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Gerrits R, Vingerhoets G. Brain functional segregation, handedness and cognition in situs inversus totalis: A replication study. Neuropsychologia 2023; 191:108731. [PMID: 37949213 DOI: 10.1016/j.neuropsychologia.2023.108731] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/25/2023] [Revised: 10/04/2023] [Accepted: 11/06/2023] [Indexed: 11/12/2023]
Abstract
Situs inversus totalis (SIT) is a rare congenital anomaly in which the arrangement of the visceral organs is completely left-right mirrored. A previous study by our lab suggests that SIT (N = 15) correlated with more heterogeneous asymmetrical brain organization and increased left-handedness. In addition, visceral reversal correlated with poorer cognitive performance, especially when hemisphere organization was atypical. The current study sought to replicate these findings in a larger sample. We scanned 23 volunteers with SIT as well as an equal number of controls with usual organ arrangement, and used fMRI to determine their hemisphere dominance for two left hemisphere functions (language and manual praxis) and two right hemisphere functions (spatial attention and face recognition). Effects of SIT etiology were explored by pooling data from the original cohort with the replication sample. Our results reveal that each of those four cognitive functions demonstrated the expected population dominance in SIT, albeit they were less pronounced - but not significantly so - compared to controls. Unusual patterns of hemispheric crowding and mirror-reversal of functional brain organization was observed more often in SIT (48%) than in the controls (30%), but this difference also did not reach statistical significance. However, left-handedness was found to be significantly more common in SIT (26%) than in the overall population (10.6%). Finally, cognitive ability, as assessed by a neuropsychological test battery, was not associated with organ situs or hemisphere organization. Taken together, our data adds to the growing evidence that the determinants of visceral and neural asymmetries are largely independent from one another and that complete situs inversus does not co-occur with an obligatory transposition of the brain's functional architecture. There nevertheless might be instances in which (genetic) mechanisms could simultaneously cause complete visceral reversal and atypical brain laterality.
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Affiliation(s)
- Robin Gerrits
- Department of Experimental Psychology, Ghent University, Ghent, Belgium
| | - Guy Vingerhoets
- Department of Experimental Psychology, Ghent University, Ghent, Belgium.
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2
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Chen XQ, Lin SJ, Wang JJ, Long S, Kong FX, Guo ZK. "Reverse life": A rare case report of situs inversus totalis combined with cardiac abnormalities in a young stroke. CNS Neurosci Ther 2022; 28:1458-1460. [PMID: 35702947 PMCID: PMC9344078 DOI: 10.1111/cns.13879] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/19/2022] [Revised: 05/20/2022] [Accepted: 05/24/2022] [Indexed: 12/01/2022] Open
Affiliation(s)
- Xue-Qin Chen
- Fourth Clinical Medical College of Guangzhou University of Traditional Chinese Medicine, Shenzhen, China
| | - Song-Jun Lin
- Department of Encephalopathy and Phychology, Shenzhen Traditional Chinese Medicine Hospital Shenzhen, Guangdong, China
| | - Jian-Jun Wang
- Department of Encephalopathy and Phychology, Shenzhen Traditional Chinese Medicine Hospital Shenzhen, Guangdong, China
| | - Sha Long
- Department of Ultrasound, Shenzhen Traditional Chinese Medicine Hospital Shenzhen, Guangdong, China
| | - Fan-Xin Kong
- Fourth Clinical Medical College of Guangzhou University of Traditional Chinese Medicine, Shenzhen, China.,Department of Encephalopathy and Phychology, Shenzhen Traditional Chinese Medicine Hospital Shenzhen, Guangdong, China
| | - Zhou-Ke Guo
- Department of Encephalopathy and Phychology, Shenzhen Traditional Chinese Medicine Hospital Shenzhen, Guangdong, China
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Abstract
Asymmetries in the functional and structural organization of the nervous system are widespread in the animal kingdom and especially characterize the human brain. Although there is little doubt that asymmetries arise through genetic and nongenetic factors, an overarching model to explain the development of functional lateralization patterns is still lacking. Current genetic psychology collects data on genes relevant to brain lateralizations, while animal research provides information on the cellular mechanisms mediating the effects of not only genetic but also environmental factors. This review combines data from human and animal research (especially on birds) and outlines a multi-level model for asymmetry formation. The relative impact of genetic and nongenetic factors varies between different developmental phases and neuronal structures. The basic lateralized organization of a brain is already established through genetically controlled embryonic events. During ongoing development, hemispheric specialization increases for specific functions and subsystems interact to shape the final functional organization of a brain. In particular, these developmental steps are influenced by environmental experiences, which regulate the fine-tuning of neural networks via processes that are referred to as ontogenetic plasticity. The plastic potential of the nervous system could be decisive for the evolutionary success of lateralized brains.
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Abstract
The alignment of visceral and brain asymmetry observed in some vertebrate species raises the question of whether this association also exists in humans. While the visceral and brain systems may have developed asymmetry for different reasons, basic visceral left–right differentiation mechanisms could have been duplicated to establish brain asymmetry. We describe the main phenotypical anomalies and the general mechanism of left–right differentiation of vertebrate visceral and brain laterality. Next, we systematically review the available human studies that explored the prevalence of atypical behavioral and brain asymmetry in visceral situs anomalies, which almost exclusively involved participants with the mirrored visceral organization (situs inversus). The data show no direct link between human visceral and brain functional laterality as most participants with situs inversus show the typical population bias for handedness and brain functional asymmetry, although an increased prevalence of functional crowding may be present. At the same time, several independent studies present evidence for a possible relation between situs inversus and the gross morphological asymmetry of the brain torque with potential differences between subtypes of situs inversus with ciliary and non-ciliary etiologies.
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Kienast P, Schwartz E, Diogo MC, Gruber GM, Brugger PC, Kiss H, Ulm B, Bartha-Doering L, Seidl R, Weber M, Langs G, Prayer D, Kasprian G. The Prenatal Origins of Human Brain Asymmetry: Lessons Learned from a Cohort of Fetuses with Body Lateralization Defects. Cereb Cortex 2021; 31:3713-3722. [PMID: 33772541 DOI: 10.1093/cercor/bhab042] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/14/2020] [Revised: 01/13/2021] [Accepted: 02/01/2021] [Indexed: 11/14/2022] Open
Abstract
Knowledge about structural brain asymmetries of human fetuses with body lateralization defects-congenital diseases in which visceral organs are partially or completely incorrectly positioned-can improve our understanding of the developmental origins of hemispheric brain asymmetry. This study investigated structural brain asymmetry in 21 fetuses, which were diagnosed with different types of lateralization defects; 5 fetuses with ciliopathies and 26 age-matched healthy control cases, between 22 and 34 gestational weeks of age. For this purpose, a database of 4007 fetal magnetic resonance imagings (MRIs) was accessed and searched for the corresponding diagnoses. Specific temporal lobe brain asymmetry indices were quantified using in vivo, super-resolution-processed MR brain imaging data. Results revealed that the perisylvian fetal structural brain lateralization patterns and asymmetry indices did not differ between cases with lateralization defects, ciliopathies, and normal controls. Molecular mechanisms involved in the definition of the right/left body axis-including cilium-dependent lateralization processes-appear to occur independently from those involved in the early establishment of structural human brain asymmetries. Atypically inverted early structural brain asymmetries are similarly rare in individuals with lateralization defects and may have a complex, multifactorial, and neurodevelopmental background with currently unknown postnatal functional consequences.
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Affiliation(s)
- Patric Kienast
- Department of Biomedical Imaging and Image-guided Therapy, Medical University of Vienna, Vienna 1090, Austria
| | - Ernst Schwartz
- Department of Biomedical Imaging and Image-guided Therapy, Medical University of Vienna, Vienna 1090, Austria
| | - Mariana C Diogo
- Department of Biomedical Imaging and Image-guided Therapy, Medical University of Vienna, Vienna 1090, Austria
| | - Gerlinde M Gruber
- Department of Anatomy and Biomechanics, Karl Landsteiner University of Health Sciences, Krems, Lower Austria 3500, Austria
| | - Peter C Brugger
- Center for Anatomy and Cell Biology, Medical University of Vienna, Vienna 1090, Austria
| | - Herbert Kiss
- Department of Obstetrics and Gynecology, Medical University of Vienna, Vienna 1090, Austria
| | - Barbara Ulm
- Department of Obstetrics and Gynecology, Medical University of Vienna, Vienna 1090, Austria
| | - Lisa Bartha-Doering
- Department of Pediatrics and Adolescent Medicine, Medical University of Vienna, Vienna 1090, Austria
| | - Rainer Seidl
- Department of Pediatrics and Adolescent Medicine, Medical University of Vienna, Vienna 1090, Austria
| | - Michael Weber
- Department of Biomedical Imaging and Image-guided Therapy, Medical University of Vienna, Vienna 1090, Austria
| | - Georg Langs
- Department of Biomedical Imaging and Image-guided Therapy, Medical University of Vienna, Vienna 1090, Austria
| | - Daniela Prayer
- Department of Biomedical Imaging and Image-guided Therapy, Medical University of Vienna, Vienna 1090, Austria
| | - Gregor Kasprian
- Department of Biomedical Imaging and Image-guided Therapy, Medical University of Vienna, Vienna 1090, Austria
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Bieder A, Einarsdottir E, Matsson H, Nilsson HE, Eisfeldt J, Dragomir A, Paucar M, Granberg T, Li TQ, Lindstrand A, Kere J, Tapia-Páez I. Rare variants in dynein heavy chain genes in two individuals with situs inversus and developmental dyslexia: a case report. BMC MEDICAL GENETICS 2020; 21:87. [PMID: 32357925 PMCID: PMC7193346 DOI: 10.1186/s12881-020-01020-2] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 11/17/2019] [Accepted: 04/05/2020] [Indexed: 02/08/2023]
Abstract
Background Developmental dyslexia (DD) is a neurodevelopmental learning disorder with high heritability. A number of candidate susceptibility genes have been identified, some of which are linked to the function of the cilium, an organelle regulating left-right asymmetry development in the embryo. Furthermore, it has been suggested that disrupted left-right asymmetry of the brain may play a role in neurodevelopmental disorders such as DD. However, it is unknown whether there is a common genetic cause to DD and laterality defects or ciliopathies. Case presentation Here, we studied two individuals with co-occurring situs inversus (SI) and DD using whole genome sequencing to identify genetic variants of importance for DD and SI. Individual 1 had primary ciliary dyskinesia (PCD), a rare, autosomal recessive disorder with oto-sino-pulmonary phenotype and SI. We identified two rare nonsynonymous variants in the dynein axonemal heavy chain 5 gene (DNAH5): a previously reported variant c.7502G > C; p.(R2501P), and a novel variant c.12043 T > G; p.(Y4015D). Both variants are predicted to be damaging. Ultrastructural analysis of the cilia revealed a lack of outer dynein arms and normal inner dynein arms. MRI of the brain revealed no significant abnormalities. Individual 2 had non-syndromic SI and DD. In individual 2, one rare variant (c.9110A > G;p.(H3037R)) in the dynein axonemal heavy chain 11 gene (DNAH11), coding for another component of the outer dynein arm, was identified. Conclusions We identified the likely genetic cause of SI and PCD in one individual, and a possibly significant heterozygosity in the other, both involving dynein genes. Given the present evidence, it is unclear if the identified variants also predispose to DD and further studies into the association between laterality, ciliopathies and DD are needed.
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Affiliation(s)
- Andrea Bieder
- Department of Biosciences and Nutrition, Karolinska Institutet, Hälsovägen 7, 141 83, Huddinge, Sweden.
| | - Elisabet Einarsdottir
- Department of Biosciences and Nutrition, Karolinska Institutet, Hälsovägen 7, 141 83, Huddinge, Sweden.,Stem Cells and Metabolism Research Program (STEMM), University of Helsinki, Helsinki, Finland.,Folkhälsan Institute of Genetics, Helsinki, Finland.,Science for Life Laboratory, Department of Gene Technology, KTH-Royal Institute of Technology, Solna, Sweden
| | - Hans Matsson
- Department of Women's and Children's Health, Karolinska Institutet, Solna, Sweden.,Center for Molecular Medicine, Karolinska Institutet, Stockholm, Sweden.,Department of Immunology, Genetics and Pathology, Uppsala University, Uppsala, Sweden
| | - Harriet E Nilsson
- Department of Biosciences and Nutrition, Karolinska Institutet, Hälsovägen 7, 141 83, Huddinge, Sweden.,Department of Biomedical Engineering and Health Systems, School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, Huddinge, Sweden
| | - Jesper Eisfeldt
- Center for Molecular Medicine, Karolinska Institutet, Stockholm, Sweden.,Department of Molecular Medicine and Surgery, Karolinska Institutet, Stockholm, Sweden.,Science for Life Laboratory, Karolinska Institutet Science Park, Solna, Sweden
| | - Anca Dragomir
- Department of Pathology, Uppsala University Hospital, Uppsala, Sweden.,Department of Immunology, Genetics and Pathology, Uppsala University, Uppsala, Sweden
| | - Martin Paucar
- Department of Clinical Neuroscience, Karolinska Institutet, Stockholm, Sweden
| | - Tobias Granberg
- Department of Clinical Neuroscience, Karolinska Institutet, Stockholm, Sweden.,Department of Radiology, Karolinska University Hospital, Stockholm, Sweden
| | - Tie-Qiang Li
- Department of Clinical Science, Intervention and Technology, Karolinska Institutet, Stockholm, Sweden
| | - Anna Lindstrand
- Center for Molecular Medicine, Karolinska Institutet, Stockholm, Sweden.,Department of Molecular Medicine and Surgery, Karolinska Institutet, Stockholm, Sweden.,Department of Clinical Genetics, Karolinska University Hospital, Stockholm, Sweden
| | - Juha Kere
- Department of Biosciences and Nutrition, Karolinska Institutet, Hälsovägen 7, 141 83, Huddinge, Sweden.,Stem Cells and Metabolism Research Program (STEMM), University of Helsinki, Helsinki, Finland.,School of Basic and Medical Biosciences, King's College London, Guy's Hospital, London, UK
| | - Isabel Tapia-Páez
- Department of Medicine, Solna, Karolinska Institutet, Solnavägen 30, 171 76 Solna, Stockholm, Sweden.
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Jakab A, Meuwly E, Feldmann M, Rhein MV, Kottke R, O'Gorman Tuura R, Latal B, Knirsch W. Left temporal plane growth predicts language development in newborns with congenital heart disease. Brain 2020; 142:1270-1281. [PMID: 30957841 DOI: 10.1093/brain/awz067] [Citation(s) in RCA: 18] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/03/2018] [Revised: 01/17/2019] [Accepted: 01/27/2019] [Indexed: 01/07/2023] Open
Abstract
Congenital heart defects are the most common congenital anomalies, accounting for a third of all congenital anomaly cases. While surgical correction dramatically improved survival rates, the lag behind normal neurodevelopment appears to persist. Deficits in higher cognitive functions are particularly common, including developmental delay in communication and oral-motor apraxia. It remains unclear whether the varying degree of cognitive developmental delay is reflected in variability in brain growth patterns. To answer this question, we aimed to investigate whether the rate of regional brain growth is correlated with later life neurodevelopment. Forty-four newborns were included in our study, of whom 33 were diagnosed with dextro-transposition of the great arteries and 11 with other forms of severe congenital heart defects. During the first month of life, neonates underwent corrective or palliative cardiovascular bypass surgery, pre- and postoperative cerebral MRI were performed 18.7 ± 7.03 days apart. MRI was performed in natural sleep on a 3.0 T scanner using an 8-channel head coil, fast spin-echo T2-weighted anatomical sequences were acquired in three planes. Based on the principles of deformation-based morphometry, we calculated brain growth rate maps reflecting average daily growth occurring between pre- and postoperative brain images. An explorative, whole-brain, threshold-free cluster enhancement analysis revealed strong correlation between the growth rate of the Heschl's gyrus, anterior planum temporale and language score at 12 months of age, corrected for demographic variables (P = 0.018, t = 5.656). No significant correlation was found between brain growth rates and motor or cognitive scores. Post hoc analysis showed that the length of hospitalization interacted with this correlation, longer hospitalization resulted in faster enlargement of the internal CSF spaces. Our longitudinal cohort study provides evidence for the early importance of left-dominant perisylvian regions in auditory and language development before direct postnatal exposure to native language. In congenital heart disease patients, the perioperative period results in a critical variability of brain growth rate in this region, which is a reliable neural correlate of language development at 1 year of age.
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Affiliation(s)
- Andras Jakab
- Centre for MR Research, University Children's Hospital Zurich, Zurich, Switzerland.,Neuroscience Center Zurich, University of Zurich, Zurich, Switzerland
| | - Eliane Meuwly
- Child Development Center, University Children's Hospital Zurich, Zurich, Switzerland
| | - Maria Feldmann
- Child Development Center, University Children's Hospital Zurich, Zurich, Switzerland
| | - Michael von Rhein
- Child Development Center, University Children's Hospital Zurich, Zurich, Switzerland
| | - Raimund Kottke
- Department of Diagnostic Imaging, University Children's Hospital Zurich, Zurich, Switzerland
| | - Ruth O'Gorman Tuura
- Centre for MR Research, University Children's Hospital Zurich, Zurich, Switzerland.,Children's Research Center, University Children's Hospital Zurich, Zurich, Switzerland
| | - Beatrice Latal
- Child Development Center, University Children's Hospital Zurich, Zurich, Switzerland.,Children's Research Center, University Children's Hospital Zurich, Zurich, Switzerland
| | - Walter Knirsch
- Children's Research Center, University Children's Hospital Zurich, Zurich, Switzerland.,Division of Pediatric Cardiology, Pediatric Heart Center, University Children's Hospital Zurich, Zurich, Switzerland
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Postema MC, Carrion-Castillo A, Fisher SE, Vingerhoets G, Francks C. The genetics of situs inversus without primary ciliary dyskinesia. Sci Rep 2020; 10:3677. [PMID: 32111882 PMCID: PMC7048929 DOI: 10.1038/s41598-020-60589-z] [Citation(s) in RCA: 20] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/11/2019] [Accepted: 02/13/2020] [Indexed: 12/14/2022] Open
Abstract
Situs inversus (SI), a left-right mirror reversal of the visceral organs, can occur with recessive Primary Ciliary Dyskinesia (PCD). However, most people with SI do not have PCD, and the etiology of their condition remains poorly studied. We sequenced the genomes of 15 people with SI, of which six had PCD, as well as 15 controls. Subjects with non-PCD SI in this sample had an elevated rate of left-handedness (five out of nine), which suggested possible developmental mechanisms linking brain and body laterality. The six SI subjects with PCD all had likely recessive mutations in genes already known to cause PCD. Two non-PCD SI cases also had recessive mutations in known PCD genes, suggesting reduced penetrance for PCD in some SI cases. One non-PCD SI case had recessive mutations in PKD1L1, and another in CFAP52 (also known as WDR16). Both of these genes have previously been linked to SI without PCD. However, five of the nine non-PCD SI cases, including three of the left-handers in this dataset, had no obvious monogenic basis for their condition. Environmental influences, or possible random effects in early development, must be considered.
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Affiliation(s)
- Merel C Postema
- Department of Language & Genetics, Max Planck Institute for Psycholinguistics, Nijmegen, The Netherlands
| | - Amaia Carrion-Castillo
- Department of Language & Genetics, Max Planck Institute for Psycholinguistics, Nijmegen, The Netherlands
| | - Simon E Fisher
- Department of Language & Genetics, Max Planck Institute for Psycholinguistics, Nijmegen, The Netherlands.,Donders Institute for Brain, Cognition and Behaviour, Radboud University, Nijmegen, The Netherlands
| | - Guy Vingerhoets
- Department of Experimental Psychology, Ghent University, Ghent, Belgium
| | - Clyde Francks
- Department of Language & Genetics, Max Planck Institute for Psycholinguistics, Nijmegen, The Netherlands. .,Donders Institute for Brain, Cognition and Behaviour, Radboud University, Nijmegen, The Netherlands.
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9
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Vingerhoets G, Gerrits R, Bogaert S. Atypical brain functional segregation is more frequent in situs inversus totalis. Cortex 2018; 106:12-25. [DOI: 10.1016/j.cortex.2018.04.012] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/24/2017] [Revised: 02/20/2018] [Accepted: 04/26/2018] [Indexed: 12/14/2022]
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Tracing the structural origins of atypical language representation: consequences of prenatal mirror-imaged brain asymmetries in a dizygotic twin couple. Brain Struct Funct 2018; 223:3757-3767. [PMID: 30062562 DOI: 10.1007/s00429-018-1717-y] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/08/2017] [Accepted: 07/13/2018] [Indexed: 01/16/2023]
Abstract
We investigated the predictive value of prenatal superior temporal sulcus (STS) depth asymmetry in a special case of a female dizygotic twin that showed inverted prenatal asymmetry of this structure. For this purpose, we performed a follow-up investigation in this former fetus at the age of seven, where we assessed the functional language lateralization using task-based and resting-state functional magnetic resonance imaging (fMRI). As control group we employed her twin brother, who showed a typical folding pattern prenatally, as well as a complementary set of four age-matched children that had fetal MRI of their brains and typical STS depth asymmetry. We could show that the twin with the atypical fetal asymmetry of the STS also showed significantly differing rightward language lateralization in the frontal and temporal lobes. Additionally, resting-state data suggest a stronger connectivity between inferior frontal gyri in this case. The twin showed normal cognitive development. This result gives a first glimpse into the STS' atypical asymmetry being a very early morphological marker for later language lateralization.
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11
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Brain structural and functional asymmetry in human situs inversus totalis. Brain Struct Funct 2018; 223:1937-1952. [PMID: 29302744 DOI: 10.1007/s00429-017-1598-5] [Citation(s) in RCA: 19] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2017] [Accepted: 12/19/2017] [Indexed: 10/18/2022]
Abstract
Magnetic resonance imaging was used to investigate brain structural and functional asymmetries in 15 participants with complete visceral reversal (situs inversus totalis, SIT). Language-related brain structural and functional lateralization of SIT participants, including peri-Sylvian gray and white matter asymmetries and hemispheric language dominance, was similar to those of 15 control participants individually matched for sex, age, education, and handedness. In contrast, the SIT cohort showed reversal of the brain (Yakovlevian) torque (occipital petalia and occipital bending) compared to the control group. Secondary findings suggested different asymmetry patterns between SIT participants with (n = 6) or without (n = 9) primary ciliary dyskinesia (PCD, also known as Kartagener syndrome) although the small sample sizes warrant cautious interpretation. In particular, reversed brain torque was mainly due to the subgroup with PCD-unrelated SIT and this group also included 55% left handers, a ratio close to a random allocation of handedness. We conclude that complete visceral reversal has no effect on the lateralization of brain structural and functional asymmetries associated with language, but seems to reverse the typical direction of the brain torque in particular in participants that have SIT unrelated to PCD. The observed differences in asymmetry patterns of SIT groups with and without PCD seem to suggest that symmetry breaking of visceral laterality, brain torque, and language dominance rely on different mechanisms.
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