Subarachnoid widening is not a scary condition. The subarachnoid space refers to the space between the arachnoid membrane and the soft meninges, and widening of the subarachnoid space is usually an imaging description, usually when the patient has a CT or MRI of the head. If the widening is not severe, the patient will not have symptoms. The main cause of subarachnoid widening may be brain atrophy or subdural effusion due to head trauma, and some arachnoid cysts may also cause subarachnoid widening, which is usually asymptomatic. If the subarachnoid space is widened to a greater extent and there is a certain amount of pressure on the cerebral cortex, the patient may have certain symptoms and should be treated by surgery.