Subdural hygroma

The subdural space is a virtual one, so any collection within it is considered as abnormal.A subdural hygroma is a subdural collection of cerebrospinal fluid (CSF), without blood. Most subdural hygromas are believed to be derived from chronic subdural hematomas. It is commonly seen in elderly after minor trauma but can be seen in children after an infection. One of the common causes of subdural hygroma is sudden decrease in pressure by ventricular shunting. This can lead to leak of CSF into the subdural space especially in cases with moderate to severe brain atrophy. In these cases the symptoms such as mild fever, headache, drowsiness and confusion can be seen, which are relieved by draining this subdural fluid.

Most subdural hygromas are small and clinically insignificant. Larger hygromas may cause secondary localized mass effects on the adjacent brain parenchyma, enough to cause a neurologic deficit or other symptoms. Acute subdural hygromas can be a potential neurosurgical emergency, requiring decompression. Acute hygromas are typically a result of head trauma -- they're a relatively common posttraumatic lesion -- but can also develop following neurosurgical procedures, and have also been associated with a variety of conditions, including dehydration in the elderly, lymphoma and connective tissue diseases. It's not uncommon on head CT reports for chronic subdural hematomas (SDHs) to be misinterpreted as subdural hygromas, and vice versa. Magnetic resonance imaging (MRI) should be done to differentiate a chronic SDH from a subdural hygroma, when clinically warranted. Elderly patients with marked cerebral atrophy, and secondary widened subarachnoid CSF spaces, can also cause confusion on CT. To distinguish chronic subdural hygromas from simple brain atrophy and CSF space expansion, a gadolinium- enhanced MRI can be performed. In the majority of cases, if there hasn't been any acute trauma or severe neurologic symptoms, a small subdural hygroma on the head CT scan will be an incidental finding. If there's an associated localized mass effect that may explain the clinical symptoms, or concern for a potential chronic SDH that could rebleed, then an MRI, with or without neurologic consultation, may be useful. When in doubt, call the radiologist who reported the CT scan, and ask his/her opinion. MM (References: Radiology: Diagnosis -- Imaging -- Intervention 2000; ch. 37: 9-13., Brain Inj 1998 Jul;12(7):595-603.)