医学院神经解剖学教案脑脊髓被膜脑脊液循环及相关临床问题

Clinical Correlations The dura may become partly calcified or even ossified with age. In some cases, often in association with longstanding hydrocephalus, the falx is fenestrated. Several types of herniation of the brain can occur. The tentorium separates the supratentorial and the infratentorial compartments, and the two spaces communicate by way of the incisura that contains the midbrain. Both the falx and the tentorium form incomplete separations, and a mass or expanding lesion may displace a portion of the brain around these septa, resulting in either a subfalcial or a transtentorial herniation.,The latter type may be downward (uncal, or caudal transtentorial, herniation) or upward (rostral transtentorial herniation). The herniation of the cerebellar tonsils into the foramen magnum by a lesion is often called coning. Transtentorial herniations, especially the caudal type, are potentially lifethreatening because they can distort or compress the brain stem and damage its vital regulatory centers for respiration, consciousness, blood pressure, and other functions (see Chapters 18 and 20).,Clinical Correlations Blocking the circulatory pathway of cerebrospinal fluid usually leads to dilatation of the ventricles upstream (hydrocephalus), because the production of fluid usually continues despite the obstruction (Figs 11-10 to 11-14 and Table 11-2). There are two types of hydrocephalus: noncommunicating and communicating.,In noncommunicating (obstructive) hydrocephalus, which occurs more frequently than the other type, the cerebrospinal fluid of the ventricles cannot reach the subarachnoid space because there is obstruction of one or both interventricular foramina, the cerebral aqueduct (the most common site of obstruction, Figs 11-9 and 11-10), or the outflow foramina of the fourth ventricle (median and lateral apertures).,A block at any of these sites leads rapidly to dilatation of one or more ventricles. The production of cerebrospinal fluid continues, and in the acute obstruction phase, there may be a transependymal flow of cerebrospinal fluid. The gyri are flattened against the inside of the skull. If the skull is still pliable, as it is in most children under 2 years of age, the head may enlarge (see Fig 11-11).,In communicating hydrocephalus, the obstruction is in the subarachnoid space and can be the result of prior bleeding or meningitis, which caused thickening of the arachnoid with a resultant block of the return-flow channels (see Fig 11-14). If the intracranial pressure is raised because of excess cerebrospinal fluid (more production than reabsorption), the central canal of the spinal cord may dilate.,In some patients, the spaces filled by cerebrospinal fluid are uniformly enlarged without an increase in intracranial pressure. This normal-pressure hydrocephalus may be accompanied by a gait disorder, incontinence, and dementia in the elderly.,Various procedures have been devel