The status quo and advances in categorization of congenital cataract

来源期刊： Eye Science | 2024年1月第1卷第1期： 50-61 发布时间：2024-01-01 收稿时间：2024/1/3 16:46:30 阅读量：4598

作者：: 邹颖诗,

李芸倩,

刘臻臻,

关键词：: congenital cataract categorization morphology etiology genotype-phenotype; congenital cataract categorization morphology etiology genotype-phenotype

DOI：: 10.12419/es23110805

Congenital cataract (CC) is one of the most common causes of pediatric visual impairment.As our understanding of CC's etiology, clinical manifestations, and pathogenic genes deepens,various CC categorization systems based on diferent classifcation criteria have been proposed.Regrettably, the application of the CC category in clinical practice and scientifc research is limited. It is challenging to obtain preciseinformation that could guide the timely treatment decision-making for pediatric cataract patients or predict their prognosis from a specificCC classification. This review aims to discuss the statusquo of CC categorization systems and the potential directions for future research in this field, focusingon categorization principles and scientificapplication in clinical practice.Additionally, it aims to propose the potential directions for future research in this domain.

INTRODUCTION

Congenital cataract (CC) is defined as the occurrence of cataract within the first year of a child’s life.^[1] The overall prevalence of CC ranges from 0.63 to 9.74 per 10,000 children.^[2] It is oneof the leading causes of visual disability in children, accounting for approximately 10% of childhood blindness around the world, which has brought significant burdens to their families as well as global socioeconomic status.^[3] The etiology and clinical features of CC are multifaceted.The condition can manifest in one or both eyes,with variations in morphology, anatomical location, and severity of lens opacities. Most CC cases remain stable, while some may progress overtime.^[4] Moreover, CC can occur in isolation or alongside other ocular or systemic developmental anomaliessuch as microphthalmia, microcornea, and/or iris anomalies.^[5-6]

CC can be classified into different types based on the anatomical landmarks of the lens, cataract morphology, and etiology. However, there is a lack of a unified categorization method. Relying onone specificcategorization systems in clinical practice, it is challenging forcliniciansinmakingprecise treatment strategies. Therefore, the objective of this review is to provide an overview of the status quo of CC categorization systems, including the categorization principle and applications, while highlighting areas in need of further study and future directions.

The principle of categorization and its development progress

By far, there has been no unified standard for the categorization of CC. In the early stage, specifc cataracts were described based upon the name of investigating doctor or the patients’pedigree, such asMarner cataract^[7]and Coppock cataract^[8]. In 1942, Adamas frst described Coppock cataract as a circular spotted disc in the center of the lens under slit-lamp examination.^[9]Subsequently, scholars began to classify CC based on the morphology andanatomical locations of lens opacities.^[10] Meanwhile, in pursuit of understanding the pathogenesis of cataracts and pursuing etiological treatment, the etiology of CC was also used as a classification criterion.With the advancement of medical technology, innovative approaches such as genome-wide arrays,whole genome sequencing technologies, and advanced ocular examinations had been employed to classify CC.Gene sequencing technology facilitated the exploration of the relationship between causative genes and cataracts, and scholars triedto classify CC based on the correlation between genotype and phenotype.^[11] Recently, advancements in ultrasonic and optical measurement technology have enabled the identification of more detailed characteristics of the anterior segmentby various measuring instruments.^[12-14] Classifcation systems for CC based on the outcomes of anterior segment measuring instruments have emergedto investigate the association between CC and other anterior segment abnormalities.
Obviously, current categorization systemsfor CC are complexity and diversity, due to diferent principles of classification (Figure 1). From the perspective of ophthalmologists, these sytems possess both strengths and weaknesses and their respective areas of application are diferent.

Categorization system based upon morphology and locatio

The morphology and locations of CC exhibit considerable variation.^[15] In terms of morphology, CC can be classified as solid, pulverulent, or blue dot, among others. With respect toanatomical locations, CC can be categorized as nuclear, cortical and more, as depicted in Figure 2. Multiple classifications have been the morphology or locations of CC.^[4,15-19] Reddy et al.^[20] presented a relatively comprehensive classifcation method by integrating the morphology and locations of opacities within the lens. According to this classifcation, CC is divided into anterior and posterior polar, nucleus, lamellar, nucleus and cortex, and cortex based on their repective locations. Among these, the nucleus and cortex can be further subcategorized into solid, pulverulent, blue dot and crystalline based on the type of opacity. This classification system is widely employed in clinical practicedue to its convenience in facilitating directdiferential diagnosisby ophthalmologists.

Many reports have described the distribution of different morphological types of CC. In Marshall et al.’s study,^[21] nuclear cataract was the most prevalent type (30.5%) among their cohort, followed by lamellar cataract (25.9%) and posterior lenticonus (19.0%). In Haargaard et al.’s research,^[6] nuclear/zonular cataract was identified as the most frequent morphological type, accounting for 34% of all cases.Similary, Yang et al.^[22] oberseved a significantly elevated prevanlence of nuclear cataract (48.4%), followed by total cataract (22.1%) and posterior polar cataract (13.8%). In general, nuclear cataracts are the predominant morphological type within the CC cohort. Moreover, the distribution of CC morphology difers between bilateral and unilateral cases. Nagamoto et al.^[23] observed that total cataract (29.3%), nuclear cataract (19.4%), and lamellar cataract (18.5%) were prevalent in bilateral cases, whereas unilateral cases predominantly exhibited posterior subcapsular/polar cataract (34.7%), nuclear cataract (23.5%), and total cataract (21.8%). Long et al.^[19] arrived at a comparable conclusion, with nuclearcataract (35.8%), total cataract (32.3%), and lamellar cataract (11.1%) being predominant among bilateral cases, while unilateral cases mostly presented with polar cataract (42.3%), total cataract (32.3%), and nuclear cataract (17.7%).This phenomenon is thought to potentially associated with gene mutations or embryonic development.
The morphology of cataracts plays an important role in predicting surgical outcomes. Marshall et al.^[21] observed significant variations in that postoperative visual acuity among different cataract types, with lamellar and posterior lenticonus groupsexhibiting the most favorable outcomes, while nuclear group demonstratedrelatively poorer results. Li et al.^[24] documented suboptimal visual outcomesin patients with bilateral total congenital cataracts, with 93/176 (52.8%) eyes achievinga postoperative best-corrected visual acuity (BCVA) of < 20/200. Kim et al.^[25] found that postoperative BCVAfor posterior polar cataracts (0.34±0.12 logMAR) surpassedthat of nuclear (0.39±0.22 logMAR) and total (0.58±0.42 logMAR) cataracts. Mistr et al.^[26] reported surgery outcomes of 30 eyes with posterior polar cataracts and 32 eyes with posterior lenticonus, revealing that 84% and 68% of eyes in the respective groups achieved acuities better than 20/40. Chen et al.^[27] reported similar findings, with 21/30 (70%) of eyes diagnosed with posterior polar cataracts or lenticonus achieving a BCVA better than 20/40.Conversely, Kekunnaya et al.^[28] found poorer visual outcomes in cases of posterior lenticonus, with 29/59 (49%) of eyes exhibiting a BCVAworse than 20/100 or eccentric fixation 6 months postoperatively. Generally, total and nuclear cataracts are related to unsatisfactory visual prognosis, requiring aggressive surgical intervention as early as possible. The varying prognoses observed in posterior polar cataracts and lenticonus indicate that the influence of other factors such as age at surgery, potential fundus lesions and preoperative visual acuity.^[27-28] Treatment strategies should be tailored according to the specifc morphology of cataract.
Whilethis classification is a simple and useful tool in clinical practice, it is not without its drawbacks. Firstly, due to the absence of a detailed classification standard, ophthalmologists may encounter difficulty in precisely discerning the specific locations or morphology of CC sometimes.Furthermore, this classification seems to overlook other coexisting ocular conditions. Common ocular comorbidities in congenital cataracts include aniridia, microphthalmos, persistent fetal vasculature (PFV), nystagmus, and strabismus. Nagamoto et al.^[23] found that compared to unilateral cases, bilateral cases exhibited a notablyhigh frequency of nystagmus, while in unilateral cases, strabismus emerged as the most prevalentassociated ocular diseases. Additionally, strabismus, nystagmus and microphthalmos were frequently observedin cases of total cataract, withPFV often presenting in eyes afected by posterior subcapsular/polar cataracts. Haargaard et al.^[6] highlighted the frequent occurrence of PFVin unilateral cases, while noting thatmicrophthalmos and microcornea were more prevalent in bilateral cases. Ocular comorbidities hold the potential to influence the formulation of treatment strategies and patients’prognosis. For example, in cases of PFV, the fetal vessels may strongly adhere to the posterior capsule, resultingin preoperative or intraoperative posterior capsule rupture^[29] and intraoperative hemorrhage^[4]. To avoid severe complications, comprehensive preoperative examinations are required,and surgical procedures must be meticulously planned. Futhermore, individuals with CC may also present various multisystem disorders, suchasDown’ssyndrome^[30], Oculo-Facio-Cardio-Dental (OFCD) syndrome^[31], Trisomy 21, Lowe syndrome and many others^[32], all of which have the potential to signifcantly impact patients’prognosis.
Recently, there have been rapid advancements in anterior segment measurement instruments, such as anterior segment Optical Coherence Tomography or Pentacam connected to a digital Scheimpfug rotational camera. Based on the locations of lens opacities shown in slit-lamp and Pentacam examinations, Liu et al.^[33] established a classification for CC into four groups: total, anterior, interior, and posterior cataracts. In this classifcation, total cataract is defned as opacity of entire lens, whileinterior cataract refers to denotes opacity of the interior lens without involvement of anterior or posterior capsules. Anterior and posterior cataracts indicate lens opacities with involvement of anterior or posterior capsules, respectively. This classification method, being straightforward, is bothsimple and practical for ophthalmologists to employ in in clinical settings. Using this classification, they explored the relationship between locations of CC and other anterior segmental characteristics. It was observed that signifcant corneal astigmatism (CA) was found in all types of CC, with anterior cataract showing the strongest association with corneal astigmatism.As the locations of CC became more posterior, the value of CAdecreased. Additionally, anterior chamber depths (ACD) varied across the different types, with interior and posterior cataract had greater ACD, while total and anterior cataracts exhibited shallowerACD. Other studies also focusedon the abnormal anterior segment structure in CC. Han et al.^[34] observed the distribution of anterior corneal astigmatism (ACA), posterior corneal astigmatism (PCA), and total corneal astigmatism (TCA) of patients with CC. They also found large CAin patients with CC, with 70.3% of cataractous eyes (378/538) displaying an ACAexceeding1.25 D, and 72.8% of cataractous eyes (392/538) exhibiting a TCAexceeding1.25 D. Among cataractous eyes, 70.6% of cataractous eyes (380/538) had PCAranging from 0.25 to 0.75 D. In addition to anterior segmental characteristics, Liu and coworkers^[35] further into the link between visual function and ocular structure in patients diagnosed with posterior cataract. The discovered that compared with their contralateral healthy eyes, cataractous eyes exhibited a prolongedpeak time of P100 of PVEP-60’, along with a diminished amplitude of P100 of PVEP-60’.
In contrast to the traditional 2-dimensional image-based classification, this modified classification system leverages 3-dimensional images for categorization of CC, yielding heightened precision in clinical applications. Furthermore, it facilitates the anticipation of potential anterior segment irregularities based on the locations of lens opacities and ofers more pragmatic guidance in formulating treatment approaches for ophthalmologists.Nonettheless, despite its simplicity and practicality in clinical settings, this method overlooks the varied morphologies and severity gradientsof CC, alongside potential ocular or systemic comorbidities. Moreover, in instances where the morphology of CCis intricate, this classifcation method may not be applicable.

Categorization system based upon etiology

The etiology of cataracts presents a range of causes. Cataracts can be categorized into several types based on their origins, including idiopathic, hereditary, metabolic, infectious cataracts.^[36] Reports indicate that idiopathic cataract represent themost prevalent form, accounting for up to 50% of all the cases.^[6,37-38] Patients diagnosed as idiopathic CC can have other ocular or systemic disorders, thereby requiring the exclusion of hereditary, infectious and other etiologies. Notably,unilateral cases are more likely to be idiopathic in comparision to bilateral instances. Whilethe risk factors of idiopathic CCremain unconfirmed, earlier reports have suggested a potential correlation with prenatal or perinatal causive factors.^[37] Following idiopathic cataracts, hereditary cataract representthe second common type, comprising over 20% of all the cases.^[39] Hereditary modes of CC include autosomal dominant, autosomal recessive, and X-linked recessive inheritance, with autosomal dominant inheritance being the most prevalent.^[40] Hereditary cataractsdemonstrate considerable clinical and geneticheterogeneity.^[41-42] Additionally, metabolic disorders such as galactosemia, hypoglycemia, hypocalcemiacan lead to the development of cataracts.
Galactosemia arises from a deficiency in any of the three enzymes, including galactokinase, galactose-1-phosphate uridy1 transferase, or uridine diphosphate-galactose-4-epimerase.^[43] Galactose is converted into galactitol in the lens, leading to the accumulation of galactitol and subsequent osmotic changes.Initially, the lens exhibitsoil-droplet opacification, which mayprogress to cortical or nuclear opacification. This process is reversible if galactose is promptly excluded from the diet during the early stages.Additionally, intrauterineinfections, particularly those caused by toxoplasma gondii, rubella virus, syphilis, cytomegalovirus, herpes simplex virus (TORCH), can lead to CC. Among these, rubella virus infection is the mostprevalent. Besides cataract, the clinical manifestations of congenital rubella syndrome include glaucoma, pigmentary retinopathy, heart disease, and brain dysfunction. However,the precise pathogenesis of CC resulting fromintrauterine rubella infectionremains elusive. Nguyen et al.^[44] reported that theviral infection within the ciliary body may underlie the development of cataract.
To a certain extent, classifying cataractsbased on their etiology can serve as a guide for clinicians in determining treatment strategies. Forinstance, patients diagnosed with hereditary CC, may require genetic testing and counselingfor their families.Individuals with galactosemia would benefit from a galactose-free diet. In cases where cataracts are caused by rubella virus, clinicians should consider the patients’general health to rule out severe systemic comorbidities. However, achieving the preciseetiologicaldiagnosisofCC can be challenging at times. In clinical practice, etiology classification is not used independently but rather combined with other CC classifications to facilitate improveddiagnosis andtreatment.

Categorization system based upon genotype-phenotype correlation

With the emergence and application of genetic technology, researchers have embarked on the exploration ofpotential genetic abnormalities associated withhereditary cataract. Presently, the literature reports an association of more than 100 genes with both nonsyndromic or syndromic hereditary cataract.^[40,45] Hereditary cataracts showsignificant variation within and between families. It is noteworthy that a single mutation can give rise to diverse froms of cataracts, while identical types of cataracts can result frommutations in diferent genes. To elucidate the the suspected geneticcausesbased on phenotype, some scholars have categorized the implicatedgenesinto four groups, guided by the genotype-phenotype correlation: the genes implicatedin syndromic cataracts, genes associated specifcally with congenital cataracts within syndromic cases, genes exclusively associated with cataracts, and genes associated with both cataract and eye anomalies.^[11]
The extent to which currently known genes can account for the occurrence of cataracts has varied across differentstudies. More than 50 genes have beenidentified in association with nonsyndromic cataract, including crystalline genes, membrane transport protein genes, developmental regulator and transcription factor genes, cytoskeletal protein genes, and transmembrane protein genes.^[46] Crystalline genes are particularly prominent in nonsyndromic hereditary cataracts, demonstratinga range of phenotypicvariations. Berry et al.^[47] reported 10 different crystallin variants in CRYAA, CRYBA1, CRYBB1, CRYGD, and CRYGC, withnuclear or lamellar phenotypes being predominant. A variant in CRYBB2 was found to be associated withnuclear-sutural phenotype.^[48] Rogaev et al. indentifed variant in the γ-crystallin gene.^[49] that may contribute to non-nuclear phenotypes. Cai et al.^[50] discovered avariant in theCRYGD geneassociated with coralliform phenotype. Disease-causing variants in CRYAB, CRYBA2, CRYBA3, CRYBA4, and CRYGS have been reported, and these variants exhibit association with various cataract phenotypes.^[48,51] Mutations in membrane transport protein genes can result in a diverse array of phenotypes, including total, nuclear, Y-sutural with nuclear, pulverulent, lamellar, lamellar sutural, pearl box, coralliform, punctate, Coppock-likecataracts.^[11,52-53] A Variant in LIM2 gene, in particular, has been linked to membranous cataract.^[54] Mutation in developmental regulator and transcription factor genes, such as of PAX6 and HSF4, are associated with a wide range of phenotypes, includinglamellar, punctate, coralliform, anterior and posterior polar, nuclearcataracts.^[52,55] Whereas, variants in PITX3 are associated with narrower range of cataract phenotypes, including posterior polar, total, and nuclear phenotypes.^[55] It is worth noting that variants in some of the above-mentioned genes can give rise to other ocular anomalies. For instance, Zin et al.^[56]
Undeniably, this classification system has the potential to enhance our understanding of the genotype-phenotype correlation in hereditary cataracts, thereby aiding in the genetic counseling for patients and their families. However, due to its clinical and genetic heterogeneity of the condition, there are challenges in establishing a precise genotype-phenotype correlation of hereditary cataract. Therefore, it is difficult to solely rely on complex phenotypes for gentic classification of hereditary cataracts.

Conclusion

Thereexist multiple categorization systemsfor CC, some of which are utilized in genetic and basic researches. However, the practical application of CC categorizationsystems in clinical practice is limited, such as appearing as only baseline data in clinical studies or neglecting eye and systemic symptoms other than cataract itself in clinical practice. At present, obtaining precise data on morphology, etiology, and genetics within a specific classification remains challenging,and not all classifcations reliably predict the visual outcomes of patients. Therefore, to comprehensively characterized CC conditions and facilitate in treatment decisions and prognostic assessment, it is important for clinicians to adopt a comprehensive approach by utilizing multipleCC classifications instead of relying solely on a specific one. By incorporating various classifcation systems, clinicians can gain a more complete understanding of the condition, leading to improve management strategis and patient care.

Future research

Currently, many CC categorization systems lack the ability to precisely predict the visual prognosis of pediatric cataract patients with varying characteristics.While some studies have reported the postoperative visual acuity based on morphological categorization of different CC types, further research is needed to investigatethe correlation between variousCC categorization systems and visual prognosis from multiple clinical perspectives,encompassing factors such as visual acuity, surgical complications and others.
In the context of CC classification, slit lamps and anterior segment measurement instruments are commonlyused. However, young children often face challenges in cooperating with these examinations. To overcome this issues, the utilization of handheld eye examination instrument during sedation has proven to be a more feasible approach for collecting eye information from young children. The developmetof additional handheld eye examination instruments in the future will further facilitate the classification of CC.
In addition, future research should focus on integrating dimensions for classifyingCC. The phenotypic characteristics of CC patients, including ocular biometry of anterior and posterior segments, usually often exhibit variations among individuals. The current CC categorizationsystems primarily rely on singlestandards, which may not adequately capture the comprehensive ocularcharacteristics of patients.Therefore, it is important to develop new categorization systems that incorporate multiple dimensions. For exampleutilizing ocular biometry and considering the conditions of anterior and posteriorsegments as classification criteria. Furthermore, it is crucial to explore the correlation between this new classifcation system and the visual prognosis of children with CC. By doing so, we can enhance the clinical value and applicability of the classifcation system. The ultimate objective of future research is to improve the clinical and scientific utility of CC categorization systems, leading to more accurate diagnoses, effective treatment approaches, and better outcomes for patients.

Correction notice

None

Acknowledgement

Our heartfelt appreciation to Ms. Yin Qiuxia for her dedicated eforts in meticulously improving the language and grammar of this article with her precious time.

Author Contributions

(I) Conception and design: Zhenzhen Liu
(II)Administrative support: Zhenzhen Liu
(III) Provision of study materials or patients: Yingshi Zou and Yunqian Li
(IV) Collection and assembly of data: Yingshi Zou and Yunqian Li
(V) Data analysis and interpretation: Yingshi Zou and Yunqian Li
(VI) Manuscript writing:All authors
(VII) Final approval of manuscript:All authors

Funding

This work was supported by the Joint Funding Project of Municipal Schools (Colleges) of Science and Technology Program of Guangzhou, China (2023A03J0188) and the Guangzhou Municipal Science and Technology Project (202201011815).
The funding organizations had no role in the following aspects: design and conduct of the study; the collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and the decision to submit the manuscript for publication.

Confict of Interests

None of the authors has any conflicts of interest to disclose. All authors have declared in the completed the ICMJE uniform disclosure form.

Patient consent for publication

None

Ethical Statement

This study does not contain any studies with human or animal subjects performed by any of the authors

Provenance and Peer Review

This article was a standard submission to our journal. The article has undergone peer review with ouranonymous review system

Data Sharing Statement

None

OpenAccess Statement

This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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1、This work was supported by the Guangzhou Municipal Science and Technology Project(202201011815)

2、This work was supported by the Natural Science Foundation of Guangdong Province, China(2023A1515011102)