In our clinical work, we often meet parents who are anxiously inquiring about their condition with the CT reports. Because these terms are mentioned on those cranial CT reports: external hydrocephalus, subdural effusion, and extracerebral effusion, they anxiously ask the doctor: what does this mean; does it affect the child; and does it need to be treated. This short article gives you a brief introduction to these questions. What do these terms mean? External hydrocephalus and subdural fluid all fall under the category of extracerebral fluid. Simply put, an external hydrocephalus is an accumulation of fluid, blood, or pus in the space between the surface of the brain and the dura mater. They are often caused by backward brain development, head trauma, brain hemorrhage, and intracranial infection. Two separate cavities exist in the space between the brain surface and the dura mater, the subdural and subarachnoid cavities. The subarachnoid space contains a large amount of cerebrospinal fluid in circulating flow, while the subdural space contains a small amount of cerebrospinal fluid, which usually only increases in the subdural space under abnormal conditions. An increase in fluid in either of these two cavities, or both, is called an extracerebral effusion. This appears on cranial CT as a widening of the space between the brain surface and the dura mater. To find out which of these two spaces is widened, cranial magnetic resonance imaging (MRI) is a good test. There are many terms used for this condition, such as benign external hydrocephalus, external hydrocephalus, benign obstructive hydrocephalus, infantile extracerebral effusion, benign subarachnoid widening, subdural effusion, subdural hydrocephalus, and so on. If a child has a widened subarachnoid space and increased fluid, parents may notice that their head circumference is larger than normal, but there is no significant bulge in fontanelle. These children may be behind in motor development compared to their peers, such as learning to roll over or stand up later than other children. As the child grows, this lag will often disappear. However, if a hemorrhage occurs in the subarachnoid space, whether due to trauma or spontaneous bleeding, some of these children may develop hydrocephalus and may require surgery. Subdural hemorrhage due to trauma or spontaneous bleeding is the most common cause of widening of the subdural space. Children who have just started bleeding often do not experience any discomfort and therefore do not attract the attention of their parents. It is only when some of these children have an enlarged head circumference and a bulge behind the fontanelle that parents think to go to the hospital for examination. Here, the physician should draw attention to the fact that these children are likely to have bleeding cramps or reduced movement of their arms and legs. These phenomena do not usually occur at the time of bleeding. Therefore, we often encounter children who first come to the hospital with cramps and then have a cranial CT or MRI that reveals a hemorrhage in the subdural space. By following up with the parents, some are able to recall that the child had an injury not too long ago. Does it require treatment? Treatment options vary from person to person and from disease to disease. Most extracerebral effusions do not require treatment. They require regular visits to a neurosurgery specialist clinic for a follow-up cranial CT or MRI to see if the fluid has increased. However, for extracerebral fluid that tends to increase, hospitalization is often necessary for children with headaches, vomiting, bulging fontanelle, cramping or abnormal movement of the arms and legs, and even surgery is required.