The development of the posterior cranial fossa, including the brainstem, occurs early in pregnancy. Brainstem folding, a distinctive characteristic, occurs between weeks 3 and 8, while cerebellar development typically reaches completion by week 16. The presence of a kinked or "z-shaped" brainstem suggests a genetic or environmental insult around 7 weeks gestational age, interrupting brainstem and cerebellum development[2]. Thus, brainstem kinking often indicates severe CNS disorders and may be an initial manifestation in some cases[2].
Brainstem abnormalities are not clearly defined on fetal ultrasound, and in such cases, fetal MRI is crucial for revealing brainstem structure and narrowing potential diagnoses[1,7]. The Z-shaped appearance of the brainstem was previously reported by Stroustrup Smith et al., who utilized fetal MRI scans between 19 and 34 weeks of gestation in a unique series comprising seven cases[2]. They identified a correlation between the "Z"-shaped appearance of the brainstem and severe neurodysgenesis occurrence. Subsequently, this kinked brainstem was recognized as a characteristic manifestation of WWS, which can be detected as early as 14 weeks[8], particularly when accompanied by agyria and hydrocephalus. Literature has also documented associations between Z-shaped brainstems with L1CAM mutation or tubulinopathy [9, 10]. In our case series, both cases exhibited hydrocephalus and cobblestone lissencephaly, with one diagnosed with WWS and the other with tubulinopathy. Thus, when detecting a "Z"-shaped brainstem with agyria and hydrocephalus, WWS, L1CAM mutation, or tubulinopathy should be highly suspected, with implications for prenatal counseling and prognosis.
The most likely diagnosis in cases with a kinked brainstem is WWS, a lethal autosomal recessive disorder characterized by hydrocephalus, agyria, retinal dysplasia with or without encephalocele. It has been reported that patients with WWS typically do not survive beyond infancy[11]. Therefore, early detection and diagnosis of WWS are crucial for the prevention of birth defects. However, prenatal detection of WWS remains challenging due to some diagnostic criteria, such as muscular hypotonia and elevated creatine kinase levels, cannot be reliably identified prenatally.. Additionally, lissencephaly, cerebellar malformations, and retinal abnormalities observed on antenatal imaging are often nonspecific and insensitive indicators, found in less than one-quarter of reported cases[11]. In our case series, we present a prenatal diagnosis of WWS in the second trimester using prenatal ultrasonography, MRI scans, and pMES. The fetus exhibited hydrocephalus, agyria, brainstem dysplasia, and a distinctive "Z"-shaped appearance on prenatal MRI scans, along with bilateral hyperechogenic kidneys and generalized skin edema.
Hyperechogenic kidneys often accompany additional abnormalities in the renal tract and extra-renal structures, as well as chromosomal and genetic disorders like autosomal recessive polycystic kidney disease (ARPKD), autosomal dominant polycystic kidney disease (ADPKD), and Beckwith-Wiedemann syndrome. These factors influence the overall outcome more significantly than the hyperechogenicity of the kidneys[12]. In our case series, case 1 presented with bilateral hyperechogenic kidneys. Previous studie have reported urinary malformations characterized by large cystic kidneys associated with POMT2-related WWS[13]. Gasser et al. (1998) reported prenatal diagnosis of WWS in three siblings, where subsequent pregnancies revealed fetal hydrocephalus during ultrasound examination. Termination occurred at 20 weeks for the third female fetus, which exhibited postmortem findings including dilated ventricles, thin cortex, type II lissencephaly with microscopic evidence of disorganized architecture, retinal dysplasia, cystic changes, and stenosis at the pyeloureteral junction[14]. Fetal generalized skin edema, often an initial manifestation of hydrops fetalis, is valuable for detecting genetic abnormalities such as aneuploidy or Noonan syndrome, and structural malformations, especially in early pregnancy stages[15]. Notably, previous literature on WWS symptoms has not mentioned skin edema, thus it broadening our comprehension of the clinical spectrum to encompass hyperechogenic kidneys and generalized skin edema.
Approximately one-third of WWS cases involve mutations in genes like FKRP, FKTN, LARGE, POMT1, and POMT2. However, the genetic cause for most WWS patients remains unknown[16]. Exome sequencing has become essential for uncovering molecular-level fetal malformations by identifying single nucleotide variations (SNVs) and insertions/deletions (indels) in gene coding regions. Recent advancements in exome sequencing have increased studies investigating pathogenic variations associated with WWS. In our current case series, trio medical exome sequencing followed by Sanger sequencing revealed variants in the FKRP gene in fetuses with WWS.
FKRP variations are associated with a spectrum of muscular dystrophies, including limb-girdle muscular dystrophy 2I (LGMD2I), congenital muscular dystrophies type 1C (MDC1C), CMD with mild structural brain involvement, muscle-eye-brain disease (MEB), and Walker-Warburg syndrome (WWS)[17]. Case 1 carried a homozygous nonsense mutation at c.815_816del (p.Leu272ArgfsTer117) in the FKRP gene, inherited from both parents. This variant, classified as likely pathogenic, is de novo. The homozygous nonsense mutation in the FKRP gene can explain the anatomopathological features of the fetus, including WWS features, with hydrocephalus, cobblestone lissencephaly, brainstem anomalies, bilateral hyperechogenic kidneys, and generalized skin edema.
Tubulinopathies represent another less common genetic etiology associated with cases featuring a kinked brainstem. Previous literature has identified two distinct prenatal imaging patterns linked to tubulinopathies[17]. The more severe presentation is characterized by voluminous germinal matrices, indicating trapped precursor cells, leading to significant parenchymal reduction and a smooth cortical surface, often referred to as "microlissencephaly," along with severe brainstem dysgenesis. In our case, the thin and kinked brainstem was better visualized using MRI rather than ultrasound. This pattern was also observed in Case 2, diagnosed prenatally with tubulinopathy, exhibiting hydrocephalus, cobblestone lissencephaly, absence of midline structures (including septum pellucidum and corpus callosum), cerebellar dysplasia, and a distinctive "Z"-shaped appearance of the brainstem with brainstem dysplasia.
Tubulin proteins play a critical role in cortical development, influencing neuronal proliferation, migration, differentiation, and cortical lamination. Mutations in tubulin genes often affect patients with complex malformations of the cortex, commissures, posterior fossa, and varying degrees of ventriculomegaly. Tubulins are structural subunits that form microtubules, essential for cell structure, intracellular transport, and cell division[19]. Dysfunctional tubulins and microtubule-associated proteins, known as tubulinopathies, can lead to a wide range of complex brain malformations. The alpha and beta tubulins are the most common isoforms in humans responsible for assembling microtubules. Since their first description in 2007, at least eight genes encoding α- (TUBA1A, TUBA8), β- (TUBB2A, TUBB2B, TUBB3, TUBB4A, TUBB), and γ-tubulins (TUBG1) have been clinically reported. Mutations in these tubulin genes, highly expressed during central nervous system development, result in cortical malformations[19].
The TUBA1A gene (OMIM #605529) encodes tubulin α-1A, a crucial protein involved in the function and stability of microtubules[21]. Mutations in this gene can lead to various malformations, primarily due to impaired neuronal migration and/or proliferation. Cerebral cortex development relies on a sequential process involving neurogenesis, cell migration, and terminal differentiation, facilitated by the dynamic nature of the neuronal cytoskeleton. Mutations in the tubulin gene can disrupt cytoskeletal dynamics, affecting aggregation, stability, and association with microtubule protein isolates. Malformations associated with TUBA1A mutations include lissencephaly, microlissencephaly, cerebellar hypoplasia, agenesis of the corpus callosum, pachygyria, and polymicrogyria[22,23]. Additionally, TUBA1A is expressed in both the fetal brain[24] and retina[25], contributing to ophthalmologic abnormalities such as microphthalmia, congenital cataracts, and microcephaly. Fetus 2 described in our case carried a de novo heterozygous missense mutation at c.848A > G (p.H283R) in the TUBA1A gene. Although this variant was not previously reported, it was classified as likely pathogenic, consistent with an autosomal dominant inheritance pattern associated with TUBA1A mutations. The heterozygous missense mutation in the TUBA1A gene could explain the anatomopathological features observed in the fetus, including hydrocephalus, absence of midline structures, and brainstem anomalies. However, no ocular abnormalities were evident in this case.