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MINERVA OFTALMOL 2002;44:31-8
The effect of contact lens wear on corneal sensation
A. M. NTOLA, P. J. MURPHY
Department of Optometry
and Vision Sciences
Cardiff University, Wales, UK
The corneal nerves play an important role in
the protection and maintenance of corneal
health, and the corneal epithelium has the
highest density of free nerve endings in the
body. Contact lenses are increasingly used to
correct refractive error or for cosmetic pur-
poses. It is therefore important to study the
relationship between these factors. Studies
have revealed that contact lens wear can pro-
duce a reduction in corneal sensitivity, with
the extent of sensation loss related to the type
of contact lens, the material it is made from,
and the frequency and duration of wear. In
summary, as the time of wear increases, both
in the short-term (days) and long-term
(months), the greater the loss of sensation.
Recovery to normal levels, with the cessation
of lens wear, is also prolonged with extended
durations of contact lens wear. Newer lens ma-
terials that have improved oxygen permeabil-
ity have less of an effect. The two principle
mechanisms by which the corneal nerves are
affected are the mechanical action of the lens
and interference with the metabolic function
of the cornea, as a result of the reduced oxygen
supply. The impaired metabolic function pro-
duces an increase in acidosis and a change in
corneal pH as a result of hypercapnia. Both
of these can alter nerve function, and so re-
duce corneal sensitivity.
Key words: Contact lenses - Cornea, physiology -
Cornea, metabolism.
Contact lenses have become one of the prin-
cipal methods for correcting refractive er-
ror over the last 30 years. The development
of the soft contact lens by Wichterle et al.1
created a safe and comfortable method for
patients to change from spectacle wear. New
developments in lens materials led to the in-
troduction of rigid gas permeable (RGP) lens-
es and, more recently, to silicone-hydrogel
lenses. Indeed, the silicone hydrogel lens
holds the promise of the first successful con-
tinuous wear lens.2
The clinical care of contact lens wear has
revealed a wide number of unwanted ad-
verse reactions, e.g. corneal oedema, neo-
vascularisation, papillary conjunctivitis, dry
eye, marginal ulcers.3However, each reac-
tion has encouraged the development of new
lens designs and materials to avoid/over-
come the problem.
One of the more unusual side effects of
contact lens wear is a reduction in corneal
sensation. For PMMA (poly-methylmethac-
rylate) and RGP lens wear, the practitioner is
actively encouraging a reduction in sensitiv-
ity, but this improved comfort also brings the
risk of undetected foreign bodies or pathol-
ogy. It is this paradoxical situation that we will
review in this paper.
Address reprint requests to: P. J. Murphy, Cardiff University,
Department of Optometry and Vision Sciences, Redwood
Building, King Edward VII Avenue, Cardiff CF10 3 NB, Wales, UK.
Vol. 44, N. 1 MINERVA OFTALMOLOGICA
31
NTOLA THE EFFECT OF CONTACT LENS WEAR ON CORNEAL SENSATION
Corneal nerve supply
The cornea has the highest density of free
nerve endings in the body, and these pro-
duce an exquisite level of sensitivity to nox-
ious stimulation.4They play a vital role in
the detection and prevention of damage to
the cornea and anterior ocular surface. The
cornea performs several important roles in
the eye – transmission of light to the retina,
refraction of the light as part of focussing the
image, maintaining the intraocular pressure
and protection of the internal eye. The tears,
corneal nerves and eyelids all perform roles
in the defence of these functions. The nerves
also play a role in the maintenance and health
of the corneal epithelium. Research on rab-
bits has shown that a total lack of corneal
nerve supply will result in impaired wound
healing, decreased corneal metabolism and
reduced epithelial cell adhesion.5The man-
ner in which this influence is exerted is un-
known, but may be due to axonally trans-
ported substances, such as proteins.6
The corneal nerves are derived from the
Nasociliary nerve, a branch of the Ophthalmic
nerve, a division of the Vth Cranial Nerve
(Trigeminal). The nerves supplying the cor-
nea pass along the long ciliary nerve branch
of the nasociliary nerve. They penetrate the
posterior of the eye and pass between the
sclera and choroid, coursing anteriorly to
supply the cornea, iris and the sensory fibres
of the ciliary body, trabecular meshwork and
sclera.
Upon reaching the cornea, 70-80 nerve ax-
ons (in man) enter the corneal stroma in a ra-
dial fashion from various sites around the
corneal limbal circumference. The nerves en-
ter in the middle third of the stroma and run
towards the centre of the cornea, giving rise
to branches that innervate the anterior and
mid stroma. As the axons pass towards the
epithelium, they ramify and divide to form a
poorly characterised nerve plexus beneath
Bowman’s Layer in the superficial stroma.4, 7-9
The nerves then penetrate Bowman’s Layer at
an estimated 400 sites to enter the basal epi-
thelial layer.10 As they do so, the nerve bun-
dles lose their remaining Schwann cell cov-
erings. These nerves then combine with
nerves that enter the basal epithelium from
the limbus to form the basal epithelial plex-
us.11
The nerve fibres continue to divide and
ramify anteriorly within the corneal epithelium
to distribute free nerve endings across the
whole of the cornea anterior surface. The
nerve fibres innervating the cornea are of sev-
eral different types, each responding to a dif-
ferent set of stimuli. These different nerve
types are also arranged within the corneal epi-
thelium according to their type – myelinated
A∂fibres that respond to mechanical stimuli
run parallel to the corneal surface within the
basal cell layer, unmyelinated C fibres that re-
spond to thermal and mechanical stimuli turn
upwards from the epithelial plexus towards the
surface.12 These two nerve types are the prin-
cipal moderators of the corneal nerve response
to the current corneal aesthesiometers used
to assess corneal sensitivity.
Corneal sensation measurement
The two principal methods used to assess
corneal nerve function in this review are the
Cochet-Bonnet Aesthesiometer and the Non-
Contact Corneal Aesthesiometer (NCCA). The
Cochet-Bonnet instrument uses a thin nylon
thread that is pressed against the corneal sur-
face.13 This produces a mechanical deforma-
tion in the anterior corneal surface that stim-
ulates the A∂fibres. A variation in the inten-
sity of the stimulus is achieved by varying
the length of the nylon thread, which in turn
alters the force that must be applied to pro-
duce a bend in the thread. By this indirect
method the stimulus intensity can be deter-
mined.14 In contrast, the NCCA uses a con-
trolled pulse of air, of a predetermined in-
tensity and duration, to produce a localised
cooling of the pre-corneal tear film.15 This
cooling stimulus is transferred to the corneal
epithelium and detected by the C fibres.16, 17
For both instruments, the patient is asked to
respond verbally as to whether they felt the
stimulus or not – for the Cochet-Bonnet
Aesthesiometer the subject feels a touch on
the eye, and for the NCCA the subject feels a
gentle cooling of the eye.
32
MINERVA OFTALMOLOGICA Marzo 2002
THE EFFECT OF CONTACT LENS WEAR ON CORNEAL SENSATION NTOLA
Pattern of corneal sensitivity loss
and recovery
The extent of corneal sensitivity loss and re-
covery with contact lens wear depends on
the contact lens type, the oxygen permeabil-
ity of the material, the number of hours of dai-
ly lens wear, the number of years of wear, and
the length of any recovery period. For this re-
view, we shall consider the pattern of sensi-
tivity loss and recovery of hard and soft con-
tact lenses, over both short-term and long-
term wear.
Short term effects
Hard/PMMA contact lenses.—As mentioned
earlier, Hard/PMMA lenses actually require
a reduction in corneal sensitivity to improve
their comfort and allow long-term wear. This
is most clearly seen when a lens is inserted in
the eye of a naïve wearer. An immediate lac-
rimal response occurs, which gradually re-
duces with neural adaptation. A further more
significant reduction in corneal sensitivity oc-
curs over a full days wear.
A large number of studies have investigat-
ed this effect.13, 18-33 The most interesting se-
ries of studies were completed by Millodot.
He found a reduction in corneal sensitivity of
about 110% over a 12-hour wear period. His
subjects had worn their lenses for at least 3
months and he assessed their corneal sen-
sation prior to insertion, after 4 hours, 8 hours
and 12 hours of continuous uninterrupted
wear. He found that sensitivity diminishes
progressively with the length of wear to a
maximum after the 12-hour wear period. He
also found a high correlation between central
and peripheral corneal sensitivity, although
the loss was less in the centre than in the pe-
riphery.34 This effect presumably relates to
the increased mechanical effect of the edge
of the lens. It is reasonable to assume that if
the lenses are worn for more hours without
removal, the loss will continue to some un-
known maximum level.
The recovery of sensitivity after short-term
PMMA contact lens wear is rapid. When lens-
es are removed after 8 hours of wear, a sta-
tistically significant recovery occurs within 1
hour, although complete recovery takes long-
er and is related to the number of hours of
contact lens wear.29, 34
Soft contact lenses.—Soft contact lenses do
not require any reduction in corneal sensitiv-
ity to improve their ease of wear because of
the flexibility of the material. At the same
time, investigations have shown that soft con-
tact lenses still produce a progressive decline
of corneal sensitivity, but to a much lesser
degree than hard contact lenses.
A number of studies have considered this
effect,27, 30, 37-39 although the most interesting
studies were completed by Millodot and
Velasco et al. Millodot measured the corneal
sensitivity in 15 subjects before and after 4
hours, 8 hours and 12 hours of uninterrupt-
ed HEMA soft lens wear. He observed a small,
but significant, decrease in corneal sensitiv-
ity after 8 hours of wear, and this loss in-
creased with continuing wear.
Velasco et al. found a significantly greater
decrease in corneal sensitivity with 38% water
content hydrogel lenses than with 55% water
content lenses. This reflects the influence of
lens water content on the oxygen concentra-
tion at the corneal surface - more hydrated
lenses produce a higher corneal surface oxy-
gen tension. Similar investigations found that
high water content soft lenses produce prac-
tically no change in corneal sensitivity over a
12-hour period, although different lenses and
fits cause slightly different results.35, 42, 43
The recovery of sensitivity after soft contact
lens wear is usually more rapid than that
found with hard contact lenses and depends
on the nature and duration of wear. Recovery
usually occurs within one hour of lens re-
moval.40, 41
Long term effects
Hard/PMMA contact lenses.—Most reports
of sensitivity reduction have paid attention to
short term wear of contact lenses. However,
a number of investigations have considered
the effects of longer periods of wear.13, 19, 25-
27, 30, 33, 44, 45
The most interesting studies were com-
pleted by Millodot who assessed the effect of
long-term PMMA lens wear. He found a
marked decline in sensitivity after the first
Vol. 44, N. 1 MINERVA OFTALMOLOGICA
33
NTOLA THE EFFECT OF CONTACT LENS WEAR ON CORNEAL SENSATION
few years of wear. In one study, the subjects
were divided between an experimental
group, who had worn hard contact lenses
for 1-22 years, and a control group who had
never worn lenses. Subjects who had worn
lenses for only 1-2 years had no significant
difference in corneal sensitivity when com-
pared to those subjects in the control daily
group. This indicates that the recovery which
occurs after removal of the lens is sufficient
to return sensitivity to normal levels.
However, the effect of prolonged wear is
easily seen after 5-7 years. Subjects in the ex-
perimental group have a significant decrease
in corneal sensitivity in comparison with the
control group. If we describe corneal sensi-
tivity in terms of its inverse, the corneal touch
threshold (CTT), then 5-7 years wear pro-
duces a 100% increase in CTT and after 17-22
years wear a 200% increase in CTT.
A similar study has been completed by
Sanaty and Tenel who found the same pattern
of sensitivity loss. They also found a greater
loss of sensitivity in the periphery, which
presumably again relates to the increased
mechanical edge effect of the lens.
The recovery of corneal sensitivity after
long-term wear can take many months and
depends on the length of time the subject
has worn PMMA lenses. For example, with a
subject who has worn lenses for 10 years,
recovery to normal levels can occur within 1
month, but for a subject who has worn lens-
es for 15 years, recovery takes 4 months. The
main point is that the longer the initial wear,
the longer it takes to recover.29, 46
Soft contact lenses.—A number of studies
have considered the effect of long-term dai-
ly wear of soft contact lenses on corneal sen-
sitivity. Two studies considered the effect of
high water content extended wear lenses.
Larke and Hirji followed patients who were
wearing Sauflon 85 lenses and Millodot exam-
ined people who were wearing X-Ten lens-
es. In both studies, corneal sensitivity re-
duced progressively over the weeks of wear,
with approximately a 50% increase in CTT
by the end of 3 months with the X-Ten lens-
es. From these results it is evident that, even
with lenses of high oxygen permeability,
some loss of corneal sensitivity occurs.
A more recent study by Murphy et al. as-
sessed the long-term effects of daily-wear
soft contact lenses and rigid gas permeable
(RGP) contact lenses on corneal sensitivity
using the non-invasive air-pulse stimulus (-
NCCA). Interestingly, while both lens reduced
corneal sensitivity from normal levels, no sig-
nificant difference was found between the
results of each contact lens types: soft and
RGP. A similar pattern of significance was
found when the results for the peripheral test
locations were compared, suggesting that
there is no topographical variation in the ef-
fect of the two lens types. RGP lenses gener-
ally have a higher oxygen permeability than
soft lenses and so should produce less of an
effect on corneal sensitivity as a result of an
impaired metabolic function. However, RGP
lenses also produce a mechanical adaptation
effect in the corneal nerves and this adds to
the effect from the reduced metabolic func-
tion. In contrast, soft lenses do not have a
mechanical action. When the metabolic and
mechanical effects are combined for each
lens type, they appear to produce a similar to-
tal effect on corneal sensitivity.
The second significant finding was that the
duration of lens wear for both soft and RGP
lenses doesn’t affect the extent of sensitivity
loss. It appears that with adaptation to the
metabolic change and mechanical action of
lens wear, a new balance between the meta-
bolic requirements of the corneal nerves and
their oxygen supply produces an altered cor-
neal touch threshold.
The last important finding was the lack of
topographical variation across the cornea in
corneal sensitivity change. For negative pow-
er lenses, we might expect to find greater
sensitivity loss in the periphery due to the
increased lens thickness. However, the pe-
riphery should also receive more oxygen dis-
solved in the tears, via tear exchange under
the lens, and so these effects may cancel each
other out. In contrast, RGP lenses do not cov-
er the corneal periphery, but have an in-
creased mechanical action from the edges of
the lens during blinking that may cause the
corneal sensitivity loss in the periphery.
There have been no published studies that
have considered the recovery of sensitivity af-
34
MINERVA OFTALMOLOGICA Marzo 2002
THE EFFECT OF CONTACT LENS WEAR ON CORNEAL SENSATION NTOLA
ter long-term soft lens wear. Nevertheless,
we can expect a similar pattern of recovery
to occur as with long-term PMMA lens wear.
The only main speculation might be the
length of time required to recover to normal
levels. However, since soft lenses generally
produce less of an effect on corneal sensitiv-
ity, a more rapid recovery should occur.
Possible mechanisms
of corneal sensitivity loss
There are two main answers to the ques-
tion of what causes the sensitivity loss with
contact lens wear - metabolic impairment of
the cornea or mechanical pressure on the
cornea.
Polse passed 100% nitrogen gas over a
subject’s eye via a modified swimming gog-
gle. After 2 hours wear, corneal sensitivity
was unaltered, although corneal swelling was
present. In a second experiment, he fitted
subjects with PMMA lenses and this produced
a loss of sensitivity, but no swelling. Polse
concluded that it was not the oedema which
induced the changes in corneal sensitivity,
but rather the effect from mechanical stimu-
lation.
Although there is some anectodal evidence
that lenses which produce less mechanical
stimulation give rise to a smaller decrease in
corneal sensitivity, the mechanical action of
a lens on the corneal nerves cannot be the
only mechanism for corneal sensitivity loss.
This is evident in a number of ways. Firstly,
soft lenses still produce a reduction in corneal
sensitivity. Secondly, when the eyes are
closed overnight, corneal sensitivity declines
as a result of a lower oxygen pressure at the
corneal surface and not as a result of me-
chanical stimulation.48 Thirdly, when the cor-
nea is exposed to a reduced partial pressure
of atmospheric oxygen, a reduction in sensi-
tivity occurs.49 In an experiment by Millodot
and O’Leary (1980), the cornea was exposed
to two different gas mixtures containing 2.1%
oxygen and 3.15% oxygen, (normal atmos-
pheric oxygen contains 10% oxygen). They
found a strong relationship between the time
of exposure to a reduced pressure of atmos-
pheric oxygen and a reduced corneal sensi-
tivity. They also found a time delay between
the start of the experiment and the reduc-
tion in corneal sensitivity. With the 2.1 and
3.15% oxygen pressures, it took 3 and 4 hours
respectively to produce a measurable change
sensitivity. In the study by Polse described
earlier, no change in corneal sensitivity oc-
curred with a 100% nitrogen atmosphere, but
the measurement was taken after only 2 hours
and a longer period may be needed before
any change can be detected.
The comparative impact between the me-
chanical action and the corneal oxygen sup-
ply can be demonstrated by considering the
differing effects of PMMA and RGP lens wear
on corneal sensitivity. In one experiment,
subjects were fitted with a PMMA lens in one
eye and a RGP (CAB) lens in the other. After
3 months of wear, a reduction in sensitivity
was measured in the PMMA wearing eye,
while practically no change occured in the
RGP wearing eye.36 Another experiment com-
pared the effect of three RGP lenses, each
with a different oxygen permeability, and
found a relationship between the epithelial
oxygen availability and changes in corneal
sensitivity.50 Bergenske and Polse also found
that patients who are refitted with RGP lens-
es after having worn PMMA lenses often re-
gain lens awareness.
From this series of experiments we can
conclude that corneal sensitivity reduction is
mediated by a change in the oxygen supply
to the cornea and not simply by any mechan-
ical stimulation. However, the mechanism by
which the corneal nerves are affected by a re-
duced oxygen pressure is not clear. There is
some evidence that acetylcholine is involved
in corneal sensitivity. The corneal epitheli-
um has the highest concentration of acetyl-
choline in the body. Tanelian et al. showed
that acetylcholine instilled into the eye in-
creases the action potential in the long ciliary
nerves of the rabbit cornea. Pesin and Candia
proposed that acetylcholine in the corneal
epithelium plays a role in the regulation of so-
dium positive and chloride negative trans-
port, both of which are necessary in the pro-
duction of nerve impulses. The synthesis of
choline acetyltransferase, the enzyme that
Vol. 44, N. 1 MINERVA OFTALMOLOGICA
35
NTOLA THE EFFECT OF CONTACT LENS WEAR ON CORNEAL SENSATION
synthesises acetylcholine, is interfered with
when the oxygen supply is reduced.54, 55 Since
such a situation occurs in contact lens wear,
this may be one pathway for a reduced cor-
neal nerve function.
Lastly, the reduction in corneal sensation
may be due to corneal acidosis during con-
tact lens wear. The pH of the body is careful-
ly regulated to 7.4 and even a change of 0.05
can produce severe complications. Metabolic
acidosis, and specifically lactic acidosis, can
lead to depression of neural activity ranging
from weakness and lethargy through to coma,
depression of vital functions and ultimately
death. Respiratory acidosis, due to hypercap-
nia (the accumulation of carbon dioxide),
can lead to depression of neural function as
well. The stromal pH is usually maintained at
7.54,56 which is higher than that of the body,
but closed eye wear of a PMMA lens can lead
to a decrease of pH to 7.1.57 Such a change
would cause severe depression of neural
function elsewhere in the body. Since both
lactate accumulation and carbon dioxide are
evident during contact lens wear, their in-
creased concentration may be responsible
for corneal hypoesthesia.
Conclusions
This review of the different studies has re-
vealed the gradual effect of improved lens
design on corneal sensitivity changes with
contact lens wear. Early contact lenses, par-
ticularly PMMA but also soft lenses, had a
greater impact on corneal physiology than
more recent designs. These improvements
have largely resulted from an improved oxy-
gen supply to the anterior cornea. However,
this continuing improvement in contact lens
design may produce an interesting complica-
tion for silicone-hydrogel contact lens wear-
ers. The high oxygen permeability of these
lenses has encouraged their use in extended
wear. However, the improved oxygen supply
may also ensure that a higher level of corneal
sensitivity is maintained, thereby reducing
corneal comfort with the lens. Such a situa-
tion has not been reported anecdotally and
there have been no published studies re-
porting on corneal sensation with silicone-
hydrogel lenses, but this area still merits at-
tention. Other areas of contact lens wear that
require investigation are the influence of
new generation, high oxygen permeable
RGP lenses, and the recovery of corneal sen-
sitivity after ceasing long-term daily soft lens
wear.
This review has demonstrated the useful-
ness of assessing corneal sensitivity as a meas-
ure of corneal health with contact lens wear.
Unfortunately, using the corneal sensitivity
measurement as a predictor for contact lens
wear success is not as useful. There are too
many other variables, such as patient motiva-
tion and ambient environmental conditions,
which can also have a significant impact.
Riassunto
Lenti a contatto e sensibilità corneale
Le fibre nervose corneali svolgono un ruolo im-
portante nei meccanismi di protezione e di omeo-
stasi della cornea e nell’epitelio corneale si osserva il
maggior numero di fibre nervose libere dell’intero
organismo. Le lenti a contatto vengono utilizzate sem-
pre più spesso per correggere difetti di rifrazione o ai
fini cosmetici. Di conseguenza, è importante studia-
re i rapporti fra uso delle lenti a contatto e innerva-
zione corneale. Le indagini finora condotte hanno
rilevato che l’impiego di lenti a contatto è in grado di
causare una riduzione della sensibilità corneale di
entità correlata al tipo e al materiale di costruzione del-
le lenti nonché alla frequenza e durata del loro im-
piego. Nel complesso, aumentando il periodo di tem-
po in cui le lenti vengono usate, sia in termini di
giorni che di mesi, aumenta la perdita della sensibi-
lità. Protraendo l’uso delle lenti, inoltre, si eleva il
tempo necessario per riacquistare la sensibilità cor-
neale originaria una volta sospeso l’uso delle lenti
stesse. I materiali di costruzione di recente introdu-
zione, caratterizzati da una maggiore permeabilità al-
l’ossigeno, esercitano minori effetti sulla cornea. I
sue meccanismi patogenetici principali alla base del-
l’alterazione della sensibilità corneale sono costituiti
dall’irritazione meccanica da parte delle lenti e del-
l’interferenza con le funzioni metaboliche della cor-
nea, secondarie al ridotto apporto di ossigeno. La ri-
dotta funzione metabolica porta a un aumento del-
l’acidosi e a un’alterazione del pH corneale in con-
seguenza dell’ipercapnia. Entrambi questi fattori pos-
sono alterare la funzione delle fibre nervose e quin-
di ridurre la sensibilità corneale.
Parole chiave: Lenti a contatto - Cornea, fisiologia -
Cornea, metabolismo.
36
MINERVA OFTALMOLOGICA Marzo 2002
THE EFFECT OF CONTACT LENS WEAR ON CORNEAL SENSATION NTOLA
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MINERVA OFTALMOLOGICA Marzo 2002