Various terms are used to describe subdural fluid collection, such as external hydrocephalus, subdural hygroma or effusion, benign subdural collection and extraventricular obstructive hydrocephalus. These confused terms reflect the confusion surrounding these diagnoses. Differentiation external hydrocephalus from simple subdural hygroma may be difficult, but the former appears to be a distinct clinical entity separable from the latter. In this report, we present a differential diagnostic method for differentiating external hydrocephalus from simple subdural hygroma, based on our clinical experience treating subdural fluid collection after mild head trauma.

Twenty patients with subdural fluid collection after mild head trauma were included in this study. The ventricle size was measured using a modified frontal horn index (mFHI), where the largest width of the frontal horns was divided by the bicortical distance in the same plane instead of the inner table distance Indications for surgery were the appearance of a subdural fluid collection of greater than 15 mm thickness on CT, persiting for more than 4 weeks or increasing in size, with accompanying neurological symptoms (confusion or memory impairment). During the procedure, subdural pressure was measured by manometer before opening the dura.

The subdural pressure was variable, ranging from 3 to 27.5 mmH₂O. Five patients with a subdural pressure over 15 cmH₂O were developed hydrocephalus after the burr hole trephination (p<0.05). All the patients who developed hydrocephalus after burr hole trephination, showed enlarged ventricles (mFHI more than 33%) on preoperative CT imaging.

Monitoring the subdural pressure may be a valuable tool to differentiate between subdural hygroma and external hydrocephalus in patients with mild head trauma. Additionally, the modified frontal horn index reflected the nature of the subdural collection more accurately than the standard frontal horn index.