Extrapyramidal effusion (or external hydrocephalus) is defined as a widening of the subarachnoid space on cranial imaging. The subarachnoid space on the brain surface can be as wide as 4 mm, the longitudinal fissure pool 6 mm, and the lateral fissure pool 10 mm in normal infants under 1 year of age, and gradually normalizes after 1 year of age. However, if the lumen is significantly widened, the possibility of brain hypoplasia is considered. On the question of whether to treat or not: Generally speaking, if there is no abnormality in the perinatal period and the child has normal cognitive and motor development, simple mild extracerebral fluid can be treated without the need for dynamic follow-up of development; however, if the development is abnormal, treatment is needed regardless of the amount of fluid accumulation. Treatment is mainly for functional abnormalities (e.g. developmental delay, cognitive impairment, etc.), not for methods to reduce or absorb fluid, so frequent review of cranial CT or MRI is not required. Prognosis: Most mild external hydrocephalus can be self-absorbed, but 30-40% of patients may have motor developmental delay, specific motor skill impairment, language developmental delay, learning difficulties, behavioral disorders ( However, 30-40% of patients may have motor delay, specific motor skill impairment, language development delay, learning difficulties, behavioral disorders (e.g. hyperactivity, difficulty concentrating, impulsivity, disobedience, etc.), and only a few may be left with severe neurological sequelae such as paralysis, epilepsy, and mental retardation. Differential diagnosis: It should be distinguished from cerebral atrophy, hydrocephalus and subdural effusion: 1. The head circumference of children with cerebral atrophy is not large or smaller, and on cranial CT or MRI, the cerebral sulcus is generally deepened and widened throughout the brain, sometimes the cerebellar sulcus is also deepened and the ventricles are enlarged. Previous history of disease, such as encephalitis, cerebral hemorrhage, hypoxia and other causes of brain atrophy. 2.Subdural effusion is mostly caused by meningitis and trauma. CT or MRI scan of the head shows that subdural effusion is not accompanied by enlargement of the basal pool and widening of the anterior longitudinal fissure, but mostly accompanied by ventricular compression, and the medial margin of the enlarged cavity is smoother and more asymmetric on the left and right sides. 3, hydrocephalus is mainly manifested as ventricular dilatation, increased intracranial pressure, ocular manifestation of sunset sign and cerebral nerve damage, etc.