HGM can exist alone or in combination with other brain malformations. Although this term has been officially used for more than a century, it was only discovered and recognized clinically after the introduction of CT and MR. Normal brain development is a complex process, which includes neural tube formation, primitive cell differentiation and proliferation, neuronal migration and myelin formation. During embryonic development, harmful factors such as X-rays, poisoning, ischemia and hypoxia can lead to impaired neuronal migration. If adult neuronal cells fail to migrate to the surface of the cerebral cortex in time and gather in abnormal parts of the brain, such as the deep white matter area or subventricular canal, gray matter heterotopia is formed. Internationally, gray matter heterotopia is divided into three types according to MRI characteristics and clinical manifestations: 1) ventricular canalicular heterotopia, also called nodal heterotopia; 2) subcortical heterotopia; 3) band heterotopia, also called double cortical syndrome. The disease is clinically characterized by 3 basic features: 1) recurrent and frequent seizures, most patients have generalized tonic seizures, which are difficult to control with medication; 2) mental retardation; 3) motor system impairment, such as hemiparesis; epilepsy is the most common clinical symptom, and about 80% of patients with gray matter heterotopia have seizures. The most common seizure types are generalized tonic-clonic and complex partial seizures, ranging from mild to severe and even refractory epilepsy; the first seizure can occur at several years of age, but more commonly in adolescence, and seizures are generally common after the age of 20; this type of gray matter heterotopia is mostly seen in females, while males are more often fetal This type of gray matter ectopic is more common in females, while males are more likely to die in utero. Subcortical heterotopias: Subcortical heterotopias are rare in clinical practice, and focal subcortical heterotopias present with a variety of motor and intellectual impairments, the extent of which is consistent with the extent of the damaged cortex. Patients with bilateral, large and thick subcortical heterotopias exhibit moderate to severe mental developmental delays and motor deficits, whereas extensive unilateral heterotopias present only with hemiparesis and mild mental retardation. Motor function and development can be completely normal in small or very thin subcortical heterotopias. 5. Banded ectopia: It is commonly believed that banded ectopia is a subtype of anencephalic gyrus malformation, and almost all patients present with seizures within 10 years of age; patients have cognitive and neurological disability associated with the thickness of the ectopic neuronal layer, and the thicker the ectopic neuronal band, the greater the cognitive and neurological disability. Seizures begin as partial or generalized seizures with atypical aphasic seizures and progress to generalized tonic-clonic and atonic seizures with falls. MRI can clearly show the morphology and location of gray matter ectopia, and MRI has a high degree of gray matter resolution, so MRI is the imaging test of choice to confirm the diagnosis of gray matter ectopia. 6, gray matter location abnormal nodular type: ectopic foci are nodular in shape, manifested as nodular ectopic foci of different sizes close to the surface of the lateral ventricles or protruding into the lateral ventricles; these ectopic foci can be single or multiple, distributed around one or both lateral ventricles. Subcortical type: The ectopic gray matter is connected to the cortex and extends excessively to the white matter area in the form of irregular masses, and a few are isolated nodules located in the white matter area, which are called small islands of gray matter. The subcortical edges are irregular with varying gray matter signals, the surrounding white matter signals are normal, the affected cortex is thinned, the cerebral sulcus is reduced or disappeared, and the cerebral hemisphere on the side of the lesion may be reduced in size due to the reduction of white matter, often accompanied by the corpus callosum and brainstem dysplasia. Banded type: A wide band of ectopic gray matter surrounding the cortex, separated from the cortex by a thin layer of white matter, the affected cortex is mildly thickened or normal, also called “double cortex” in the past, most of them are diffusely distributed, but some are limited to the frontal or parietal regions. 7, gray matter morphological abnormalities Gray matter ectopic foci vary in morphology, size, no specific pattern, gray matter ectopic foci can be irregular-shaped masses, nodular, pagoda, coral and ribbon-like; the large can occupy a large part of the cerebral hemisphere, the small less than 1 cm; resulting in this size disparity and no specific morphological changes may be related to the degree and time of neuronal migration disorders. 8. Ectopic gray matter signal Ectopic gray matter foci, regardless of their occurrence in any part of the brain, regardless of their size and morphological changes, and regardless of the MR scan sequence used, always have the same signal intensity as normal gray matter, even in enhanced scans, which is one of the most important signs for the diagnosis of cerebral gray matter ectopia by MRI, and it is also an important basis for differentiation from nodular sclerosis and brain tumors. 9.Characteristics of HGM in MRI 1) The location of gray matter is abnormal; 2) The morphology can be coherent, free or isolated; 3) The signal intensity of the lesion is the same as normal gray matter in T1- and T2-weighted phase and proton-weighted phase, especially in T2-weighted phase; 4) The morphology of gray matter heterotopia in the same patient can be in the form of small pieces or large masses, and can be limited to a single place or multiple places. It can be limited to a single location or multiple locations. It is worth noting that HGM should be differentiated from nodular sclerosis on MRI. In the latter, nodular foci are mostly seen in the cortex, subcortex, white matter and subventricular canal, and due to calcium deposition and gliosis, the nodular foci show high signal in the proton density-weighted phase, and calcium low signal in the center and ring-like high signal in the periphery in the T2-weighted phase. In addition, care should be taken to differentiate it from tumor dissemination to the subventricular zone, where tumor-spread lesions have a different signal intensity than gray matter and often have an enhancing effect. Currently, seizures are mainly controlled by medications, and surgical treatment can be considered at a later stage. Seizures should be treated with single or combined application of antiepileptic drugs according to the clinical seizure type. 1) Drug therapy In the early stage of the disease, seizures can be partially or completely controlled by drug therapy in most patients. Most of the seizures caused by gray matter heterotaxy are focal epilepsy, and carbamazepine is usually the first choice. Seizures should be treated with single or combined antiepileptic drugs depending on the clinical seizure type. 2) Surgical treatment Whether the relationship between gray matter ectasia and epilepsy is causal or not has been debated so far. As research progresses, more and more scholars believe that gray matter heterotopia may be the primary lesion of epilepsy. Gray matter ectopic epilepsy has become a common cause of refractory epilepsy, and surgery for gray matter ectopic refractory epilepsy requires the removal of epileptogenic ectopic gray matter masses while protecting the normal cortex, especially the functional cerebral cortex, to the greatest extent possible. The surgery can be performed with navigation guidance to locate the optimal entry point to reach the lesion, to clarify the extent of resection, to reduce collateral damage, to be minimally invasive and safe, and with repeated tracing of cortical electrodes to remove the imaging-visible epileptic pathology while maximizing the removal of the epileptogenic foci to maximize clinical efficacy.