Introduction
An aberrant structure of the cerebral cortex characterizes a varied set of disorders known as cortical formation disorders. They are brought on by faulty gene production, abnormal gene expression, or external factors like infection or ischemia that interfere with a gene's activity, interrupting the normal developmental sequences.
The abnormalities of cortical development are divided into various categories. These conditions could develop as the cortex proliferates, migrating, or organizing. These illnesses have a wide range of clinical symptoms, influenced mainly by the arrest stage. Epilepsy and developmental delays are both frequently caused by problems with cortical development. A helpful method for diagnosing these illnesses is MR imaging.
What Is Cortical Organization?
Cortical organization refers to the structure and arrangement of the cerebral cortex, which is the outermost layer of the brain. The cerebral cortex plays a crucial role in various cognitive functions, including perception, attention, memory, language, and higher-order thinking. The cortical organization is characterized by a complex and intricate arrangement of neural cells, particularly neurons. Neurons in the cerebral cortex are organized into distinct layers and columns that are specialized for different types of information processing.
Layers: The cortical layers are horizontal divisions that run parallel to the brain's surface. Typically, there are six layers in the cerebral cortex, each with its unique composition of neurons, connectivity patterns, and functional properties. These layers have different roles in information processing and communication within the cortex.
Columns: Cortical columns are vertical structures that extend across multiple cortical layers. These columns consist of a group of neurons that are functionally related and interconnected. They are organized in a columnar fashion to facilitate the processing and integration of information across different cortical layers.
Functional Areas: The cerebral cortex is divided into different functional areas, each responsible for specific cognitive functions. These areas include the primary sensory areas (such as the primary visual cortex, primary auditory cortex, and primary somatosensory cortex), as well as higher-order association areas involved in more complex cognitive processes.
Cortical Maps: The cortical organization also includes the presence of cortical maps, which are representations of sensory or motor information across specific regions of the cortex. For example, the somatosensory cortex has a body map, where different regions of the body are represented in an orderly manner. Similarly, the primary visual cortex contains a retinotopic map, where different parts of the visual field are mapped onto specific areas of the cortex.
What Are the Different Classifications of the Disorders of Cortical Formation?
The following are the different stages:
Embryology: Three steps are involved in forming the cerebral cortex, cortical organization, cell migration, and cell proliferation. Both neuronal and glial precursors are produced during cell proliferation. However, during migration, these cells move from the proliferative zone to their intended ultimate location. During the period of cortical formation, the cellular network is formed.
Between the third and fifth months of pregnancy, neurons migrate. In six successive waves, the neurons migrate from the germinal zone to their eventual location near the pial surface. Each wave of migrating neuroblasts travels to the cytoplasmic process' pial extension before detaching from it.
As a result, the cerebral cortex's deepest region is occupied by the first neurons to migrate, while the cortex's more superficial regions are home to the later migrants. Radial glial fibers and mediators control migration.
Migration abnormalities are caused by disturbances in the guiding mechanism or the timely separation of the migrating neurons in the cerebral cortex. Under migration, over-migration, or ectopic neural migration are the causes of neuronal migration disorders.
Starting at 22 weeks of fetal gestation and lasting until age two, the cortical organization depends on proper neural migration. The differentiation of neurons forms the normal cortical cytoarchitectonic pattern into different cell types, which group into horizontal laminar aggregates and vertical columns. Anomalies in gyral development and cortical organization emerge from disorders of cortical organization.
Genetics: A dominant or X-linked mutation in a particular gene frequently results in cortical development disorders. Mapped or cloned for abnormalities of cortical development are several new genes and new mutations of existing genes.
Patients with severe germline mutations, such as deletions and truncations, frequently exhibit severe symptoms, but those with somatic mosaic mutations frequently do not. Both the mosaicism and germline mutations have the mutation in some or all of the cells. When evaluating the clinical and imaging characteristics of the affected patients, the type of mutation is frequently just as crucial as figuring out which gene has been altered. In genetic counseling, understanding these pathways is crucial.
Every few years, the classification system for disorders of cortical formations changes in response to new knowledge about the neurogenetic underpinnings of the various syndromes.
The three main stages of cortex development do not occur sequentially. After the migration starts, proliferation resumes, and migration resumes as the organization begins. The cells produced by aberrant proliferation frequently do not move or are improperly organized. Too many or too few neurons or aberrant cells may cause disorders of cortical development brought on by improper cell proliferation. Too few neurons cause microlissencephaly, too many cells cause hemimegalencephaly, and aberrant cells cause focal cortical dysplasia (FCD).
Microlissencephaly: A significant microcephaly and an aberrant sulcation are the hallmarks of microlissencephaly. The patient has severe congenital microcephaly without perinatal harm. This disease is caused by either increased apoptosis or decreased cell formation in the cerebral cortex. Microcephaly is divided into two categories, severe microlissencephaly and mild microcephaly, with a streamlined gyral layout.
The congenital anomaly known as microlissencephaly is characterized by a smooth cortical surface and a thicker cortex. A moderate malformation with insufficient sulci and normal cortical thickness is microcephaly with a simplified gyral pattern, typically an isolated anomaly.
Hemimegalencephaly: A part or the entire cerebral hemisphere may be enlarged and dysplastically overgrown in hemimegalencephaly. It happens when there are too many neuromas or when apoptosis is low. It could be an isolated anomaly or linked to syndromes like tuberous sclerosis, neurofibromatosis type I, unilateral hypomelanosis of Ito, proteus syndrome, epidermal nevus syndrome, and Klippel-Trenaunay syndrome.
Patients with hemimegalencephaly have one or more cerebral hemispheres that are moderate to noticeably enlarged. The cerebral cortex could be dysplastic or normal.
FCD: In a specific area of the cerebral cortex, FCD is a diverse set of lesions distinguished by aberrant neurons and glial cells. Usually, patients arrive with uncontrollable seizures. From minor cortical disruption without cellular abnormality to severe cases with obvious cortical dyslamination, strange massive cells, and cerebral cortex astrocytosis, FCD spans a spectrum of anomalies. Taylor FCD is characterized by balloon cells in the subcortical white matter and cortex and cytoarchitectural disorder of the cortex brought on by enormous weird, disoriented neurons. There are more cortical neurons, and they are distributed atypically. Both astrocytes and neurons share features with balloon cells.
FCD manifests as a discrete gray-white matter junction and a localized area of cortical thickening. A subcortical linear, curvilinear, radial, or funnel-shaped center of aberrant signal strength can be seen from the gray-white matter junction to the superolateral edge of the lateral ventricle. There are also abnormally enlarged or deep sulci and macrogyria.
Lissencephaly: Patients with the entire form of classic lissencephaly may have a smooth brain surface. More frequently, the partial form has a smooth surface with some gyral development along the inferior frontal and temporal lobe lobes. This anomaly is the result of the migration process being stopped. Patients either appear with seizures or developmental delay in the entire form.
The abnormality is characterized by parieto-occipital agyria with frontotemporal pachygyria in the incomplete form or full agyria in the complete form. Because the radial columns of the arrested cells are contained within the cortex, it is thick. White matter in the subcortex is thin and does not interdigitate with grey matter as it should. The parieto-occipital cortex exhibits the highest signal strength in a circumferential band on T2-weighted images, corresponding to a sparse cell zone with high water content. Due to a lack of operculization or inadequate operculization, the cerebral structure is oval or hourglass-shaped with shallow Sylvian fissures.
Conclusion
MR imaging is a crucial diagnostic technique for diagnosing cortical formation abnormalities. These illnesses' morphology, distribution, and severity must be clearly shown. Additionally, it can indicate genetic problems, linked congenital malformations, and related disorders.
