Astigmatism and the advent of
photoastigmatic refractive keratotomy
Astigmatism, a refractive condition where the
surface of the cornea is not spherical, can decrease visual acuity by forming a
distorted image because light images focus on 2 separate points in the eye.
Clinicians and surgeons have searched constantly for the most successful device
or procedure to treat this refractive error. Nonsurgical devices include
spectacles and contact lenses. To date, these devices are being improved
continuously to address the complex problem of astigmatism. Initial surgical
approaches include astigmatic keratotomy, compression sutures, and wedge
resection. Recent surgical procedures involve the use of the excimer laser in
PARK and LASIK with or without wavefront-guided technology.
PRK is the application of ultraviolet
high-energy photons (193-nm wavelength) of the ultraviolet range generated by an
argon fluoride excimer laser to the anterior corneal stroma to change its
curvature and, thus, to correct a refractive error. The physical process of
remodeling by PRK is called photoablation. This surgical procedure reshapes the
central cornea to a flatter shape for people who are nearsighted and a more
curved surface for people who are farsighted. Several techniques are being used
to correct for astigmatism.
Device description
Two different methods of energy delivery are
available by the excimer laser device, a large circular beam and a scanning slit
or spot.
The earlier devices initially used large area
ablation. To date, some manufacturers still use large area ablation in their
modern devices. The circular laser beam passes through a diaphragm that slowly
enlarges to deliver more cumulative energy in the center and less in the
periphery. Some laser-induced irregularities (central islands) have been
reported in these large area systems. This method results in a shorter operating
time to deliver the necessary laser pulses versus a system that uses a scanning
slit system. The following manufacturers use circular beam lasers: Schwind
(Coherent Medical Inc, Palo Alto, Calif), Summit (Waltham, Mass), and VISX
(Santa Clara, Calif).
The scanning slit or spot is an alternative
method of energy delivery by the excimer laser. A smaller beam passes through a
beam-shaping aperture, delivering a pattern of more pulses centrally than
peripherally and resulting in greater corneal tissue ablation centrally. Less
total energy is delivered at the corneal surface; therefore, a less powerful
laser device may be used. In principle, this system is more effective in
providing different ablation patterns in the treatment of astigmatism, irregular
astigmatism, and hyperopia.
The use of scanning laser technology with its
small moving beam has resulted in reduced thermal heating. This is visualized in
a study that showed the different areas of plume production after each area of
ablation following movement of the scanning beam. Central islands have not been
reported in these systems. The smaller ablation size of the scanning laser
consequently results in a longer operating time. Maintaining fixation has always
been a problem for these scanning lasers, especially with the longer operating
time, which results from more ablations by the smaller beams. Moreover, precise
pulse-to-pulse registration of the scan is necessary to achieve a smooth and
accurate final pattern. Automatic tracking devices are provided standard in
these devices.
Manufacturers of scanning slit systems include
the following: Autonomous (Orlando, Fla), LaserSite (Orlando, Fla), Meditec (Aesculap-Meditec,
Heroldsberg, Germany), Nidek (Fremont, Calif), Novatec (Carlsbad, Calif), and
Technolas (Chiron Vision Corp, Irvine, Calif). Novatec's claim to fame is its
use of solid-state laser crystals that obviates the need for argon fluoride gas
to create its shorter ultraviolet beam. Presently, the Food and Drug
Administration (FDA) has approved only the Summit and the VISX laser systems for
commercial use within the United States. All the other systems currently are
being used in other countries.
History of the Procedure:
The history of surgical treatment for astigmatism dates
back to the late 1800s. Certain milestones in the development of this procedure
can be attributed to several individuals, and a number of parallel procedures
were in development at certain time periods.
In 1885, Schiotz performed limbal incision in
the steep meridian to reduce iatrogenic astigmatism. Faber performed anterior
transverse incisions to reduce idiopathic astigmatism. Lucciola also performed
nonperforating corneal incisions to correct astigmatism. In 1894, Bates
postulated that corneal incisions made at right angles to steeper meridians
might correct astigmatism. Later, Lans showed that flattening in the meridian
perpendicular to a transverse incision was associated with steepening in the
orthogonal meridian and that a greater effect may be achieved with deeper and
longer incisions.
In the 1940s, Sato began an extensive
investigation of radial and astigmatic keratotomy. Fyodorov is responsible for
presenting several nonperforating anterior keratotomy patterns.
Modern techniques for astigmatic keratotomy
are attributed to the works of Nordan, Thornton, and Nichamin. Nordan advocated
a simple method of straight transverse keratotomy, with a target correction of
1-4 diopters (D). Thornton proposed a technique that included up to 3 pairs of
arcuate incisions in varying optical zone sizes and with consideration of age
and timing after surgery, respectively. Nichamin developed an extensive nomogram
for astigmatic keratotomy at the time of cataract surgery, although this
technique has been modified by the use of a limbal relaxing incision during
cataract surgery. Consequently, Troutman, who fancied wedge resection for
reduction of postcorneal transplant astigmatism, developed another technique of
astigmatism reduction.
Increased interest in using lasers to ablate
tissue occurred in the late 1980s. The excimer laser initially was developed to
etch out inscriptions on microchips. The postulated application of controlled
ablation on corneal tissue led to its use in refractive correction.
In the late 1990s, wavefront aberrometry
promised both physicians and patients the potential to achieve the so-called
supervision. Initially used by astronomers, this wavefront technology reduced
unwanted wavefront distortions in the creation of land-based telescopes.
Problem:
Refractive errors (ie, myopia, hyperopia, astigmatism) can
decrease visual acuity. Astigmatism is a more challenging entity because it is
determined by regularity, amount, and orientation. It also is more difficult to
treat than myopia or hyperopia.
The quest to treat astigmatism began with the
use of nonsurgical devices, including spectacles and contact lenses. These
nonsurgical devices were followed by surgical techniques involving astigmatic
keratotomy, compression sutures, and wedge resection. Newer surgical procedures
include the use of intracorneal ring segments, PARK, LASIK, and LASEK, with or
without wavefront-guided technology.
Frequency:
The frequency of astigmatism has a wide range of values as
presented in modern literature. Naturally occurring (idiopathic) astigmatism is
common. Surgically induced (iatrogenic) astigmatism is less common yet more
problematic.
Clinically detectable refractive astigmatism
reportedly is present in as many as 9 out of 10 eyes. However, refractive
astigmatism in most of these eyes would not be clinically significant. The
incidence of clinically significant astigmatism has been reported to be 7.5-75%,
a wide range that primarily depends on the specific study and an author's
definitions. Studies have estimated that approximately 44% of the population has
more than 0.50 D of astigmatism, 10% of the population has more than 1.00 D, and
8% of the population has 1.50 D or more.
Etiology:
Aside from the previously mentioned idiopathic astigmatism that
is present, iatrogenic astigmatism may result after surgery. Visually
significant refractive astigmatism is fairly common after different kinds of
ophthalmic surgery, including cataract extraction, lamellar or penetrating
keratoplasty, other corneal and anterior segment surgeries, and trabeculectomy.
Reportedly, astigmatism of at least 1.00 D
often results after extracapsular cataract extraction, and astigmatism of at
least 3.00 D is present in as many as 20% of cases with 10-mm incisions. Even
phacoemulsification procedures, using the clear cornea technique, reportedly
cause postoperative astigmatism, thereby guiding the cataract surgeon as to the
proper placement of the corneal approach. High astigmatism usually results after
penetrating keratoplasty.
Pathophysiology:
The means of ablation of the excimer laser seem to be
photochemical in type. This removal of tissue is called photochemical ablation
or ablative photodecomposition. Photochemical ablation is an extremely confined
tissue interaction centered on the fact that every photon created by the ArF
excimer laser has 6.4 eV of energy, which is sufficient to split covalent bonds.
The intramolecular bonds of uncovered organic
macromolecules are split when a sufficient number of high-energy, 193-nm photons
are absorbed in a brief period. The resulting fragments rapidly expand and are
ejected from the exposed surface at supersonic velocities observed under
high-resolution magnification as the plume effect. This is the reason why only
the irradiated organic materials are affected and the adjacent areas are not
affected.
Clinical:
A patient with astigmatism may complain of shadowing, bending,
loss of contrast, and distortion. Astigmatism is believed to be the most common
cause of ametropia. In mild cases, it may cause blurring of vision and ghosting.
In more advanced cases of untreated astigmatism, amblyopia may be noted.
Astigmatism may occur naturally (idiopathic) or secondary to surgical procedures
(iatrogenic), such as cataract extraction and penetrating keratoplasty. Several
clinical procedures may be performed to detect astigmatism. These procedures
include automated and/or manifest refraction, keratometry, Placido ring
reflections, corneal topography, and wavefront aberrometry.
