of
overnight wear have generated. Patients
who already wear contact lenses are very
aware of the many media advertisements
for new contact lenses. Most patients
are aware that there have been dramatic
advances in the fields of technology and
Figure 5 An aligned fitting lens, illustrated by
high molecular fluorescein
medicine over the past 20 years. Patients
often welcome an opportunity to wear
the latest contact lens designs and materials. Useful reminders to patients include
discussions of providing an increased
oxygen supply to the cornea throughout long wearing periods, the increased
convenience of overnight wear, whiter
eyes and faster healing rates should an
adverse event occur.
Tips for success with overnight
wear in silicone hydrogels
Patients are quick to adopt revolutions
in technology and will welcome a new
product or modality. Ensuring that the
practice maintains appropriate literature on overnight wear products is a
good way of promoting this modality
as is inserting a feature in the practice
newsletter or including information
with reminder letters. This modality
should be mentioned to all patients, and
a discussion of the potential problems
and benefits should take place.
Advantages of overnight wear
● Convenience
● Immediate clear vision on awaking
● Less handling
● Less maintenance
● Reduced or no care products- reduced
chemical sensitivities
● Possibly a reduction in costs (depending on current regime).
Disadvantages of overnight wear
● Increased risk of corneal infection
● Potential mechanical responses, for
example tarsal plate changes
● Potential inflammatory responses
● Possibly an increase in costs (depending on current regime).
By following these steps a practice’s
overnight wear base should grow.
● Promote silicone hydrogel lenses and
overnight wear within the practice with
literature. This will help to ‘normalise’
the products and the overnight wear
modality
Figure 6 A Flat fitting lens, illustrated by high
molecular fluorescein
04.05.07 | Optician | 41
Contact lenses
● Be proactive. Discuss the benefits of
silicone hydrogel contact lenses with
all current and potential contact lens
wearers
● Ensure you provide information
about the expected benefits of the new
lenses as well as the potential problems
that may arise
● Assess the lens fit carefully especially if
there are initial visual or comfort issues,
use of high molecular sodium fluorescein is preferred
● Be aware of alternative lens designs
and also lens care incompatibilities so
that you are better prepared to deal with
any difficulties which may arise
● Make all costs and review schedules
very clear. ●
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● Jill Woods is research associate, Centre
for Contact Lens Research, University of
Waterloo
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