Isolated Microphthalmia-Anophthalmia-Coloboma

A non-syndromic group of structural developmental eye defects characterized by the variable combination of microphthalmia, ocular coloboma, and anophthalmia, either unilaterally or bilaterally, with no other associated ocular conditions in the affected/contralateral eye, and no systemic anomalies.

Epidemiology

The prevalence of microphthalmia is 1:7,000, anophthalmia is 1:30,000 and coloboma is 1:5,000 live births, with combined prevalence 3-30:100,000 births. Associated malformations affect 32-93% of the patients. There is no clear predilection for ethnicity or gender.

Clinical description

Microphthalmia-anophthalmia-coloboma (MAC) consists of phenotypic continuum of congenital eye defects that are manifest at birth. In some cases, such as retinal coloboma or mild microphthalmia, detection may occur later in life. True anophthalmia is the abortion of eye development at the developing optic vesicle stage (3-4 weeks gestation) leading to absence of the eye, optic nerve and chiasm. Commonly clinical anophthalmia (also referred to as severe microphthalmia) occurs, where a small cystic remnant is detectable on pathology/imaging. Nanophthalmos and posterior microphthalmia, are rare subsets of microphthalmia, where overall the eye is structurally normal but it has a reduced axial length of <20 mm with high hypermetropia. Ocular coloboma may involve the inferonasal aspect of the eye, including the iris, ciliary body, zonules, retina, retinal pigment epithelium (RPE), choroid and/or optic disc.

Etiology

MAC has a complex etiology, with monogenic, chromosomal and environmental causes. SOX2, OTX2 and STRA6 variants account for 75% of bilateral anophthalmia/severe microphthalmia. Chromosomal abnormalities account for 20-30% of MAC. Environmental factors associated with anophthalmia include maternally-acquired infections, smoking and antenatal exposure to certain medications. Maternal vitamin A deficiency, alcohol abuse and use of teratogenic drugs during pregnancy have been linked to coloboma and microphthalmia.

Diagnostic methods

Postnatal diagnosis can be made through clinical examination, with confirmation of true/clinical anophthalmia through MRI brain and orbit imaging. Molecular diagnosis can be made through genetic testing, such as array comparative genomic hybridization (aCGH) or whole exome/genome sequencing.

Differential diagnosis

Differential diagnoses includes aniridia, anterior segment dysgenesis, congenital corneal opacity, sclerocornea, cryptophthalmos, cyclopia and congenital cystic eye. MAC may also occur as part of various syndromes, and thus examination by specialists for the presence of systemic features (e.g. associated neurological or pituitary defects) is recommended. Genetic diagnosis may aid the identification of potential systemic anomalies.

Antenatal diagnosis

Prenatal diagnosis of anophthalmia or microphthalmia may be made through 2D or 3D ultrasonography during the second trimester (or 12 weeks post-conception with a transvaginal ultrasound) or fetal magnetic resonance imaging to visualize the orbit.

Genetic counseling

Genetic counselling can be challenging due to the range of known genetic causes and phenotypic variability. Prediction of inheritance pattern is often difficult, due to de novo changes, mosaicism and non-penetrance. If a genetic diagnosis is established, informed family planning advice can be provided including prenatal and preimplantation diagnosis.

Management and treatment

There is no treatment for MAC patients. They should be managed by a multidisciplinary team of specialists, including ophthalmologists, pediatricians and clinical geneticists. If there is visual potential, children should be monitored to maximize vision by correcting refractive error or squints, and preventing amblyopia. Fundus examinations are required in patients with chorioretinal coloboma as it is associated with a risk of retinal detachment. Low vision should be supported using visual aids. Significant microphthalmic or anophthalmic eyes may undergo socket expansion using enlarging cosmetic shells/conformers to minimize facial deformity.

Prognosis

Isolated MAC are structural birth defects with no treatment available.