CSF leakage can result from a number of underlying pathologies. Such leaks can broadly be categorized into acquired and congenital.
Congenital cerebrospinal fluid leak
Congenital causes can be due to defects in the otic capsule itself, abnormal patency of pathways associated with the otic capsule, and defects distant from the otic capsule. Although congenital sources are more common in children, they can occur in people of any age and can even be observed in the geriatric population.
Acquired cerebrospinal fluid leak
Acquired leaks result from temporal bone trauma, surgery, or infectious or neoplastic causes. Acquired spinal fluid leakage is far more common than congenital leakage. Postoperative leakage following surgery is probably the most common cause of acquired CSF leaks. It is a recognized complication of acoustic neuroma removal and other skull base surgery. These leaks are usually evident in the early postoperative period. They rarely occur more than 2 months postoperatively.
Mastoid surgery for chronic ear disease is also a potential cause of an acquired CSF leak. When the dura is violated intraoperatively, the defect should be repaired immediately if possible. Frequently, however, the dura is not injured, but a defect is left in the bony plate of the tegmen. Over the years, the continuous pulsations of the CSF cause the dura to thin, allowing the arachnoid or brain to prolapse through this defect. This dura may become thin and spontaneously rupture, resulting in a leak of CSF many years after the initial surgery. In addition to dura, portions of brain tissue may also prolapse through the defect, resulting in an encephalocele.
Even in the absence of surgical intervention, similar problems may result from middle ear disease, most notably cholesteatoma. Cholesteatoma may erode the tegmen plate and allow herniation of dura or brain to occur over time.
Spontaneous cerebrospinal fluid leak
Spontaneous leakage is leakage that occurs without an obvious antecedent pathology. Such leakage is usually due to some congenital defect in the temporal bone. Understandably, such defects are usually evident in childhood.
Spontaneous CSF leakage may result from a number of congenital deformities of the ear. Mondini deformity, an abnormal development of the cochlea, is frequently associated with CSF leakage. Such patients often have a patency at the lateral aspect of their internal auditory canal, allowing direct movement of CSF into the inner ear. A second defect, usually of the annular ring of the stapes footplate, then results in drainage of CSF into the middle ear. This usually causes loss of the remainder of hearing. A similar presentation may be observed in a patient with a widely patent cochlear or vestibular aqueduct.
Other deformities of the inner ear are also associated with spontaneous CSF leak. CSF can leak through a patent Hyrtl fissure, a congenital fusion plane found between the otic capsule and the jugular bulb (also termed the tympanomeningeal fissure). Such leakage may not be associated with any inner ear abnormalities, and the patient may have no evidence of a sensorineural hearing loss. Similar presentations may be observed in an abnormal patency of the petromastoid canal, which normally carries the subarcuate artery. Such patients may present with recurrent bouts of meningitis associated with what appears to be a unilateral serous otitis. Frequently, the true nature of the problem is not realized until a myringotomy is performed, at which time an unceasing flow of clear fluid is returned.
Although spontaneous CSF leakage is more common in children, reports of it occurring in people of greater age are increasing. In adults, spontaneous CSF otorrhea is often associated with a defect in the bone separating the middle ear space and the cranial vault. Such bony dehiscences are most commonly found on the floor of the middle fossa, along the tegmen plate. Tegmen dehiscences are found in up to 30% of autopsy specimens.
Continuous pressure of the CSF over the years results in formation of a meningocele or encephalocele. Defects in the middle fossa are associated with 88% of spontaneous leaks in adults. The remainder of spontaneous leaks are due to posterior fossa defects and arachnoid granulations, normal structures that absorb CSF. These structures are usually associated with the venous sinuses of the dura mater, but they have been found within the temporal bone on some autopsy specimens. Spontaneous leaks have recently been shown to be often associated with increased intracranial pressure. This is often manifested by a partially empty sella on the MRI scan
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