Understanding Human Hydrocephalus: Insights into Genetics and Molecular Mechanisms

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ACHAIKI IATRIKI | 2024; 43(4):205–214

Review

Dimitra Katagi1*, Vasileios Vafeiadis1*, Kyriakos Birmpas1*, Martha Assimakopoulou2

1School of Medicine, University of Patras, 26504 Patras, Greece
2Department of Anatomy, Histology and Embryology, School of Medicine, University of Patras, 26504 Patras, Greece

*These authors contributed equally to this manuscript as first authors.

Received: 08 Mar 2024; Accepted: 21 May 2024

Corresponding author: Martha Assimakopoulou, PhD, Assoc. Professor of Anatomy/Neuroanatomy, Faculty of Medicine, Department of Anatomy, Histology and Embryology, School of Medicine, University of Patras, Preclinical Medicine Department Building, 1 Asklipiou, 26504 Patras, Greece, Tel.: +30 2610969186, +30 2610969195, E-mail: massim@upatras.gr

Key words: Congenital/developmental hydrocephalus, cerebrospinal fluid, brain development, glymphatic flow, AQP1

 

Abstract

Hydrocephalus is a progressive neurological disorder associated with abnormal cerebrospinal fluid (CSF) flow resulting in an active distention of the ventricular system. Three main types of hydrocephalus have been described, including non-communicating or obstructive, communicating with reduced CSF absorption, and communicating hypersecretory. Despite common shunting procedures used for symptomatic treatment of ventricular enlargement, patients still develop symptoms, indicating the complexity of the pathogenesis of hydrocephalus, suggesting that the disease is not a mere disturbance of circulative procedure. This review aims to present the genetic and molecular aspects of human hydrocephalus associated with congenital disorders, such as X-linked hydrocephalus, the most common form of genetic hydrocephalus linked with L1-CAM mutations, and other complex pathologies including common syndromes such as primary ciliary dyskinesia and Dandy-Walker malformation. Reevaluating existing hypotheses in hydrocephalus research, such as the cilia hypothesis and glymphatic flow disruption and comprehending novel data, including downregulation of Aquaporin 1 (AQP1), a water channel involved in CSF production, and the interconnection between neurogenic defects and tissue biomechanics will pave the way for improved diagnostic and therapeutic strategies for human hydrocephalus.

INTRODUCTION

CSF production and drainage

Cerebrospinal fluid (CSF) is an ultrafiltrate of plasma accumulated in the ventricular system and the subarachnoid spaces of the cranium and spinal column. Adults acquire roughly 150ml of CSF, with a diffusion of 125ml in subarachnoid spaces and 25ml in the ventricles. Approximately 20% of CSF is primarily produced by modified ependymal cells, the choroid plexus, within the lateral, third, and fourth ventricles by percolation of plasma. These ependymal cells are highly specialized, simple, cuboidal epithelium connected with tight junctions creating the blood-CSF barrier, responsible for filtration of CSF, allowing passage only to ions and small molecules such as vitamins. CSF renewal is a persistent procedure manifesting four to five times repetitions per 24 hours in adults. This process is crucial for the proper functioning of the brain since CSF contributes to nourishment, waste removal, and protection of the brain. An impairment in CSF renewal would promote the aggregation of waste metabolites leading to aging and neurodegenerative diseases [1].

CSF passes from the lateral ventricles via the interventricular foramen (of Monro) to the third ventricle and subsequently to the fourth ventricle via the cerebral aqueduct or aqueduct of Sylvius. It exits the fourth ventricle, entering the basal cisterns, through the two lateral foramina of Luschka and the foramen of Magendie. Some of it infiltrates the subarachnoid space around the spinal cord. Arachnoid granulations, which are shaped by the arachnoid mater, project into dural venous sinuses, especially the superior sagittal sinus, and are responsible for CSF absorption. Lastly, CSF is assimilated into the venous sinuses (“bulk glow”), reentering into the systemic circulation [1] (Figure 1).

Figure 1. Schematic illustration of CSF flow and drainage. The CSF is produced by the choroid plexus, flows through the ventricles to subarachnoid space through the foramina of Luschka and Magendie, where it is subsequently drained from the dural venous sinuses through arachnoid villi. (Created with BioRender.com)
CSF: Cerebrospinal fluid

Hydrocephalus Definition and Classification

Hydrocephalus refers to a Central Nervous System (CNS) condition characterized by a surplus of CSF accumulation in the ventricles. In early 1913, Dandy was the first to propose a classification of hydrocephalus as communicating and non-communicating (obstructive). Since then, many more classifications have been demonstrated. Specifically, there are three main types of hydrocephalus: non-communicating/obstructive (↓ CSF absorption), communicating (↓ CSF absorption), and communicating hypersecretory (↑ CSF production). However, some special forms of hydrocephalus have also been described: normal pressure hydrocephalus (NPH), entrapped 4th ventricle, arrested hydrocephalus, and hydrocephalus ex vacuo [2-5].

Specifically, non-communicative or obstructive hydrocephalus is established by a blockage in CSF pathways, from the ventricles to the subarachnoid space. Different types of brain tumors constitute some of the most common obstructions at the foramina of Monro, cerebral aqueduct of Sylvius, fourth ventricle, median foramen of Magendie, etc. Other acquired causes that could lead to obstructive hydrocephalus are space-occurring lesions, such as brain abscesses, and clots due to hemorrhage. Furthermore, some congenital diseases are leading to obstruction in the ventricular system. These include Arnold-Chiari malformation, Dandy-Walker malformation, intrauterine infections, such as congenital toxoplasmosis, colloid cyst obstructing the interventricular foramen of Monro, and congenital stenosis of the cerebral aqueduct of Sylvius [2-5]. Communicating hydrocephalus occurs when the flow of CSF is obstructed in the arachnoid villi or subarachnoid cisterns after it exits the intraventricular system. This form of hydrocephalus is termed communicating because the transit passages between the ventricles remain open allowing CSF flow within the ventricular system. The most common cause of communicating hydrocephalus is infectious diseases of the central nervous system, such as meningitis or cysticercosis, leading to inflammation of arachnoid villi and eventually their abolishment. Other common causes are post-hemorrhagic, such as subarachnoid and post-intraventricular hemorrhage, and post-Traumatic Brain Injury (TBI) changes. Congenital absence of arachnoid villi could also more rarely lead to communicating hydrocephalus [2-5]. Hypersecretory communicating hydrocephalus is prompted by CSF overproduction. It is presumptively caused by choroid plexus papilloma or more seldom carcinoma. These tumors appear more often in childhood. Moreover, inflammation related to the choroid plexus can lead to hypersecretory communicating hydrocephalus [2-5].

Normal pressure hydrocephalus NPH was first described in 1965 and is a form of chronic communicating hydrocephalus, characterized by normal intracranial pressure (ICP) or slightly elevated. It can be idiopathic (iNPH) appearing most commonly in elderly people or “secondary NPH” due to chronic obstruction of CSF flow. It characteristically occurs with the distinct Hakim triad of reversible symptoms including progressive gait apraxia, urine incontinence, and dementia [4,5]. Entrapped fourth ventricle is a rare neurosurgical condition which occurs when the fourth ventricle communicates neither with the third ventricle nor with the basal cisterns. It most commonly manifests in patients with chronic lateral ventricular shunting and more seldom in Dandy-Walker malformation, intracranial masses, choroid ventriculitis [5]. Arrested or compensated hydrocephalus is a term used by physicians mostly to describe a form of hydrocephalus usually present at birth, in which there is no progression and the treatment with CSF shunt would be essential only when symptoms of intracranial hypertension occur [5]. Finally, hydrocephalus ex vacuo is often classified as a distinct form of hydrocephalus, although it is not a true hydrocephalus. It is characterized by ventricular distention provoked by cerebral atrophy either by normal aging or by the progression of certain diseases such as Alzheimer’s disease, Creutzfeldt-Jakob disease, Huntington’s disease, and TBI [5].

Epidemiology

The estimated prevalence of hydrocephalus in the general population is 1-1.5%. Communicating hydrocephalus is more common than non-communicating hydrocephalus. The incidence of congenital hydrocephalus is approximately 0.9-1.8/1000 births [5]. It is increasingly apparent that genetic factors play a fundamental role in the pathogenesis of some cases of hydrocephalus. In approximately 40% of incidents with hydrocephalus, the pathogenesis is associated with molecular and genetic variations [6]. In this study, the clinical entity of hydrocephalus will be approached regarding the perspective of the molecular etiology of the subsequent CSF accumulation rather than an anatomical viewpoint of hydrocephalus pathophysiology.

I. Unraveling the genetic and molecular profile of hydrocephalus

According to accumulating evidence, the pathogenesis of hydrocephalus is linked with many molecular changes. This phenomenon pertains to a multitude of clinical entities leading to congenital/developmental hydrocephalus which might manifest in the early years of life or later in adolescence or adult life but is not caused by acquired conditions, such as subarachnoid hemorrhage or meningitis. The molecular landscape underlying the pathogenesis of hydrocephalus unfolds in 7 primary axes. The molecular classification of congenital hydrocephalus is as follows: X-linked hydrocephalus with congenital aqueduct stenosis (AS), neural tube defects (spina bifida), Dandy-Walker syndrome, holoprosencephaly, primary ciliary dyskinesia and other ciliopathies, nonsyndromic autosomal recessive hydrocephalus, miscellaneous (less common syndromes) [7-9]. Table 1 presents a comprehensive summary of the main genes responsible for congenital/developmental hydrocephalus.

X-Linked hydrocephalus

The most common form of genetic Hydrocephalus (1/30,000) is linked with a gene mutation in the L1-CAM neural cell adhesion molecule on chromosome X [10-13]. L1-CAM is a glycoprotein, a member of the immunoglobulin-like CAM family, which is an important contributor in neural adhesion, migration, morphology, and growth, mediating cell-cell adhesion [12]. The following genotypic alterations for L1-CAM protein have been reported: Class I: mutations in the cytoplasmic domain of the protein, Class II: mutations in the extracellular domain, Class III: premature stop codon in the extracellular domain and loss of function, Class IV: mutation in noncoding regions [13]. An association between ventricular dilation and mutation class of L1-CAM has been found. Additionally, L1-CAM mutations are responsible for CRASH syndrome, a very rare inherited disorder characterized by corpus callosum hypoplasia, retardation, adducted thumbs, spastic paraplegia, and hydrocephalus which led to the acronym CRASH syndrome [14]. Another X-linked hydrocephalus disorder is called Fried-type syndrome, caused by AP1S2 encoding gene mutations [15]. This disorder is manifested with hydrocephalus, intellectual disability, and iron deposition in the basal ganglia. Moreover, in some cases, aqueduct stenosis and/or fourth ventricular or retrocerebellar cysts have been reported [10].

Neural Tube Defects

Neural tube defects occur due to incomplete closure of the neural tube during embryogenesis. Spina bifida aperta (Myelomeningocele), caused by failure of closure of the caudal neuropore, is the most frequently observed defect [13]. Hydrocephalus occurs in 80-90% of these cases and may be caused by genes related to neural tube development or genetic mutations that affect ciliary beating and ependymal cell polarity [13,16]. Nevertheless, multiple gene mutations and environmental factors seem to contribute to its development while sufficient intake of folate during pregnancy is shown to decrease the occurrence of the disease. Indeed, risk factors include genetic variants of single genes that encode the metabolism of folate-homocysteine or gene-related interactions amongst different folate molecular pathways [16-19]. Notably, in utero closure of spina bifida lowers the occurrence of post-natal hydrocephalus and the need for surgical management [20,21] whereas positive impact on Chiari II malformation and neurological deficits has been also observed [20,21].

Dandy-Walker malformation

Dandy-Walker malformation (DWM) is a common congenital malformation (1/25000-35000) which consists of hypoplasia and rotation of the cerebellar vermis, enlargement of the posterior fossa and fourth ventricle and rostrally shifted lateral sinus, tentorium and torcula herophili [13,22]. In addition to these classical findings, DWM is characterized by additional abnormalities and malformations of the CNS, including agenesis of the corpus callosum, heterotopia, occipital meningocele, visual deficits, epilepsy, schizencephaly and glial heterotopia [22,23]. Genetic and environmental factors seem to contribute to its development. At least 18 types of chromosomal abnormalities e.g., abnormalities in 2q, 5p, 8p, 9p, 13q, 16q, 17q but also mutations in several genes [Protein O-mannosyltransferase 1 (POMT1), Protein O-mannosyltransferase 2 (POMT2), protein O-mannose beta-1,2-N acetylglucosaminyltransferase (POMGNT1), fukutin-related protein (FKRP), Fukutin (FKTN), Isoprenoid synthase domain-containing gene (ISPD), LARGE Xylosyl- And Glucuronyltransferase 1 (LARGE)] have been associated with this disorder. Also, fetal viral infections (CMV, Rubella), maternal diabetes, maternal use of warfarin and alcohol use during brain development have been linked to DWM [10,13,23,24]. Hydrocephalus develops in more than 80% of DWM patients [23].

Holoprosencephaly

Holoprosencephaly (HPE) is a brain malformation in which the prosencephalon (embryonic forebrain) fails to separate into two different lobes (3-4 weeks of gestation) caused by neural differentiation abnormalities [25]. It appears in 1/10,000 births and is usually accompanied with hydrocephalus, DWM and craniofacial abnormalities [25]. According to the grade of separation, this condition is categorized as a lobar, semi-lobar and lobar holoprosencephaly [13]. Holoprosencephaly has been shown to be linked with chromosomal abnormalities in 25%-50% of cases including trisomy 13 and 18 with trisomy 13 most common. The other 50% of cases may be associated with maternal diabetes, alcohol use and smoking, anticonvulsant drugs, retinoic acid, CMV infection of the fetus, hypercholesterolemia, and mutation in 7-dehydrocholesterol reductase and a minimum of 16 other HPE-associated genes e.g., Sonic Hedgehog gene (SHH), Zic family member 2 gene (ZIC2), SIX homeobox 3 gene (SIX3), TGFB induced factor homeobox 1 gene (TGIF1) [13,26,27].

Primary ciliary dyskinesia and other ciliopathies

Primary ciliary dyskinesia (PCD) has an incidence of 1/15-30,000 births. Hydrocephalus may be present among other pathologies, like situs inversus congenital heart disease, polysplenia, or asplenia [28-30]. This disorder is caused by cilia dysfunction in which ciliary and flagellar motility or orientation is affected. Motile cilia play a crucial role in fluid flow and are composed of a basal body in order to anchor to the cell membrane and an axoneme made by 9 + 2 microtubules (a ring of nine doublets, and a single central pair). During fetal neurodevelopment, there is another type of motile cilium made of a 9 + 0 axoneme which activates a signaling cascade for the establishment of left-right sidedness and body laterality. Furthermore, dynein motor proteins play a critical role in cilia motility, composing inner and outer dynein arms on the outer microtubule doublets [28,29]. Being genetically heterogeneous, primary ciliary dyskinesia is primarily an autosomal-recessive disease while autosomal-dominant and X-linked type has been found. About 50 genes are associated with this disorder and most of the genes encode proteins that are responsible for axonemal motors, structure and regulation or assembly and preassembly of cilia e.g., dynein axonemal heavy chain 11 gene (DNAH11), NIMA related kinase 10 gene (NEK10) and growth arrest specific 2 like 2 gene (GAS2L2) [29]. However, Duy et al., (2022) have observed that the occurrence of hydrocephalus in human PCD was found to be low (1.3% in some cases) [31]. It has been reported that hydrocephalus appears primarily as a result of reduced ependyma cilia beating in narrow paths and secondary changes in CSF production via altered ependyma and choroid plexus microenvironment [32]. Other motile ciliopathies are associated with biallelic mutations in Cyclin O (CCNO) and Multiciliate Differentiation and DNA Synthesis Associated Cell Cycle Protein (MCIDAS), important proteins for centriole production. Interestingly, MCIDAS mutations demonstrate higher hydrocephalus incidence. Meanwhile de novo single mutations in Forkhead box J1 (FOXJ1), a transcription factor for cilia gene expression, result in motile cilia number reduction and hydrocephalus, laterality defects and recurrent respiratory infections in fetus [29].

Nonsyndromic autosomal recessive hydrocephalus

The 2%-11% of congenital hydrocephalus cases (2-4% sporadic/11% familial) is nonsyndromic autosomal recessive type of hydrocephalus [33]. Genetic variations of Multiple PDZ Domain Crumbs Cell Polarity Complex Component (MPDZ) and Coiled-Coil Domain Containing 88C (CCDC88C) genes have been identified in severe autosomal recessive types of hydrocephalus. MPDZ encodes Multi-PDZ domain protein 1 (MUPP-1) a tight junction protein and planar cell regulator whereas CCDC88C encodes Dishevelled -associating protein with a high frequency of leucine residues (DAPLE), a dishevelled-associated protein and negative regulator of the Wnt pathway [33,34]. Autosomal recessive hydrocephalus is usually manifested with ventricular enlargement and an interhemispheric cyst, small vermis and dilated posterior fossa [30].

Miscellaneous

Other less common syndromes include: hydrocephalus associated with intracranial arachnoid cysts impairing CSF absorption from meninges, Joubert syndrome and Meckel syndrome: mutations in the Coiled-Coil And C2 Domain Containing 2A (CC2D2A) gene, Phelan-McDermid syndrome: 22q13.3 deletion, RAS-opathies including Noonan syndrome, Cardio-facio-cutaneous syndrome, Costello syndrome: mutations in RAS pathway [e.g. neurofibromin 1 (NF1), B-Raf proto-oncogene (BRAF), KRAS proto-oncogene (KRAS), protein tyrosine phosphatase non-receptor type 11 (PTPN11)], megalencephaly syndromes: most often mutations in genes involving in PI3K-AKT pathway, craniosynostosis syndromes: mutations in fibroblast growth factor receptor (FGFR) genes, VACTERL-H syndrome: primarily mutations in the FA Complementation Group B (FANCB) gene [10,30].

II. New avenues of exploration: modern research perspectives on hydrocephalus

Hydrocephalus presents diverse challenges in modern research. The emergence of Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) and CRISPR-associated protein 9 (CRISPR/Cas9) genome editing has facilitated the creation of transgenic rat models, such as the L1-cam knockout model of X-linked hydrocephalus (XLH), providing insights into the genetic basis of this condition [35]. Additionally, diffusion tensor imaging (DTI) reveals early periventricular white matter tract injury in hydrocephalus, emphasizing the need for advanced imaging techniques. In a study by Emmert et al., (2019) CRISPR/Cas9 efficiently disrupted the L1-cam gene in rats, leading to hydrocephalus and delayed development whereas DTI unveiled significant reductions in fractional anisotropy and axial diffusivity in specific brain regions. The study emphasizes the potential of CRISPR/Cas9 in creating larger animal models, offering avenues for novel surgical and imaging techniques on a larger scale [35]. Further complicating the understanding of hydrocephalus is the intricate interplay between CSF and cerebral blood flow (CBF) dynamics. Mathematical models, such as Marmarou’s compartmental model, offer insights into these interactions. Compensatory parameters derived from CSF circulation models aid in diagnosing and managing hydrocephalus [36]. However, further studies, as reviewed by Kazimierska et al., (2012), suggest alterations in CBF dynamics, highlighting the need for comprehensive modeling approaches that consider both CSF and CBF interactions [37].

Interestingly, hydrocephalus is associated not only with genetic mutations but also with a down-regulation of AQP1 expression in choroid plexus epithelium in hydrocephalus models, suggesting a potential role for AQP1 as a regulator of CSF production [38]. Additionally, deficiency of Geminin coiled-coil domain-containing protein 1 (GemC1), a gene which regulates the balance between neural stem cell (NSC) generation and ependymal cell differentiation in the postnatal brain leads to an increased number of NSCs, contributing thus to the pathogenesis of congenital hydrocephalus [39]. Understanding the molecular mechanisms governing NSC and ependymal cell dynamics is crucial for unravelling the complexities of hydrocephalus. Recent findings by Li et al., (2023) who have investigated tumour-associated hydrocephalus (TAH), a complication of brain metastases, show that mast cells in the choroid plexus disrupt cilia and consequently increase CSF production, contributing thus to TAH [40]. Hence, a novel perspective is introduced regarding the mechanical interactions between mast cells, cilia, and CSF dynamics, shedding light on a previously unrecognized mechanism in hydrocephalus.

Another novel arena of scientific exploration regarding hydrocephalus pertains to alternative routes of CSF clearance and the glymphatic hypothesis, which postulates the existence of a process for the movement of water-soluble substances in and out of the brain, bypassing the blood-brain barrier. It posits that there is a flow of fluid transporting these substances inward through periarterial pathways, passing through the interstitial space, and then exiting through perivenous routes [41]. In that regard, it has been suggested that beyond its neuroprotective role, CSF facilitates glymphatic clearance whereas disturbances in CSF circulation, common in hydrocephalus, might involve glymphatic mechanisms as well, which can be elucidated through state-of-the-art imaging techniques [42]. Current imaging modalities, including PC-MRI and Time-SLIP, provide valuable insights into CSF flow but have limitations. Prospective research areas include comprehensive “rest-of-body” models and imaging modalities focusing on patients to better understand CSF flow disruptions in hydrocephalus [42]. Furthermore, recent findings add another layer to the intricacies of CSF drainage, presenting the nasopharyngeal lymphatic plexus as a major pathway for CSF flow towards deep cervical lymph nodes, indicating that myogenic control of the cervical lymphatics might be involved in CSF outflow regulation [43]. The subsequent interrelation between the intracranial compartment and the extracranial lymphatic system emerges as a novel control mechanism of CSF flow and a possible therapeutic target to tackle the disease.

Tissue mechanics arise as an additional area of interest regarding hydrocephalus formation. Research on the association of CSF dynamics and mechanical properties, in experimental rat models of hydrocephalus, demonstrates that changes in brain tissue mechanical properties are complex and not necessarily associated with increased brain stiffness during ventricular enlargement [44,45]. The mechanical properties of the cerebral cortex vary between different locations (non-homogeneous, anisotropic tissue) and over time, and neural cells sense and respond dynamically to these changes during development [46,47]. In the adult mammalian brain, NSCs are located mainly in the subgranular zone (SGZ) of the hippocampal dentate gyrus and the subventricular zone (SVZ) of the lateral ventricle ependymal wall (neurogenic niche). Besides these classical regions, hypothalamic neurogenesis occurring mainly along and beneath the third ventricle wall is well documented [48]. Thus, the shear stress exerted on the cells lining the neurogenic niche (ependymal cells and tanycytes) is maximized in regions residing near ventricles, where CSF is freely flowing, leading to the hypothesis that mechanical cues might regulate stem cell fate and contribute to neural tissue generation. This finding sets the scene for new avenues of scientific exploration for elucidating the interconnection between mechanotransduction and neurogenesis. The link between neurogenesis disruption and hydrocephalus is further illuminated by animal research indicating that the loss of Ccdc85c, a gene implicated in hydrocephalus in humans (nonsyndromic autosomal recessive) as well as mice and rats alike, disrupts ependymal cell development and leads to ectopic expression of immature neuro-glial cells, further consolidating the role of early neurodevelopmental stages in hydrocephalus pathogenesis [49]. Additional research has identified Tripartite motif 71/lineage defective 41 (TRIM71/lin-41) as a key gene in human hydrocephalus, specifically expressed in neuroepithelial cells, and demonstrated its role in compromising cortical neurogenesis and parenchymal-CSF biomechanics, irrespective of primary defects in CSF flow [50]. These data suggests that neurogenic defects and tissue biomechanics contribute significantly to hydrocephalus pathogenesis, indicating a shift in understanding from CSF drainage defects to intrinsic brain anomalies.

Hydrocephalus research thus enables us to explore the prospect of implementing mechanical cues to facilitate in vitro neural differentiation in bioreactors for tissue engineering purposes. It is evident that the multifaceted challenges in hydrocephalus research involve genetic, molecular, mechanical, and physiological aspects as illustrated in Figure 2, necessitating a holistic approach integrating CRISPR/Cas9 technology, advanced imaging techniques, mathematical modeling, and a reevaluation of existing hypotheses.

Figure 2. Modern perspectives in hydrocephalus pathogenesis research: Up: Contrasting evidence for the cilia hypothesis, Middle: Alterations in the molecular substrate of ependymal cells, including AQP1 downregulation, Bottom left: The glymphatic flow disruption hypothesis, Bottom middle: Addressing the issue of the mechanical properties of the brain in controlling CSF flow, Bottom right: Impairments in neurodevelopment and stem cell fate determination impacting hydrocephalus pathogenesis. (Created with BioRender.com)
CSF: Cerebrospinal fluid, AQP1: Aquaporin 1

CONCLUSIONS

Hydrocephalus is a clinical entity trademarked by ventricular dilatation caused by a multitude of diseases. Although previously mostly explored under the lens of the anatomical etiology (e.g., obstruction of CSF flow, space-occupying lesions), recent scientific investigation has unveiled an intricate molecular, genetic, and cellular landscape providing a novel perspective towards the pathogenesis of hydrocephalus. Recent findings have shed doubt on established views on hydrocephalus pathogenesis, including the ciliary flow hypothesis while proposing new hypotheses such as AQP downregulation or neural development disruption and capitalizing on the rheological profile of CSF flow as proposed in the glymphatic flow hypothesis. Notably, the mechanical environment in the ventricles seems to be pivotal in regulating the dynamics of CSF-ependyma interrelations, even guiding stem cell fate. In addition, human and animal evidence suggests that motile ciliopathies infrequently cause hydrocephalus in humans, and certain hydrocephalus cases associated with ciliary gene mutations may result from altered neurodevelopment rather than the loss of cilia-generated CSF flow, as ciliary genes are shown to also affect neural stem cell fate. This prompts a reevaluation of the link between motile cilia, CSF physiology, and brain development, crucial for understanding hydrocephalus and related neurodevelopmental disorders. It is thus evident that the etiological substrate underlying hydrocephalus pathogenesis constitutes a kaleidoscope of cellular, molecular, and genetic factors with nontrivial relationships. Further research on the complex interplay of factors leading to hydrocephalus is anticipated to further illuminate the mechanisms of hydrocephalus formation.

Conflict of interest: None to declare

Declaration of funding sources: None to declare

Author contributions: D Katagi, V Vafeiadis, K Birmpas and M Assimakopoulou contributed to the study conception and design; K Birmpas, D Katagi and V Vafeiadis performed the material preparation, data collection, and analysis; V Vafeiadis, K Birmpas and D Katagi prepared the first draft of the manuscript; all authors commented on previous versions of the manuscript; M Assimakopoulou critically edited and reviewed the manuscript; all authors read and approved the final manuscript.

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