Iris Coloboma
A congenital keyhole-shaped defect of the inferior iris resulting from failure of embryonic fissure closure at 5–7 weeks of gestation. May occur in isolation or as part of a broader ocular or systemic syndrome such as CHARGE.
Last updated: March 2026
Panel A — Anterior View (Slit-Lamp)
Panel B — Cross-Section (Embryonic Fissure Closure)
Panel A: Anterior view showing classic inferonasal keyhole-shaped pupil with absent iris tissue. Panel B: Cross-section comparing normal globe (left) with colobomatous eye (right) demonstrating iris, chorioretinal, and optic disc involvement from incomplete embryonic fissure closure.
Iris coloboma is a congenital structural defect characterised by a sector of absent iris tissue, classically in the inferonasal quadrant, giving the pupil a distinctive keyhole or pear-shaped appearance. It results from incomplete closure of the embryonic (choroidal) fissure during the 5th to 7th week of gestation.
The prevalence of uveal coloboma (including iris coloboma) is approximately 0.5–2.2 per 10,000 live births, making it one of the most common congenital ocular anomalies. Iris coloboma may be unilateral or bilateral, and isolated or part of a broader uveal coloboma spectrum involving the ciliary body, choroid, retina, and optic disc.
When isolated to the iris, visual prognosis is generally excellent, as the macula and optic disc are unaffected. However, the keyhole appearance of the pupil can cause glare and photophobia in bright light, and cosmetic concerns are common. When coloboma extends posteriorly to involve the chorioretinal tissue or optic disc, visual acuity may be substantially impaired and the risk of retinal detachment is significantly elevated.
A thorough systemic evaluation is essential in all cases, as iris coloboma may be the presenting feature of CHARGE syndrome, trisomy 13, Cat Eye syndrome, or other chromosomal abnormalities.
Iris coloboma arises from failure of the embryonic (choroidal) fissure to close completely during the 5th to 7th week of gestation. The fissure normally closes in a zipper-like fashion from mid-point toward both poles. Failure of closure leaves a gap in the developing eye structures.
- Sporadic (majority): Most cases occur without family history; may represent de novo mutations or multifactorial inheritance.
- Familial: Autosomal dominant inheritance with variable expressivity and incomplete penetrance; autosomal recessive forms also reported.
- PAX2 mutations: Renal coloboma syndrome (optic disc coloboma + renal anomalies); less commonly iris coloboma.
- CHD7 mutations: CHARGE syndrome (Coloboma, Heart defect, Atresia choanae, Retardation of growth, Genital abnormalities, Ear anomalies); CHD7 at 8q12 is the most commonly identified gene.
- SHH, FZD5, ABCB6, GDF3, GDF6: Various genes implicated in isolated or syndromic coloboma.
- Teratogens: Maternal thalidomide exposure (historical), alcohol, retinoic acid — associated with coloboma in animal models.
- Chromosomal: Trisomy 13 (Patau syndrome), trisomy 18, Cat Eye syndrome (inv dup 22p), 4p deletion (Wolf-Hirschhorn).
During normal ocular embryogenesis, the optic vesicle invaginates to form the optic cup, creating a groove on the inferior surface — the embryonic (choroidal or hyaloid) fissure — through which the hyaloid vasculature enters. The fissure normally closes between weeks 5 and 7 of gestation by fusion of the neuroectodermal lips.
Closure proceeds from the equatorial region anteriorly and posteriorly simultaneously. Failure at the anterior extent leaves an iris coloboma; failure at the posterior extent results in chorioretinal or optic disc coloboma; failure throughout results in a complete coloboma spanning iris, ciliary body, choroid, retina, and disc.
Because the fissure runs in the inferonasal direction, all colobomas are characteristically located inferonasally. The persistent gap creates a sector of absent iris stroma and pigment epithelium. In the posterior segment, unclosed fissure tissue is replaced by fibrous or neuroectodermal stalk tissue forming the pale white excavation visible ophthalmoscopically.
The thin dysplastic retina at the margins of a posterior coloboma predisposes to retinal breaks and detachment — the most serious long-term complication.
By Extent (Anatomical)
Anterior iris coloboma
Iris only; pupil deformed inferonasally; vision typically unaffected; cosmetic and glare concerns.
Posterior chorioretinal coloboma
White excavated lesion inferior to optic disc; variable visual impairment; high retinal detachment risk.
Optic disc coloboma
Excavation of the optic disc; can cause significant visual impairment and visual field defects.
Complete uveal coloboma
Involves iris, ciliary body, choroid, retina, and disc; most extensive form; worst visual prognosis.
By Laterality and Association
- Unilateral vs bilateral — bilateral involvement more commonly associated with systemic syndromes.
- Isolated vs syndromic — isolated coloboma has better prognosis; syndromic requires multidisciplinary evaluation.
- Complete vs incomplete — refers to extent along the inferonasal meridian from pupil to limbus.
- Family history: Autosomal dominant forms confer 50% offspring risk; may present with variable expression (one parent may have only mild iris notch).
- CHD7 mutation: De novo or inherited; associated with CHARGE syndrome — the most commonly identified syndromic cause.
- Chromosomal abnormalities: Trisomy 13, Cat Eye syndrome (22p duplication), 4p deletion — all associated with coloboma.
- Maternal teratogen exposure: Thalidomide (historical), alcohol, high-dose vitamin A/retinoic acid during first trimester.
- Assisted reproduction / low birth weight: Some studies suggest increased prevalence in premature infants and those born via IVF, possibly due to epigenetic factors.
- Consanguinity: Increases risk of autosomal recessive forms.
- Often asymptomatic (isolated iris coloboma): Many patients with isolated anterior iris coloboma have no visual symptoms and are identified incidentally or on routine screening.
- Photophobia and glare: The coloboma creates an additional uncontrolled aperture; bright light enters through the gap, causing glare and photosensitivity particularly outdoors.
- Reduced visual acuity: Depends entirely on macular or optic disc involvement; isolated iris coloboma leaves visual acuity intact; macular coloboma causes significant central vision loss.
- Visual field defect: Superior visual field loss corresponds to an inferior chorioretinal coloboma (floor of the visual field represented superiorly on retina).
- Cosmetic concern: The keyhole pupil shape is noticeable and may cause psychosocial distress, particularly in children and adolescents.
- Diplopia: If strabismus develops secondary to unilateral or asymmetric vision loss.
- Retinal detachment: The most serious complication; occurs at dysplastic retinal margins of chorioretinal coloboma; risk is 10–40% lifetime in eyes with chorioretinal coloboma; often presents as a tractional or rhegmatogenous detachment originating from the coloboma margin. Regular surveillance is mandatory.
- Amblyopia: From anisometropia (colobomatous eyes are often more myopic), visual deprivation, or media opacity; requires early detection and patching therapy in children.
- Nystagmus: Develops when bilateral extensive coloboma causes bilateral poor central vision; reflects absent foveal fixation.
- Macular hypoplasia: When the coloboma extends to the foveal region, irreversible central vision loss results.
- Secondary glaucoma: Uncommon but reported; may result from angle anomaly associated with coloboma or cyst formation in coloboma.
- Choroidal neovascularisation: Rare complication at coloboma margins; may cause rapid deterioration of residual vision.
CHARGE Syndrome
Most common syndromic association. Caused by CHD7 mutations (8q12). Mnemonic:
- C — Coloboma (most commonly chorioretinal; iris coloboma also present)
- H — Heart defects (tetralogy of Fallot, VSD, ASD)
- A — Atresia choanae (nasal blockage; bilateral causes respiratory distress in neonates)
- R — Retardation of growth and/or development
- G — Genital abnormalities (hypogonadotropic hypogonadism, micropenis, cryptorchidism)
- E — Ear anomalies (external ear deformity, deafness — sensorineural and/or conductive)
Trisomy 13 (Patau Syndrome)
Midline defects including coloboma, cyclopia variants, cleft lip/palate, polydactyly; poor survival.
Cat Eye Syndrome
Trisomy or tetrasomy of chromosome 22p11; iris coloboma is characteristic; also preauricular tags/pits, anal atresia.
Wolf-Hirschhorn (4p deletion)
Coloboma, intellectual disability, cardiac defects, characteristic facies.
Goldenhar Syndrome
Oculoauriculovertebral spectrum; coloboma + preauricular tags + vertebral anomalies; usually unilateral.
- Slit-lamp biomicroscopy: Characterises the iris defect morphology — location, extent, sector involvement; documents whether defect extends to the ciliary body; assesses lens for coloboma-related notch.
- Dilated fundus examination: Essential — indirect ophthalmoscopy to assess for posterior extension (chorioretinal or optic disc coloboma), retinal detachment, or macular involvement; wide-field imaging provides comprehensive documentation.
- Optical coherence tomography (OCT): Macular OCT to assess foveal structure; wide-field OCT or OCT-A to image coloboma margins and detect subretinal fluid.
- Wide-field fundus photography: Documents extent of chorioretinal coloboma for baseline and serial comparison.
- Electroretinogram (ERG) and VEP: Assess retinal function in children; useful when fundus cannot be fully visualised; helps predict visual potential.
- Chromosomal karyotype and microarray: Recommended in all cases; detects trisomy 13, Cat Eye syndrome, and other chromosomal anomalies.
- CHD7 sequencing: If clinical features suggest CHARGE syndrome; sequencing + deletion/duplication analysis.
- Systemic assessment: Cardiac evaluation, audiological testing, renal ultrasound, and paediatric review when syndromic cause is suspected.
Optical and Visual Rehabilitation
- Full cycloplegic refraction; correct any significant anisometropia promptly to prevent amblyopia.
- Tinted soft contact lenses or painted prosthetic iris contact lenses: reduce glare through the coloboma sector and improve cosmetic appearance. Coloured cosmetic lenses should be fitted professionally to match the fellow iris and mask the defect.
- Tinted spectacle lenses (grey/brown UV-blocking) for photophobia.
- Amblyopia patching therapy — gold standard for unilateral amblyopia; initiate as early as possible in children under 7.
- Low vision aids if posterior segment involvement reduces acuity.
Retinal Detachment Surveillance and Intervention
- Annual dilated fundus examination for all patients with chorioretinal coloboma; more frequent if symptoms of floaters/flashes develop.
- Prophylactic laser demarcation of the coloboma margins is controversial — no universal consensus; consider in high-risk eyes.
- Surgical repair of retinal detachment involving the coloboma: vitrectomy with tamponade; outcomes are less predictable than conventional RD repair.
Surgical and Genetic
- No surgical repair for isolated iris coloboma — prosthetic iris implants (artificial iris, Morcher or HumanOptics devices) are reserved for traumatic aniridia and are not routinely used for congenital coloboma.
- Strabismus surgery when angle and magnitude are appropriate.
- Genetic counselling for families — prognosis, recurrence risk, prenatal diagnosis options.
- Systemic management: cardiology, ENT, audiology, and genetics as indicated by syndrome.
Singapore Optometry Scope Note: Optometrists are well-positioned to detect iris coloboma during routine anterior segment examination and to provide ongoing optical management. Fit cosmetic prosthetic iris soft contact lenses (painted lenses) to reduce glare and improve appearance where desired. Prescribe refractive correction promptly and manage amblyopia with patching under appropriate supervision. Refer all patients for dilated fundus examination to exclude posterior segment coloboma — this is essential even if the patient is asymptomatic. If symptoms of floaters, flashes, or curtain-like vision loss are reported, refer urgently to ophthalmology for retinal detachment assessment. Coordinate paediatric and genetic referral in children where CHARGE syndrome or chromosomal anomaly is suspected. Patient education should emphasise the importance of lifelong surveillance for retinal complications.
The visual prognosis of iris coloboma is almost entirely dependent on the extent of posterior segment involvement. Isolated anterior iris coloboma carries an excellent visual prognosis — visual acuity is unaffected, and the main issues are cosmetic and light-related.
When chorioretinal coloboma is present, prognosis depends on whether the macula is spared. Extramacular coloboma preserves central vision but creates a superior field defect. Macular coloboma causes permanent central visual impairment, often 6/60 or worse, which cannot be corrected.
Retinal detachment risk persists lifelong for all patients with chorioretinal coloboma; cumulative lifetime risk is estimated at 10–40%. Prompt surgical intervention improves outcomes but anatomical and functional results are less predictable than conventional retinal detachment repair.
In the context of CHARGE syndrome, the overall prognosis is influenced by associated cardiac, airway, and hearing complications, which require multidisciplinary long-term management.
| Condition | Key Differentiator |
|---|---|
| Iridodialysis | Traumatic separation of iris root from ciliary body; history of blunt trauma; typically temporal/inferior tear, not keyhole shape; anterior chamber angle recession often present. |
| Sector iris atrophy (ICE syndrome) | Acquired; progressive; unilateral; middle-aged women; endothelial changes on specular microscopy; secondary glaucoma; no classic keyhole shape. |
| Surgical peripheral iridectomy | History of intraocular surgery (cataract, glaucoma); full-thickness iris hole at periphery; often at 12 o'clock; surgical scar at limbus. |
| Aniridia | Near-total bilateral iris absence; PAX6 mutation; large exposed pupil; no keyhole configuration; foveal hypoplasia + nystagmus. |
| Iris sphincter rupture | Traumatic; unilateral; irregular pupil margin with notching; history of blunt injury; other signs of anterior segment trauma. |
| Persistent pupillary membrane | Strands crossing pupil from minor arterial circle; not a sector defect; does not extend to limbus; benign remnant of foetal membrane. |
- Shah SP, Taylor AE, Sowden JC, et al; Surveillance of Eye Anomalies (SEA-UK) Special Interest Group. Anophthalmos, microphthalmos, and coloboma in the United Kingdom: clinical features, results of investigations, and early management. Ophthalmology. 2011;118(8):1546–1551. doi:10.1016/j.ophtha.2011.01.055
- Pagon RA. Ocular coloboma. Surv Ophthalmol. 1981;25(4):223–236. doi:10.1016/0039-6257(81)90092-5
- Chang L, Blain D, Bertuzzi S, Brooks BP. Uveal coloboma: clinical and basic science update. Curr Opin Ophthalmol. 2006;17(5):447–470. doi:10.1097/01.icu.0000243020.82380.f6
- Pagon RA, Graham JM Jr, Zonana J, Yong SL. Coloboma, congenital heart disease, and choanal atresia with multiple anomalies: CHARGE association. J Pediatr. 1981;99(2):223–227. doi:10.1016/s0022-3476(81)80454-4
- Williamson KA, FitzPatrick DR. The genetic architecture of microphthalmia, anophthalmia and coloboma. Eur J Med Genet. 2014;57(8):369–380. doi:10.1016/j.ejmg.2014.05.002
- Bermejo E, Martínez-Frías ML. Congenital eye malformations: clinical-epidemiological analysis of 1,124,654 consecutive births in Spain. Am J Med Genet. 1998;75(5):497–504.
- Jesberg DO, Schepens CL. Retinal detachment associated with coloboma of the choroid. Arch Ophthalmol. 1961;65:163–173. doi:10.1001/archopht.1961.01840020165011