Macular hypoplasia
Frequency: Occurs in nearly all cases.
Impact on visual prognosis: The most significant factor for reduced visual acuity. Presents with absence of the foveal pit and abnormal macular vascular pattern.
Congenital aniridia is a rare disease with complete or partial absence of the iris. The prevalence is 1 in 64,000 to 96,000 people, and it was classified as a designated intractable disease in Japan in 20171).
The cause is loss-of-function mutations in the PAX6 gene on chromosome 11p13. PAX6 is a master control gene for eye development, and haploinsufficiency leads to the disease. Biallelic abnormalities are lethal in utero1). Inheritance is autosomal dominant in two-thirds of cases, and sporadic in one-third. 60–90% are bilateral, with a slight male predominance.
Glaucoma is considered an acquired complication of aniridia, occurring in 50–75% due to angle dysgenesis1). Onset in infancy is rare; it progressively develops after adolescence. The Japanese Glaucoma Guidelines (5th edition) classify it as “3-iii Aniridia” under secondary childhood glaucoma. Management follows the treatment principles of primary congenital glaucoma (PCG), but angle dysgenesis specific to aniridia requires special consideration in treatment strategy.
Aniridia is a pan-ocular disease accompanied by various ocular and systemic complications in addition to glaucoma, and frequently includes macular hypoplasia, aniridia-associated keratopathy (AAK), cataract, and nystagmus. In sporadic cases, deletion of the WT1 gene adjacent to the PAX6 gene can cause WAGR syndrome, requiring attention for Wilms tumor (nephroblastoma).
Glaucoma occurs in 50–75% of cases, but not all patients develop it. Onset in infancy is rare, and progressive intraocular pressure elevation often becomes apparent after adolescence, so lifelong regular intraocular pressure monitoring is important.
The main complaint in aniridia is often horizontal nystagmus observed from early infancy due to associated macular hypoplasia. Because the iris is absent, the amount of light entering the eye cannot be regulated, causing severe photophobia (glare). Poor fixation is also observed, and the condition is often detected relatively early in life.
Symptoms related to glaucoma are as follows:
Slit-lamp microscopy reveals varying degrees of iris dysplasia, from partial iris atrophy to complete iris absence1). In cases of severe absence, the lens equator and Zinn’s zonules may be visible.
Glaucoma-related findings:
Main ocular complications:
Macular hypoplasia
Frequency: Occurs in nearly all cases.
Impact on visual prognosis: The most significant factor for reduced visual acuity. Presents with absence of the foveal pit and abnormal macular vascular pattern.
Corneal disease (AAK)
Frequency: Develops progressively.
Characteristics: Conjunctival tissue invades the cornea due to limbal stem cell deficiency. Leads to pannus formation and corneal opacity.
Cataract
Frequency: Occurs in about 80% of cases1).
Characteristics: Develops in 50–85% of patients by age 20. Surgery is challenging due to fragile zonules of Zinn.
Nystagmus
Frequency: Seen in all cases.
Characteristics: Mainly horizontal nystagmus. Often the chief complaint in early infancy. Affects visual development.
Association with long-term prognosis of glaucoma:
In a case series of 306 patients, no patient with glaucoma had visual acuity better than 20/60 (0.33)15). In another study of 30 patients, 10 (30%) had glaucoma, which was the main cause of vision loss. Among them, 2 patients (6%) became blind16). Visual field and optic nerve damage due to glaucoma is irreversible, and early detection and appropriate intraocular pressure management are directly linked to maintaining visual function.
Extraocular findings: WAGR syndrome (11p13 deletion syndrome) includes Wilms tumor, genitourinary abnormalities, and intellectual disability. PAX6 is also expressed in the central nervous system, pancreatic islets of Langerhans, and olfactory epithelium; associated conditions include agenesis of the corpus callosum, epilepsy, higher brain dysfunction, anosmia, and glucose intolerance1).
PAX6 is expressed from the early eye and is a master control gene for eye development, regulating various transcription factors. Abnormalities in PAX6 cause congenital anomalies throughout the eye (aniridia, Peters anomaly, macular hypoplasia, optic nerve hypoplasia, etc.).
The types of genetic mutations and associated diseases are shown below.
| Gene | Chromosome | Associated Disease |
|---|---|---|
| PAX6 | 11p13 | Aniridia, macular hypoplasia, Peters anomaly |
| WT1 | 11p13 (adjacent to PAX6) | Wilms tumor |
| PITX2 | 4q25 | Axenfeld-Rieger syndrome type 1 |
| FOXC1 | 6p25 | Axenfeld-Rieger syndrome type 3 |
PAX6 mutations are often premature truncated codon (PTC) types such as nonsense and frameshift, and missense mutations have also been reported1). Sequencing detects some mutation in nearly 85% of isolated aniridia cases2).
In a large registry study, genetic diagnosis was obtained in 56.5% of glaucoma associated with non-acquired ocular developmental anomalies, and PAX6 is shown to be one of the major causative genes10).
PAX6 and WT1 (the causative gene for Wilms tumor) are adjacent on 11p13, and chromosomal deletions that delete both result in aniridia complicated by Wilms tumor (WAGR syndrome: Wilms tumor, Aniridia, Genitourinary abnormalities, mental Retardation).
Chromosomal microarray (CMA) or FISH testing is recommended to detect deletions in the 11p13 region including PAX6 and the adjacent WT1 gene. If a WT1 gene deletion is confirmed, the risk of Wilms tumor is high, so regular abdominal ultrasound examinations until age 6 are necessary. Testing should be performed under genetic counseling2).
Clinical diagnosis is easy by confirming iris defects with slit-lamp microscopy. The diagnostic criteria established by the Ministry of Health, Labour and Welfare research group are classified into the following categories1):
Severity classification is based on corrected visual acuity and visual field constriction1).
| Severity | Criteria |
|---|---|
| Grade I | Unilateral involvement, fellow eye normal |
| Grade II | Bilateral involvement, best corrected visual acuity in the better eye ≥ 0.3 |
| Grade III | Bilateral involvement, best corrected visual acuity in the better eye ≥ 0.1 and < 0.3 |
| Grade IV | Bilateral involvement, best corrected visual acuity in the better eye < 0.1 |
If grades I–III are accompanied by visual field constriction due to secondary glaucoma, the severity shifts up one grade1). Grade III or higher qualifies for medical expense subsidies for designated intractable diseases.
The diagnostic criteria for childhood glaucoma are as follows: glaucoma is diagnosed when two or more of the following items are met.
In aniridia, corneal thickness may differ from normal (tendency to thicken, thinning in aphakic eyes), and caution is needed when interpreting intraocular pressure values. There is no interchangeability between different tonometers.
Diseases that may be confused with aniridia are listed below1). These are disease groups that should be excluded as criterion C (differential diagnosis) of the diagnostic criteria.
Treatment for high intraocular pressure and glaucoma in aniridia is strongly recommended (evidence strength C)2). Treatment is considered stepwise in the following order.
① Medication Therapy
First step: Intraocular pressure reduction using eye drops or oral medication.
Medications used: Beta-blockers, carbonic anhydrase inhibitors (CAIs), prostaglandin analogs.
② Outflow Reconstruction Surgery
Second step: When medication therapy is insufficient.
Procedures: Goniotomy, trabeculotomy. Indication depends on the degree of angle dysgenesis.
③ Filtration Surgery / Tube
Third step: When outflow reconstruction is difficult or ineffective.
Procedures: Trabeculectomy, long tube surgery (Ahmed/Baerveldt).
④ Cyclophotocoagulation
Last resort: When all other treatments are ineffective.
Caution: High risk of phthisis bulbi (loss of eye function); ciliary body hypoplasia has been reported in aniridia.
Intraocular pressure (IOP) control with eye drops or oral medication is the first-line treatment. A long-term observational study of 60 cases reported that 31 developed glaucoma, and in 12 of these, IOP was managed with medication alone3).
The main medications used are as follows:
In cases with corneal epithelial stem cell deficiency (AAK) or when worsening of AAK is a concern, consider using preservative-free formulations7).
Goniotomy has been reported effective for controlling high IOP and glaucoma4), and outflow reconstruction surgery can be recommended as an initial procedure2). Trabeculotomy has also been reported effective as an initial surgery5). However, note the following:
This is selected when outflow reconstruction is difficult or unsuccessful. There are reports of achieving intraocular pressure control11), but the following issues exist.
Baerveldt glaucoma implant and Ahmed glaucoma implant are available. This is selected when trabeculectomy is ineffective or when filtration surgery is expected to have poor outcomes due to angle pathology2).
A meta-analysis of Ahmed and Baerveldt implants for pediatric glaucoma (32 studies, 1,221 eyes) showed a decrease in mean preoperative IOP from 31.8 mmHg to 16.5 mmHg (95% CI: 15.5–17.6) at 12 months. Success rates were 87% (95% CI: 0.83–0.91) at 12 months and 77% (95% CI: 0.71–0.83) at 24 months, but declined to 37% (95% CI: 0.32–0.42) at 120 months in the long term9).
Arroyave et al. (2003) summarized the use of GDD for glaucoma associated with aniridia and reported a certain degree of IOP-lowering effect14). Recent reviews also indicate that glaucoma drainage devices are a major option when outflow reconstruction or filtration surgery is insufficient13). In Japan, there are case reports of effective Baerveldt implant use12). In aniridic eyes, due to the absence of the iris, it is recommended to insert the tube tip tangentially rather than toward the center of the cornea7). In phakic eyes, attention must be paid not only to the corneal endothelium but also to contact with the lens.
This is a last resort when all surgical treatments are ineffective2).
In the management of glaucoma associated with aniridia, lifelong regular intraocular pressure monitoring is essential. Follow-up should be performed with attention to the following points.
First, medication therapy is attempted; if insufficient, outflow reconstruction surgeries such as goniotomy or trabeculotomy are considered. If these are difficult or ineffective, trabeculectomy is performed, followed by long-tube surgery (Ahmed or Baerveldt implant). Only when intraocular pressure control cannot be achieved with any treatment is cyclophotocoagulation considered as a last resort 2).
PAX6 encodes a transcription factor that regulates gene expression and is a master control gene expressed from the early eye. It is involved in optic cup formation, lens differentiation, and development of the corneal epithelium and retina. PAX6 haploinsufficiency affects not only the iris but also the macula, corneal limbal stem cells, optic nerve, and the entire eye.
PAX6 is also expressed in the central nervous system, pancreatic islets of Langerhans, and olfactory epithelium, and hypoplasia of these tissues leads to various extraocular complications 1).
Pathologically, smooth muscle is absent except for the iris root, and angle development is incomplete. The pathogenesis of glaucoma progresses stepwise as follows 3):
This progressive angle change was reported by Grant and Walton in 1974 3), and in recent years has been confirmed by UBM as anterior displacement of the ciliary body 6). Depending on the degree of angle dysgenesis, both open-angle and closed-angle types can occur.
PAX6 mutations also affect corneal limbal stem cells, leading to dysfunction of corneal epithelial stem cells.
AAK often develops and progresses with growth even if normal in early childhood1). There are two types of keratopathy: congenital central corneal opacity (CCO) and AAK, and it has been reported that the rate of glaucoma complication is higher in cases with CCO than in those with AAK16). Antimetabolites and preservative-containing eye drops used for glaucoma treatment carry a risk of worsening AAK, affecting the choice of treatment strategy. Since corneal and glaucoma management interact with each other, an integrated evaluation of both is required.
As an anatomical finding specific to aniridia, hypoplasia of the ciliary body has been reported on UBM6). This finding increases the risk of eyeball perforation during cyclophotocoagulation and also suggests that the amount of aqueous humor produced by the ciliary body may be less than normal.
Chen and Walton (1999) reported that based on the natural course of progressive angle changes in aniridia, prophylactic goniotomy performed before the onset of ocular hypertension or glaucoma may prevent the development of glaucoma4). However, this is a descriptive study without a control group, and the evidence is limited2).
Future prospective studies are awaited regarding the effectiveness of prophylactic surgical intervention.
For the treatment of limbal stem cell deficiency (LSCD), allogeneic limbal transplantation and cultured oral mucosal epithelial transplantation are being considered. Clinical practice guidelines weakly recommend surgical treatment, and it is reported that ocular surface reconstruction can be achieved with a certain probability 2). In cases complicated by corneal stromal opacity, combined corneal transplantation may be useful.
Large-scale registry studies are revealing the genetic profile of childhood and early-onset glaucoma 10). Correlation analysis between PAX6 mutation types (e.g., PTC type, missense type) and glaucoma risk/severity is expected to lead to personalized medicine based on risk stratification.