Hydrocephalus is a series of symptoms that occur when excessive cerebrospinal fluid collects in the ventricular system, causing it to dilate and compress brain tissue. There are many reasons for hydrocephalus, but from a pathological point of view, there are only three causes: (1) excessive secretion of cerebrospinal fluid (secretion is greater than absorption), such as choroid plexus papilloma, which causes an abnormal increase in cerebrospinal fluid secretion; (2) impaired absorption of cerebrospinal fluid (imbalance of secretion and absorption due to normal secretion and absorption), which causes excessive cerebrospinal fluid to collect in the ventricles and form hydrocephalus; (3) obstruction or narrowing of the cerebrospinal fluid circulation pathway (such as congenital midbrain aqueduct stenosis). (3) obstruction or stenosis of the cerebrospinal fluid circulation pathway (e.g., congenital midbrain aqueduct stenosis is most common). Etiologically, hydrocephalus can be caused by congenital abnormalities, trauma, tumors, infections, intracranial hemorrhage, and many other causes. It is also referred to as external hydrocephalus, which refers to the accumulation of cerebrospinal fluid in the subdural space, resulting in the widening of the extracerebral space, while the ventricular system is normal, so the standard diagnosis should be “subdural fluid”. （Therefore, the standard diagnosis should be “subdural effusion” (commonly known as “external hydrocephalus” or “external hydrocephalus”), which shows that there is a fundamental difference between hydrocephalus and the so-called “external hydrocephalus”, and there is also a world of difference in treatment. Hydrocephalus should be seen in neurosurgery, while “external hydrocephalus” should be seen in neurology. Most hydrocephalus requires surgery, while most external hydrocephalus does not require surgery. Many patients and even many physicians have misconceptions about the concepts of hydrocephalus and external hydrocephalus. In clinical practice, we encounter a large number of children in neurosurgery clinics who present with hydrocephalus, but in fact it is all subdural fluid (extracerebral fluid). Because of this misconception, children are first registered with neurosurgery for hydrocephalus, but then they are referred to neurology for subdural effusion, which wastes valuable consultation time. Therefore, we remind all patients that when the doctor diagnoses your child as “hydrocephalus” and recommends that you take your child to a specialized clinic, you must ask whether it is hydrocephalus or subdural effusion (external hydrocephalus). You should see a neurologist! One more kind word: In medicine, “neurology” usually means neurology, not neurosurgery! We encounter too many patients in the clinic who should see a neurologist but are mistakenly registered as neurosurgeons!