Clinical Study Scleral Lenses in the treatment of Neurotrophic | Page 7

challenges. She required many hours
of practice before being proficient
with the insertion and removal of the
lenses. Proper hygiene and ade-quate
cleaning of the lenses also had to be
confirmed before dispensing them.
Her parents also became
proficient with these procedures
in order to help her, if needed.
RGP cleaner and conditioning
solution (Boston ™ ) were recom-
mended. Non-preserved artificial
tears were prescribed (Refresh ™ ,
Allergan) to fill the lens before
insertion. The patient was instructed
to begin with 4 hours of wear on the
first day and to increase by
2-hour increments every day, until
reaching a maximum of 12 hours
per day. At her 1- and 2-month
follow-up visits, the patient’s
comfort and vision remained excel-
lent. Anterior segment evaluation
did not show any corneal staining,
conjunctival redness, or any other
signs of contact lens intolerance.
She was then referred back to her
ophthalmologist for further follow-
ups. However, annual exams at the
contact lens clinic were recom-
mended to monitor contact lens
fitting and parameters.
Discussion
Neurotrophic keratopathy is always a
challenging disease to manage. In the
case summarized here, the treatment
plan was decided based on the ocular
surface condition and the potential to
restore visual acuity. It is
recommended to have a back-up pair
of glasses when prescrib-ing
specialty contact lenses. These both
improve the vision when the
patient is not wearing the lenses and
provide a temporary solution if the
patient has a problem with them.
An important consideration for
prolonged contact lens wear is oxygen
delivery to the cornea. 8 RGP are
permeable to oxygen and allow it to
pass through the lens. Tear flow
underneath the lens, if present, can
also bring tears rich in oxygen to the
cornea. In LDRGP, the lens vaults the
limbus so oxygen from the con-
junctival and limbal vessels can also
contribute to the oxygen supply. 8 One
can calculate the overall oxygen
transmissibility using the lens and
fluid layer thicknesses and perme-
ability to oxygen. In this system, the
fluid layer represents the limiting
factor, its Dk being 80 × 10 -11 (cm 2 /
sec)(mlO 2 /ml × mmHg). In this case,
we can estimate the oxygen
transmissibility with the following
formula:
Dk =
t
scl
1
(t 1 /Dk 1 +t 2 /Dk 2 )
where Dk 1 and Dk 2 represent the
permeability of lens and fluid layer
whereas t 1 and t 2 are the lens and fluid
layer thicknesses. Assuming
that the lens is 300 µm thick, made
with Tyro 97 material and fitted
with a clearance of 125 µm centrally
and 40 µm peripherally, this gives:
Dk /t =1/ (3.0/97+1.25/80)
= 21.5×10 -9 [cm/sec][mlO 2 /
(ml×mmHg)] (central)
= 1/ (3.0/97+0.4/80) = 27.8×10 -9
[cm/sec][mlO
/(ml×mmHg)]
(peripheral) 2
These values meet the Holden-
Mertz criteria to avoid corneal
hypoxia for daily wear. 14 This does
not necessarily occur when the
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lens is fitted with higher
clearance, wh