Comprehensive Analysis of Congenital Aniridia and Differential Diagnoses: Genetic Insights and Clinical Manifestations

In CA, the extent of iris tissue absence varies significantly between individuals, even among those with the same causative genetic variant. This variability ranges from mild iris hypoplasia, increased transillumination, abnormal pupil configuration, and atypical coloboma or ectropion uveae to the complete absence of the iris [2]. However, even in severe cases that appear to involve a complete absence of the iris, a remnant of iridian tissue is typically present. This can be identified through gonioscopy, anterior-segment optical coherence tomography (AS-OCT), and histopathological evaluation [11, 22, 23].

Findings of keratopathy in CA are commonly referred to as aniridia-associated keratopathy (AAK) (Fig. 1). AAK is prevalent, affecting 70–80% of patients, and can significantly impair visual acuity [7]. The prevalence and severity of AAK increase with patient age [24]. Limbal stem cell deficiency (LSCD) is recognized as a critical factor in its development. The Palisades of Vogt, a clinical marker for LSCD, are initially altered and eventually absent in eyes with congenital aniridia [25]. At birth, the cornea is either clear or exhibits peripheral thickening and peripheral pannus [26, 27]. Corneal neovascularization initially presents more prominently in the superior and inferior regions but may later progress to a circumferential pattern [28]. Over time, progressive corneal opacification extending from the periphery to the center, pannus formation, and epithelial erosions become apparent [29, 30]. The migration of blood and lymphatic vessels, infiltration of inflammatory cells, and alterations in corneal nerves can be detected. These changes are observable through slit-lamp biomicroscopic examination and in vivo confocal microscopy (IVCM) [31, 32].

Bild vergrößern

Furthermore, intraocular surgery in patients with congenital aniridia has been shown to exacerbate the severity of keratopathy (IVCM) [31, 33]. Additionally, microcornea is more commonly observed in congenital aniridia eyes.

The prevalence of dry eye syndrome is notably high among individuals with diagnosed congenital aniridia. Clinical studies have demonstrated a loss and dysfunction of meibomian glands, along with increased osmolarity and tear film instability [34, 35]. Dry eye disease (DED) and AAK appear to be interrelated, with each condition contributing to the progression of the other [35].

The incidence of secondary glaucoma in patients with CA is notably high, affecting more than half of all patients, often manifesting during adolescence or early adulthood [7, 36]. The reported prevalence of aniridia-associated glaucoma varies widely, ranging from 6 to 75% [37]. Glaucoma is typically diagnosed at a mean age of 15 years, but approximately 15% of patients with CA are diagnosed before the age of 10 [38]. In children with CA, gonioscopy reveals progressive changes in the iridocorneal angle. Over time, attachments of the iris stroma gradually obstruct the trabecular meshwork, forming a membrane-like structure that leads to elevated intraocular pressure (IOP) [23, 39]. Prolonged elevated baseline IOP and repeated surgical interventions are associated with poorer visual outcomes [40]. In most cases of aniridia-associated glaucoma, topical treatments eventually become inadequate, necessitating surgical intervention. Undetected secondary glaucoma poses the greatest risk for irreversible vision loss in patients with aniridia [28].

Minor lens opacities are often detectable from birth in congenital aniridia, typically located at the anterior or posterior pole [28]. However, cataract-associated visual impairment tends to manifest later. According to a large survey, the median age of cataract diagnosis in congenital aniridia is 14 years, highlighting juvenile cataracts as a common feature of the condition [41]. Most cataracts develop within the first two decades of life, as evidenced by a recent study involving 556 eyes with CA [7]. Overall, lens opacities are observed in 50–90% of patients with congenital aniridia [37, 42]. In the cohort of 556 eyes, with a mean age of 20 years, cataracts were found in 40% of eyes, pseudophakia in 23%, and aphakia in 6%. Additional lens abnormalities included lens subluxation, occurring in approximately 2% of eyes, and aphakia in 6% [7]. Rarely reported lens abnormalities include lens coloboma, posterior lenticonus, and microspherophakia in congenital aniridia [43].

It has been reported that the risk of complications during cataract surgery in patients with congenital aniridia is elevated due to factors such as lens capsule fragility, weak zonules, or lens subluxation [44]. Additionally, the lens capsule in younger patients with CA appears to be significantly thinner [45]. Furthermore, cataract surgery in patients with CA may trigger the onset or exacerbation of AAK and ocular hypertension [33, 46].

Foveal hypoplasia (FH) is the most common finding in CA following iris anomalies occurring in approximately 70 to 90% of patients [7, 46‐48]. Foveal hypoplasia is associated with nystagmus and contributes to decreased visual acuity [38]. The underdevelopment of the fovea leads to characteristic features such as a diminished foveal reflex, absence of a foveal pit, macular hypopigmentation, and retinal vessels encroaching upon the avascular zone [2]. Additionally, the retina in congenital aniridia eyes typically exhibits hypopigmentation [43]. Patients with CA demonstrate varying degrees of foveal hypoplasia on macular optical coherence tomography (OCT), which can serve as a predictor of visual acuity in children. Notably, foveal minimum and central macular thickness are increased in these patients [49, 50]. Both full-field and multifocal electroretinography (ERG) are affected in individuals with CA, reflecting widespread retinal dysfunction [51, 52].

The prevalence of nystagmus in congenital aniridia is remarkably high, affecting up to 95% of patients [7, 48]. Most cases manifest as pendular horizontal nystagmus [26]. Nystagmus in CA is strongly associated with the grade of foveal hypoplasia and with a significantly reduced visual outcome [7, 48].

Optic nerve hypoplasia is observed in approximately 10% of patients with congenital aniridia. It may occur in isolation or in conjunction with foveal hypoplasia [53]. In 1974, the first authors to describe optic nerve anomalies in CA suggested that this condition might result from incomplete retinal development, as the iris pigment epithelium, iris musculature, and retina all originate from the neuroectoderm [54]. The specific impact of optic nerve hypoplasia on visual impairment in CA remains unclear. Other optic nerve anomalies, such as optic nerve dysplasia and a small optic nerve head, are also frequently reported in congenital aniridia [7, 28, 55].

In over 95% of cases, CA is caused by haploinsufficiency of the transcription factor Paired box-6 gene (PAX6), typically resulting from heterozygous pathogenic variants that lead to reduced gene dosage [57]. Identified in 1991, PAX6 plays a pivotal role in neurogenesis and oculogenesis across evolutionary scales [58]. In ocular development, PAX6 is expressed early in the optic vesicle and optic cup, contributing to the formation of the neural and pigmentary retina. It is also expressed in the surface ectoderm of the crystalline vesicle and differentiating cells of the conjunctiva, cornea, ciliary body, and lens [59]. Beyond the eye, PAX6 expression extends to the forebrain, ventral spinal cord, olfactory system, and endocrine pancreas, underlining its broad developmental significance. Haploinsufficiency of PAX6 results in diverse phenotypes. In mouse models, such as the Small eye (Sey) model, this manifests as microphthalmia, whereas in humans, it predominantly causes hypoplasia of the iris and fovea. These findings underscore the critical regulatory function of PAX6 in both eye-specific and systemic developmental processes [60].

The PAX6 gene is located in 11p13.3 [61] and consists of 14 exons (11 of which are coding) that produce at least two main protein isoforms comprising 422 and 436 amino-acids, respectively. These isoforms feature two DNA-binding domains: a conserved 128-amino-acid paired domain characteristic of the Pax family of transcription factors and a homeodomain [57, 62].

The expression of PAX6 is tightly regulated by numerous regulatory elements located downstream (3’), intragenically, and upstream (5’). The most critical regulatory elements for ocular development are in the downstream regulatory region (DRR), which resides in introns 7–9 of the adjacent ELP4 gene, approximately 150 kb downstream of PAX6.

PAX6-associated aniridia results from heterozygous pathogenic variants or chromosomal rearrangements within the 11p13.3–11p14 region. These pathogenic variants are either inherited, displaying autosomal-dominant inheritance in about two-thirds of cases, or de novo with a sporadic appearance in approximately one-third of the cases [2]. Biallelic pathogenic variants in PAX6—either compound heterozygous or homozygous—completely disrupt eye development, leading to anophthalmia and in utero death, due to associated central nervous system defects [62‐64].

To date, over 700 PAX6 pathogenic variants have been identified. The former Human PAX6 Mutation Database listed 491 distinct variants in its last update in August 2018. Since then, more than 250 additional variants have been reported [9, 65‐70]. Unfortunately, no comprehensive pathogenic variants database currently exists to document the full mutational spectrum of PAX6-associated congenital aniridia.

Approximately 96% of disease-causing variants are intragenic mutations, found in 70–80% of patients. The remaining 4% involve microdeletions of varying sizes, which may encompass partial or complete deletions of PAX6 and potentially affect neighboring genes [8]. These may be associated with syndromic conditions such as WAGR syndrome (involving the WT1 gene) or the rarer WAGRO syndrome (involving the BDNF gene).

In some patients, PAX6 haploinsufficiency is associated with transcriptional deregulation mediated by the DRR. This is most commonly linked to 3' microdeletions of approximately 500–600 kb, affecting the DRR within the ELP4 gene, as well as other downstream genes such as IMMP1L, DNAJC24 and DCDC1 [71‐75]. Additionally, PAX6 deregulation can arise through other mechanisms. These include direct disruption of DRR-associated regulatory elements, "positional effects" caused by chromosomal translocations or inversions that physically separate the DRR from PAX6, and point variants within specific regulatory elements such as SIMO (a DRR-located regulatory element) [76, 77]. Rarely, chromosomal rearrangements, such as inversions involving intragenic regions of PAX6, have also been described [76].

Overall, chromosomal rearrangements involving the PAX6 locus at 11p13 are found in 25–30% of patients, including cases with WAGR-associated microdeletions [8]. These rearrangements underscore the intricate mechanisms underlying PAX6-associated aniridia and its related syndromic conditions.

Overall, approximately 80% of PAX6 pathogenic variants are coding variants that result in a premature termination codon (PTC), leading to haploinsufficiency and to the manifestation of classical aniridia [48, 78].

Approximately 96% of PAX6 pathogenic variants are intragenic. The remaining 4% involve complete gene deletions or pathogenic variants located in the 5' and 3' regulatory regions. These non-intragenic alterations can disrupt the gene's expression and contribute to the development of PAX6-associated conditions [8].

Using the last updated version of the discontinued Human PAX6 Mutation Database, Lima-Cunha et al. reported the frequency distribution of intragenic pathogenic variants associated with the aniridia phenotype as follows: nonsense variants (39%), frameshift indels (27%), splice variants (15%), missense variants (12%), C-terminal extension (CTE) mutations (2%), and in-frame indels (2%) [8]. These non-intragenic alterations can disrupt the gene's expression and contribute to the development of PAX6-associated conditions. Studies by Plaisancie & Tarilonte et al. uncovered variants in 5' UTRs and intronic regions in several PAX6-negative cases [77]. Further research has confirmed that these variants disrupt transcriptional regulation or splicing processes, contributing to the development of aniridia and related phenotypes [66, 77, 79].

Even though PAX6 proteins variants with C-terminal deletions show dominant negative activity, the associated phenotypes are not suggestive of increased severity [78, 80, 81]. Another report suggests that CTE pathogenic variants as well as loss-of-function pathogenic variants cause more severe phenotypes [48]. In a few reports, exudative retinopathy or chorioretinal degenerative abnormalities have been documented in patients with CTE pathogenic variants. On the other hand, missense pathogenic variants were found to be associated to rather mild phenotypes [48, 82, 83]. Very few patients have been described with biallelic PAX6 variants [62, 84, 85]. This condition is typically lethal or associated with severe central nervous system (CNS) abnormalities and other systemic disorders, reflecting the critical role of PAX6 in developmental processes.

The severity of aniridia phenotypes can vary widely, even among family members carrying the same disease-causing PAX6 variants, while showing less variability between the two eyes of an individual patient [83]. This variable intrafamiliar expressivity may be explained by several hypotheses, including mosaicism—the presence of somatic PAX6 variants at variable frequencies (< 50%) that arise post-zygotically in an individual [86, 87]. Some mosaic patients exhibit atypical or incomplete forms of congenital aniridia or may even remain asymptomatic. These asymptomatic individuals can still transmit the pathogenic variants to one or more offspring, contributing to the variability observed in familial cases [86].

Initial and progressive signs and symptoms of congenital aniridia (CA) significantly overlap with those of other congenital ocular diseases [88]. In particular, atypical forms of CA may be associated with severe iris hypoplasia, which can also occur in certain cases of anterior segment dysgenesis (ASD). This overlap underscores the importance of careful differentiation, as will be further discussed in the next section.

WAGR syndrome derives its name from the primary clinical features: Wilms tumor (nephroblastoma), aniridia, genitourinary abnormalities, and mental retardation/range of developmental delays. This association was first described in 1964 [14]. It is a contiguous gene syndrome resulting from the deletion of genes on chromosome 11p13, most notably PAX6 and WT1 (Wilms' tumor) [132, 133].

Congenital aniridia is present in almost all cases. The clinical diagnosis of WAGR is confirmed when at least one additional primary WAGR feature accompanies congenital aniridia [133]. In patients with WAGR, the prevalence of specific features is as follows: Wilms tumor in 47%, congenital aniridia in 98%, genitourinary anomalies in 59% (males 85%, females 59%) and cognitive issues in 89% of the patients [134]. In cases where Wilms tumor develops, it typically manifests between the first and fifth year of life in 80–100% of affected individuals, with a median age of around 19 months at diagnosis [28, 134].

The proportion of sporadic aniridia is notably higher than familial cases in patients with WAGR. Individuals with sporadic aniridia face a 67-fold increased risk of developing Wilms tumor compared to the general population [135]. Notably, about 30% of sporadic aniridia cases are eventually diagnosed as WAGR syndrome [37, 75, 133].

WAGRO syndrome extends the WAGR acronym by including obesity as a common feature. Obesity is observed in a significant proportion of patients with WAGR, with prevalence ranging from 19 to 67%, depending on the study [133, 134, 136]. These findings highlight the complex and variable clinical presentation of WAGR syndrome and its related forms.

Axenfeld–Rieger syndrome (ARS) encompasses a spectrum of anomalies that were originally described independently by Axenfeld, who identified posterior embryotoxon in 1920, and Rieger, who documented mesodermal dysgenesis in 1935 [119, 120]. The condition is rare, with a prevalence estimated at 1 in 50,000 to 100,000 [137].

ARS is characterized by bilateral ocular manifestations involving the iris, peripheral cornea, and chamber angle. A hallmark feature is posterior embryotoxon (Fig. 3), which results from anterior displacement of Schwalbe’s line and can vary in prominence. While the cornea is typically normal, abnormalities such as megalocornea or microcornea are sometimes observed. Gonioscopy often reveals tissue strands forming iridocorneal adhesions [138]. Iris abnormalities are a key feature of ARS, with findings such as iris hypoplasia, corectopia (pupil displacement frequently associated with high myopia or ectopia lentis), and polycoria (multiple pupil openings) [137]. Congenital or early-onset cataracts are also common in ARS [139]. Unlike CA, patients with ARS often exhibit posterior embryotoxon, relatively good visual acuity, and an absence of nystagmus, which can aid in distinguishing between the two conditions [83]. Glaucoma is a significant complication in ARS, affecting approximately 50% of patients with ARS, often leading to progressive visual loss [140]. In addition to ocular findings, ARS is associated with systemic abnormalities, including craniofacial features such as hypertelorism, dental anomalies such as hypodontia or microdontia, and distinctive periumbilical skin abnormalities [141]. Genetically, FOXC1 and PITX2 pathogenic variants are the most frequently identified causes of ARS. However, in approximately 60% of cases, the genetic basis remains undetected, highlighting the complexity of the condition [137]. The variability in both clinical presentation and genetic findings emphasizes the need for comprehensive evaluations to differentiate ARS from other anterior segment dysgenesis disorders, including classical CA.

Peters anomaly (PA) was first described in 1906 [18] and has since been categorized into subtypes based on clinical presentation. Type I is the milder form, characterized by central corneal opacity and iris strands that extend to the corneal endothelium. Type II, a more severe form, presents with central corneal opacity and lens abnormalities, including cataracts or adhesions between the lens and cornea [142]. Type I is more common than type II, and the ocular manifestations of PA can occur either unilaterally or bilaterally [143]. A frequent complication in PA is developmental glaucoma, which significantly impacts visual outcomes [144, 145].

Peters-plus syndrome is a condition in which PA is combined with systemic features such as craniofacial abnormalities, short stature, and developmental delay [146].

While PAX6 pathogenic variants are implicated in some cases of PA, pathogenic variants in other genes such as CYP1B1 and PITX2 are also known to cause the condition. Interestingly, a rare case has been reported in which PA co-occurred with congenital aniridia (CA) in a patient with a causative PAX6 pathogenic variants [147]. Most cases of PA are sporadic; however, it can be inherited in both autosomal-dominant and autosomal-recessive patterns [141, 148‐152]. These findings highlight the genetic and phenotypic heterogeneity of PA, as well as its overlap with other anterior segment dysgenesis disorders.

Gillespie syndrome (GS) was first described by Frederick Gillespie in 1965. It is characterized by the combination of congenital aniridia/iris hypoplasia, cerebellar ataxia, intellectual disability (oligophrenia), and congenital hypotonia [20].

The aniridia associated with GS is typically bilateral and partial, characterized by aplasia of the centrally located iris tissue. Affected individuals often have fixed dilated pupils due to the impaired development of the iris sphincter and stromal tissues, leading to functional deficits in iris control [116]. GS is a rare disorder, with just over twenty families reported to date, and is caused by pathogenic variants in the ITPR1 gene [153].

Unlike classic aniridia, which is associated with a broader range of ocular abnormalities, iris abnormalities are the sole ocular manifestation in GS [116]. This distinction underscores the importance of genetic and clinical evaluation to differentiate GS from other forms of aniridia.

The presence of ring chromosomes is a rare phenomenon, with only a few reported cases of patients with Ring Chromosome 6 syndrome (RC6S). This condition is associated with a wide range of clinical features, including growth retardation, microcephaly, psychomotor retardation, and various ocular abnormalities [154]. Among the ocular manifestations, iris coloboma, strabismus, nystagmus, microphthalmia, optic nerve atrophy, posterior embryotoxon, and megalocornea have been reported [155‐158]. In one notable case, Zhang et al. identified FOXC1 pathogenic variants associated with RC6S, further linking the condition to anterior segment dysgenesis disorders [154]. These findings highlight the genetic and phenotypic complexity of RC6S, which encompasses both systemic and ocular abnormalities.