Congenital malformations - Edocr

Chapter 18
Coloboma
BRAD ANGLE
INTRODUCTION
A coloboma is an ocular malformation consisting
of a cleft, notch, gap, hole, or fissure caused by
absent tissue in the eye. All layers of the eye can
be involved, including the cornea, iris, ciliary
body, choroids, retina, and optic nerve. This section
will focus on iris coloboma. Iris coloboma
can occur as an isolated malformation, in conjunction
with other ocular malformations, or with
other congenital anomalies and malformation
syndromes. The most frequently associated ocular
anomaly is microphthalmia (small globe).
EMBRYOLOGY AND INCIDENCE
Congenital ocular colobomata are caused by
defects in embryogenesis. The eye derives from
three embryological germ layers: neuroectoderm,
which gives rise to optic vesicle; neural
crest cells, which migrate to the anterior chamber
of the developing eye; and the ectoderm,
from which forms the lens placode. Linear invagination
of the optic vesicle at approximately
30 days gestation results in the formation of a
double-layered optic cup and gives rise to the
fetal or choroidal fissure, allowing blood vessels
from the vascular mesoderm to enter the developing
eye. The fetal fissure narrows and closes
during the fifth or sixth week of gestation.
Ocular colobomata result from failure of a
portion of the fetal fissure to close. 1 The defect
may appear as a coloboma of one or more ocular
structures including the iris, lens, ciliary
body, choroid, retina, and the optic nerve. The
incidence of ocular coloboma is approximately
1–2.5 per 10,000 births. 2
ASSOCIATED ANOMALIES AND
SYNDROMES
Although most cases of iris coloboma are isolated
congenital anomalies, they frequently occur
in association with other ocular anomalies
or with multiple congenital anomalies in many
well-defined monogenic disorders, chromosome
abnormalities, and recognized malformation
syndromes. 3 Some of the more common
disorders are listed in Table 18-1 and details of
several of those are discussed below.
Iris Coloboma with Primary Ocular
Abnormalities
Iris coloboma may occur in association with aniridia
(complete or partial iris hypoplasia). Aniridia may,
in turn, occur as part of the Wilms tumor-aniridiagenital
anomalies-retardation (WAGR) syndrome
caused by deletion of chromosome 11p13 involving
genes associated with anirida (PAX6) and
121
Copyright © 2008 by The McGraw-Hill Companies, Inc. Click here for terms of use.