0521779407-18 CUNY1086/Karliner 0 521 77940 7 June 13, 2007 8:1
Refractive Disorders (Ametropias) 1275specific therapy
■Myopia
➣Optical correction – Myopia can be corrected with a minus power,
or concave spectacle or contact lens. Degree of myopia measured
by the power of the concave lens to correct it, as expressed in units
of diopters (D).
➣Surgical correction – Myopia and myopic astigmatism can be
corrected by a variety of methods:
➣Keratorefractive procedures that flatten the central cornea or
otherwise reduce the effective convex power of the cornea, or
lens implant-based procedures that either replace the crystalline
lens or add a lens implant that has reduced convex power.
Keratorefractive methods of myopia correction include:
■LASIK (laser in situ keratomileusis) – a microkeratome or a femtosec-
ond laser creates a thin flap of corneal tissue that is then lifted, and
the underlying corneal stromal bed is precisely photoablated by an
ArF 193 nm excimer laser.
■PRK (photorefractive keratectomy) – the corneal tissue is ablated
directly, without first creating and lifting a corneal flap
■LASEK (laser subepithelial keratectomy) – in this variant of PRK, the
epithelium is elevated manually or with a modified microkeratome,
and excimer ablation is performed on the underlying stroma
■RK (radial keratotomy) – deep incisions in the midperipheral and
peripheral cornea made in radial fashion cause central flattening.
Radial keratotomy, while common in the past, is uncommonly per-
formed today.
■ICRS (intrastromal corneal ring segment implantation) – arc-like seg-
ments of clear polymethylmethacrylate (Intacs) are inserted into the
peripheral corneal mid-stroma, inducing central corneal flattening
Lens-based methods of myopia correction include:
■Phakic intraocular lens implantation – an artificial lens is placed in
the anterior or posterior chamber, but the natural crystalline lens is
left intact
■Lens extraction with intraocular lens implantation – in patients with
cataract, a spherical or toric intraocular lens can be implanted after
the cataract has been removed, thereby correcting any co-existing
ametropia. Clear lens extraction in the absence of cataract is gen-
erally avoided as a means of correcting high myopia because of the
elevated risk of postoperative retinal detachment in these patients.
■Hyperopia
➣Keratorefractive procedures that steepen the central cornea or
otherwise increase the effective convex power of the cornea, or