External hydrocephalus is a widening of the symmetrical subarachnoid space (extracerebral space) in the frontal or frontoparietal lobe within a certain range during infancy. The normal range of the subarachnoid space in infants is 0-4 mm in width between the frontoparietal surface of the cerebral cortex and the inner plate of the skull, and 0-5 mm in width at the anterior part of the longitudinal fissure; when these two gaps exceed the normal range, it is called external hydrocephalus. The pathogenesis is not well understood, but most scholars believe that it is related to impaired absorption of arachnoid granules. (1) Perinatal factors play an important role, such as asphyxia, intracranial hemorrhage, prematurity, and neonatal hyperbilirubinemia; (2) A few of them may be related to intracranial infection and trauma; (3) In addition, some cases are related to cesarean section, but the reasons for this need to be further discussed. Infants and neonates have a wide extracerebral gap, showing a small→large→small evolutionary pattern. In neonates, there is no periaqueductal space and no cerebral sulcus, only a narrow longitudinal fissure, and in infants 2 to 12 months of age, the periaqueductal space (subarachnoid space) can be significantly widened, most significantly at 2 to 6 months of age, and by about 2 years of age, the periaqueductal space and cerebral sulcus disappear in most infants, with only a narrow longitudinal fissure. In addition, with the myelination of cerebral white matter, the water content of brain tissue decreases sharply from birth to 6 months of age and the brain volume shrinks; then the nerve cells proliferate again and the brain volume increases, which may also be a reason for the enlargement of the extracerebral space and its subsequent gradual reduction. This is a physiological change. Therefore, it is wrong to diagnose external hydrocephalus as a widening of the subarachnoid space (extracerebral space) in the frontal or parietal lobes of infants and children within the normal range. External hydrocephalus (EH) is diagnosed in infants and newborns when only the symmetrical subarachnoid space (extracerebral space) in the frontal or parietal lobes exceeds the normal physiologic range and the subarachnoid space in other areas is not wide or slightly wide. The criteria for determining abnormal widening of the subarachnoid space are: the width between the frontoparietal surface of the cerebral cortex and the inner plate of the skull is greater than 5 mm, and the width of the anterior longitudinal fissure is greater than 6 mm. EH. The prognosis of idiopathic external hydrocephalus is generally good, and most of the hydrocephalus can be reduced after 2-3 months and can be completely absorbed by 2-3 years of age, which is a benign self-healing disease. However, for those who cannot find the cause but have clinical neurological symptoms or developmental delays in motor and speech, early consultation with a neurological rehabilitation specialist is indicated. Secondary EH may vary depending on the cause and clinical presentation. If the damage is severe or the imaging suggests cerebral atrophy, the infant may develop neurodevelopmental abnormalities such as psychomotor retardation, transient convulsive seizures, behavioral problems, mood disorders, or even cerebral palsy after birth from 2 months to 2 years of age, so aggressive treatment is necessary for more severe external hydrocephalus.