Corneal wavefront-guided LASIK very effective in eyes with highly

Refractive Laser
Corneal wavefront-guided LASIK very effective in
eyes with highly irregular corneas
Cyres K Mehta
LASIK ablations directed at correcting
corneal wavefront error can bring about
dramatic improvements in vision in
patients who have undergone penetrating
keratoplasty or who have amblyopia due
to corneal aberrations, according to Cyres
K Mehta, MS(Ophth) FASCRS(US),
Mumbai, India.
Speaking at the 2006 EyeAdvance
Congress in Mumbai, Dr Mehta noted
corneal wavefront-guided ablations are
particularly well-suited to eyes with highly
irregular corneas. He noted that 80 per
cent of the aberrations of the eye
originate in the cornea and that the
corrections of corneal aberrations are not
limited by pupil size, allowing optical
zones of up to 8.5mm.
a sphere of -2.0D and a cylinder of -3.0 D,
a highly distorted wavefront and a BCVA
of 6/18.
Three weeks postoperatively the
patients had only 0.75 D of cylinder an
uncorrected visual acuity of 6/6 and
minimal amount of aberrations.
The third patient was an 11-year-old
male anisometropic amblyope with a preoperative UCVA of 6/6 in his right eye but
a BCVA of only 6/18 in his left eye, which
had a sphere of -6.75 D and -0.5 D of
cylinder. One day postoperatively he had
a UCVA of 6/9.
Dr Mehta noted that his findings raise
the question of whether all anisometropic
amblyopes should undergo corneal
wavefront analysis to see if they might
benefit from laser correction of their
higher order aberrations. Also
“The term ‘corblyopia’ refers to corneal amblyopia, where
the eye seems normal and has normal corneal topography
but is significantly aberrated on corneal wavefront
analysis. In such cases BCVA is typically between 6/12 and
6/36 and cylinder is 2.5 D or more”
Cyres K Mehta, MS(Ophth) FASCRS(US)
Dr Mehta noted that one group of
patients heretofore neglected in the field
of refractive surgery are those with a high
degree of unilateral higher order
aberrations, a condition he calls
“corblyopia”.
“The term ‘corblyopia’ refers to corneal
amblyopia, where the eye seems normal
and has normal corneal topography but is
significantly aberrated on corneal
wavefront analysis. In such cases BCVA is
typically between 6/12 and 6/36 and
cylinder is 2.5 D or more,” Dr Mehta
explained.
He added that he has achieved good
results using a corneal wavefront guided
approach in 14 patients with corneal
amblyopia. He noted that while their preoperative BCVA ranged from 6/12 to
6/24, their postoperative BCVA ranged
from 6/9 to 6/6.
Dr Mehta described in detail the preoperative findings and postoperative
results in three corneal amblyopia cases.
The first patient was an 18-year-old
amblyope with 6.0D of cylinder an RMS of
6.0 microns and a BCVA of 6/24.
Following corneal wavefront guided LASIK
he had an RMS of only 2.0 microns and
BCVA improved to 6/12, a gain of two
lines.
The second patient he described was a
27-year-old anisometropic amblyope with
unanswered is the question of what the
youngest age should be for patients
undergoing such procedures.
“Is there a cut-off age? We have
performed corneal wavefront LASIK on
patients as young as eight years with
spectacular results,” he said.
Corneal wavefront technique
When performing the LASIK procedures
Dr Mehta used the Carriazo Pendular
microkeratome and a Schwind Esiris
200Hz flying spot laser, which has a
0.8mm spot size with a Gaussian profile
and a 330 Hz eyetracker.
To obtain corneal wavefront
measurements Dr Mehta uses the
Keratron Scout topographer (Optikon).
The device is a placido disc-based system
with 28 border mires and generates an
arc step reconstruction of the corneal
surface that is accurate to within one
micron, Dr Mehta said.
The accuracy of the topographer
enables the calculation of the optical path
difference between the corneal wavefront
and an ideal wavefront. The distortion of a
wavefront as it emerges from the eye is
equal and opposite to the cornea’s
variance from an ideal shape but only
about one third of its magnitude, he
explained.
“The concept of
optical path
difference is the
keystone of this
whole exercise. It
is the “rule of 3”.
For every three
microns of
distortion on the
corneal surface
there is one
micron of corneal
wavefront error.”
Good results in
Post-PK and
post-RK eyes
Dr Mehta has also
conducted a trial
using corneal
wavefront-guided
treatment of 60
eyes with high
amounts of
aberrations
following
penetrating
keratoplasty and
radial keratotomy.
The study
showed that after
one month’s
follow-ups 15 eyes
(25 per cent)
gained three lines
of BCVA, while 42
eyes (70 per cent)
gained two lines
and three patients
gained four lines. In
addition, the patients’ corneal aberration
RMS decreased by 41 per cent to 56 per
cent. Furthermore, 40 per cent had a
postoperative cylinder less than 1.0 D.
As an example of the kind of results
that can be achieved with corneal
wavefront-guided LASIK in eyes that have
undergone penetrating keratoplasty, Dr
Mehta described the case of a patient
who underwent the procedure five years
after receiving a corneal graft.
The patient had a pre-operative sphere
of -4.0D and a cylinder of -5.0 D and a
BCVA of 6/12. At one month’s follow-up
the treated eye had a plano sphere, only
half a dioptre of astigmatism and a UCVA
of 6/6.
Dr Mehta noted that the rationale
behind using corneal wavefront-guided
LASIK in eyes that have undergone
penetrating keratoplasty is that
approximately eight per cent to 20 per
cent of eyes have significant irregular
astigmatism that cannot be satisfactorily
managed with glasses or contact lenses.
Courtesy of Cyres K Mehta, MS(Ophth) FASCRS(US)
Roibeard O’hEineachain
in Mumbai
He added that while there is no
established time interval between the
initial keratoplasty and LASIK procedures,
most investigators recommend a period of
approximately three to six months after
suture removal, and 12 to 18 months after
the corneal graft procedure. Other factors
to consider are the stability of the cornea
and the integrity of the endothelium.
“Using corneal wavefront guided LASIK
to treat post-keratoplasty and post-RK
eyes has proved very successful, and all
eyes with aberrated corneas whether
post-keratoplasty or post-RK should have
corneal wavefront guided LASIK
performed for optimum improvement in
BCVA and drastic reduction in
aberrations,” he added.
cyresmehta@yahoo.com
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