Microphthalmos
Definition
Microphthalmos (or microphthalmia) is a congenital eye malformation characterized by a reduction in the volume of the eye
- Corneal diameter typically < 10 mm
- Anteroposterior globe diameter less than 20 mm (or an axial length below 2 standard deviations for chronological age).[1][2]
- One or both eyes may be affected.
- Severe cases the eyeball may appear absent, though residual eye tissue is generally present; this severe form should be distinguished from true anophthalmia, in which no eyeball forms at all.[3]
- Microphthalmos exists on a phenotypic continuum with anophthalmia and coloboma, collectively referred to as the MAC spectrum (microphthalmia/anophthalmia/coloboma).[4][2]
- Combined birth prevalence of anophthalmia and microphthalmia is estimated at up to 30 per 100,000 population,
- Microphthalmia reported in up to 11% of blind children.[5]
- Isolated (nonsyndromic) 66%
- Part of a broader syndrome (syndromic) 33%.[3][5]
Associated Conditions
Ocular Associations
- Coloboma — missing tissue in the iris, retina, choroid, or optic nerv*[3][4]
- Congenital cataract — the most common associated ocular anomaly, found in approximately 32% of cases in one large series[4]
- Persistent fetal vasculature (PFV) and other posterior segment anomalies[4]
- Microcornea — small, abnormally curved cornea[3]
- Anterior segment dysgenesis — corneal opacification, abnormal irides, small abnormal lenses[4]
- Narrowed palpebral fissure[3]
- Ocular motility disorders[4]
Systemic Associations
Approximately 34% of patients have an associated systemic abnormality.[4] Common systemic associations include:
- Neurological abnormalities — most common in the anophthalmic subgroup[4]
- Musculoskeletal and facial anomalies[4]
- Urological and genital anomalies — particularly seen in cases with optic fissure closure defects[4]
- Cardiac defects[6]
- Chromosomal syndromes — trisomy 13, trisomy 18, and others are found in over 20% of cases[6]
Named Syndromes
Numerous syndromes include microphthalmos as a feature (over 269 listed in the London Dysmorphology Database).[2] Notable examples include:
- PDAC syndrome (Pulmonary hypoplasia/agenesis, Diaphragmatic hernia, Anophthalmia/microphthalmia, Cardiac defects)
- AEG syndrome (Anophthalmia–Esophageal–Genital)[2]
Complications
- Visual impairment and blindness — microphthalmos accounts for approximately 12% of permanent blindness in children[6]
- High hyperopia (spherical equivalent >3D)[7]
- Angle-closure glaucoma[7]
- Uveal effusion syndrome[7]
- Retinal detachment[7]
- Chorioretinal folds[7]
- Post-surgical complications — affected eyes present significant surgical challenges with increased risk of intraoperative and postoperative sequelae[7]
- Orbital and facial growth abnormalities — the small globe fails to stimulate normal bony orbital growth, leading to facial asymmetry[5]
Causes
The etiology of microphthalmos is complex and heterogeneous, encompassing genetic, chromosomal, and environmental factors.[5][8]
Genetic Causes
- SOX2 — the single most commonly implicated gene and the only major causative gene identified to date*[5][8]
- PAX6 and OTX2 — important transcription factors in eye development*[9][5]
- Other monogenic causes — CHX10 (VSX2), RAX, FOXE3, STRA6, ALDH1A3, RARβ, BMP4, BMP7, SMOC1, BCOR, YAP1, TFAP2α, GDF3, GDF6, SHH, MFRP, PRSS56, among others*[9][8][3]
- Approximately 50% of patients have identifiable mutations in known genes, though more than half currently remain without a molecular diagnosis even after whole exome/genome sequencing*[6][8]
Chromosomal Causes
- Chromosomal duplications, deletions, and translocations[5]
- Recognized aneuploidies including trisomy 13 and trisomy 18[6]
Environmental Causes
- Gestational infections — rubella and other congenital infections (strongest environmental evidence)[5][3]
- Maternal vitamin A deficiency[5]
- Radiation exposure (X-rays)[5]
- Teratogen exposure — thalidomide, solvent misuse[5][3]
Pathogenesis
Microphthalmos is believed to arise from disruption of early embryonic eye development, including failure of optic vesicle formation, lens induction failure (SOX2, PAX6 pathway), failure of retinal differentiation (OTX2, CHX10, RAX pathway), or abnormalities in optic fissure closure.[5][6] The condition typically originates early in pregnancy, though at least one report has suggested identification of microphthalmia development in midpregnancy.[1]
Management
- Maximizing existing vision and improving cosmesis through stimulation of soft tissue and bony orbital growth.
- Mild to moderate cases are managed conservatively with conformers
- severe microphthalmos and anophthalmia may require endo-orbital volume replacement with implants, expanders, or dermis-fat grafts and soft tissue reconstruction.[5]
- Genetic counselling is indicated but can be challenging due to the extensive genetic heterogeneity and variable phenotypic expression.[5][8]