Aniridia

Aniridia is a congenital bilateral panocular disorder characterised by near-total absence of the iris, resulting from haploinsufficiency of the PAX6 gene located on chromosome 11p13. The condition presents at birth and affects approximately 1 in 64,000–100,000 live births.

Despite its name, aniridia is not a purely iris condition. It is a panocular developmental disorder affecting the iris, cornea, lens, fovea, and optic nerve. The rudimentary iris stump that remains is typically only visible on gonioscopy or ultrasound biomicroscopy (UBM).

Associated ocular features include foveal hypoplasia (limiting central vision), nystagmus (present from birth in the majority), glaucoma (developing in 50–75% of patients over time), anterior polar or lamellar cataract, and aniridic keratopathy due to limbal stem cell deficiency. Vision is commonly limited to 6/24–6/60 due to foveal and optic nerve hypoplasia.

Approximately one-third of cases are sporadic (de novo mutations). When a large chromosomal deletion at 11p13 includes the adjacent WT1 tumour suppressor gene, WAGR syndrome results — a serious multisystem condition associated with Wilms tumour (nephroblastoma), genitourinary anomalies, and intellectual disability. All sporadic cases require chromosomal microarray testing to exclude WAGR syndrome.

The overwhelming majority of aniridia cases result from mutations in the PAX6 gene (paired box gene 6) at chromosome 11p13. PAX6 encodes a transcription factor critical for ocular, pancreatic, and neural development. Over 500 pathogenic variants have been described, the majority being loss-of-function mutations (nonsense, frameshift, splice-site).

• Autosomal dominant familial aniridia (~2/3 of cases): Heterozygous point mutations or small intragenic deletions in PAX6; penetrance is near-complete with variable expressivity. Positive family history is present.
• Sporadic de novo mutations (~1/3 of cases): No family history; may represent new point mutations or large chromosomal deletions. Chromosomal microarray is mandatory in sporadic cases.
• WAGR syndrome: Large contiguous deletion at 11p13 encompasses both PAX6 and the adjacent WT1 (Wilms tumour 1) gene, conferring a 30–50% lifetime risk of Wilms tumour. Occurs exclusively in sporadic cases.
• Gillespie syndrome: A distinct clinical entity caused by ITPR1 mutations (not PAX6), presenting with partial aniridia, cerebellar ataxia, and intellectual disability.

PAX6 is the master transcription factor for vertebrate eye development, acting as a dosage-sensitive regulator during embryogenesis. A single functional copy of PAX6 is insufficient to drive normal ocular development — haploinsufficiency is the pathomechanism.

During normal development, the optic cup induces formation of the lens placode, which in turn signals iris and ciliary body formation from the neuroectoderm and neural crest. PAX6 haploinsufficiency disrupts this inductive cascade, resulting in:

• Iris: Arrested development of the iris at an early stage, leaving only a small peripheral rudimentary stump. Trabecular meshwork dysgenesis contributes to glaucoma risk.
• Cornea: Limbal stem cell niche disruption leads to progressive limbal stem cell deficiency (LSCD). Conjunctival epithelium migrates centripetally onto the corneal surface, causing aniridic keratopathy — vascularisation, pannus formation, and eventual opacification.
• Lens: Disordered lens epithelial cell organisation produces anterior polar or lamellar cataracts, often progressing to posterior subcapsular cataract.
• Fovea: PAX6 is required for foveal pit formation and photoreceptor differentiation; haploinsufficiency arrests foveal development, causing foveal hypoplasia with absent foveal avascular zone and absent foveal reflex.
• Optic nerve: Optic nerve hypoplasia with reduced axon number contributes to permanent visual impairment independent of other structural anomalies.

The progressive nature of glaucoma, keratopathy, and cataract means that visual function often deteriorates throughout life even with optimal management.

Aniridia is associated with progressive, sight-threatening complications that require lifelong surveillance:

• Aniridic keratopathy (limbal stem cell deficiency): Progressive conjunctivalisation of the corneal surface — vascularisation, pannus, stromal scarring, and eventual opacification. The most common cause of acquired corneal blindness in aniridia. Frequency increases with age.
• Glaucoma: Develops in 50–75% of patients; most common cause of severe progressive visual loss in aniridia. Results from angle dysgenesis and progressive closure of the trabecular meshwork by the rudimentary iris stump in later life.
• Cataract: Anterior polar cataract is near-universal; posterior subcapsular cataract develops in many adults. Cataract surgery is challenging due to absent iris, zonular weakness, and risk of vitreous prolapse.
• Foveal hypoplasia: Permanent structural limitation; central vision rarely recovers regardless of treatment. OCT demonstrates absent foveal pit and abnormal inner retinal layer persistence.
• Amblyopia: Occurs if anisometropia, media opacity, or uncorrected refractive error is not managed in early childhood.
• Optic nerve hypoplasia: Contributes to reduced visual potential independent of other structural anomalies.
• Wilms tumour (WAGR): Life-threatening renal malignancy in 30–50% of patients with WT1 deletion; presents typically before age 5.

Other reported associations: Type 2 diabetes mellitus (PAX6 role in pancreatic beta-cell development), anosmia, brain malformations (rare), and mild cognitive differences in familial aniridia (PAX6 expression in developing brain).

Diagnosis is usually clinical but must be confirmed genetically, particularly in sporadic cases:

• Slit-lamp biomicroscopy: Reveals absent or near-absent iris with visible rudimentary peripheral stump; corneal pannus assessment; lens status.
• Gonioscopy: Demonstrates trabecular dysgenesis, iris stump insertion, and angle configuration; essential for glaucoma risk stratification.
• Ultrasound biomicroscopy (UBM): Best modality to image the residual iris stump, ciliary body, and anterior angle structures; useful when media opacity precludes direct view.
• OCT (optical coherence tomography): Foveal OCT confirms hypoplasia — absent foveal pit, persistence of inner retinal layers at foveal centre, reduced cone density on HS-OCT.
• Electrodiagnostics: Visual evoked potentials (VEP) and electroretinogram (ERG) assess retinal and cortical function; useful in children too young for subjective testing.
• Genetic testing: PAX6 sequencing (Sanger or next-generation sequencing) for point mutations; chromosomal microarray (chromosomal SNP array) to detect large deletions including WT1 — mandatory in sporadic cases.
• Renal ultrasound: If WAGR syndrome suspected or WT1 deletion identified; repeated every 3 months until age 8.
• IOP measurement and optic disc assessment: Regular Goldmann applanation tonometry or iCare rebound tonometry; OCT-RNFL for glaucoma monitoring.

Management is multidisciplinary and lifelong, targeting photophobia, corneal surface disease, glaucoma, cataract, amblyopia, and refractive error:

Visual prognosis in aniridia is guarded. The structural limitations imposed by foveal hypoplasia and optic nerve hypoplasia result in permanent reduction in best-corrected visual acuity, typically 6/60 to 6/24, regardless of treatment. Correction of refractive error and amblyopia management may achieve modest improvement but central vision rarely reaches 6/12.

Glaucoma is the most common cause of severe progressive visual loss and can reduce vision to counting fingers or hand movements if uncontrolled. Early and lifelong glaucoma surveillance is essential.

Aniridic keratopathy (LSCD) has a progressive natural history; without intervention, corneal opacification and vascularisation advance over decades, potentially leading to corneal blindness. Limbal stem cell transplantation can arrest progression but success rates are variable and repeat surgery may be required.

Nystagmus persists lifelong. In WAGR syndrome, the prognosis is substantially influenced by the onset, stage, and treatment of Wilms tumour; with early detection (renal ultrasound surveillance), tumour prognosis is markedly improved.