Subdural Effusion: Leakage and accumulation of CEREBROSPINAL FLUID in the subdural space which may be associated with an infectious process; CRANIOCEREBRAL TRAUMA; BRAIN NEOPLASMS; INTRACRANIAL HYPOTENSION; and other conditions.Subdural Space: Potential cavity which separates the ARACHNOID MATER from the DURA MATER.Hematoma, Subdural: Accumulation of blood in the SUBDURAL SPACE between the DURA MATER and the arachnoidal layer of the MENINGES. This condition primarily occurs over the surface of a CEREBRAL HEMISPHERE, but may develop in the spinal canal (HEMATOMA, SUBDURAL, SPINAL). Subdural hematoma can be classified as the acute or the chronic form, with immediate or delayed symptom onset, respectively. Symptoms may include loss of consciousness, severe HEADACHE, and deteriorating mental status.Empyema, Subdural: An intracranial or rarely intraspinal suppurative process invading the space between the inner surface of the DURA MATER and the outer surface of the ARACHNOID.Intracranial Hypotension: Reduction of CEREBROSPINAL FLUID pressure characterized clinically by HEADACHE which is maximal in an upright posture and occasionally by an abducens nerve palsy (see ABDUCENS NERVE DISEASES), neck stiffness, hearing loss (see DEAFNESS); NAUSEA; and other symptoms. This condition may be spontaneous or secondary to SPINAL PUNCTURE; NEUROSURGICAL PROCEDURES; DEHYDRATION; UREMIA; trauma (see also CRANIOCEREBRAL TRAUMA); and other processes. Chronic hypotension may be associated with subdural hematomas (see HEMATOMA, SUBDURAL) or hygromas. (From Semin Neurol 1996 Mar;16(1):5-10; Adams et al., Principles of Neurology, 6th ed, pp637-8)Hematoma, Subdural, Chronic: Accumulation of blood in the SUBDURAL SPACE with delayed onset of neurological symptoms. Symptoms may include loss of consciousness, severe HEADACHE, and deteriorating mental status.Pleural Effusion: Presence of fluid in the pleural cavity resulting from excessive transudation or exudation from the pleural surfaces. It is a sign of disease and not a diagnosis in itself.Hematoma, Subdural, Acute: Accumulation of blood in the SUBDURAL SPACE with acute onset of neurological symptoms. Symptoms may include loss of consciousness, severe HEADACHE, and deteriorating mental status.Pericardial Effusion: Fluid accumulation within the PERICARDIUM. Serous effusions are associated with pericardial diseases. Hemopericardium is associated with trauma. Lipid-containing effusion (chylopericardium) results from leakage of THORACIC DUCT. Severe cases can lead to CARDIAC TAMPONADE.Pleural Effusion, Malignant: Presence of fluid in the PLEURAL CAVITY as a complication of malignant disease. Malignant pleural effusions often contain actual malignant cells.Hematoma, Subdural, Spinal: Subdural hematoma of the SPINAL CANAL.Otitis Media with Effusion: Inflammation of the middle ear with a clear pale yellow-colored transudate.