Diagnostic method for differentiating external hydrocephalus from simple subdural hygroma

Object. The various terms used to describe subdural fluid collection - "external hydrocephalus," "subdural hygroma," "subdural effusion," "benign subdural collection," and "extraventricular obstructive hydrocephalus" - reflect the confusion surrounding the diagnoses of these diseases. Differentiating external hydrocephalus from simple subdural hygroma may be difficult, but the former appears to be a distinct clinical entity separate from the latter. In this report, the authors present a diagnostic method for differentiating external hydrocephalus from simple subdural hygroma, based on their clinical experience in treating subdural fluid collection after mild head trauma. Methods. Twenty patients with subdural fluid collection after mild head trauma were included in this study. Ventricle size was measured using a modified frontal horn index (mFHI); that is, the largest width of the frontal horns divided by the bicortical distance in the same plane, instead of the inner table distance. Bur hole trephination was performed on the appearance of a subdural fluid collection thicker than 15 mm on computed tomography (CT), persistent (longer than 4 weeks) or increasing in size, and accompanied by neurological symptoms (confusion or memory impairment). During the procedure, subdural pressure was measured using a manometer before opening the dura mater. Subdural pressure varied among the patients, ranging from 3 to 27.5 cm H₂O. Four patients with a subdural pressure greater than 15 cm H₂O had hydrocephalus after surgery (p < 0.05). Hydrocephalus developed in a pediatric patient (2 years old) with a subdural pressure of 12 cm H ₂O. All of the patients in whom hydrocephalus developed after bur hole trephination had had enlarged ventricles (mFHI ≥ 33%) on preoperative CT scans. Conclusions. Monitoring subdural pressure may be a valuable tool for differentiating subdural hygroma from external hydrocephalus in patients with mild head trauma. Additionally, the mFHI reflects the nature of the subdural collection more accurately than the standard frontal horn index.